Apparatus and method

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 18166955.7 which was filed on April 12, 2018 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an apparatus comprising an electrostatic clamp, and a method of its operation. More particularly, but not exclusively, the apparatus may comprise a lithographic tool, the electrostatic clamp being configured to clamp a component such as a patterning device during lithographic patterning.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask, or reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A lithographic apparatus may typically use high-voltage electrostatic clamps in order to clamp patterning devices, for example, during patterning operations. Electrostatic clamps and patterning devices are often maintained in a low pressure hydrogen rich environment. This environment is non- conductive. It will be understood, therefore, that electric charge can build up on dielectric or ungrounded surfaces. For example, during operation, electric charge can accumulate on dielectric or ungrounded surfaces by touching parts (e.g. mask clamping) or by particle collisions during gas flow.

[0006] It will, also be understood that EUV radiation may cause the hydrogen rich environment to become conductive due to the creation of an EUV-induced hydrogen plasma. Free charges generated within the EUV-induced hydrogen plasma may be attracted to (or repelled by) electric fields generated by the electrostatic clamp. On the other hand, in the absence of the EUV-induced plasma, or in regions distant or well screened from any EUV-induced plasma, electric charges may accumulate on dielectric or ungrounded surfaces, and may remain after any electric field has been removed.

[0007] In addition to the accumulation of electric charges, very strong electrostatic fields (e.g. in the region of -1-100 kV/cm) may be generated between parts of the electrostatic clamp and other system components. In particular, the high voltages applied to electrodes of the electrostatic clamps result in nearby conductors (e.g. conductive coatings which may be present on surfaces of the mask) being polarised. As such, strong electrostatic fields are developed, especially at sharp features (e.g. edges of conductive mask coatings). The voltages applied to the electrostatic clamp electrodes may be frequently switched in polarity to avoid breakdown within the insulation of the electrostatic clamp. During such transitions, the electrostatic fields in the regions around the clamps may change rapidly.

SUMMARY

[0008] It is an object of the present invention to obviate or mitigate one or more problems associated with the accumulation of charge within a lithographic apparatus, and/or the generation and switching of electrostatic fields within a lithographic apparatus.

[0009] According to a first aspect of the invention there is provided an apparatus comprising an electrostatic clamp for clamping a component, and a mechanism for generating free charges adjacent to the electrostatic clamp. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp during a transition from a first energisation state of the electrostatic clamp to a second energisation state of the electrostatic clamp.

[00010] By providing free charges (e.g. via EUV induced ¾ plasma) around the electrostatic clamp during repolarisation, a relatively conductive medium will be provided in the vicinity of the clamp and any clamped patterning device. As such, as the clamp is repolarised, the abundance of free charges will effectively screen any external fields generated by the clamp, and in particular any external fields generated by regions of the clamp which extend beyond the clamped patterning device (e.g. leads and contacts), thereby reducing the likelihood of particles being released from surfaces these regions of the clamp.

[00011] It will be appreciated that, in use, when a component is clamped by the electrostatic clamp, the component will obscure parts of the electrostatic clamp from the generated free charges. Thus, while the mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp, such charges will, in use, typically be prevented from reaching regions of the clamp which are directly obscured by the component being clamped. That is, in use, generated free charges will provide screening to regions of the clamp that are not contributing to the clamping force. Indeed, it will be understood that, during clamping, the clamped component will be clamped by a field generated between the clamp and the component, and that the free charges which are present in the vicinity of the clamp and the clamped component will not interfere with this clamping effect. Rather, the free charges will not typically extend into the region between the clamp and the clamped components (which may, for example, in places have a maximum separation of around 10 pm, and in other places be in direct contact). [00012] The electrostatic clamp may comprise a clamping region configured to clamp said component. When a component is clamped, a clamping electric field may be generated between said clamping region and said component.

[00013] The electrostatic clamp may further comprise a non-clamping region. When a component is clamped by the clamping region, a secondary electric field may be generated around said non clamping region.

[00014] For example, in the absence of a grounded conductive medium around the non-clamping region, a secondary electric field may be generated between the non-clamping region and one or more of a portion of said apparatus and/or a portion of said clamped component.

[00015] The clamp may comprise a first region which is configured to support a patterning device, and a second region which is not configured to support a patterning device. The first region may comprise one or more clamping electrodes. The second region may comprise one or more secondary electrodes. Each of the secondary electrodes may correspond to a respective one of the clamping electrodes. The second region may comprise a plurality of non-contiguous sub-regions. For example, the second region may comprise protrusions which extend on either side of the first region.

[00016] Each of said (clamping and secondary) electrodes may be coated with a dielectric material. The dielectric material may have a thickness of around 100 pm.

[00017] The first region may comprise a clamping region. The second region may comprise a non clamping region. It will be appreciated, of course, that in some embodiments, a clamped component may have a size which is substantially similar to the size of the first region. However, in alternative embodiments, a clamped component may have a size which is smaller than the first region such that, when clamped, some portions of the first region are covered by the clamped component, while other portions of the first region are not covered by the clamped component. In such an arrangement, the uncovered portions of the first region may be considered to comprise a non-clamping region.

[00018] The apparatus may comprise a mechanism for generating free charges adjacent to the non clamping region. The mechanism for generating free charges may be configured to generate free charges adjacent to said non-clamping region during a transition from said first energisation state of the electrostatic clamp to said second energisation state of the electrostatic clamp.

[00019] The electrostatic clamp comprises at least one electrode, wherein, when a component is clamped by the electrostatic clamp, a clamping voltage is applied to the at least one electrode such that the clamping electric field is generated between said clamping region and said component.

[00020] The first region may comprise a plurality of clamping electrodes. In said first energisation state of the electrostatic clamp, a first clamping voltage may be applied to a first one of said plurality of clamping electrodes, and a second clamping voltage may be applied to a second one of said plurality of clamping electrodes. The first and second clamping voltages may have opposite polarities.

[00021] The apparatus may further comprise a voltage source. The voltage source may be configured to supply said clamping voltage. The clamping voltage may, for example, be a voltage of around plus or minus 1 to 10 kV. The clamping voltage may, for example, be a voltage of around plus or minus 2 kV.

[00022] The electrostatic clamp may further comprise at least one contact configured to provide an electrical connection to said at least one electrode. The mechanism for generating free charges may be configured to generate free charges adjacent to said at least one contact during said transition from said first energisation state of the electrostatic clamp to said second energisation state of the electrostatic clamp.

[00023] In the first energisation state, a voltage having a first polarity may be applied to the at least one electrode. In the second energisation state, a voltage having a second polarity opposite to the first polarity may be applied to the at least one electrode.

[00024] The voltage source may be configured to supply said voltage having a first polarity and/or said voltage having said second polarity.

[00025] The electrostatic clamp may comprise at least two electrodes. In the first energisation state, a voltage having the first polarity may be applied to a first one of the electrodes and voltage having the second polarity may be applied to a second one of the electrodes. In the second energisation state, a voltage having the second polarity may be applied to the first one of the electrodes and voltage having the first polarity may be applied to the second one of the electrodes.

[00026] The electrostatic clamp may further comprise at least two secondary electrodes. In the first energisation state, said voltage having the first polarity may be applied to a first one of the secondary electrodes and said voltage having the second polarity may be applied to a second one of the secondary electrodes. In the second energisation state said voltage having the second polarity may be applied to said first one of the secondary electrodes and said voltage having the first polarity may be applied to said second one of the secondary electrodes.

[00027] The first one of the electrodes may be electrically connected to the first one of the secondary electrodes. The second one of the electrodes may be electrically connected to the second one of the secondary electrodes.

[00028] The clamp may be configured such that, in each of the first and second energisation states, a component can be clamped by the electrostatic clamp.

[00029] The mechanism for generating free charges adjacent to the electrostatic clamp may comprise a source of gas, and a source of ionising radiation configured to ionise gas provided by the source of gas.

[00030] The source of ionising radiation may comprise a source selected from the group consisting of: an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[00031] There may also be also provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus may comprise an apparatus according to the first aspect of the invention. The patterning device may comprise said component to be clamped. [00032] The lithographic apparatus may further comprise an illumination system configured to condition a radiation beam. The electrostatic clamp may be configured to clamp said patterning device. The patterning device may be capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam. The lithographic apparatus may further comprise a substrate table constructed to hold a substrate. The lithographic apparatus may further comprise a projection system configured to project the patterned radiation beam onto the substrate.

[00033] The lithographic apparatus may be configured to perform a plurality of imaging exposures during which the radiation beam is incident upon the patterning device, and during which the patterned radiation beam is projected onto the substrate. The electrostatic clamp may be configured to clamp said patterning device during said imaging exposures. Between consecutive ones of said plurality of imaging exposures the electrostatic clamp may be configured to transition from said first energisation state to said second energisation state.

[00034] Each imaging exposure may comprise exposures of a plurality of dies on a wafer. The clamp may be re -polarised during a period between the exposure of a first wafer and a second wafer.

[00035] There may also be provided a lithographic system comprising said lithographic apparatus.

[00036] The lithographic system may further comprise a radiation source configured to generate said radiation beam. The mechanism for generating free charges may comprise a secondary source of ionising radiation selected from the group consisting of: an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[00037] The lithographic system may further comprise a radiation source configured to generate said radiation beam, wherein the mechanism for generating free charges comprises said radiation source. Said radiation source may be an EUV source.

[00038] The lithographic system may be further configured to perform at least one non-imaging exposure during which the radiation beam is incident upon the patterning device and during which no radiation is projected onto the substrate; said non-imaging exposure being performed between consecutive ones of said plurality of imaging exposures.

[00039] During a series of imaging exposures, the lithographic apparatus may be further configured to perform a non-imaging exposure between each consecutive imaging exposure.

[00040] Said transition from said first energisation state of the electrostatic clamp to said second energisation state of the electrostatic clamp may be performed during said non-imaging exposure.

[00041] By providing the radiation beam at the patterning device during a non-imaging exposure, it is possible to provide a source of free charges, by virtue of a plasma which will be created by ionization of gas molecules (e.g. Hydrogen) which are present around the patterning device. The plasma will be generated both in regions which are directly illuminated by the radiation beam, and in adjacent regions (e.g. due to diffusion, and secondary electrons). In this way, a single radiation beam (e.g. an EUV radiation beam) can be used both for imaging purposes, and to provide a source of free charges during re-polarization of the electrostatic clamp. [00042] The lithographic system may be controlled such that amount of radiation incident upon the patterning device is greater during each imaging exposure, than during said non-imaging exposure.

[00043] It will be understood that the amount of radiation incident upon the patterning device may be controlled in a number of ways. For example the amount of radiation may be controlled by varying the intensity of radiation generated by the radiation source. Alternatively, or additionally, the amount of radiation may be controlled by varying the spatial extent of the radiation beam incident upon the patterning device (e.g. by using shutters or masking blades). Alternatively, or additionally, the amount of radiation may be controlled by varying the number or frequency of pulses of radiation generated by the radiation source.

[00044] For example, it may be that the radiation dosage (during a predetermined period of time) required to provide sufficient free charges to provide useful screening is less than the radiation dosage required to perform imaging exposures.

[00045] The amount of radiation incident upon the patterning device may be gradually increased from a non-imaging exposure to one of said plurality of imaging exposures.

[00046] In the above described embodiment, a shutter or masking blade or masking blade arrangement may thus be used to control the amount of free charges that are generated around the electrostatic clamp, e.g. by partially blocking the radiation beam.

[00047] It is worth mentioning that the part of the radiation beam that impinges on the shutter or masking blade arrangement may also result in the generation of free charges. As such, an alternative manner for generating free charges adjacent to the electrostatic clamp is to provide the radiation beam at a surface different from the patterning device, e.g. a surface of a shutter or masking blade. As mentioned above, a shutter or masking blade arrangement may be used to block the radiation beam from reaching the patterning device or to control the spatial extend of the radiation beam incident on the patterning device. The application of the radiation beam, or a part thereof, onto said shutter or mask blade(s) may also result in the generation of free charges, due to an interaction of the radiation beam with gas molecules (e.g. Hydrogen) which are present or provided around, at, or near the shutter or mask blade arrangement. The free charges will be generated both in regions which are directly illuminated by the radiation beam, and in adjacent regions (e.g. due to diffusion, and secondary electrons). As the shutter or mask blade arrangement is typically comparatively close to the patterning device, and thus the electrostatic clamp, the free charges generated by irradiating the shutter or mask blade arrangement may also result in free charges adjacent to the electrostatic clamp. The generation of free charges by irradiating the shutter or mask blade arrangement may thus also be considered a mechanism for generating free charges adjacent to the electrostatic clamp.

[00048] Regarding this manner of generating free charges adjacent to the electrostatic clamp, it can be pointed out that, in case the shutter or mask blade arrangement is entirely shut during the generation of the free charges, the risk of any unwanted irradiation reaching the substrate can be avoided. [00049] According to a second aspect of the invention there is provided a method of operating an apparatus, the apparatus comprising an electrostatic clamp, and a mechanism for generating free charges adjacent to the electrostatic clamp. The method comprises: controlling the electrostatic clamp to have a first energisation state; controlling the electrostatic clamp to have a second energisation state; and during a transition from the first energisation state to the second energisation state, controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp.

[00050] The method may further comprise providing a component adjacent to the electrostatic clamp. When the electrostatic clamp is one or both of the first and/or second energisation states, the component may be clamped by the electrostatic clamp.

[00051] Controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp and/or component may comprise providing a gas adjacent to the electrostatic clamp and/or component and controlling a source of ionising radiation to provide ionising radiation adjacent to the electrostatic clamp and/or component such that the gas is ionised.

[00052] It will, of course, be appreciated that any of the features described above in combination with the apparatus of the first aspect of the invention may be combined with the features of the method of the second aspect of the invention.

[00053] According to a third aspect of the invention there is provided a lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a support structure constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the patterning device; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate. The lithographic apparatus is configured to perform a pre-imaging exposure during which the radiation beam is incident upon the patterning device and during which no radiation is projected onto the substrate. The amount of radiation incident upon the patterning device is gradually increased during the pre-imaging exposure. The lithographic apparatus is configured to perform an imaging exposure in which the radiation beam patterned by the patterning device is projected onto the substrate.

[00054] The gradual, or soft, ramp up of EUV power results in a gradual increase in the conductivity of the region around the patterning device, and the electrostatic clamp. Such a gradual increase in the medium conductivity does not cause the electrostatic fields to collapse in an abrupt manner, but rather allows charge to leak towards the various surfaces according to the pre-existing field lines. Such a process allows any surfaces which had become charged as a result of a previous clamp polarisation state to be neutralized. Similarly, any charged particles on surfaces of the clamp or patterning device can thus be neutralized. This gradual increase in EUV power can significantly reduce the change of electrostatic discharge occurring, and can therefore reduce the rate of generation of particles (which are often generated by discharge events). [00055] The imaging exposure may comprise comprises a burst, said burst comprising a plurality of radiation pulses. Each of said pulses may comprise a substantially constant radiation dosage. It will be understood that during an imaging exposure the radiation beam may be pulsed, such that the instantaneous radiation intensity is not uniform at all times. However, during the imaging exposure, the pulse rate may be sufficiently high that the radiation incident upon the patterning device and patterned substrate are substantially uniform within a frame of reference which includes tens, hundreds or thousands of pulses. Moreover, the pulse rate is sufficiently high that, any plasma generated by the EUV radiation will likely persist for longer than gap between adjacent pulses, such that, once established, an equilibrium plasma density will be reached and maintained by the ongoing pulses.

[00056] The pre-imaging exposure may immediately precede the imaging exposure.

[00057] The pre-imaging exposure may comprise a burst, said burst comprising a plurality of radiation pulses.

[00058] Each pulse of the imaging exposure and/or the pre-imaging exposure may, for example, have a duration of around 100 ns, and a pulse pitch of around 20 ps (i.e. a pulse frequency of around 50 kHz).

[00059] The pulse rate during the pre-imaging exposure may be sufficiently high that a hydrogen plasma created (either directly or indirectly) by the incident EUV photons in each pulse of the pre imaging exposure persists for longer than the gap between adjacent ones of the pulses. Said burst may comprise a plurality of mini-bursts, each mini-burst comprising a plurality (e.g. up to 10) of radiation pulses.

[00060] The gradual increase in the amount of radiation during said pre-imaging exposure may be configured to be provided over a plurality of said radiation pulses.

[00061] In this way, as the amount of radiation incident upon the patterning device is gradually increased during the pre-imaging exposure the plasma density can be gradually increased, bringing about the desired gradual increase in the conductivity of the region around the patterning device. The gradual increase may be configured to be provided over a plurality of said mini-bursts.

[00062] The gradual increase of radiation during said pre-imaging exposure may be configured to be provided over at least 1000 radiation pulses.

[00063] In a system having a pulse frequency of around 50 kHz, 1000 radiation pulses will be delivered in around 20 ms. The gradual increase of radiation during said pre-imaging exposure may be configured to be provided over up to around 50,000 radiation pulses (i.e. up to around 1 second). Preferably, the gradual increase of radiation during said pre-imaging exposure may be configured to be provided over up to around 10,000 radiation pulses (i.e. up to around 0.2 seconds).

[00064] It will be understood that there is a compromise between the time taken to increase the radiation intensity to the full intensity required for imaging (which, if too long, can reduce wafer throughput), and achieving the benefit of reducing the likelihood of discharge and/or particle generation or release. [00065] The gradual increase of radiation during said pre-imaging exposure may be configured to be provided substantially linearly over a plurality of said radiation pulses.

[00066] The radiation intensity may increase by a predetermined amount for each of a plurality of consecutive radiation pulses, such that the radiation intensity incident upon the patterning device increases gradually, in a substantially linear manner.

[00067] During a first portion of the pre-imaging exposure having a predetermined duration, the radiation beam may be controlled to deliver a first dosage of radiation to the patterning device, said first dosage comprising less than around 10 percent of an imaging dosage of radiation delivered to the patterning device during a first portion of the imaging exposure, said first portion of the pre-imaging exposure having said predetermined duration.

[00068] It will be understood that the actual durations of the first portion of the pre-imaging exposure and the first portion of the imaging exposure is not important. Rather, the apparatus is controlled such that dosage of radiation delivered during a predetermined time period (e.g. 200 ps) during the first portion of the pre-imaging exposure is less than the dosage of radiation delivered during a corresponding predetermined time period (e.g. 200 ps).

[00069] The first dosage may comprise a non-zero dosage. The first dosage may comprise around 5 percent of an imaging dosage of radiation delivered to the patterning device during the first portion of the imaging exposure.

[00070] The first portion of the pre-imaging exposure may comprise a plurality of pulses. As such, the first dosage may comprise a total dosage delivered by said plurality of pluses within the first portion of the pre-imaging exposure. The first portion of the imaging exposure may also comprise a plurality of pulses. As such, the imaging dosage of radiation delivered to the patterning device during said first portion of the imaging exposure may comprise a total dosage delivered by said plurality of pluses within the first portion of the imaging exposure.

[00071] Prior to the first portion of the pre-imaging exposure, the radiation intensity radiation delivered to the patterning device by the radiation beam may be substantially zero. Thus, the first portion of the pre-imaging exposure may constitute a jump from around zero percent of the imaging dosage to around 5 percent of the imaging dosage.

[00072] The amount of radiation incident upon the patterning device may be gradually increased from said first portion of the pre-imaging exposure to said imaging exposure.

[00073] During a second portion of the pre-imaging exposure having said predetermined duration, the second portion following said first portion, the radiation beam may be controlled to deliver a second dosage of radiation to the patterning device, said second dosage being greater than said first dosage and less than said imaging dosage of radiation delivered to the patterning device during said first portion of the imaging exposure.

[00074] At least 1000 radiation pulses may be delivered between a start of said first portion of the pre-imaging exposure and a start of said imaging exposure. [00075] In this way, the radiation dosage can be gradually increased from the first portion of the pre-imaging exposure to the imaging exposure over a period spanning at least 1000 radiation pulses.

[00076] The lithographic apparatus may be further configured to perform a post-imaging exposure during which the radiation beam is incident upon the patterning device; and during which no radiation is projected onto the substrate, wherein the amount of radiation incident upon the patterning device is gradually decreased during the post-imaging exposure and wherein said post-imaging exposure follows an imaging exposure.

[00077] The lithographic apparatus may be further configured to perform a first imaging exposure and a second imaging exposure. Said pre-imaging exposure may immediately precede said second imaging exposure. Between said first imaging exposure and said pre-imaging exposure, the lithographic apparatus may be further configured to perform a non-imaging exposure during which the radiation beam is incident upon the patterning device and during which no radiation is projected onto the substrate.

[00078] During a first portion of the non-imaging exposure having said predetermined duration, the radiation beam may be controlled to deliver a third dosage of radiation to the patterning device, said third dosage comprising less than around 10 percent of said imaging dosage of radiation. The apparatus may be configured to cause the electrostatic clamp to transition from a first energisation state to a second energisation state during said non-imaging exposure.

[00079] That is, the clamp may be repolarised during the non-imaging exposure between imaging exposures.

[00080] The lithographic apparatus may be further configured to cause the electrostatic clamp to transition from a first energisation state to a second energisation state during said pre-imaging exposure.

[00081] That is, the clamp may be repolarised during the ramp up of EUV power prior to an imaging exposure. This allows a low level of radiation (and thus relatively low plasma density) to be provided during a clamp repolarisation, with the full radiation intensity only being provided during imaging exposures. The low level of radiation may be sufficient to mitigate some of the negative consequences associated with clamp repolarisation (e.g. particle release), and may reduce the load on the radiation source. More general, the application of the low level of radiation may also be advantageously applied during any voltage change as applied to the clamp. As an example, the application of a low level of radiation may advantageously be applied when an object is loaded onto the clamp or unloaded from the clamp.

[00082] During a series of imaging exposures, the lithographic apparatus may be further configured to perform a non-imaging exposure between each consecutive imaging exposure.

[00083] It will be understood that the amount of radiation incident upon the patterning device may be controlled in a number of ways. For example the amount of radiation may be controlled by varying the intensity of radiation generated by the radiation source. Alternatively, or additionally, the amount of radiation may be controlled by varying the spatial extent of the radiation beam incident upon the patterning device (e.g. by using shutters or masking blades). Alternatively, or additionally, the amount of radiation may be controlled by varying the number or frequency of pulses of radiation generated by the radiation source.

[00084] For example, it may be that the radiation dosage required to provide sufficient free charges to provide useful screening is less than the radiation dosage required to perform imaging exposures.

[00085] There may also be provided a lithographic system comprising the lithographic apparatus of the third aspect of the invention. The lithographic system may further comprise a radiation source configured to generate said radiation beam. Said radiation source may be an EUV source.

[00086] According to a fourth aspect of the invention there is provided a method of operating a lithographic apparatus. The lithographic apparatus comprises: an illumination system configured to condition a radiation beam; a support structure constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the patterning device; ₌ a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate. The method comprises causing the lithographic apparatus to perform a pre-imaging exposure during which the radiation beam is incident upon the patterning device, and during which no radiation is projected onto the substrate, wherein the amount of radiation incident upon the patterning device is gradually increased during the pre-imaging exposure. The method further comprises causing the lithographic apparatus to perform an imaging exposure in which the radiation beam patterned by the patterning device is projected onto the substrate.

[00087] It will, of course, be appreciated that any of the features described above in combination with the apparatus of the third aspect of the invention may be combined with the features of the method of the fourth aspect of the invention.

[00088] According to a fifth aspect of the invention there is provided an apparatus comprising an electrostatic clamp for clamping a component, and a mechanism for generating free charges adjacent to the electrostatic clamp, the apparatus having a first configuration in which a component is clamped by the electrostatic clamp; and a second configuration in which the component is spaced apart from the electrostatic clamp. The apparatus is configured to be in said first configuration at a first point in time and to be in said second configuration at a second point in time after said first point in time. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp and/or component at a third point in time between said first point in time and said second point in time.

[00089] Free charges can be used in the apparatus during handling operations, for example during the removal of a patterning device in a lithographic apparatus, to prevent negative effects associated with the amplification of voltages due to the change in capacitance associated with the increasing separation between various electrically isolated system components. In particular, as the clamped component is removed from the clamp, the capacitance therebetween will reduce (in inverse proportion to the separation). The generated free charges can be used to transfer charge to reduce the effects of voltage amplification, thereby reducing the likelihood of discharge (e.g. via hydrogen breakdown if the Paschen limit is exceeded).

[00090] The mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp and/or component so as to prevent a potential difference between said electrostatic clamp and said component exceeding a predetermined threshold.

[00091] It will be understood that both the timing and extent of the free charge generation is somewhat flexible. In particular, should the change in capacitance associated with the increasing separation between the electrostatic clamp and the component result in the potential difference between the electrostatic clamp and the component exceeding a threshold (e.g. the breakdown voltage of hydrogen according to Paschen’ s law) then discharge may occur. However, it will also be understood that the minimum discharge voltage will be a function of both distance and gas pressure. As such, the voltage threshold will vary from one configuration to another.

[00092] The predetermined threshold is determined based upon a pressure in the apparatus. The pressure may be a pressure of hydrogen.

[00093] The third point in time may be selected so as to prevent a potential difference between said electrostatic clamp and said component exceeding said predetermined threshold.

[00094] The free charges may be generated at a time (i.e. the third point in time) which is selected so as to provide charge to reduce (or limit) the potential difference between the electrostatic clamp and said component before the potential difference exceeds the threshold.

[00095] The predetermined threshold may be around 250 V or lower, e.g. around 130 V.

[00096] The apparatus may be configured to be in said second configuration at said third point in time.

[00097] In particular, the free charges may be generated shortly after the clamping voltage is removed, and the component begins to be separated from the clamp. In this way, the generated free charges will be easily able to reach surfaces of the clamp and the clamped component.

[00098] At the third point in time a minimum separation between a surface of the clamp and a surface of the component may be greater than around 10 micrometer.

[00099] The surface of the clamp may be a surface of the clamp which, when the component is clamped, is in contact with the component. It will be appreciated that the clamp may comprise a generally planar surface which is provided with protrusions (which may be referred to as burls). The protrusions may ensure that, even during clamping, the separation between the generally planar surface of the clamp and a clamped surface of the component exceeds a minimum value (e.g. 10 micrometer). However, during clamping, it will be understood that the surfaces of the protrusions will be in contact with the clamped component, and thus, during clamping a minimum separation between a surface of the clamp and a surface of the component is zero. [000100] At the third point in time the minimum separation between a surface of the clamp and a surface of the component may be greater than or equal to around 100 micrometer.

[000101] At the third point in time, a minimum separation between a surface of the clamp and a surface of the component may be less than a predetermined separation. Said predetermined separation may be around 200 micrometer.

[000102] The mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp and/or component when the apparatus is configured to be in said first configuration.

[000103] The apparatus may be configured to be in said first configuration at said third point in time.

[000104] It will be appreciated that, in use, when a component is clamped by the electrostatic clamp, the component will obscure parts of the electrostatic clamp from the generated free charges. Thus, while the mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp, such charges will, in use, typically be prevented from reaching regions of the clamp which are directly obscured by the component being clamped. However, as the component is separated from the clamp (i.e. after the clamping voltage is removed), it will be understood that free charges may diffuse to the surfaces of the clamp and clamped component, providing compensation for the voltage amplification effect. Therefore, free charges generated adjacent to the clamping device and/or component when the component is clamped may provide an effective reduction in the voltage amplification.

[000105] The electrostatic clamp may comprise a clamping region configured to clamp said component. When a component is clamped, a clamping electric field may be generated between said clamping region and said component.

[000106] The electrostatic clamp may comprise at least one electrode. When a component is clamped by the electrostatic clamp, a clamping voltage may be applied to the at least one electrode such that the clamping electric field is generated between said clamping region and said component.

[000107] The apparatus may further comprise a voltage source.

[000108] The mechanism for generating free charges adjacent to the electrostatic clamp may comprise a source of gas, and a source of ionising radiation configured to ionise gas provided by the source of gas.

[000109] The source of ionising radiation may comprise a source selected from the group consisting of: an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[000110] The apparatus may further comprise a component exchange assembly configured to remove said component from the electrostatic clamp.

[000111] The component exchange assembly may be configured to control the separation between said component and the electrostatic clamp. [000112] There may also be provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus may comprise an apparatus according to the fifth aspect of the invention. The patterning device may comprise said component to be clamped.

[000113] The lithographic apparatus may further comprise: an illumination system configured to condition a radiation beam. The electrostatic clamp may be configured to clamp said patterning device. The patterning device may be capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam. The lithographic apparatus may further comprise a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate.

[000114] There may also be provided a lithographic system comprising the lithographic apparatus. The lithographic system may further comprise a radiation source configured to generate said radiation beam.

[000115] The mechanism for generating free charges may comprise a secondary source of ionising radiation selected from the group consisting of: an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[000116] The mechanism for generating free charges may comprise said radiation source. Said radiation source may be an EUV source.

[000117] According to a sixth aspect of the invention there is provided a method of operating an apparatus. The apparatus comprises an electrostatic clamp, and a mechanism for generating free charges adjacent to the electrostatic clamp. The method comprises: providing a component adjacent to the electrostatic clamp; controlling the electrostatic clamp to have a first configuration in which the component is clamped by the electrostatic clamp at a first point in time; controlling the electrostatic clamp to have a second configuration in which the component is spaced apart from the electrostatic clamp at a second point in time after said first point in time; and controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp and/or the component at a third point in time between said first point in time and said second point in time.

[000118] It will, of course, be appreciated that any of the features described above in combination with the apparatus of the fifth aspect of the invention may be combined with the features of the method of the sixth aspect of the invention.

[000119] According to a seventh aspect of the invention there is provided an apparatus comprising an electrostatic clamp for clamping a component, and a mechanism for generating free charges adjacent to the electrostatic clamp. The apparatus has a first configuration in which a voltage having a first polarity is applied to at least one clamp electrode and no component is clamped by the electrostatic clamp; and a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to the at least one clamp electrode. The apparatus is configured to be in said first configuration at a first point in time and to be in said second configuration at a second point in time after said first point in time. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the first configuration, and not generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration.

[000120] By providing free charges adjacent to the electrostatic clamp when the clamp is polarised in a first energisation state, any particles trapped on the clamp surface can be charged. Then, when the polarisation is reversed, and the free charges are no-longer present, the charged particles can be released from the clamp surface by electrostatic repulsion. This cleaning process for an electrostatic clamp can avoid, or at least reduce, negative consequences associated with particles being trapped on the surface of the clamp.

[000121] The apparatus may be an apparatus for cleaning electrostatic clamps.

[000122] A component may be provided adjacent to the electrostatic clamp during a transition from said first configuration to said second configuration.

[000123] By providing a component adjacent to the electrostatic clamp during said transition, particles released by the clamp due to the change in polarisiation can be captured by the component, rather than contaminating other surfaces of the apparatus. The component may be referred to as a sacrificial component or a cleaning component. The component may comprise a patterning device. The component may have a size which is substantially equal to the size of a component intended to be clamped by the electrostatic clamp during lithographic operations.

[000124] The component may be clamped by the electrostatic clamp when the clamp is in the second configuration. That is, the electrostatic clamp may be configured to clamp the component when the clamp is in the second configuration.

[000125] The apparatus may have a third configuration in which no voltage is applied to the at least one clamp electrode and no component is clamped by the electrostatic clamp. The apparatus may be configured to be in said third configuration at a third point in time after said second point in time.

[000126] Once particles have been released by the application of a reverse polarisation (during the second configuration), the clamp can be returned to a neutral configuration (i.e. where no voltage is applied to the clamp electrode(s)).

[000127] The mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the third configuration.

[000128] Once particles have been released by the application of a reverse polarisation (during the second configuration), the clamp can be return to a neutral configuration (i.e. where no voltage is applied to the clamp electrode(s)) and any residual charge on the clamp surface removed by providing free charges adjacent to the clamp.

[000129] No component may be provided adjacent to the electrostatic clamp in the third configuration. That is, the apparatus may be controlled such that no component is provided adjacent to the electrostatic clamp in the third configuration. [000130] The apparatus may further comprise a component exchange assembly configured to support a component adjacent to the electrostatic clamp.

[000131] The component exchange assembly may be configured to control the separation between said component and the electrostatic clamp.

[000132] The component exchange assembly may be configured to provide said component adjacent to the electrostatic clamp between said first point in time and said second point in time.

[000133] The component exchange assembly may be configured to remove said component from adjacent to the electrostatic clamp between said second point in time and said third point in time.

[000134] The electrostatic clamp may comprise a clamping region configured to clamp a component. When a component is clamped, a clamping electric field may be generated between said clamping region and said component.

[000135] Said clamping electric field may be generated when said clamping voltage is applied to the at least one electrode. The apparatus may further comprise a voltage source.

[000136] The mechanism for generating free charges adjacent to the electrostatic clamp may comprise a source of gas, and a source of ionising radiation configured to ionise gas provided by the source of gas.

[000137] The source of ionising radiation comprises a source selected from the group consisting of: an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[000138] There may also be provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate. The lithographic apparatus may comprise an apparatus according the seventh aspect of the invention. The electrostatic clamp may be configured to clamp said patterning device during a lithographic operation.

[000139] The lithographic apparatus may further comprise an illumination system configured to condition a radiation beam. The electrostatic clamp may be configured to clamp said patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam. The lithographic apparatus may further comprise a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto the substrate.

[000140] There may also be provided a lithographic system comprising the lithographic apparatus. The lithographic system may further comprise a radiation source configured to generate said radiation beam.

[000141] The mechanism for generating free charges may comprise a secondary source of ionising radiation selected from the group consisting of: an EUV source, a VUV source, a soft-x-ray source and a radioactive source.

[000142] The mechanism for generating free charges may comprise said radiation source. Said radiation source may be an EUV source. [000143] According to an eighth aspect of the invention there is provided a method of operating an apparatus, the apparatus comprising an electrostatic clamp, the electrostatic clamp comprising at least one clamp electrode, and a mechanism for generating free charges adjacent to the electrostatic clamp. The method comprises: controlling the electrostatic clamp to have a first configuration in which a voltage having a first polarity is applied to the at least one clamp electrode and no component is clamped by the electrostatic clamp at a first point in time; controlling the electrostatic clamp to have a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to the at least one clamp electrode at a second point in time after said first point in time; controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp when the clamp is in the first configuration; and controlling the mechanism for generating free charges to not generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration.

[000144] The method may further comprise providing a component adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration.

[000145] The component may be clamped by the electrostatic clamp, when the clamp is in the second configuration.

[000146] The method may further comprise controlling the apparatus to have a third configuration in which no voltage is applied to the at least one clamp electrode at a third point in time after said second point in time.

[000147] The method may further comprise removing said component from adjacent to the electrostatic clamp between said second point in time and said third point time.

[000148] The method may further comprise controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the third configuration.

[000149] It will, of course, be appreciated that any of the features described above in combination with the apparatus of the seventh aspect of the invention may be combined with the features of the method of the eighth aspect of the invention.

[000150] According to a ninth aspect of the invention, there is provided a method of operating an apparatus, the apparatus comprising an electrostatic clamp having a first electrode, the method comprising: a) providing a component adjacent to the electrostatic clamp; b) controlling the electrostatic clamp so as to provide a first clamping voltage to the first electrode such that the component is clamped by the electrostatic clamp; c) measuring a voltage associated with a portion of the component; and d) determining an adjustment to the first clamping voltage on the basis of the measured voltage.

[000151] The electrostatic clamp comprises a first electrode to which is supplied a first clamping voltage. The first clamping voltage generates an electric field between the clamp and the component such that the component can be clamped at its back side to a clamping region of the clamp. The component also comprises a front side arranged opposite the back side. The component may accumulate charge through electrostatic attraction of particles (i.e. electrons or ions), in particular at the back side, which is adjacent to the clamp, and thus be at a different voltage compared to other parts of the system. By measuring the voltage associated with a portion of the component (e.g. the back side of the component), it is possible to determine an adjustment to the first clamping voltage such that the voltage associated with the portion of the component becomes zero. It will be appreciated that the adjustment may be determined in terms of an additional voltage to be added to or subtracted from the first clamping voltage, or as a percentage change in the first clamping voltage, or in any other suitable terms as apparent to the skilled person. By determining and applying an adjustment to the first clamping voltage, virtual grounding of the component with respect to the surrounding system can be achieved. As a result, electrostatic attraction of charges (and charged particles) to the component can be reduced or eliminated. The component may be a patterning device (e.g. a reticle) which is clamped by the electrostatic clamp. However, it will be appreciated that the component may be another component to be clamped by the clamp.

[000152] The method may further comprise: bl) while the component is clamped by the electrostatic clamp, exposing the component to radiation; b2) controlling the electrostatic clamp such that the component is released from the electrostatic clamp; and b3) removing the component from the vicinity of the electrostatic clamp.

[000153] In this way, the voltage measurement may be carried out in an offline process. In this implementation, the component is clamped by the electrostatic clamp and exposed to radiation. During exposure, charge may be deposited on the back side of the component (that is, the side of the component which is clamped to the clamp). When the clamping voltage is removed in order to release the component, the accumulated charge remains on the surface of the component. The component may be removed from the vicinity of the clamp for the associated voltage to then be measured. Any adjustment to the first clamping voltage can then be determined on the basis of the measured voltage.

[000154] The method may further comprise: e) adjusting the first clamping voltage in accordance with the determined adjustment; and f) repeating steps a) to c) in order to verify the adjustment.

[000155] Once the adjustment to the first clamping voltage has been determined, steps a) to c) may be repeated in order to verify that the adjustment successfully reduces the voltage associated with the component to zero volts. If there is still found to be a voltage associated with the portion of the component, further adjustments and verification steps may be performed. As a result, problems associated with voltage amplification during unloading of a previously clamped component may be mitigated since no net charge will remain on the virtually grounded component after exposure.

[000156] The method may further comprise, after repeating steps a) to c), determining a further adjustment to the first clamping voltage on the basis of the measured voltage. The measuring and adjustment process may be repeated a plurality of times. The measuring and adjustment process may be repeated until the voltage associated with a portion of the component satisfies a predetermined criterion (e.g. it is within a predetermined tolerance of zero volts). [000157] The repetition of steps a) to c) may be performed using a further component. In particular, in some implementations, the verification step may be performed using the same component as used for the initial measurement. However, in other implementations, it may be desirable to use a further component which was not used for the initial measurement.

[000158] The method may further comprise: subsequent to determining the adjustment to the first clamping voltage, adjusting the first clamping voltage in accordance with the determined adjustment; and measuring the voltage associated with the portion of the component.

[000159] As an alternative to the offline measurement process, the measurement may be performed online, i.e. while the component is clamped. In this implementation of the method, the adjustment is determined immediately following the measurement step without the component being released and removed from the clamp. It may be desirable to perform the adjustment and subsequent measurement steps while the component remains clamped in place. In this way, processing time may be reduced since there is no need to remove the component from the clamp in order to measure the associated voltage. In addition, the method may be repeated periodically. Alternatively or in addition, the method may be implemented as a continuous feedback loop in which the voltage associated with the component is monitored and the clamping voltage(s) adjusted automatically as a result.

[000160] The electrostatic clamp may further comprise a second electrode and the method may further comprise: determining an adjustment to a second clamping voltage to be supplied to the second electrode on the basis of the measured voltage.

[000161] The first and second clamping voltages may have different values. In particular, the first and second clamping voltages may have different polarities. For example, the first clamping voltage may be approximately +1 to 10 kV and the second clamping voltage may be approximately -1 to 10 kV. In particular, the first clamping voltage may be approximately +2 kV and the second clamping voltage may be around -2 kV. Of course, it will be appreciated that there may also be a difference in the absolute value of the first and second clamping voltages. Furthermore it will be appreciated that the determined adjustment to the first and/or second clamping voltage may be an adjustment to the first clamping voltage, or to the second clamping voltage, or to both the first and second clamping voltages. In addition, the determined adjustment may be, for example, an adjustment of a difference between the first and second clamping voltages or an adjustment to an average value of the first and second clamping voltages.

[000162] According to a tenth aspect of the invention, there is provided a system for virtually grounding a component, comprising: an apparatus comprising an electrostatic clamp configured to clamp the component, the electrostatic comprising a first electrode configured to receive a first clamping voltage; a voltage monitor configured to measure a voltage associated with a portion of the component; and a calculation unit configured to determine an adjustment to the first clamping voltage on the basis of the measured voltage. [000163] The system may further comprise a support assembly configured to support the component, wherein the support assembly comprises the voltage monitor. The support assembly may be, for example, an exchange assembly configured to convey the component towards or away from the vicinity of the clamp.

[000164] The electrostatic clamp may comprise a second electrode configured to receive a second clamping voltage and the calculation unit may also be configured to determine an adjustment to the second clamping voltage on the basis of the measured voltage.

[000165] The first and second clamping voltages may have different values.

[000166] The voltage monitor may be an electrostatic voltmeter.

[000167] It will, of course, be appreciated that any of the features described above in combination with the method of the ninth aspect of the invention may be combined with the features of the system of the tenth aspect of the invention.

[000168] Further, it will be appreciated that any of the features described above in combination with any of the first to tenth aspects of the invention may be combined with features described in the context of different ones of the above described aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

[000169] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

Figures 2a-2c depict an electrostatic clamp used within the lithographic apparatus shown in Figure 1 in cross-section, plan, and cross section views, respectively;

Figure 3 depicts a simulation of plasma density around the electrostatic clamp of Figure 2 during exposure to EUV radiation;

Figures 4a and 4b depict an exposure sequence and clamp polarisation sequence of a prior art lithographic apparatus, respectively;

Figures 5a and 5b depict an exposure sequence and clamp polarisation sequence of a lithographic apparatus according to an embodiment of the invention, respectively;

Figures 6a and 6b depict an alternative exposure sequence and clamp polarisation sequence of a lithographic apparatus according to an embodiment of the invention, respectively;

Figures 7a and 7b depict a further alternative exposure sequence and clamp polarisation sequence of a lithographic apparatus according to an embodiment of the invention, respectively;

Figures 8a-8c depict an electrostatic clamp used within a lithographic apparatus according to an alternative embodiment of the invention in cross-section, plan, and cross section views, respectively;

Figures 9a and 9b depict an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention during a patterning device unloading process; Figure 10 depicts a movement sequence of an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention during a patterning device unloading process;

Figure 11 depicts an equivalent circuit model of an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention;

Figures 12a-12c depict simulated characteristics of the equivalent circuit of Figure 9, during the movement sequence of Figure 10;

Figure 13 depicts an equivalent circuit model of an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention;

Figure 14 depicts an electrostatic clamp and patterning device used within a lithographic apparatus according to embodiments of the invention; Figures 15a-15e depict a cleaning process for electrostatic clamp used within a lithographic apparatus according to embodiments of the invention;

Figure 16 depicts a flow chart of a method for providing virtual grounding of a patterning device according to an embodiment;

Figure 17 depicts a flow chart of an alternative method for providing virtual grounding of a patterning device according to another embodiment; and

Figure 18 depicts an electrostatic clamp and patterning device with a support assembly comprising a voltage monitor according to embodiments of the invention.

DETAILED DESCRIPTION

[000170] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[000171] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[000172] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[000173] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[000174] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[000175] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[000176] Figure 2a shows a cross section of the support structure MT in more detail. The cross section is in an x-plane, extending vertically in the z-direction and horizontally in the y-direction in the orientation shown. The support structure MT comprises an electrostatic clamp 100 which is configured to clamp the patterning device MA during lithographic operations. The clamp 100 comprises a clamp body 102, and clamp electrodes 104A-104D disposed within the clamp body 102. The electrodes 104A- 104D are separated from the generally planar clamping surface of the clamp 100 by a dielectric coating. Burls 106 protrude from the clamping surface of the clamp body 102, and act to separate the clamped patterning device MA from the clamp body 102. The burls 106 may, for example, have a height of around 10 pm, and may, collectively, cover around 1% of the surface of the clamp 100. It will be appreciated that many features of the clamp 100 (e.g. wiring, additional electrodes) are omitted for simplicity.

[000177] The patterning device MA comprises a substrate 120 which may typically be formed from a material having an ultra-low coefficient of thermal expansion (e.g. ULE® manufactured by Corning, or Zerodur® manufactured by Schott AG). The substrate 120 is generally planar, and has first and second planar surfaces 122, 124 opposed to one another. In use (e.g. as shown in Figure 1) the first surface 122 is configured to reflect the radiation beam B, and to cause a pattern to be imparted to the beam B. In particular, a region of the first surface 122 may be patterned so as to cause the radiation beam B to become patterned. The patterning region of the first surface is provided with a conductive coating 126.

[000178] To enable the electrostatic clamp 100 to clamp the patterning device MA, the second surface 124 is provided with a conductive coating 128 which typically covers a majority of the second surface 124. [000179] It will be understood that the electrostatic clamp 100 may use voltages in the order of several kV in order to clamp the patterning device MA. For example the clamp 100 may be a bipolar electrostatic clamp, in which a first subset 102A, 102C of the electrodes 104A-104D are connected to a voltage supply (not shown) of around +1...10 kV (e.g. +2 kV), and a second subset 102B, 102D of the electrodes 104A-104D are connected to a voltage supply of around -1...10 kV (e.g. -2 kV). As such, a high electric field may be established between the clamp 100 and the patterning device MA, causing the patterning device MA to be attracted to the clamp 100. In particular, a charge is induced in the regions of conductive coating 128 adjacent to the electrodes 104A-104D having an opposite sign to the applied voltages, and an attractive force established between the opposite charges at various positions across the clamp 100 and patterning device MA. The region of the clamp 100 which is configured to support the patterning device MA may be referred to as a support region. Moreover, when the clamp is operated to clamp the patterning device MA, the region of the clamp which is configured to generate a clamping force may be referred to as a clamping region.

[000180] Masking blades 140, 142 are provided adjacent to the patterning device MA. The masking blades 140, 142 are configured to selectively mask the patterning device MA from the radiation beam B during the exposure sequence. In particular, the masking blades 140, 142 may be moved in the y- direction (i.e. left and right in figure 2a) so as to scan the beam B of radiation across the surface of the patterning device MA during an exposure. Moreover, the masking blades 140, 142 may be moved towards and away from each other in the y-direction to provide different levels of masking for the patterning device MA. For example, the masking blades 140, 142 can be closed to obscure the entire patterning device MA, or partially closed to allow radiation to pass through a narrow slit.

[000181] In general terms, it will be understood that the masking blades 140, 142 can be used to modulate the total dose of radiation incident upon the patterning device MA. The blades 140, 142 may, for example, be spaced apart from the patterning device in the z-direction by around 5-10 mm (e.g. 10 mm).

[000182] Figure 2b shows the clamp 100 in plan-view (as compared to the cross-section shown in Figure 2a). . The plan- view is in a z-plane, extending vertically in the x-direction and horizontally in the y-direction in the orientation shown. The cross-section of figure 2a is taken along the line A-A’ shown in figure 2b.

[000183] The electrodes 104A-104D each have a rectangular shape, and are arranged so as to be generally parallel to one another. In such an arrangement the four electrodes illustrated each span the width of the clamped patterning device MA in the x-direction, and each cover around a quarter of the length of the patterning device MA in the y-direction. In Figure 2b, the position of the patterning device MA is indicated by a dashed line. The masking blades 140, 142 are shown in outline with dotted lines.

[000184] The electrostatic clamp 100 further might comprise regions 108 and 110 which protrude from the upper and lower sides of the electrostatic clamp 100 in the orientation shown in Figure 2b (although are not visible in the cross section view shown in Figure 2a). The protrusions 108, 110 are generally in the plane of the electrostatic clamp 100 and do not therefore protrude further above or below the body of electrostatic clamp 102 as shown in Figure 2a. The protrusions 108, 110 may be referred to as clamp‘ears’.

[000185] The protrusion 108 includes secondary electrodes 114A and 114B. The protrusion 110 comprises secondary electrodes 114C and 114D. Each of the electrodes 104A - 104D has a corresponding secondary electrode 114A - 114D. Each of the electrodes 104A - 104D and the corresponding secondary electrodes 114A - 114D are connected together electrically. That is, electrode 104A is electrically connected to 114A, and so on (although these connections are not shown). However, as can be seen in Figure 2b, the protrusions 108, 110 extend beyond the perimeter of the clamped patterning device MA, and do not therefore contribute to the clamping force between the electrostatic clamp 100 and the patterning device MA. The protrusions 108, 110 may, therefore, be referred to as non-clamping regions of the clamp.

[000186] The secondary electrodes 114 A - 114D provide a convenient way to connect the voltage supply to the primary electrodes 104 A - 104D. It will be understood that maintaining flatness of the clamp 100 is desirable so as to improve imaging performance. However, in some circumstances, providing external electrical connections to the clamp electrodes 104 A - 104D can lead to distortions in the clamp flatness. As such, by providing internal connections between electrodes within the clamp body (as described above), external connections (not shown) can be made to the secondary electrodes 114A - 114D without interfering with the critical regions of the clamp 100 which support the patterning device MA. This can improve the overall flatness of the patterning device MA during imaging operations. The external connections may comprise leads (not shown) which are connected to contacts provided by the secondary electrodes 114A - 114D. The leads may be connected to voltage sources (not shown) which are configured to supply clamping voltages as required.

[000187] Figure 2c shows a side-on view of the clamp 100 in cross-section in a y-plane. This illustrated view extends vertically in the x-direction and horizontally in the z-direction. The cross- section is taken along the line B-B’ shown in figure 2b. Thus, only electrode 104B, and secondary electrodes 114B and 114C can be seen. The protrusions 108, 110 can be seen to extend further in the x- direction than the patterning device MA.

[000188] Additional masking blades 144, 146 are provided adjacent to the patterning device MA. The masking blades 144, 146 are configured to selectively mask the patterning device MA from the radiation beam B during the exposure sequence. In particular, the masking blades 144, 146 may be moved in the x-direction so as to control the width of the radiation beam B. Such control can be used, for example, to adapt the lithographic apparatus for different sized die exposures, and, as with the blades 140, 142, to modulate the total dose of radiation incident upon the patterning device MA. The blades 144, 146 may, for example, be spaced apart from the patterning device in the z-direction by around 5- 10 mm (e.g. 6 mm). [000189] It will be understood that in normal use the electrodes 104 A - 104D are screened by the overlying patterning device MA. However, the secondary electrodes 114A - 114D are not screened in the same way. This is due to the patterning device MA not extending over the secondary electrodes 114A - 114D (as best seen in figures 2b and 2c). Of course, it will be understood that the secondary electrodes 114A - 114D are insulated from the clamp surface in the same way as the primary electrodes 104A - 104D.

[000190] During use, the polarity of voltage applied to the electrode 104A - 104D is switched regularly. Switching may, for example, occur between each wafer exposure (e.g. after around 100 individual die exposures), or at longer intervals (e.g. after each 10, or 50 wafer exposures). This switching reduces the likelihood that the insulation around and between the electrodes will break down during prolonged use. This switching of the polarity of voltage applied to the electrode may be referred to as repolarisation. Each of the polarisation states may be referred to as an energisation state of the clamp. For example, in a first energisation state, a positive voltage may be applied to the electrode 104A and, in a second energisation state, a negative voltage may be applied to the electrode 104A. Alternatively, an energisation state of the clamp may be referred to, generally, as a configuration of the clamp. A clamp configuration may include an energisation state, and, optionally, further configuration details (e.g. whether or not a component is clamped by the device)

[000191] A consequence of the lack of screening of the secondary electrodes 114A - 114D by the patterning device MA is that external electric fields may be established between adjacent and oppositely polarised ones of the secondary clamp electrodes (for example between electrodes 114A - 114B) or between the secondary electrodes and other nearby components. For example, electric fields may be established between the secondary electrodes 114A-114D and the conductive coatings 126, 128 on either side of the patterning device MA, or between the secondary electrodes 114A-114D and the masking blades 140, 142, 144, 146. Such field lines are shown schematically, in figures 2b and figure 2c.

[000192] Of course, it will be understood that during normal operation of the clamp 100, an electric field will be established between surfaces of the clamp 100 and patterning device MA. Moreover, due to the close separation between various charged surfaces, (including other components within the lithographic apparatus, such as, for example masking blades 140, 142, 144, 146), electrostatic discharge can occur. That is, electrostatic discharge can occur between any charged surfaces with the likelihood of discharge increasing as the electric field strength increases. Electrostatic discharges can generate particles from surfaces, and can also release particles which were previously attached to surfaces within the lithographic apparatus. It will be understood that such release of particles is undesirable in a lithographic apparatus as particles can land on critical regions of the apparatus, possibly leading to patterning defects in processed substrates.

[000193] Moreover, Fichtenberg patterns, which are known to be associated with electrostatic discharge damage, have been observed in tests performed with Cr coated test patterning devices. Similarly, Cr particles having a morphology which indicates creation through a high energy process (e.g. nano-spheres which have been molten and re-solidified) have also been observed in these tests. It will be understood, therefore, that electrostatic discharge can generate unwanted particles.

[000194] It will, of course, be understood that reducing the clamp voltage can lead to a reduction in the electrostatic field, and therefore lead to a reduction in the defects on patterning devices as a result of electrostatic discharge. Such a reduction has been observed in systems in which patterning devices have a front side coating containing Mo. That is, a reduction in clamp voltage has been demonstrated to significantly reduce the number of Mo particle generated defects on a reticle. However, such a reduction in clamp voltage may not be practical in some circumstances where this may lead to a reduction in clamping force.

[000195] In some circumstances, it is possible to improve shielding around the clamp electrodes, so as to reduce the number of defects observed on a reticle. Such shielding may significantly reduce Mo and Cr defects. It will be appreciated, however, that the protrusions 108 and 110 shown in Figures 2b, 2c are not shielded in normal use.

[000196] Electrostatic discharge can also be problematic for pellicles. Ultra- thin pellicle films may be used to prevent the movement of particles between various regions of a lithographic apparatus. However, it will be understood that an ultra-thin pellicle film (which may, for example, contain a layer of metal with a thickness of the order of a few nanometers) may over-heat and rupture if any significant current flows through the metal layer. It will of course be understood that electrostatic discharge may result in significant currents flowing through such a film. As such, electrostatic discharge also poses a risk to pellicles.

[000197] Further still, as noted above, electrostatic clamps are regularly repolarised. Each repolarisation will result in the sign of the electric field around any un-shielded electrodes changing. If charged particles are attracted to that electrode during a first polarisation state (e.g. the application of a negative voltage), they may be repelled once the polarisation state has changed. Furthermore, particles on a clamp surface when the clamp is in the first polarisation state (e.g. when a negative voltage is applied to an electrode) may become charged. However, such charged particles may be repelled once the polarisation state has changed. That is, strong electrostatic forces may overcome attractive forces and cause any trapped particles to be released, which can potentially lead to more particles being incident upon patterning devices or other system components.

[000198] In more detail, the high electric fields generated by electrostatic clamps strongly attract any free charges. By free charges, it is meant charges (positive - e.g. ions, or negative - e.g. electrons) that are not bound to a physical substrate, but are free to move according to electric field lines. Moreover, abundant free charges are generated during EUV exposure. For example, electrons may be generated by photo-emission and also from an EUV-induced plasma which is typically generated in the presence of hydrogen gas (which is often present in lithography tools). Positive ions may also be generated within an EUV-plasma. Thus, during lithographic exposures, un-shielded clamp ears 108, 110 are likely to attract free charges, resulting in any free-space electric fields collapsing (i.e. being compensated by the free charges), and meaning that the electric fields are restricted to the internal parts of the clamps (i.e. between the clamp electrodes, and the now-charged clamp surfaces). The resulting high charge density at the clamp surface will be likely to transfer charge to any trapped particles, with the particles being then attracted more strongly by the electrostatic field.

[000199] Then, when the clamp polarity is reversed (with the EUV plasma no longer being present), the trapped charged particle will now have a charge of equal sign to the electrode, which will result in a strong repelling electric force away from the surface.

[000200] For example, it can be estimated that each of the secondary electrodes (i.e. each one of the electrodes 114A-114D) will have a capacitance in the region of 10 around 10 pF to around 500 pF (e.g. around lOOpF). The electrode capacitance can be calculated based upon the (known) area of each electrode, the (known) permittivity of the clamp dielectric, and (known) dielectric thickness.

[000201] Assuming that a clamping voltage of 2 kV is used, the charge on each ear is 200 nC (Q=CV), which corresponds to a surface charge density of around 720 pC/m ² . Such a charge density will generate a field strength E of around 4.10 ⁷ V/m (according to E = s/2e ₀ , and assuming that the field radiates in both directions, resulting in the field strength being divided by a factor of two).

[000202] The plasma generation process, which will now be discussed in more detail. It is understood that EUV photons within the beam B will ionise hydrogen molecules, generating H2 ⁺ ions, and free electrons. In an example using 13.5 nm EUV radiation, each photon may have an energy of around 92 eV, with the ionisation energy of molecular hydrogen being around 15 eV. Thus, the generated free electrons may have sufficient energy (e.g. >75 eV) and range to create a secondary plasma relatively far from the initial ionisation event. Further, an electron released in this way (i.e. having an energy of around 75 eV) may ionise one, two, or even three further hydrogen molecules. Thus, even if the primary plasma is created only where EUV photons are incident (which generally does not include the clamp ears 108, 110), a secondary plasma may be created in the vicinity of the clamp ears 108, 110. As described above, the masking blades 140-146 are spaced apart from the patterning device MA by around 5-10 mm. Thus, any plasma which reaches the clamp ears 108, 110 should either diffuse through the slit formed between the blades 140-146 and the patterning device MA, or be generated in situ as a secondary plasma.

[000203] Figure 3 shows a modeled plasma density in the region of the patterning device during an EUV exposure. The horizontal axis shows the distance R (in cm) from the centre of the patterning device MA, while the vertical axis shows distance z (in cm) from the clamp surface. This model assumes an EUV source having an output power of 40W, and a hydrogen pressure of 5 Pa. In the modelled environment, the patterning device MA is shown blocking the radiation path, with the masking blades 140, 142 being opened by around 10 mm to enable EUV radiation to reach the central part of the patterning device MA. [000204] It can be seen that adjacent to the centre region of the patterning device MA, the plasma density reaches around 10 ⁸ ions/cm ³ . However, this density decreases under the masking blades (where EUV photons do not reach directly) to around 10 ⁷ ions/cm ³ at around 1.5 cm from the centre of the patterning device MA and to around 10 ⁶ ions/cm ³ by around 4 cm from the centre of the patterning device MA.

[000205] Following the calculations above, and assuming a particle diameter of around 100 nm, having received around 10-100 charges in a plasma environment having a density of 10 ⁶ -10 ⁷ ions/cm ³ , the resulting repulsive electrostatic force on the charged surface particle after clamp repolarisation (in the absence of the plasma) would be around 10 ⁸ - 10 ⁷ N. The attractive Van de Waal’s and other adhesive forces may be expected to be around 10 ⁹ - 10 ⁸ N.

[000206] As such, according the assumptions and approximate calculations set out above, trapped particles on the clamp surface may thus experience strong repelling forces, which will generally be able to overcome weaker adhesion forces. Any particles released in this way will be accelerated along the electric field lines, and may land on the masking blades 140-146, the patterning device MA, or other system components.

[000207] Further, significant and abrupt increases in free charge density shortly after the onset of EUV exposure due to the creation of a hydrogen plasma (as illustrated in figure 3), and the corresponding increase in the conductivity of the medium surrounding the clamp 100 and clamped patterning device MA, may lead to high transient currents flowing between system components.

[000208] It will be understood that, during clamping, electric fields may established both within the clamp dielectric, and between the clamp surfaces and the clamped patterning device, or other system components (which other system components could be several centimeters away from the clamp surface). However, the sudden increase in conductivity in the region surrounding the clamp due to the creation of a hydrogen plasma can cause any established electric fields which extend beyond the clamp surface (e.g. between the clamp and the patterning device or other system components) to collapse, since the conductive plasma will be unable to support an electric field. This can result in the electric field strength within the clamp dielectric increasing rapidly, and possibly exceeding the threshold for field emission. Moreover, once field emission occurs, the high current can cause heating of the clamp electrode. Such heating can cause the threshold for field emission to be reduced, causing an increase in current. This can, in turn, cause the electrode to be heated further, and the threshold for field emission to be reduced further still. Thus, in this way, abrupt increases in free charge density shortly after the onset of EUV exposure can lead to high transient currents flowing between system components.

[000209] In some circumstances, such abrupt changes in conductivity can preferentially cause field emission from sharp features of the clamped patterning device MA in the presence of such high fields. For example, it will be understood that the coating 128 may have sharp edges with a curvature radius of around for example 100 nm. Similarly, field emission can occur from surfaces of the clamp. [000210] Further, it will be understood that the thickness of a dielectric coating which is substantially uniform in thickness across the planar surface of a clamp electrode can be reduced at edges of the electrodes. Thus, electric field strength can be increased at electrodes edges, increasing the risk of field emission in this area.

[000211] The extent to which a particular component or feature is susceptible to field emission may depend upon the nature of the material and/or surface. Moreover, the extent to which electrostatic discharge is damaging to surfaces will also depend upon material properties. For example, relative poorly conductive coating (e.g. hardened metals such as CrN, TaN) which may be used as coatings of patterning devices may be susceptible to damage by electrostatic discharge.

[000212] Referring now to figure 4, a mode of operation of the lithographic apparatus illustrated in figures 1 and 2a to 2c is now described in more detail. Figure 4a illustrates the EUV dose received at the patterning device. In particular, the magnitude of EUV dose is shown schematically in the vertical axis, and time is shown in the horizontal axis.

[000213] As noted above, the voltage applied to the electrodes 104A - 104D of the electrostatic clamp 100 is regularly reversed in order to preserve the integrity of the electrical insulation. The voltage applied to one of those electrodes is shown schematically in Figure 4b, which shows the voltage in the vertical axis and time in the horizontal axis. In particular, the voltage rises from 0 at time to to a clamping voltage +Vc at time ti. The voltage is then maintained at +Vc for an exposure cycle until time X2 after which the voltage is ramped down to 0 at time t3, and then to -Vc at time U- After this, the voltage is maintained at -Vc during another exposure cycle. At time tv the voltage is then increased to 0 at time t ₆ . Thereafter, this cycle of positive and negative energisation of the clamp electrodes continues, with stable periods of energisation being provided between each repolarisation event.

[000214] Lithographic exposures are performed during the stable periods of clamping. However, it will be understood that exposures can occur during either positive or negative polarisations of the illustrated electrode. Moreover, it will be understood that different electrodes will follow a different energisation cycle. For example, for each positive to negative transition illustrated for the electrode 104A, electrode 104B may follow a reverse transition. Moreover, other electrodes may be re-polarised at different times (so as to ensure that there is some clamping force applied to the patterning device MA at all times).

[000215] Referring again to figure 4a, during periods T _A , T _B , TC and T _D , the EUV beam is primarily used for imaging. It will be noted that each of time periods T _A , T _B , T _C and T _D correspond to a period during which the clamp polarisation does not change. However, immediately prior to period T _A , there is a period T _A ’ during which EUV energy is incident upon the patterning device MA. Similarly, immediately following time period T _A there is a short period T _A ’’ during which EUV energy is incident upon the patterning device MA. The time period T _A may be referred to as an exposure burst. The time periods T _A ’ and T _A ’’ may be referred to as a pre-exposure and post-exposure bursts, respectively. The exposure bursts T _A -T _D correspond to the EUV beam being used for imaging (i.e. exposing a substrate), or metrology (e.g. alignment, setting of mirrors, etc.). Thus, the intensity of radiation reaching the patterning device MA (and being provided to the wafer stage) is critical during this period. However, during the pre-exposure and post-exposure bursts T _A ’, T _A ” etc., EUV power is still received at the patterning device, but is not used for imaging. Thus, the intensity of radiation reaching the patterning device MA (and being provided to the wafer stage) is less critical, and may be subject to variation.

[000216] It will be noted that in the example shown in figure 4, the pre-exposure and post-exposure bursts T _A ’ and T _A ” are both also contained in within the periods of stable clamping (i.e. periods T _A ’, T _A , and T _A ’’ are all between ti and t2), during which the clamp polarisation does not change. That is, the entire EUV pulse is contained between clamp repolarisation events.

[000217] Moreover, it will be understood that the illustrated illumination and polarisation sequence is schematic, and does not include all events. For example, even during periods of apparently continuous EUV power, there may be pulses applied to the EUV source, and there may be optional blackouts for example between the imaging of different dies, or during wafer swaps.

[000218] In more detail, each of the bursts T _A ’ , T _A , and T _A ’’ may include many separate EUV pulses, and may also include periods when no pulses are provided. For example, the exposure burst T _A may correspond to a full wafer exposure (e.g. comprising around 100 die exposures), during which there may be around 10 ⁶ pulses in total (e.g. 10 ⁴ pulses per die exposure). Between each die exposure, there may be a wafer positioning period during which the EUV beam is not present. Such a period may last for around 30-40 ms. Further, prior to the start of imaging, various metrology actions may be performed during which the beam B is incident upon the patterning device MA, and is provided to the substrate table WT, but is not incident upon the substrate itself (the substrate table WT may be moved such that the substrate W does not lie in the beam path).

[000219] During the pre- and post-exposure bursts T _A ’ and T _A ” transient effects within the source SO may be monitored and controlled (e.g. to ensure that the radiation intensity experienced by the wafer during the exposure burst T _A is as uniform as possible). Further, the pre- and post-exposure bursts T _A ’ and T _A ” may be used to perform calibration, alignment or metrology operations, rather than wafer exposures. However, regardless of their use and impact upon the substrate, the bursts T _A ’, T _A , and T _A ” each involve EUV radiation being incident upon the patterning device MA.

[000220] It will be understood that as the EUV power is increased rapidly shortly after time E (i.e. once the patterning device has been properly clamped) to begin the pre-exposure burst T _A ’, the conductivity of the environment around the electrostatic clamp will rapidly change from a non conducting environment (comprising low density hydrogen), to a high conductivity environment (comprising an EUV-induced hydrogen plasma). As noted above, such a rapid change in conductivity can lead to electrostatic discharge. Moreover, during clamp repolarisation events, particles trapped on the clamp protrusions 108, 110 (which may have become charged in the plasma environment during the bursts T _A ’, T _A , and T _A ”), can be ejected from the clamp surface by the sudden change in electric field. [000221] Figure 5 illustrates a modified illumination sequence according to an embodiment of the invention. Figure 5b illustrates a polarisation sequence for the electrostatic clamp which generally corresponds to that described above with reference to Figure 4b. However, Figure 5a shows an illumination sequence which is modified as compared to that described above with reference to Figure 4a.

[000222] In particular, during the pre-exposure periods T _A ’, rather than the EUV power being abruptly switched from an OFF state to an ON state, the EUV power is gradually ramped up. That is, the EUV power incident upon the patterning device MA is gradually increased while the clamp is already polarised (or repolarised). Similarly, during the post-exposure periods T _A ’’ , rather than the EUV power being abruptly switched from an ON state to an OFF state, the EUV power may be gradually ramped down. Note that during the pre-exposure periods T _A ’ and the post-exposure periods T _A ”, EUV power is being applied to the patterning device but is not used for imaging. Within the meaning of the present invention, this may be referred to as a non-imaging exposure of the patterning device. Such a non-imaging exposure thus refers to an exposure during which an EUV radiation beam is incident upon the patterning device and during which no radiation is projected onto the substrate; said non-imaging exposure can e.g. be performed between consecutive ones of said plurality of imaging exposures.

[000223] In an embodiment of the present invention, such a non-imaging exposure is applied during a polarization or repolarization of the clamp.

[000224] The soft ramp up of EUV power, e.g. applied during the non-imaging exposure, results in a gradual increase in the conductivity of the region around the patterning device MA, and the electrostatic clamp 100. Such a gradual increase in the medium conductivity does not cause the electrostatic fields to collapse in an abrupt manner, but rather allows charge to leak towards the various surfaces according to the pre-existing field lines. Such a process allows any surfaces which had become charged as a result of a previous clamp polarisation state to be compensated. Similarly, any charged particles on surfaces of the clamp or patterning device can to be compensated.

[000225] A soft ramp may be expected to extend over a number of EUV pulses. For example, a soft ramp up may take less than a thousand pulses, or less than around 20 milliseconds. Preferably, a soft ramp up may take less than a 10,000 pulses, or less than around 200 milliseconds. In general terms, the ramp period will be small when compared to the exposure time of a wafer. A wafer exposure may, for example, last for around 30 seconds. A ramp period of less than around 1 second (or around 50,000 pulses), when compared to a wafer exposure of around 30 seconds, may be considered to be a soft ramp, or a gradual increase.

[000226] A soft ramp up such as that described above may be beneficial in one or more of the following situations:

a. after a first start-up of the EUV apparatus;

b. after loading a new patterning device MA; and

c. after reversal of the polarity of an electrostatic clamp electrode (repolarisation). [000227] Such a soft ramp-up may be performed in several different ways. For example, a ramp up may be carried out by gradually ramping up a pulse energy of the source. Alternatively, or in addition, the soft ramp-up may be implemented by slowly increasing the number of pulses in each of a plurality of mini-bursts that together contribute to the total radiation energy (or dosage) delivered to the wafer. Each mini-burst may, for example comprise around 10 pulses, each having a duration of around 100 ns, and being delivered at a frequency of around 50 kHz. In case an LPP radiation source is used, the number of EUV pulses as generated may be controlled by controlling an operation of the applied laser. In such an LPP radiation source, EUV pulses are generated by irradiating fuel targets such as Tin (Sn) targets with one or more laser beams. The amount of EUV radiation as generated may e.g. be controlled by controlling the number of fuel targets that are irradiated. The pulsed laser beam as applied may e.g. be controlled to only irradiate one in two or one in three fuel targets, thus reducing the amount of EUV radiation to 50 % or 33 %. Alternatively, or in addition, the soft ramp-up may be implemented by inserting and slowly retracting one or more of the masking blades 140, 142, 144, 146. Of course, alternative mechanisms may also be implemented for such a soft ramp-up of EUV energy.

[000228] The soft ramp may, for example, begin with a small jump (e.g. to around 5-10 % of the full imaging power), before gradually increasing from this level to the 100 % imaging power at the start of an imaging exposure. Alternatively, the radiation power may increase linearly from 0 % to 100 %. During a gradual increase in radiation power, there may be several small steps, which result in an overall increase which is substantially linear. For example, the power could be increased in steps of, e.g. 5 of 10 %. Such an increase would still result in a gradual change in the conductivity of the medium around the clamp and patterning device.

[000229] In general terms, it will be understood that providing a gradual change in conductivity reduces the risks of discharge associated with abrupt increases in conductivity.

[000230] In some cases the EUV energy may be ramped-down in a similar (i.e. symmetric) way as illustrated in figure 4a. However, in other embodiments, the EUV energy may be abruptly stopped as is customary in the state of the art.

[000231] As illustrated in Figure 5, clamp electrode repolarisation may occur during the EUV ramp up or ramp down. Alternatively the clamp electrode repolarisation may occur during EUV OFF periods. It may also be pointed out that EUV may be ON during a voltage build-down, i.e. a reduction of the applied voltage to zero, as e.g. done prior to an unloading of an object. Period tn-ti2 as shown in Figure 5b may represent such a voltage build-down. As mentioned, it may be advantageous to have at least a low level of radiation applied during said period as well.

[000232] Figure 6 shows a further embodiment in which the EUV power energisation is modified with respect to that which is customary. Figure 6b illustrates the clamp polarisation sequence, which is similar to that described above with reference to figure 4b. However, in figure 6a it can be seen that between each of the EUV exposure pulses T there is constant EUV illumination of the patterning device MA. That is, the EUV radiation is provided to the patterning device MA even during the periods when the electrostatic clamp is repolarised. This requires that during the periods between exposure pulses, the EUV energy output by the source SO is maintained, and the masking blades 140-146 are controlled so as to allow at least some of the EUV radiation to reach the patterning device MA. As mentioned above, the application of EUV power to the patterning device without the use of said power for imaging is referred, within the meaning of the present invention, a non-imaging exposure. Such a non-imaging exposure may result in the generation of an EUV-induced plasma.

[000233] By providing EUV illumination to the patterning device MA during repolarisation (i.e. during a transition from a first energisation state of the clamp to second energisation state), it will be understood that a relatively conductive medium (i.e. EUV-induced hydrogen plasma) will be provided in the vicinity of the clamped patterning device MA, and the electrostatic clamp 100 at all times. As such, as the clamp 100 is repolarised, the free space charge provided by the EUV-induced hydrogen plasma (which, as shown in Figure 3 will be present around the clamp protrusions 108, 110) will allow charge to redistribute rapidly. The abundance of free charges will effectively screen the fields generated by the secondary electrodes 114A-114D, and reduce the likelihood of particles being released by the surface. The free charge will also act to reduce dangerously high strength fields. This will result in the reduction of high transient currents between dielectric surfaces and the field-concentrating features of the edges of the patterning device coatings, via the EUV induced plasma. That is, by providing a conductive medium at all times, the high transient currents and risk of discharge associated with the abrupt onset of EUV energy at the beginning of each exposure pulse, can be reduced, or eliminated entirely. It may also be pointed out that EUV may be ON during a voltage build-down, i.e. a reduction of the applied voltage to zero, as e.g. done prior to an unloading of an object. Period tn-ti as shown in Figure 6b may represent such a voltage build-down. As mentioned, it may be advantageous to have at least a low level of radiation applied during said period as well.

As shown in Figure 7, in a yet further mode of operation, the EUV power (Figure 7a) is modulated during repolarisation (Figure 7b) such that before each exposure burst T pulse there is a gradual ramp up RU, after each exposure pulse there is a gradual ramp down RD, and between each EUV burst T there is a low EUV power L applied to the reticle. That is, rather than removing the EUV power entirely between exposure pulses, the EUV power is gradually reduced and maintained at a low level during repolarisation. This arrangement, reduces the load on the EUV source as compared to the arrangements described above with respect to Figure 6, but also provides the advantage that the medium around the electrostatic clamp is maintained in a conductive state at all times, so as to minimise the effect of high transient currents which may be observed during abrupt EUV switch-ons, and allowing charge compensation to occur during each re-polarisation event. It may also be pointed out that EUV may be ON during a voltage build-down, i.e. a reduction of the applied voltage to zero, as e.g. done prior to an unloading of an object. Period tn-ti as shown in Figure 7b may represent such a voltage build-down. As mentioned, it may be advantageous to have at least a low level of radiation applied during said period as well. [000234] It will be understood that in each of the arrangements described above with reference to Figures 5, 6 and 7, the performance of the lithographic apparatus may be improved by reducing defects caused by electrostatic discharge events, and the emission of charged particles without changing any of the hardware. In particular, existing features of the EUV illumination system are used in a novel way, in order to reduce the high free space fields seen during and after repolarisation events.

[000235] In each of the above described embodiments, it will be understood that an EUV induced plasma is required to be generated, which provides a source of free charges in the vicinity of the patterning device MA, and in particular in the vicinity of the clamp protrusions 108, 110. However, it will also be understood that to be effective in screening the fields around the electrodes 114A-114D, sufficient free charge density will be required. Of course, the density of generated charge at any particular location will depend upon the particular characteristics of any system, including EUV intensity, gas density, and system geometry. Moreover, the amount of free charge required will also depend on manage parameters (e.g. field strength, electrode geometry).

[000236] Considering the quantity of charge expected to be accumulated at dielectric surface adjacent to the clamp secondary electrodes 114A-114D (as discussed in more detail above), it will be understood that charge must be supplied at at least a minimum rate in order to adequately prevent external fields from being established during repolarisation events.

[000237] For example, a clamp repolarisation may be expected to occur in around 200ms, or around 10 ⁴ EUV pulses. Moreover, the clamp repolarisation slew rate may be, for example, around 16 kV/s. At this rate, the clamp voltage will change at a rate or around 0.3 V for each EUV pulse. Further, as discussed above, the number of charges required to screen a clamp ear with a clamping voltage of 2 kV may be of the order of 10 ¹² (200 nC ~ 1.25xl0 ¹² charges).

[000238] Assuming that both primary plasma ions, and secondary remote plasma ions (caused by electrons removed during ionisation) contribute to the charge present at the clamp ears (as shown in figure 3), and that the masking blades 140-146 are fully open (allowing the full patterning device to be exposed to the radiation beam B), it is believed that 10 ⁴ EUV pulses will provide sufficient free charges to substantially prevent external fields being established during repolarisation events. An ion density of around 10 ⁷ /cm ³ may be expected at the clamp ears in this situation, assuming 40 W source power.

[000239] However, if the flux of free charges is reduced, then a greater number of EUV pulses may be required to provide sufficient re-balancing charge. For example, if the primary plasma is not able to diffuse to the clamp ear locations, resulting in only secondary plasma contributing to free charges at the clamp ear locations, an ion density of around 10 ⁶ /cm ³ may be expected at the clamp ears. This ion density may not provide sufficient free charges to fully compensate for the repolarising fields.

[000240] If this is determined to be the case, then it may be possible to reduce the rate at which repolarisation occurs. For example, if the repolarisation rate was reduced by a factor of five (to 3.2 kV/s), a clamp repolarisation would last around 5xl0 ⁴ EUV pulses, providing a significantly increased supply of free charge to compensate for the changing field. It will be understood, of course, that such an adjustment might cause a productivity reduction. However, a reduction in clamp switching speed by a factor of five as discussed above would be expected to cause a reduction of <0.5 % in overall productivity.

[000241] It will, of course, be understood by the skilled person that the quantity of free charge required will depend on the clamping voltage and geometry, and that the proportion of free charges generated which can provide a screening effect will also depend on many factors (some of which are discussed above). However, the skilled person will be able to modify various parameters (e.g. EUV intensity, EUV pulse duration, EUV pulse number, ¾ gas pressure, clamp voltage, switching rate, masking blade positions, etc.) as required to ensure that the clamp ears are adequately screened. For example, empirical studies may be performed to establish the level of free charge required and/or provided in a particular arrangement. Alternatively, charge density modeling may be carried out in order to determine what level of charge density will be experienced at the clamp ear locations.

[000242] In the embodiments described with reference to Figures 6 to 8, an EUV induced plasma is generated by a so-called non-imaging exposure of the patterning device, i.e. an exposure during which the radiation beam is incident upon the patterning device and during which no radiation is projected onto the substrate; said non-imaging exposure being performed between consecutive ones of said plurality of imaging exposures.

[000243] By providing the radiation beam at the patterning device during a non-imaging exposure, it is possible to provide a source of free charges, by virtue of a plasma which will be created by ionization of gas molecules (e.g. Hydrogen) which are present around the patterning device. The plasma will be generated both in regions which are directly illuminated by the radiation beam, and in adjacent regions (e.g. due to diffusion, and secondary electrons). In this way, a single radiation beam (e.g. an EUV radiation beam) can be used both for imaging purposes, and to provide a source of free charges during re-polarization of the electrostatic clamp.

[000244] In an alternative embodiment, the EUV power may be applied as a source of free charges by having the EUV power impinge on a surface near the patterning device. As such, an alternative manner for generating free charges adjacent to the electrostatic clamp is to provide the radiation beam at a surface different from the patterning device, e.g. a surface of a shutter or masking blade. As mentioned above, a shutter or masking blade arrangement may be used to block the radiation beam from reaching the patterning device or to control the spatial extend of the radiation beam incident on the patterning device. The application of the radiation beam, or a part thereof, onto said shutter or mask blade(s) may also result in the generation of free charges, due to an interaction of the radiation beam with gas molecules (e.g. Hydrogen) which are present or provided around, at, or near the shutter or mask blade arrangement. The free charges will be generated both in regions which are directly illuminated by the radiation beam, and in adjacent regions (e.g. due to diffusion, and secondary electrons). As the shutter or mask blade arrangement is typically comparatively close to the patterning device, and thus the electrostatic clamp, the free charges generated by irradiating the shutter or mask blade arrangement may also result in free charges adjacent to the electrostatic clamp. The generation of free charges by irradiating the shutter or mask blade arrangement may thus also be considered a mechanism for generating free charges adjacent to the electrostatic clamp.

[000245] In some embodiments, a secondary ionisation source may be provided thereby allowing a plasma to be created in the vicinity of the electrostatic clamp by means other than the EUV source SO. Such an arrangement may reduce the overall output load of the EUV source SO. It will be understood that the above described embodiments may put additional requirements on the EUV source SO, by requiring additional EUV output than that which is required for imaging. Further, in some embodiments, it may not be possible for an EUV source to generate power continuously (as illustrated in figure 6a). Similarly, it may not be possible and/or desirable for an EUV source to provide arbitrary EUV pulse energy in the range 0-100% of the nominal output power, while also ensuring clean collector operation and pulse energy stability.

[000246] As such, in some embodiments it may be preferred to provide an alternative mechanism for the creation of a region of increased gas conductivity than a primary EUV source..

[000247] For example, a source may be provided close to the electrostatic clamp 100 and clamped patterning device MA. The source may preferably be placed in the vicinity of the clamp protrusions 108, 110. A plurality of sources may be used. The source may, for example, be a soft x-ray source or a VUV light source which is capable of operating at pressures below one bar in a clean environment. The source may comprise a low power ioniser having a power of around 0.1-1 W. One such suitable device may be a VUV ioniser L12542 as manufactured by Hamamatsu Photonics K.K., Hamamatsu City, Shizuoka, Japan. In some embodiments, the source may comprise a radioactive source or an electron-beam source.

[000248] As shown in Figures 8a- 8c (in which the sub-figures and illustrated parts generally correspond to those shown in figures 2a-2c) a source S is provided adjacent to the electrostatic clamp 100 and the patterning device MA. In an embodiment, the source S may, for example, be energised during the repolarisation of the electrostatic clamp 100, thereby ensuring that the hydrogen gas in the region of the clamp 100 and patterning device MA is ionised to create a hydrogen plasma. As discussed above, providing free charge during repolarisation effectively screens the fields generated by the electrostatic clamp 100 (especially around the secondary electrodes 114A-114D).

[000249] For example as illustrated in Figures 8a-8c (which shows a time at which the source S is emitting VUV radiation), a plasma P is generated as a result of the emitted VUV radiation. The plasma P causes a cloud of free charge to be provided throughout the clamp and patterning device environment. As such, while there may be electric fields established between each of the electrodes 104 A to 104D, and adjacent regions of the clamped patterning device MA, the unscreened electrodes 114A to 114D attract a cloud of charge from the plasma thereby forming screening charges QA, QB, QC and QD. Each of the screening charges QA - QD has a sign opposite to that of the corresponding secondary electrode 114A - 114D. Consequently, there are no significant and unwanted electric fields established around the electrostatic clamp 100, reducing the effect of fields FI, F2 and F3 illustrated in figures 2b and 2c.

[000250] The source S can therefore be used instead of the EUV radiation generated by the source SO to provide free space charges to screen unwanted electric fields, effectively collapsing free space fields (by providing mobile charges that can compensate any free-space fields). This screening results in the electric fields generated by the clamp 100 being substantially constrained to the inside of the insulator surrounding the electrostatic clamp electrodes 114A-D.

[000251] It will be appreciated that the secondary ionisation source S may be provided in particular vicinity of the protrusions 108, 110 of the clamp 100 so as to provide localized free charges to the regions in which it is most needed. As such, it will be understood that the secondary source S can be used to provide an alternative to the illumination schemes described above with reference to figures 6 and 7, in which EUV energy generated by source SO is provided to the patterning device during repolarisation.

[000252] In general terms, the EUV source SO, and the source S (which may, for example, comprise a soft x-ray source, or a VUV ioniser), may each be considered to be examples of a source of ionising radiation. Further still, such sources, in combination with a source of hydrogen (or other) gas, may be considered to be a mechanism for generating free charges. That is, the hydrogen plasma, which contains both positive ions, and free electrons, can be considered to be a cloud of free charges. Further still, such free charges comprise both positive and negative free charges. This allows the free charges to compensate for and screen fields of both polarities. It will be understood that fields of both polarities are experienced at different electrodes in each clamp polarisation state, and at each of the electrodes during different polarisation states.

[000253] Furthermore, as described briefly above, a significant voltage can be established between the clamp 100 and the patterning device MA upon removal of the patterning device MA from the clamp 100. A patterning device MA removal process is now described in more detail with reference to Figures

9.

[000254] Figure 9a shows the clamp 100 with patterning device MA clamped thereto. The clamped patterning device MA is shown spaced apart from an exchange assembly 150. The exchange assembly comprises an exchange device 152, which supports a support structure 154. The support structure 154 comprises a small number of protrusions 156 which extend from the surface of the support structure 154 towards the patterning device MA. The protrusions 156 may, for example, have a height of around 200 pm. The exchange assembly 150 is configured to move the patterning device MA towards and away from the clamp 100, for example to enable the patterning device MA to be exchanged for alternative patterning devices, or to be cleaned. The exchange device 152 may comprise a robotic arm which moves with respect to the rest of the lithographic apparatus so as to move the patterning device MA to and from a load lock (not shown). The support structure 154, and in particular the protrusions 156, support the patterning device MA during transport. It can be seen from Figure 9a that the protrusions 156 have a small area as compared to the total area of the patterning device MA. As such, when the patterning device MA is supported by the support structure 154 there is only a small area of contact. In the configuration shown in Figure 9a, there is a small gap g between the electrostatic clamp 100 and the patterning device MA. This gap g may, for example, be around 10 pm (which corresponds to the height of the burls 106 provided on the surface of the electrostatic 100).

[000255] The surface of the patterning device MA is shown spaced apart from the surface of the support structure 154 by a gap b. In the configuration shown in Figure 9a, the gap b is in excess of 200pm (the height of protrusions 156). As such, when the patterning device MA is spaced apart from the top surface of the protrusions 156, there must be a gap of at least 200pm between the planar surfaces of the patterning device MA and the support structure 154.

[000256] Conversely, in the configuration shown in Figure 9b (which includes the same components shown in Figure 9a), the patterning device MA is shown resting upon the top of the protrusions 156. Therefore, the gap b is 200 pm, and the gap g (that is the gap between the electrostatic clamp 100 and the patterning device MA), is greater than 10pm, meaning that there is no physical contact between the burls 106 (or indeed any part of the clamp 100, and the patterning device MA.

[000257] Figure 10 shows a time sequence which illustrates the changes in the gaps b and g through the removal of the patterning device MA. At time t20 the gap g is initially 10pm (which corresponds to the height of the burls 106, meaning that there is contact between the top of the burls 106 and the surface of the patterning device MA). The gap b at time t20 is well in excess of around 900pm, and therefore there is no contact between any part of the exchange assembly 150 and the patterning device MA.

[000258] At time t21 the exchange assembly 150 approaches the patterning device MA, as can be seen by the reduction in the distance b. This movement stops at time t22, when the exchange assembly 150 is configured to hover below the patterning device MA, maintaining a gap b of around 900 pm (i.e. the 200 pm height of protrusions 156, plus a gap of 700 pm). Then at time t23 the electrostatic clamp 100 and clamped patterning device MA are lowered gradually towards the exchange device 150. This movement slowly continues until around t24 when the distance b has been reduced to around 200pm. That is, the electrostatic clamp 100 and clamped patterning device MA are lowered until the lower surface of the patterning device MA (as shown in Figure 8a and 8b) makes contact with the upper surface of the protrusions 156. This configuration remains until around time t25. During this time period the voltages supplied to the clamp electrodes 104A-D, 114A-D are removed, such that the electrostatic clamp 100 is no longer clamping the patterning device MA.

[000259] At time t25 the electrostatic clamp is moved upwards to move away from the patterning device MA (which is now fully supported by the protrusions 156 of the exchange assembly 150), resulting in the gap g increasing. This movement continues until around time t26 when the gap g has been increased to around 700 pm. At this time, the exchange assembly 150 supports the patterning device MA and hovers below the electrostatic clamp 100. At time t27 the exchange assembly 150 is caused to move the supported patterning device MA away from the electrostatic clamp 100 such that the gap g increases from 700 pm to a greater distance. This movement continues until time t28 (after which the exchange assembly 150 can be moved to a patterning device exchange area, such as a load lock).

[000260] It will be understood that capacitances exist between several of the above described system components. As shown in Figure 11 the system can be modelled as a number of capacitances are arranged in series. In particular the capacitance between the exchange apparatus 150 and the lower surface (in the orientation shown in Figures 9a, 9b) of the patterning device MA may be considered to be a variable capacitance C _b , which varies as a function of the gap b. The capacitance C _b can be calculated as follows:

where: eo is the permittivity of free space;

A is the area of the patterning device MA; and

b is the separation between the surfaces of the b.

[000261] The capacitance of the patterning device MA itself can be considered to be a fixed capacitance C _r , which can be calculated as follows:

A

C _r ^£ rr ¾

where: e, _t is the relative permittivity of the substrate 120; and

r is the separation between the front and back surfaces of the patterning device MA.

[000262] The gap between the topside of the patterning device MA and the electrostatic clamp 100 can be considered to be a variable capacitance C _g which can be calculated as follows:

r _ A

Lg ^{~ e} o _b ’

where b is the separation between the back surfaces of the patterning device MA and the clamp 100.

[000263] Finally, a clamp capacitance Ca is provided between the electrodes of the clamp 100 (which on average have a voltage of 0 V), and the surface of the electrostatic clamp. This capacitance Ca is fixed, and may be calculated as follows:

where: a _r a is the relative permittivity of the clamp dielectric; and

d is the separation between the clamp electrodes 104A-D and the clamp surface.

[000264] Of course, it will be understood that the above expressions represent a simplification of the actual capacitances, and neglect various parasitic and additional components. Further, the permittivity of the low pressure hydrogen gas environment is assumed to be similar to the permittivity of free-space (e ₀ ). Similarly, the area of each capacitance is assumed to the equal to A, while effect of burls 106 and protrusions 156 are ignored. However, the model described serves to illustrate the general trends of capacitance and charge distribution. It would be understood by the skilled person that more accurate modelling could be performed if required.

[000265] As illustrated in Figure 11, the capacitances Ca, C _g , C _r , and C _b are arranged in series. Moreover, the capacitances Ca, and C _r are fixed, while the capacitances C _g and C _b are variable. It will be understood, therefore, that in a closed system, with no charge able to enter or leave the system, and for a given initial charge state, any variation in the separation g between the clamp 100 and the patterning device MA, and the separation b between the patterning device MA and the exchange apparatus 150, will result in the variable capacitances C _g and C _b changing. Moreover, this change in capacitance will also result in the voltages across the various capacitances changing, possibly significantly, in accordance with the changes in separation.

[000266] In particular, the relationship Q = CV must be maintained at all times for each capacitance (assuming no charge is injected). Therefore if a capacitance C is changed, and the amount of charge Q contained in that capacitance is maintained the same, the voltage V must change in inverse proportion to the changing capacitance C. This can result in significant voltage amplification.

[000267] It will, of course, be appreciated that charge can be injected into the various nodes between capacitors in some circumstances. In particular, charge injected at the clamp surface may be modelled as charges Q _s . Charge injected into the backside of the patterning device may be modelled as charge Q _b . Charge injected into the front of the patterning device may be modelled as charge Q _f . Charge sources Q _s , Q _b and Q _f are shown in Figure 11.

[000268] Figures 12a-12c show changes in various parameters of a model of the clamp environment during an unload sequence. In particular, the Figure 12a shows the changes in separations b, and g during a similar series of movements to those described above with reference to Figure 10, as the patterning device MA is removed from the clamp 100. The x-axis shows time in seconds, while the y- axis shows (on a logarithmic scale) distance in metres. First, the distance b reduces from around 100mm to around 1mm at time 3s, before the distance b reduces further to around 200 pm (i.e. contact between protrusions 156 and patterning device MA) at around time 6s. Then, the distance g increases from around 10 pm at time 8s, to around 700 pm at time 9s, before increasing again from around time 11s.

[000269] As described above with reference to Figure 11, the capacitances associated with various system components also change in accordance with the separations b and g. The changing capacitance associated with these various movements is shown in Figure 12b in which the x-axis shows time in seconds and the y-axis shows (again on a logarithmic scale) the capacitance of each of capacitors Ca, C _g , C _r , and C _b . As expected, it can be seen that the capacitances Ca and C _r do not change across the time period. However, it can be seen that the capacitance C _b is increased by approximately three orders of magnitude between time 2 seconds and time 6 seconds (which corresponds to the movement of the exchange assembly 150 towards the patterning device MA). Conversely, from time 8 seconds to time 12 seconds the capacitance C _g (which corresponds to the capacitance between the patterning device MA and the electrostatic clamp 100), reduces, in two stages, by approximately 4 orders of magnitude. In particular, the capacitance C _g first decreases from around 20nF to around 300pf at time 9s, and then again to around 20pF at time 12 s.

[000270] Figure 12c shows the voltages at various points within the equivalent circuit shown in Figure 11 during the removal sequence discussed above. In particular, a voltage Va at the clamp dielectric surface (which is also equal to the voltage across capacitor Ca) is shown to change only minimally during the sequence.

[000271] A voltage VFS at the front side of the patterning device MA can be understood to be a sum of voltages Va, V _g and V _r (which are the voltages across capacitors Cd, Cg and Cr respectively). The voltage VFS is also equal to the voltage V _b across capacitor C _b . The VFS can be seen to rise from a relatively small amount to around 500 volts at time 11 seconds. This corresponds to the time when the gap g between the patterning device MA and the electrostatic clamp 100 rises significantly.

[000272] However, by far the most significant change in voltage occurs at the back side of the patterning device MA, which is represented by the sum of the voltages Va and V _g (i.e. the sum of voltages across capacitors Ca and C _g ). This voltage can be seen to increase to over 1000 volts during the initial small separation between the patterning device MA and the electrostatic clamp 100 (i.e. the separation to around 700pm at around 8 seconds). However, there is then an even more significant rise to a voltage to around over 4300 volts at around time 11 seconds, which corresponds to the significant increase in distance g from the patterning device MA to the electrostatic clamp 100.

[000273] It should be noted that the modelled voltage changes are based upon the assumption that some charge will be injected to the back side of the patterning device MA from the electrostatic clamp 100. This modelled charge injection is illustrated in Figure 13, which is generally similar to the equivalent circuit described above with reference to Figure 11, and includes a single charge source Q which is modelled to inject approximately 500nC of charge at around time 1 second. This charge injection can also be seen in the inset of Figure 1 lc, which shows the voltages between time Is and 8s on an expanded vertical scale. In particular, it can be seen that, at time Is, the injection of the 500 nC charge causes the voltages VBS, VFS to increase abruptly to around 80 V.

[000274] In reality, rather than a charge being injected precisely at time Is, charge can be accumulated at the back side of the patterning device MA during clamping. Also, while the patterning device is clamped, field emission may occur from sharp features (e.g. trapped particles) on the surface of the clamp. This may result in the patterning device becoming (often negatively) charged.

[000275] Assuming that no further charge is introduced to the system, the equivalent circuit model illustrated in Figures 11 and 13 can be used to model the evolution of voltages in response to the changes in capacitances described above. The high resulting voltages shown in Figure 12c will be understood to significantly increase the risk of discharge due to break down of the hydrogen gas (e.g. due to the voltage at the patterning device MA surface exceeding the lowest Paschen limit for hydrogen, which is around 250V) in the vicinity of the electrostatic clamp 100 and patterning device MA. [000276] Thus, a further challenge associated with electrostatic discharge within lithographic apparatus is during unloading of patterning devices after clamping. As described above, charge can become trapped at the dielectric surfaces of the clamp 100. Furthermore, residual charge can remain on a clamped patterning device MA once it has been released. As the unclamped patterning device MA is moved away from the clamp surface, the increasing separation between the clamp surface and the patterning device surface can lead to a decrease in capacitance, and an amplification of the voltage. That is, given the proportional relationship between charge and voltage (i.e. Q = C.V) in a closed system, when the capacitance changes (in inverse proportion to the separation between parallel plates), any reduction in capacitance will result in a proportional increase in voltage. Thus, as the patterning device MA and clamp 100 are separated, it is possible that the voltage of the patterning device will rise sufficiently to cause electrical breakdown of the hydrogen gas to occur. Such discharge can result in particle generation, which can lead to subsequent defects.

[000277] However, it has been realised that the effects of the varying capacitances can be mitigated to some extent by the introduction of free charges during the unload process. For example, the separate ionisation source S, or indeed the EUV source SO, can be used to generate a hydrogen plasma, which provides free charges (as described in detail above), and allows the fields established across the various dielectric components (and gaps) to be relaxed during the removal process.

[000278] The provision of free charges may result in a significant reduction of the voltages established between the various system components. That is, the established electric fields which result from the high voltages illustrated in Figure 12c can be compensated by the introduction of additional free charges. Such charges may be considered to be provided to surfaces of the patterning device and clamp by charge sources Q _s , Q _b , and Q _f as illustrated in Figure 11. These charge sources are effectively provided by the hydrogen plasma, which provides sufficient charge to each of the nodes within the clamp 100 and patterning device MA equivalent circuit so as to ensure that they are maintained in a neutral state. That is, the free charges within the plasma are driven by any electric fields as they begin to be established, and cause those fields to collapse.

[000279] In this way, the potential problems associated with significant voltages being established across the patterning device MA upon removal from the electrostatic clamp 10 can be mitigated or avoided entirely. As noted above, it will be understood that this effect is not binary, and that if insufficient charge is provided, some (reduced strength) fields may still be established. It will be understood, however, that even a reduction in voltage amplification (rather than a complete avoidance) may be beneficial, especially if the voltages are thus always maintained below the lowest Paschen limit for hydrogen (of about 250V).

[000280] Moreover, the free charges may be provided at various times during the separation of the clamp 100 and patterning device MA. Indeed, it will be understood that, when the patterning device MA is clamped, it may be difficult for the free charges to penetrate between the adjacent surfaces. Thus, there may be an effective minimum separation at which free charges are to be optimally provided. This separation will depend on a required penetration depth (i.e. a distance which the free charges should penetrate between the separating surfaces of the clamp and the patterning device to provide effective charge neutralisation).

[000281] The minimum separation may also be a function of the voltage (which voltage increases during increasing separation, as described above). For example, where the voltage between the separating surfaces is small (e.g. zero), charges will not penetrate far into small gaps due to random diffusion and surface recombination. However, where there is a significant voltage between the separating surfaces, there will also be a field created between the (neutral) environment around the clamp 100 and patterning device MA and the surfaces of the clamp 100 and patterning device MA which have an increased voltage. Any such field may act to cause free charges of the appropriate sign to be drawn into the volume between the separating surfaces. The effect of such a field will generally be stronger than random diffusion of free charges, resulting in charges penetrating deeper into the volume than where no field exists. For a penetration depth of order of 1 mm, a minimum gap to allow charges to penetrate to clamp area may, for example, be of the order of 100 micrometer.

[000282] It will be understood that there may be a distance between the physical edge of the patterning device MA (or a chamfered edge of the patterning device) and the start of the conductive coating 128. This distance may be around 1 mm. As such, a plasma penetration depth of less than 1 mm would provide free charges only to the non-conductive surface of the patterning device MA, but not to the conductive coating 128. The conductive coating 128 may act as a reservoir for charges, which can be released locally and in a very short time during a discharge. As such, it is beneficial to provide free charges to the conductive coating 128 during separation, so as to neutralise any charge imbalance.

[000283] It will also be understood that it may be beneficial to provide free charges before the voltage difference between the separating components exceeds a dangerous level (e.g. 250 V). Thus, there may be an effective maximum separation at which free charges are to be optimally provided. The maximum separation distance at which charge should be provided will vary in dependence upon a number of factors.

[000284] For example, the initial separation between the parallel surfaces of the clamp 100 and the patterning device MA is one such factor. This initial separation may be of the order of 10 micrometer (which corresponds to the height of the burls 106, meaning that there is direct contact between the top of the burls 106 and the surface of the patterning device MA). It will be understood that the capacitance will change in inverse proportion to the separation of the the parallel surfaces of the clamp 100 and the patterning device MA (rather than the separation between the burls and the patterning device MA, which is initially zero). Therefore, the separation which corresponds to a dangerous voltage level will depend upon the initial separation level. That is, the ratio of the separation between the parallel surfaces of the clamp 100 and the patterning device MA when the burls contact the patterning device (e.g. 10 micrometer‘separation’) and when the patterning device has moved away from the clamp, will be substantially equal to the inverse of the ratio of capacitances in those two configuration. [000285] Further, the separation which corresponds to a dangerous voltage level will also depend upon voltage imbalances between parts of the clamp and clamped patterning device. For example, it will be understood that clamping is typically achieved by multiple electrodes, to which opposite voltages are applied. These voltages effectively balance each other out, resulting in the patterning device being maintained at a nominal overall voltage of zero (even while a clamping voltage of ±l-10kV is applied to each clamp electrode). However, it will also be understood that variations can occur between the exact clamping voltage per electrode, or in the capacitance per electrode. Either of these factors can result in a deviation from voltage neutrality for the patterning device as a whole. Alternatively or in addition, there can, for example, be charge transfer by micro-discharges from charged particles which are present between the burls of the clamp. Further, various alternative charge transfer mechanisms can result in a net charge and associated voltage remaining on the patterning device once the clamping voltage has been removed from the clamp 100.

[000286] In an embodiment, if it is assumed that any voltage imbalance is less than around ~10 V (or ~0.5 % of a 2kV clamping voltage), and that the voltage between components should be prevented from exceeding 200 V (to minimise the risk of discharge), a voltage amplification of less than 20 times can be permitted. This would require that compensating free charges should be provided before the separation (between parallel surfaces, rather than burl tips) has increased by a factor of 20. In such an example, where the nominal separation during clamping is 10 pm, a separation of around 200 pm can be considered to be an effective maximum separation at which free charges are to be optimally provided.

[000287] Of course, it will be understood that the effective maximum and minimum separations at which free charges are provided will depend upon many characteristics, and will vary between different apparatus configurations and operating conditions. More generally, it will be understood that free charges may be generated at a time which is selected so as to provide charge to reduce (or limit) the potential difference between the electrostatic clamp and a previously clamped (and subsequently released) component before the potential difference exceeds a threshold. In an embodiment, free charges are provided from before the separation has started to increase until a point after the minimum separation has been passed. In an alternative embodiment, free charges are provided from before the separation has exceeded the maximum separation until a point after the maximum separation has been passed. In an alternative embodiment, free charges are provided from before the separation has reached the minimum separation until a point after the maximum separation has been passed.

[000288] In addition to the use of free charges associated with hydrogen plasma as a charge source described above, in an alternative embodiment the free charges can be used to enable cleaning of an electrostatic clamp 100.

[000289] It will be understood that particles can become deposited upon the surface of the electrostatic clamp 100 for example between the burls 106. Such particles can be detrimental to the clamp performance. For example such particles may promote the transfer of charge between the insulator of the electrostatic clamp 100, and the surfaces of the clamped patterning device MA. Such charge transfer can create extra sticking of the electrostatic clamp 100, even when the clamping voltage has been removed from the electrodes. For example, particles can become deposited onto the insulator, leading charge to remain at the clamp surface. Such trapped charge can cause a corresponding mirror charge to be induced in the polarised coating of the clamped patterning device MA.

[000290] It will be understood that any particles which are trapped on the surface of the electrostatic clamp 100 can result in field emission from the tips of such particles (which may have sharp features), resulting in a stream of electrons being emitted towards the positively polarised coating 128 of the patterning device MA (assuming a negative bias voltage is supplied to that electrode which is adjacent to the particle). Alternatively, charge can be transferred by positive or negative ions of hydrogen (or indeed any other gas that is supplied in the gap between the patterning device and the electrostatic clamp 100). It will be appreciated that tunneling ionisation may occur in fields which may have a strength in excess of 100 MV/m. Ions created in this way can then be attracted to the polarised coating 128 of the patterning device MA.

[000291] It is also noted that any particle present at the surface of the conductive coating 128 of the patterning device MA would be likely to be transferred to the insulating surface of the electrostatic clamp 100. In particular, particles will be understood to adhere more strongly to insulating surfaces than conducting surfaces in presence of alternating electric fields.

[000292] Figure 14 shows some of the interactions between the particles and the charged surfaces of the electrostatic clamp 100 and patterning device MA in more detail. In particular, Figure 14 shows a cross section of a part of the electrostatic clamp 100 and a part of the patterning device MA. Several different particle related events are illustrated in more detail at different parts of the clamp/patterning device interface. In the illustrated embodiment, the electrode 104 A is negatively biased while the electrode 104B is positively biased. Field lines are indicated by solid arrows F.

[000293] In a first region X, a particle XI is initially associated with the surface of the patterning device MA. The particle XI is urged by the electrostatic field created between the electrode 104 A and the surface 208 of the patterning device MA to move towards the body of the electrostatic clamp 100. It would be understood that the particle XI is initially positively charged and is thus attracted towards the negatively biased electrode 104A.

[000294] Similarly, a further particle Y 1 (this time negatively charged) is illustrated at region Y. The particle Y1 is initially associated with the surface 128 of the patterning device MA, and is initially negatively charged. Under the influence of the positive voltage at the electrode 104B, the particle Y1 is urged towards the insulating surface of the electrostatic clamp 100.

[000295] At a region W, field omission from a sharp feature on a particle W 1 can be seen causing electrons to be emitted to the conductive surface 128 of the patterning device MA. Such a process may result in a region of positive charge W2 remaining at the clamp surface, and may also result in an attractive force being generated between the charged particle and the electrode. [000296] At region V, a particle VI is located on the insulating surface of the electrostatic clamp 100. Tunneling ionisation can occur at the particle VI, resulting in a negative ion being generated. In particular, electrons can tunnel from the clamp surface through the potential barrier of the particle under the field created by the polarisation of electrode 104A. Such a process may result in a negative ion being created, and a positive charge V2 remaining at the clamp surface.

[000297] In a further region Z, a particle Z1 which is associated with the clamp 100 surface can be subject to tunneling ionisation (e.g. by electrons tunneling from the particle to the clamp surface), leading to a positive ion being generated, and a negative charge Z2 remaining at the clamp surface. Any such positive ion will be attracted to the patterning device MA under the influence of the field established between the electrode 104B and the patterning device MA. Therefore, the positive ion (and the associated charge) may be transferred to the patterning device.

[000298] It will be understood that the various scenarios described above with reference to Figure 14 are illustrative of several different situations which can occur at the interface between the patterning device MA and electrostatic clamp 100, but are not exhaustive. However, each of these scenarios (and others that are not illustrated) can be mitigated to some extent by the use of source of free charges. For example, an ionisation source (e.g. VUV source S, or EUV source SO) may be used as a source of mobile charges to compensate for some of the accumulated charges associated with the various scenarios described above. Moreover, charge compensation may also be used to clean trapped particles from an electrostatic clamp.

[000299] A process by which an electrostatic clamp can be cleaned will now be described with reference to Figures 15a to 15e. In figure 15a, part of the electrostatic clamp 100 is again shown. In this example only two electrodes 104 A, 104B are shown. However, it will be understood that the electrostatic clamp 100 may correspond to that described above with reference to Figure 3. In the initial state in shown in figure 15a, the electrostatic clamp 100 is unbiased. Particles PI and P2 are trapped on the surface of the clamp 100. The particle PI is adjacent to the electrode 104A, while the particle P2 is adjacent to the electrode 104B. Ionisation source S is provided adjacent to the clamp 100, and is initially de-energised such that no VUV is emitted, and no hydrogen plasma is generated.

[000300] Referring now to figure 15b, which shows a first step in the cleaning process of the electrostatic clamp 100, the source S is energised, so as to generate a plasma P. The plasma P extends across the surface of the electrostatic clamp 100, and has a decreasing intensity as the distance from the source increases. At the same time, the electrodes 104A and 104B are energised such that the electrode 104A is negatively biased, and the electrode 104B is positively biased.

[000301] The free charges of the plasma P, in combination with the bias applied to the clamp 100, results in charges being attracted to the surfaces of the clamp 100 adjacent to each of the electrodes 104A, 104B. A positive charge Q1 is formed adjacent to the electrode 104A, while a negative charge Q2 is formed adjacent to the electrode 104B. The trapped particles PI, P2 will be charged by the charges Ql, Q2 attracted to the clamp surface. [000302] The voltages applied to the electrodes 104 A 104B during the process described above with reference to figure 15b may be lower than the typical clamping voltages. For example, whereas the clamping voltages may be the order of 1 to 5 kV, the voltage supplied to the electrodes during the cleaning operation may be, for example, between around 0.1 to 2 kV.

[000303] Figure 15c illustrates a further stage of the cleaning process of the electrostatic clamp 100. A cleaning reticle 160 is provided adjacent to the electrostatic clamp 100. The cleaning reticle 160 comprises a body 162, which is coated with a conducting layer 164, and finally an insulating layer 166. The insulating layer may be formed from an insulating material such as, for example, Kapton. In order to clamp the cleaning reticle 160 to the electrostatic clamp 100, a zero voltage may be applied to the electrodes 104 A and 104B. In order to release the particles PI, P2, a voltage may briefly be applied to the electrodes 104A, 104B, as shown in figure 15c. Note however that the bias voltages supplied in the configuration shown in figure 15c are reversed with respect to those shown in figure 15b. That is, a positive voltage supplied to the electrode 104 A, while a negative voltage is supplied to the electrode 104B. The source S is no longer energized.

[000304] Once these new electric fields have been established, any charge previously present at the surface of the clamp 100 will be repelled by the reversed bias voltages applied to the electrodes 104A, 104B. This will result in the particles PI, P2 being repelled from the clamp surface, and being deposited on the surface of the cleaning reticle 160. That is, the charge accumulated on the particles PI, P2 during the charging process at figure 15b is used to cause the particles to move in the step shown in figure 15c. The charges Ql, Q2 will also be repelled by the voltages applied to the electrodes 104A, 104B. However, in the absence of a conductive medium through which to move, the charges Ql, Q2 may remain at the surface of the clamp 100.

[000305] Referring now to figure 15d, the cleaning reticle 160 is removed from the electrostatic clamp 100 along with the particles PI and P2 which are deposited on the surface of the cleaning reticle 160. In order to remove, i.e. unclamp the cleaning reticle 160, the voltages as shown in Figure 15b may be applied, i.e. the electrodes 104A and 104B are energized such that the electrode 104A is negatively biased, and the electrode 104B is positively biased. Subsequently, as shown in Figure 15d, the voltages applied to the electrodes 104 A, 104B are removed. It is noted that trapped surface charges Ql, Q2 may still remain at the surface of the electrostatic clamp 100.

[000306] Turning now to figure 15e, in a final stage of the cleaning process, any residual charges which are present on the surface of the clamp 100 may be removed by once again generating an ionisation field by source S, so as to provide the plasma P. When the plasma P is provided in the absence of any bias applied to the electrodes 104A 104B, residual charges Ql, Q2 on the surface of the electrostatic clamp 100 can be neutralised by the free space charges of the plasma. That is, any residual charges on the insulator of the clamp 100 can be removed.

[000307] The cleaning process described above could also be performed with an EUV source SO that is also used for lithographic exposures. However, the use of a separate ionisation source S, rather than the primary EUV source SO in the process described above with reference to figures 14a-14e may be particularly beneficial since this allows the generation of mobile charges (e.g. hydrogen plasma) to be decoupled from the operation of the EUV source SO. Further, the use of a separate source S allows compensating mobile charges to be generated in regions where EUV is not available, or in volumes which are isolated from the EUV source. This allows charge compensation to be performed at a lower pressure (for example at pressures of between around 0.0001 to 1 Pa) than is typical for an EUV source SO (which may, for example, operate at pressures of around 1 to 10 Pa). As such, the use of a secondary ionisation source may be intrinsically cleaner (on the basis that there are higher vacuum levels available). Moreover, the power of the secondary ionisation source can be readily tuned and optimised for cleaning performance.

[000308] It will also be understood that de-coupling the cleaning of an electrostatic clamp from the operation of the EUV source SO allows the cleaning process to be carried out in a smaller volume of the lithographic apparatus, rather than in the projection system. This operation contributes to the ability to work in a lower vacuum environment given that the entire volumes of the source SO, illumination system IL, and projection system PS do not need to be evacuated, and only the region in which the clamp is cleaned needs to be maintained at the lower level of vacuum.

[000309] The cleaning process described above includes several steps. It will be appreciated, however, that these steps are not all essential. For example, the cleaning reticle may be omitted. Any trapped charges may be repelled by inverting the polarity of the clamp electrodes while no reticle is present. Such a process may result in particles being thrown off into the environment around the clamp. As such, this process may preferably be carried out in an environment other than an operational lithographic apparatus. Such cleaning could be carried out within a specialised cleaning tool.

[000310] Moreover, the cleaning reticle described above is described as comprising a metal surface layer covered by a thin dielectric layer facing the clamp. However, both the metal layer, and dielectric layer are optional. It will be understood, however, that the use of a dielectric layer facing the clamp may provide maximum particle adhesion for any particles incident upon the cleaning reticle. In particular, such charged particles will be attracted to the dielectric surface by coulomb attraction, and will maintain their charge (due to the dielectric surface), resulting in a mirror charge being induced in the metal surface below the dielectric layer.

[000311] In the foregoing description various embodiments are described in which an EUV inducted plasma, or a plasma generated by a secondary ionisation source, can be used to reduce the extent to which high free space fields in a lithographic apparatus (or associated tools) can cause problem. Such free-space fields can become problematic during the ramp up of EUV power and during the repolarisation of electrostatic clamps. However, by using free charges generated at the appropriate times, as described above, negative effects associated with these events can be mitigated or avoided.

[000312] Moreover, a gradual increase in the EUV power provided to an electrostatic clamp and patterning device can be used to allow gradual collapse of any free space fields, rather than providing a sudden influx of charge carriers which can lead to high transient currents, and associated problems (e.g. discharge, particle generation). Such an effect (i.e. the gradual increase of EUV power at the clamp) can be brought about by the gradual movement of the masking blades to block the EUV from impinging upon the patterning device MA. Such control can be used to modulate the supply of free charges (that is ions or electrons) to the clamp during polarisation of the clamp, or during repolarisation of the clamp. Again this allows free space fields to be removed or reduced in a smoothly controlled manner, rather than being abruptly changed.

[000313] Further still, EUV induced plasmas or plasmas generated by a secondary ioniser, can be used during patterning device handling procedures (for example patterning device removal) to prevent negative effects associated with the amplification of voltages due to the change in capacitance associated with the increasing separation between various isolated system components.

[000314] Furthermore, the use of free space charges has been demonstrated to provide a mechanism by which an electrostatic clamp can be cleaned so as to remove trapped particles from the surface of the clamp. Such process can be performed in combination with a specially designed sacrificial cleaning reticle.

[000315] It will be understood that the embodiments described above include a number of significant advantages. Moreover, in some embodiments the advantages described above can be brought about without the need to modify the structure of existing lithographic systems. That is, in some embodiments, an existing EUV source can be used in a new way to provide energy at different times within an exposure cycle than is ordinarily the case. Such an arrangement can be implemented in existing apparatus without the need for significant hardware modification, with only the change in the control procedures. Furthermore, the nature of the modifications to the operating protocol described above can be implemented without significantly impacting upon the throughput of existing apparatus. As such, the changes in operating procedures can be carried out between existing exposure cycles without significant impact upon cycle length.

[000316] Further still the above described cleaning mechanism for cleaning an electrostatic clamp can avoid, or at least reduce, negative consequences associated with particles being trapped on the surface of the clamp. That is, trapped particles (which can also result in there being trapped charges) can lead to unpredictable or drifting clamp forces when particles are present between clamped surfaces.

[000317] Further still, the above described clamp cleaning sequence can be used to clean a clamp while it is not being used within an active lithographic apparatus. That is, the use of an external or secondary plasma source can ensure that the clamp can be effectively cleaned without interfering with the normal operation of the lithographic apparatus. Further still, it also allows the clamp cleaning to be performed while the lithographic apparatus is offline for other purposes (e.g. routine maintenance) given that there is no reliance on the EUV source being used to assist with the cleaning process.

[000318] As discussed above, the electrostatic clamp comprises an electrode which is supplied with a clamping voltage in order to generate an electric field which allows the patterning device (i.e. reticle) to be clamped. The electrostatic clamp may comprise a pair of electrodes: one positive electrode and one negative electrode. The clamping voltage supplied to the electrodes may be around plus or minus 1-10 kV, for example plus or minus 2 kV. The pairing of a positive and a negative electrode means that the voltage at the surfaces of the patterning device is approximately zero, that is, the surfaces are held at a voltage approximately in the middle of the positive and negative electrode voltages. However, tolerances associated with various factors may mean that the voltage at the patterning device surface is not actually at zero volts compared to the rest of the system. Instead, a voltage imbalance may be present which can lead to a build-up of charge on the patterning device. It will be appreciated that the patterning device may be referred to more generally as a component.

[000319] The electrodes are coated with a material having an ultra-low coefficient of thermal expansion (e.g. ULE® manufactured by Corning) having a thickness of approximately 100 pm. As described above, the clamp may comprise a generally planar surface which is provided with protrusions (which may be referred to as burls). The protrusions may ensure that, even during clamping, the separation between the generally planar surface of the clamp and a clamped surface of the patterning device exceeds a minimum value (e.g. 10 micrometer). However, during clamping, it will be understood that the surfaces of the protrusions will be in contact with the clamped patterning device, and thus, during clamping a minimum separation between a surface of the clamp and a surface of the patterning device is zero. The spacing between the generally planar surface of the clamp and the clamped surface of the patterning device determines a capacitive coupling of the electrodes to the patterning device. As a result of tolerances in either the outputs of high voltage amplifiers to the electrodes or in the individual electrode capacitances (for example owing to varying thickness of coating material, e.g. ULE, in the micrometer range), the surface potential of the patterning device may deviate from zero by up to around 10 V. As an example, a deviation within a 0.1% tolerance of a high- voltage-amplifier output of 2 kV to the first clamping electrode may result in up to 4 V being present on the surface of the patterning device. Similarly, a deviation of plus or minus 1 pm in the thickness of the coating material (100 pm) on the electrode(s), i.e. 1%, can lead to even more significant imbalances.

[000320] One problem associated with the patterning device being at a non-zero voltage is that charged particles can be attracted to the patterning device surface, which may lead to imaging defects. Further, the patterning device being at a non-zero voltage during EUV exposure can cause charge to be accumulated at the back side of the patterning device. For example, during EUV-ON periods, plasma (e.g. hydrogen plasma) may be created by the interaction of the EUV radiation and gases present in the scanner. The plasma comprises free charges (ions) which can transfer to the surfaces, in particular the back side, of the patterning device as a result of the electric field generated by the clamp. That is, if surfaces of the patterning device are at a non- zero voltage during EUV exposure, the potential field established between grounded parts of the system and the non-zero patterning device will cause the free charges generated within the plasma to flow towards (or away from) the patterning device, resulting in it becoming charged (either positively or negatively). Subsequently, when the clamping voltage is removed (and when the plasma is no-longer present), residual charge accumulated on the surface(s) of the patterning device can remain, resulting in a potential difference between the patterning device and other parts of the system (such as, for example, parts of the system connected to ground).

[000321] Another problem associated with the patterning device being at a non-zero voltage is that, as discussed above, during unload of a patterning device, any charge which remains on the patterning- device surface induces a voltage therein which is amplified significantly as the patterning device is moved away from the clamp (and the capacitance is thus reduced). This can lead to discharges between the back side of the patterning device (that is, the side facing the clamp) and nearby grounded parts of the system. Furthermore, particles present on the front surface of the patterning device may be ejected from the surface at high speeds and cause damage within the system. It is therefore desirable to provide methods for mitigating one or more of the problems set out above.

[000322] In order to address the challenges of electrostatic attraction of particles to the patterning device (reticle) and/or voltage amplification and discharge during unloading, it is proposed to ground (i.e. earth) the patterning device such that the net voltage associated with the patterning device is zero volts. In particular, it may be preferable to provide“virtual grounding” of the patterning device. In other words, rather than providing the patterning device with a physical ground connection, it may be desirable to balance the voltage induced across the patterning device to yield a total of zero volts by adjusting the individual voltages of the electrodes in the electrostatic clamp. In this way, no additional hardware is required and there is no risk of a physical ground connection being lost.

[000323] In a first method for providing virtual grounding, as illustrated in figure 16, a patterning device is provided adjacent to an electrostatic clamp, for example using the exchange assembly described with reference to figures 9a and 9b, at a step 200. The electrostatic clamp is then controlled, at a step 202, such that the patterning device is clamped to the electrostatic clamp. In particular, a first clamping voltage may be supplied to a first electrode of the clamp, thereby to induce an electric field which acts to clamp the patterning device to the clamp.

[000324] The patterning device is subject to an exposure of EUV radiation at a step 204 and then released and removed from the vicinity of the electrostatic clamp at a step 206. As described above, charge can be accumulated at the back side of the patterning device during clamping and exposure. As also described, in particular with reference to figures 10 to 13, a voltage associated with a surface of the patterning device, for example the back side of the patterning device, can increase significantly as the patterning device is moved away from the clamp. This voltage is measured at a step 208. It will be appreciated that a voltage associated with a different portion of the patterning device may alternatively be measured. For example, a voltage associated with a front side of the patterning device may be measured. As discussed above, in particular with reference to figure 12c, the front side voltage may be smaller than the back side voltage. For example, owing to the internal capacitance of the patterning device, the front side voltage may be around five times smaller than the back side voltage. [000325] On the basis of the measured voltage, an adjustment to the first clamping voltage is determined at step 210. It will be understood that any measured offset voltage may be used to calculate an adjustment voltage. In the event that the determined adjustment is non-zero, the first clamping voltage may be adjusted, in a step 212, in accordance with the determined adjustment. For example, the output of a high-voltage amplifier connected to the first electrode may be adjusted. It will be appreciated that the adjustment may be determined in terms of a voltage to be added to or subtracted from the electrode voltage, or a percentage change to the present voltage, or a change in the gain of the high- voltage power amplifier, or any other suitable terms as apparent to the skilled person. In this way, it is possible to ensure that the voltage induced in the patterning device is balanced at zero volts.

[000326] It will be appreciated that the electrostatic clamp may comprise more than one electrode, in particular the clamp may comprise two to n electrodes. In this case, step 210 also comprises determining an adjustment to the second to nth clamping voltages and step 212 may comprise adjusting any or all of the second to nth clamping voltages in accordance with the determined adjustments. For example, if it is determined that, during clamping, the reticle surface is caused to be held at a voltage of +4 V, 4 V could be subtracted from the clamping voltage of each of the positive and negative clamping electrodes. Alternatively, an adjustment could be made to only the positive (or negative) clamping electrodes of a magnitude and direction which is configured to cause the voltage at the reticle surface, which, as noted above, is caused to be at a voltage substantially midway between the potentials of the clamping electrodes, to approach zero volts.

[000327] In order to verify that the correct adjustment has been made, the method may be repeated. It will be appreciated that the same patterning device may be used during the verification method. Alternatively, a further patterning device may be used. Such a verification process may be repeated until a satisfactory outcome is determined (e.g. until the voltage induced in the patterning device during clamping is substantially zero, or below a predetermined threshold).

[000328] In some implementations, the measurement of the voltage is carried out while the patterning device is clamped by the clamp. An example flowchart of this implementation is schematically shown in figure 17. The method may be carried out using the system shown in figure 18.

[000329] The system illustrated in figure 18 corresponds largely to the one shown in figure 9a and discussed in detail above. In particular, the system of figure 18 comprises a clamp 100 with patterning device MA clamped thereto. In this particular configuration, the clamp 100 comprises four electrodes 104A, 104B, 104C and 104D. The clamped patterning device MA is shown spaced apart from an exchange assembly 150. The patterning device MA comprises a front side 126 facing towards the exchange assembly 150 and a back side 128 which is adjacent the clamp 100. The exchange assembly comprises an exchange device 152, which supports a support structure 154. The exchange assembly 150 is configured to move the patterning device MA towards and away from the clamp 100, for example to enable the patterning device MA to be exchanged for alternative patterning devices, or to be cleaned. [000330] Referring back to figure 17, at a step 300, the patterning device is provided adjacent to the clamp. Again, this may be achieved using the exchange assembly 150. At a subsequent step 302, the clamp is controlled to clamp the patterning device. In particular, a first clamping voltage may be supplied to a first electrode of the clamp, thereby to induce an electric field which acts to clamp the patterning device to the clamp. The system comprises a voltage monitor 180 configured to measure a voltage associated with a portion of the patterning device. In some implementations of this method, the measured voltage associated with a portion of the patterning device is the front side voltage. In some implementations, the voltage monitor 180 may be a part of the exchange assembly 150 which is in contact with the front side of the patterning device, as illustrated in figure 18. However, the voltage monitor 180 may be located at any suitable location within the system. In some implementations, the voltage monitor 180 may be an electrostatic voltmeter. The electrostatic voltmeter may be arranged to measure the voltage associated with the front side of the patterning device without contacting the front side of the patterning device, thereby avoiding charge being transferred to (or from) the patterning device during the measurement. A similar measurement arrangement (i.e. using an electrostatic voltmeter) may be used to measure the voltage of the front side or the back side of the patterning device during the offline method described above. Of course, it will be appreciated that what is shown in figure 18 is merely a schematic illustration. In particular, it will be understood that the depicted connection from the voltage monitor 180 to the front side of the patterning device serves to illustrate that the voltage of the front side of the patterning device is being measured but does not necessarily imply a physical connection from the voltage monitor 180 to the patterning device. As discussed above, the voltage monitor 180 may be an electrostatic voltmeter arranged to measure the voltage associated with the front side of the patterning device without making physical contact therewith.

[000331] In contrast to the offline method described above, in this implementation, the voltage measurement occurs while the patterning device is clamped by the clamp, during a step 304. On the basis of the measured voltage, an adjustment to be made to the first clamping voltage is determined at a step 306. If the determined adjustment is non-zero, the first clamping voltage is adjusted at a step 308. Subsequent to the adjustment step 308, the method returns to the step of measuring the voltage associated with the portion of the patterning device. The voltage is measured and an adjustment is determined. In this way, it can be verified whether or not a suitable adjustment has been made to the first clamping voltage. It will be appreciated that the method may end after step 306 if the determined adjustment is zero. Alternatively, further voltage measurements may be taken at appropriate intervals to ensure that the virtual grounding remains correct. For example, further measurements may be performed periodically between EUV exposures by providing the exchange assembly adjacent to the patterning device, but without removing the patterning device from the clamp.

[000332] It will be appreciated that, in the event that the electrostatic clamp has more than one electrode, for example two to n electrodes, each having their own clamping voltage, step 306 may also comprise determining an adjustment to the second ... nth clamping voltage on the basis of the measured voltage. Likewise, step 308 may comprise adjusting any or all of the second ... nth clamping voltages in accordance with the determined adjustment.

[000333] It will further be appreciated that the steps 300, 302, 304 and 306 correspond to steps 200, 202, 208 and 210 respectively of the method illustrated in figure 16. Since the method illustrated in figure 17 is performed in real time, there is no need to subject the patterning device to EUV exposure in order to“fix” the charge in place on the patterning device for measurement, unlike in the offline method illustrated in figure 16. However, it will of course be appreciated that the method of figure 17 could include EUV exposure as an optional step. In this case, if the voltage monitor 180 formed part of the exchange assembly 150, the exchange assembly 150 would have to comprise an aperture in order to allow EUV radiation to reach the clamped patterning device. Alternatively, if the voltage monitor 180 formed part of another system component, the exchange assembly could be moved away from the patterning device such that the EUV radiation could reach the patterning device without being impeded by the exchange assembly. It will further be appreciated that free charges generated by alternative means than an EUV exposure may be used to provide charge to the clamped patterning device (e.g. a secondary ionisation source).

[000334] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid- crystal displays (LCDs), thin-film magnetic heads, etc.

[000335] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions. Indeed, embodiments of the invention may form part of any apparatus in which an electrostatic clamp is used.

[000336] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[000337] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[000338] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.