CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application no. 17173276.1, which was filed on 29 May 2017 and which is incorporated herein its entirety by reference.

FIELD

[0002] The present invention relates to a lithographic apparatus and a device manufacturing method.

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 from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

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

[0005] A projection system of the lithographic apparatus may be held in a first chamber and a substrate table of the lithographic apparatus may be held in a second chamber. The first and second chambers may be held under vacuum conditions to reduce unwanted absorption of EUV radiation. The first chamber may be kept at a lower pressure than the second chamber because the projection system is very sensitive to contaminants whereas the second chamber is a source of contaminants e.g. through outgassing of the resist or through outgassing of cables that connect to movable parts such as the substrate table.

[0006] A channel may be provided between the first chamber and the second chamber, a perimeter of the channel being defined by a wall. The channel may be provided with a flow of purging fluid that is configured to act as a purging fluid curtain between the first chamber and the second chamber. The purging fluid curtain may be configured to reduce the amount of contamination reaching the first chamber from the second chamber. It may be desirable to provide a channel which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.

SUMMARY [0007] According to a first aspect of the invention, there is provided a lithographic apparatus comprising a first chamber comprising a projection system, the projection system being configured to project a patterned radiation beam onto a substrate, a second chamber comprising a substrate table, the substrate table being constructed to hold a substrate, a channel extending between the first chamber and the second chamber, the channel being configured to receive a flow of purging fluid, a perimeter of the channel being defined by a wall, and a cooling system configured to cool the wall of the channel.

[0008] Cooling the wall of the channel advantageously cools the purging fluid with greater efficiency and allows greater control of the temperature of the purging fluid across a greater range of temperatures compared with a wall which is not cooled. Cooling the wall of the channel reduces heat transfer from the purging fluid to the substrate thereby reducing lithographic errors related to thermal expansion of the substrate. Cooling the wall of the channel also reduces the variation of lithographic errors between different lithographic apparatus that are caused by heat transfer from the purging fluid to the substrate. Cooling the wall of the channel advantageously increases a mass flow rate of purging fluid flowing towards the second chamber and decreases a diffusion coefficient of the purging fluid for contaminants diffusing from the second chamber to the first chamber, thereby reducing the flow rate of purging fluid that is required to reach a desired level of contaminant suppression. Cooling the wall of the channel advantageously reduces the dependency of the cooling system on the flow rate of purging fluid provided to the channel and reduces the vacuum requirements of the first chamber and/or the second chamber, thus reducing operational costs of the lithographic apparatus.

[0009] The cooling system may comprise a dedicated thermal conductor in thermal communication with the wall of the channel.

[00010] The word "dedicated" as used herein is intended to indicate that the sole function of the thermal conductor is to conduct heat.

[00011] The dedicated thermal conductor may comprise a heat pipe.

[00012] A heat pipe advantageously provides highly efficient heat transfer away from the wall of the channel compared to other known thermal conductors.

[00013] The dedicated thermal conductor may comprise two or more heat pipes connected in parallel.

[00014] The cooling system may further comprise a heat exchanger in thermal communication with the dedicated thermal conductor.

[00015] The cooling system may be configured to cool the purging fluid before the purging fluid travels via a conduit to the channel.

[00016] Cooling the purging fluid before the purging fluid travels via the conduit to the channel and cooling the wall of the channel such that the wall of the channel cools the purging fluid in the channel advantageously reduces the dependency of the cooling system on the flow rate of purging fluid provided to the channel. [00017] The cooling system may comprise a mount configured to provide a thermally conductive path between the dedicated thermal conductor and the wall of the channel.

[00018] The mount may comprise an attachment feature, the attachment feature being configured to enable removal and reattachment of the cooling system to the wall of the channel.

[00019] The attachment feature may comprise a thread for receiving a bolt.

[00020] The cooling system may further comprise a controller and a temperature sensor, the temperature sensor being configured to measure a temperature of the wall of the channel and output a signal indicative of the temperature of the wall of the channel, the controller being configured to receive the signal from the temperature sensor and adjust the cooling provided by the cooling system based on the signal received from the temperature sensor.

[00021] The controller may be a proportional integral derivative controller.

[00022] The cooling system may be configured to cool a portion of the wall of the channel.

[00023] The portion of the wall of the channel may be located beneath a purging fluid inlet of the wall of the channel.

[00024] Cooling the purging fluid beneath the purging fluid inlet increases a density of the purging fluid beneath the purging fluid inlet, which advantageously increases a fraction of flow of purging fluid travelling toward the second chamber thereby providing a greater suppression of contaminants.

[00025] The lithographic apparatus may further comprise a cooling apparatus configured to cool a region of the substrate, and the cooling system may be configured to cool a part of the cooling apparatus.

[00026] The lithographic apparatus may further comprise a heating system configured to heat a portion of the wall of the channel located above a purging fluid inlet of the wall of the channel.

[00027] Heating a portion of the wall of the channel located above the purging fluid inlet may advantageously reduce the amount of contamination reaching the first chamber from the second chamber. This is because thermal expansion of the purging fluid above the purging fluid inlet induces an increased flow of purging fluid towards the second chamber. Increasing the fraction of flow of purging fluid travelling towards the second chamber advantageously reduces the amount of contaminants reaching the first chamber from the second chamber.

[00028] According to a second aspect of the invention, there is provided a lithographic apparatus comprising a first chamber comprising a projection system, the projection system being configured to project a patterned radiation beam onto a substrate, a second chamber comprising a substrate table, the substrate table being constructed to hold a substrate, a channel extending between the first chamber and the second chamber, the channel being configured to receive a flow of purging fluid, a perimeter of the channel being defined by a wall, the wall of the channel comprising a purging fluid inlet, wherein the purging fluid inlet comprises a flow restrictor configured to allow different sections of the purging fluid inlet to provide different flow rates of purging fluid to the channel.

[00029] Providing a flow restrictor to allow different sections of the purging fluid inlet to provide different flow rates of purging fluid to the channel advantageously reduces an inhomogeneity of purging fluid flow within the channel. This reduces the amount of contamination reaching the first chamber from the second chamber. Reducing the inhomogeneity of the flow of purging fluid within the channel may allow a lower total flow rate of purging fluid to be used.

[00030] The flow restrictor may comprise a baffle configured to separate the purging fluid inlet into the different sections.

[00031] The flow restrictor may comprise a varying purging fluid inlet opening size such that the different sections of the purging fluid inlet have different opening sizes.

[00032] Using a varying purging fluid inlet opening size advantageously allows a more continuous variation of the flow rate of purging fluid about the purging fluid inlet of the channel compared to the use of baffles.

[00033] According to a third aspect of the invention, there is provided a lithographic apparatus comprising a first chamber comprising a projection system, the projection system being configured to project a patterned radiation beam onto a substrate, a second chamber comprising a substrate table, the substrate table being constructed to hold a substrate, a channel extending between the first chamber and the second chamber, the channel being configured to receive a flow of purging fluid, a perimeter of the channel being defined by a wall, the wall of the channel comprising an angled purging fluid inlet, wherein an interior wall of the angled purging fluid inlet is curved.

[00034] The phrase "angled purging fluid inlet" as used herein is intended to indicate that an injection angle of the purging fluid inlet (i.e. the angle between a line which bisects the angled purging fluid inlet and an optical axis of the lithographic apparatus) may not be perpendicular at all positions around the purging fluid inlet. For example, the injection angle may be acute. Having an angled inlet with a curved interior wall advantageously increases the momentum of purging fluid travelling towards the second chamber, thereby reducing the amount of contamination reaching the first chamber from the second chamber.

[00035] The interior wall of the purging fluid inlet may be curved such that there is a smooth transition from the interior wall of the purging fluid inlet to the wall of the channel.

[00036] Having a smooth transition from the interior wall of the purging fluid inlet to the wall of the channel advantageously reduces a separation of the flow of purging fluid into an upward flow towards the first chamber and a downward flow towards the second chamber. That is, a greater fraction of the flow of purging fluid may exit the angled purging fluid inlet and travel towards the second chamber, which may in turn advantageously reduce the amount of contamination reaching the first chamber from the second chamber.

[00037] The interior wall of the purging fluid inlet may be curved such that the purging fluid inlet defines a converging flow path for the purging fluid.

[00038] The converging flow path of the purging fluid defined by the curved interior wall of the purging fluid inlet advantageously encourages acceleration of the purging fluid towards the second chamber. This reduces the amount of contamination reaching the first chamber from the second chamber.

[00039] The interior wall of the purging fluid inlet may be curved such that an opening size of the purging fluid inlet is narrowest at or before an end of the purging fluid inlet.

[00040] Having the narrowest opening size of the purging fluid inlet located at or before an end of the purging fluid inlet advantageously encourages further acceleration of the purging fluid towards the second chamber, which reduces the amount of contamination reaching the first chamber from the second chamber.

[00041] A distance between a lower wall of the purging fluid inlet and the substrate table may be greater than about 15 mm.

[00042] According to a fourth aspect of the invention, there is provided a lithographic apparatus comprising a first chamber comprising a projection system, the projection system being configured to project a patterned radiation beam onto a substrate, a second chamber comprising a substrate table, the substrate table being constructed to hold a substrate, a channel extending between the first chamber and the second chamber, the channel being configured to receive a flow of purging fluid, a perimeter of the channel being defined by a wall, the wall of the channel comprising a first purging fluid inlet and a second purging fluid inlet, the second purging fluid inlet being located closer to the substrate than the first purging fluid inlet, and a controller configured to control the flow rate of purging fluid provided through the first purging fluid inlet and the flow rate of purging fluid provided through the second purging fluid inlet, wherein the controller is configured to control the flow rate of purging fluid provided through the second purging fluid inlet based on a flow rate of purging fluid that passes from the channel through to the second chamber when substantially no flow is provided through the second purging fluid inlet.

[00043] Providing a flow rate of purging fluid that depends upon a flow rate of purging fluid that passes from the channel through to the second chamber when substantially no flow is provided through the second purging fluid inlet advantageously reduces recirculation of purging fluid within the channel. This reduces the amount of contamination travelling up the wall of the channel from the second chamber to the first chamber.

[00044] The controller may be configured to provide a flow rate of purging fluid through the second purging fluid inlet that is between about 50% and about 200% of a flow rate of purging fluid that passes from the channel through to the second chamber when substantially no flow is provided through the second purging fluid inlet. This range of flow rates was found to provide improved suppression of contamination.

[00045] According to a fifth aspect of the invention, there is provided a device manufacturing method using a lithographic apparatus, the method comprising projecting a patterned beam of radiation onto a substrate using a projection system in a first chamber, supporting the substrate using a substrate table in a second chamber, providing a channel that extends between the first chamber and the second chamber, a perimeter of the channel being defined by a wall, providing a flow of purging fluid to the channel and cooling the wall of the channel.

[00046] The wall of the channel may be cooled by a dedicated thermal conductor. The dedicated thermal conductor may comprise one or more heat pipes.

[00047] The method may further comprise cooling the purging fluid before the purging fluid reaches the channel.

[00048] The method may further comprise measuring a temperature of the wall of the channel and controlling the cooling of the wall of the channel based on the measured temperature of the wall of the channel.

[00049] The method may further comprise cooling a portion of the wall of the channel located beneath a purging fluid inlet of the wall of the channel.

[00050] The method may further comprise heating a portion of the wall of the channel located above a purging fluid inlet of the wall of the channel.

[00051] The method may further comprise cooling the wall of the channel to a temperature between about 8°C and about 15°C.

[00052] The method may further comprise calibrating cooling of the wall of the channel such that a relationship between the temperature of the wall of the channel and an overlay error of a lithographic exposure that takes place during cooling of the wall of the channel is determined, wherein the calibrating comprises a first step of cooling the wall to a desired temperature, a second step of carrying out a lithographic exposure of the substrate, and a third step of processing the substrate and measuring an overlay error of the lithographic exposure.

[00053] According to a sixth aspect of the invention, there is provided a device manufacturing method using a lithographic apparatus, the method comprising projecting a patterned beam of radiation onto a substrate using a projection system in a first chamber, supporting the substrate using a substrate table in a second chamber, providing a channel extending between the first chamber and the second chamber, a perimeter of the channel being defined by a wall, and providing different flow rates of purging fluid to the channel through different sections of a purging fluid inlet of the wall of the channel.

[00054] The flow rates of purging fluid provided to different sections of the purging fluid inlet may depend at least in part upon a ratio of the size of a cross-sectional area of the channel to a total length of purging fluid inlet section that provides the cross-sectional area of the channel with purging fluid. This method advantageously achieves a more homogeneous flow of purging fluid within the channel, thereby reducing the amount of contamination reaching the first chamber from the second chamber.

[00055] According to a seventh aspect of the invention, there is provided a device manufacturing method using a lithographic apparatus, the method comprising using a projection system in a first chamber to project a patterned beam of radiation onto a substrate, the substrate being supported by a substrate table in a second chamber, the patterned beam passing from the first chamber to the second chamber via a channel, a perimeter of the channel being defined by a wall, an angled purging fluid inlet having a curved interior wall is provided in the wall of the channel, and wherein the method further comprises providing purging fluid through the angled purging fluid inlet into the channel.

[00056] The interior wall of the purging fluid inlet may be curved such that the purging fluid flows along a converging flow path.

[00057] According to an eighth aspect of the invention, there is provided a device manufacturing method using a lithographic apparatus, the method comprising projecting a patterned beam of radiation onto a substrate using a projection system in a first chamber, supporting the substrate using a substrate table in a second chamber, providing a channel extending between the first chamber and the second chamber, a perimeter of the channel being defined by a wall, providing purging fluid to the wall of the channel from a first purging fluid inlet and a second purging fluid inlet, the second purging fluid inlet being located closer to the substrate than the first purging fluid inlet, and providing purging fluid through the second purging fluid inlet with a flow rate that is based on a flow rate of purging fluid that passes from the channel through to the second chamber when substantially no purging fluid is provided through the second purging fluid inlet.

[00058] The method may further comprise providing purging fluid through the second purging fluid inlet at a flow rate that is substantially equal to the flow rate of purging fluid that passes from the channel through to the second chamber when substantially no purging fluid is provided through the second purging fluid inlet. BRIEF DESCRIPTION OF THE DRAWINGS

[00059] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus, a radiation source and a channel which may comprise an embodiment of the invention;

- Figure 2 schematically depicts a known channel and a purging fluid pre-cooling system;

Figure 3 schematically depicts a cooling system and a channel according to an embodiment of the invention;

Figure 4 schematically depicts a cooling system and a channel according to an embodiment of the invention;

- Figure 5 schematically depicts a view of an opening of a known channel;

Figure 6 schematically depicts a view from above a channel comprising a purging fluid inlet having a flow restrictor according to an embodiment of the invention;

Figure 7 schematically depicts a cross-sectional view of a section of a purging fluid inlet comprising a flow restrictor according to an embodiment of the invention;

- Figure 8 schematically depicts a cross-sectional view from the side of a known channel;

Figure 9 schematically depicts a cross-sectional view from the side of a channel comprising an angled purging fluid inlet according to an embodiment of the invention; Figure 10 schematically depicts a perspective view in cross-section of a channel comprising an angled purging fluid inlet according to an embodiment of the invention;

Figure 11 schematically depicts the known channel depicted in Figure 8 with a visible flow of purging fluid exiting the purging fluid inlet;

- Figure 12 schematically depicts a channel comprising multiple purging fluid inlets according to an embodiment of the invention; and,

Figure 13 schematically depicts a perspective view of a channel comprising multiple purging fluid inlets and a cooling system according to an embodiment of the invention. DETAILED DESCRIPTION

[00060] Figure 1 shows a lithographic system including a channel which may comprise an embodiment of the invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. 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. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[00061] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00062] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source). A laser 1, which may for example be a CO ₂ laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma. [00063] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[00064] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.

[00065] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[00066] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. 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.

[00067] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. The reduction factor may vary for different directions, i.e. the reduction factor in the x-direction may be different to the reduction factor in the y-direction. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00068] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[00069] The projection system PS of the lithographic apparatus is held in a first chamber 15 and the substrate table WT is held in a second chamber 16. The first chamber 15 and the second chamber 16 are held under vacuum conditions. Contaminants may be generated in the second chamber 16 and diffuse towards the first chamber 15. For example, hydrocarbons and/or metals such as tin may outgas from the resist on the substrate W and travel towards highly sensitive optical components within the projection system PS. Exposing the substrate to EUV radiation may cause an increase in the amount of contaminants generated in the second chamber 16. The contaminants may diffuse from the second chamber 16 to the first chamber 15. The contaminants may accumulate on optical components in the projection system PS thereby negatively affecting the performance of the optical components. For example, contaminants may accumulate on a reflective surface of a mirror in the projection system PS and reduce the reflectivity of the mirror. A reduction of the reflectivity of a mirror in the projection system PS may reduce the amount of EUV radiation reaching the substrate W, which may in turn reduce a throughput of the substrate because a longer amount of time is needed for the same EUV dose to be applied to the substrate W.

[00070] Known lithographic apparatus comprise a channel 17 extending between the first chamber 15 and the second chamber 16. The channel 17 has a perimeter that is defined by a wall 19. The channel 17 may be provided with a flow of purging fluid through a purging fluid inlet (not shown) provided in the wall 19 of the channel 17. The purging fluid may, for example, comprise Hydrogen gas. Other fluids may be used, e.g. Helium, Nitrogen, Argon and/or any mixture thereof. A purging fluid may be selected that has a low diffusion coefficient for contaminants present in the lithographic apparatus (e.g. lower than the diffusion coefficient of Hydrogen gas).

[00071] When in use, a portion of the purging fluid flows through the channel 17 towards the second chamber 16 and another portion of the purging fluid flows through the channel 17 towards the first chamber 15. A first exhaust system (not shown) is provided in the first chamber 15 to remove purging fluid from the first chamber 15. A second exhaust system (not shown) is provided in the second chamber 16 to remove purging fluid from the second chamber 16. The portion of the flow of purging fluid that flows through the channel 17 towards the second chamber 16 forms a purging fluid curtain. The purging fluid curtain is configured to reduce the amount of contaminants reaching the second chamber 16 from the first chamber 15, thereby protecting the projection system PS from contamination. The channel 17 and the purging fluid curtain are both configured to allow EUV radiation to pass from the first chamber 15 to the second chamber 16 such that a lithographic exposure may take place.

[00072] Some lithographic apparatus have a larger numerical aperture than others. Increasing the numerical aperture of a lithographic apparatus may negatively affect the ability of the channel 17 and the purging fluid curtain to reduce contamination of the first chamber 15. Increasing the numerical aperture of a lithographic apparatus may result in a larger opening between the first chamber 15 and the second chamber 16, which may result in a wider channel 17. Increasing the width of the channel 17 may reduce the amount of flow of purging fluid travelling towards the second chamber 16. Increasing the width of the channel 17 increases the cross-sectional area of the channel 17, thus giving contaminants a larger cross-sectional area to diffuse across from the second 16 chamber to the first chamber 15.

[00073] Some lithographic apparatus comprise a cooling apparatus (not shown) configured to cool a region of the substrate W that is in the vicinity of an exposure target area of the substrate W. The presence of the cooling apparatus may restrict at least part of the outflow of purging fluid exiting the second chamber 16 via the second exhaust system. Restricting a part of the outflow of purging fluid from the second chamber 16 may reduce the ability of the purging fluid curtain to reduce the amount of contaminants reaching the first chamber 15 from the second chamber 16.

[00074] A lithographic apparatus having a larger numerical aperture (e.g. 0.5) and/or a cooling apparatus restricting flow of the purging fluid in the second chamber 16 may have a reduced ability to suppress contaminants compared to a lithographic apparatus having a smaller numerical aperture (e.g. 0.3) and lacking a cooling apparatus.

[00075] Figure 2 schematically depicts a known channel 17 and a purging fluid pre-cooling system 20. The channel 17 extends between the first chamber 15 and the second chamber 16. A perimeter of the channel 17 is defined by a wall 19. The projection system PS of the lithographic apparatus is held in the first chamber 15 and the substrate table WT is held in the second chamber 16. In some embodiments a membrane 18 may extend across the channel 17. The channel 17 is configured to receive a flow of purging fluid 23. The purging fluid 23 is passed from a source 21 through a heat exchanger 22. The heat exchanger 22 is configured to cool the purging fluid 23 before the purging fluid reaches the channel 17. The purging fluid 23 may, for example, have an ambient temperature of about 22°C before reaching the heat exchanger 22. The heat exchanger 22 may, for example, pre-cool the purging fluid 23 such that the temperature of the purging fluid decreases from about 22°C to about - 10°C. Pre-cooled purging fluid 23 then travels to the channel 17 via a conduit 24. The purging fluid 23 enters the channel via a purging fluid inlet 29. The purging fluid inlet 29 extends along a perimeter of the channel 17. The purging fluid inlet 29 has only been shown on one side of the channel 17 depicted in Figure 2 for ease of illustration. The pre-cooled purging fluid flows through the channel 17 and forms a purging fluid curtain 25 configured to reduce the amount of contamination reaching the first chamber 15 from the second chamber 16. The flow rate of purging fluid 23 provided to the channel 17 may depend at least in part upon a numerical aperture of the lithographic apparatus. For example, the flow rate of the purging fluid 23 provided to a lithographic apparatus having a relatively small numerical aperture (e.g. about 0.3) may, for example, be between about 10 mbarLs ^"1 and about 150 mbarLs ^"1 , e.g. about 115 mbarLs ^"1 . The flow rate of the purging fluid 23 provided to a lithographic apparatus having a larger numerical aperture (e.g. about 0.5) may, for example, be up to about 300 mbarLs ^"1 . Increasing a flow rate of purging fluid provided to the channel 17 may reduce the amount of contamination reaching the first chamber 15 from the second chamber 16. However, the flow rate of purging fluid 23 provided to the channel 17 may be limited by the vacuum requirements of the first chamber 15 and/or the vacuum requirements of the second chamber 16.

[00076] Pre-cooled purging fluid 23 may gain heat energy via one or more mechanisms when the purging fluid travels from the heat exchanger 22 to the channel 17. For example, heat may transfer via heat leaks 26 from hotter parts of the lithographic apparatus 28 to the pre-cooled purging fluid 23. As another example, friction induced heating of the purging fluid 23 may occur when the purging fluid flows from the heat exchanger 22 to the channel 17. Fluid dynamics may also result in some parts of the flow of purging fluid 23 being at a higher temperature than other parts of the flow of purging fluid. For example, parts of the flow of purging fluid that are relatively stagnant may be at a higher temperature than other parts of the flow of purging fluid.

[00077] Due to the effects of the heat transfer mechanisms discussed above the purging fluid 23 reaches the channel 17 at a higher temperature than desired. For example, the purging fluid 23 may exit the heat exchanger 22 with a temperature of about -10°C and gain heat energy when travelling from the heat exchanger 22 to the channel 17 such that the purging fluid has a temperature in the range of about 0°C to about 22°C when the purging fluid is in the channel 17. The amount of heat energy gained by the purging fluid 23 when the purging fluid travels from the heat exchanger 22 to the channel 17 may vary between different lithographic apparatus.

[00078] Some heat energy may transfer from the purging fluid 23 to the substrate W when the purging fluid reaches the substrate W. The amount of heat energy that transfers from the purging fluid 23 to the substrate W depends upon a number of case-specific variables such as, for example, the ambient temperature of the lithographic apparatus 28 and the purging fluid 23, the temperature to which the purging fluid 23 is cooled, the heat transferred to the purging fluid 23 via heat leaks 26 when the purging fluid travels from the heat exchanger 22 to the channel 17, the flow rate and type of fluid used as purging fluid, etc.

[00079] A variable that affects the amount of heat transfer from the purging fluid to the substrate W is the presence of the membrane 18 in the channel 17. In the example of Figure 2, the membrane 18 is present. The membrane 18 is configured to reduce the amount of radiation having an unwanted wavelength reaching the substrate W. For example, the membrane 18 may be configured to absorb infrared radiation and transmit EUV radiation. The membrane 18 may stop contaminants from reaching the first chamber 15 via the channel 17 or significantly reduce the amount of contamination reaching the first chamber 15 from the second chamber 16. The presence of the membrane 18 in the channel 17 is optional. The membrane 18 may substantially stop the flow of purging fluid 23 from entering the first chamber 15. A flow of purging fluid 23 may be provided in the channel 17 when the membrane 18 is present to reduce contamination of the membrane 18. Contamination of the membrane 18 may result in reduced transmission of desired wavelengths of radiation through the membrane 18. The flow rate of purging fluid 23 provided to the channel 17 when the membrane is present 18 may be lower than the flow rate of purging fluid 23 provided to the channel 17 when the membrane 18 is not present. The reduction of the flow of purging fluid 23 in the channel 17 when the membrane 18 is present may cause an increase in the amount of heat that transfers from the purging fluid to the substrate W. For example, when the membrane 18 is present and the flow rate of purging fluid is consequently lower, the purging fluid 23 may transfer a power of up to about 400 mW to the substrate W. When the membrane 18 is not present, and the flow rate of purging fluid is consequently higher, the purging fluid 23 may transfer a power of up to about 100 mW to the substrate W.

[00080] The substrate W may be at an ambient temperature of the lithographic apparatus. For example, the lithographic apparatus and the substrate W may both have an ambient temperature of about 22°C. The heat energy transferred from the purging fluid 23 to the substrate W may cause the substrate to undergo a thermal deformation which may result in a lithographic error such as an overlay error during a lithographic exposure. If the membrane 18 is not present then the heat energy transfer from the purging fluid 23 to the substrate W may, for example, result in an overlay error of between about 0.1 nm and about 0.8 nm. If the membrane 18 is present then the heat energy transfer from the purging fluid 23 to the substrate W may, for example, result in an overlay error of between about 0.1 nm and about 1.5 nm.

[00081] Figure 3 schematically depicts a cooling system 30 and a channel 31 according to an embodiment of the invention. The cooling system 30 is configured to cool a wall 36 of the channel 31. The cooling system 30 comprises a heat exchanger 32 having a cool region 33 in thermal communication with a wall 36 of the channel 31. The heat exchanger 32 may, for example, comprise a Peltier device. The heat exchanger 32 may be in thermal communication with the wall 36 of the channel 31 via dedicated thermal conductors 34. The dedicated thermal conductors 34 may be configured to provide a thermally conductive path between the cool region 33 of the heat exchanger 32 and the wall 36 of the channel 31. The dedicated thermal conductors 34 may, for example, comprise heat pipes 34. In the example of Figure 3 the dedicated thermal conductors 34 comprise two heat pipes connected in parallel. A greater or smaller number of dedicated thermal conductors 34 may be provided. The two heat pipes are in thermal communication with a cool region of a Peltier device. The wall 36 of the channel 31 may be formed from a thermally conductive material such that a cooling power provided by the heat exchanger 32 via the dedicated thermal conductors 34 diffuses across the wall 36 of the channel 31. This enables parts of the wall 36 that are not in direct contact with the thermal conductor 34 to be cooled by the cooling system 30. The wall 36 of the channel 31 may be formed from a metal, e.g. steel, aluminium and/or titanium.

[00082] Selection of the type of heat pipe 34 to be used with the cooling system 30 may depend at least in part upon the desired range of operating temperatures of the heat exchanger 32. For example, if a desired operating temperature of the heat exchanger 32 is equal to or less than 0°C then a water- based heat pipe 34 is not suitable because the water in the heat pipe may freeze during operation. An alcohol-based heat pipe 34 may therefore be desirable when operating temperatures of the heat exchanger 32 equal to or lower than 0°C are desired. Alternatively ammonia-based or carbon dioxide- based heat pipes may be used.

[00083] The cooling system 30 may be configured to cool the wall 36 of the channel 31 from an ambient temperature of, for example, about 22°C to a temperature of between about 8°C and about 15°C. Once cooled by the cooling system 30, the wall 36 of the channel 31 reduces the temperature of purging fluid 35 that flows along the wall 36 of the channel 31. That is, a purging fluid curtain 43 formed by a flow of purging fluid 35 within the channel 31 is cooled by the wall 36 of the channel. The purging fluid 35 enters the channel 31 via a purging fluid inlet 48. The purging fluid inlet 48 extends along a perimeter of the channel 31. The purging fluid inlet 48 has only been shown on one side of the channel 31 depicted in Figure 3 for ease of illustration. The cooled wall 36 of the channel 31 may, for example, reduce the temperature of the purging fluid 35 from an ambient temperature of about 22°C to a temperature of between about 5°C and about 8°C.

[00084] A membrane (not shown) may be provided in the embodiment of the channel depicted in Figure 3. The wall 36 of the channel 31 may be cooled to a lower temperature when a membrane is present within the channel 31 to account for the lower flow rate of purging fluid provided to the channel 31 compared to when a membrane is not present within the channel 31 and a higher flow rate of purging fluid is provided to the channel. For example, a desired temperature set point of the wall 36 of the channel 31 may be between about 5°C and about 15°C depending, at least in part, upon the flow rate of purging fluid in the channel 31. Cooling the wall 36 of the channel 31 rather than just pre-cooling the purging fluid 35 may cool the purging fluid 35 with greater efficiency, thereby advantageously allowing control of the temperature of the purging fluid 35 across a greater range of temperatures.

[00085] The cooling system 30 may optionally be configured to cool the purging fluid 35 before the purging fluid travels via a conduit 37 to the channel 31. In the example of Figure 3, a portion of a conduit 37 that directs the purging fluid 35 from a source 39 through the heat exchanger 32 to the channel 31 is in thermal communication with the cool region 33 of the heat exchanger 32 such that the portion of the conduit 37 and the purging fluid 35 contained therein are cooled by the cool region 33 of the heat exchanger 32. Cooling the purging fluid 35 before the purging fluid travels via the conduit 37 to the channel 31 and cooling the wall 36 of the channel 31 such that the wall 36 of the channel 31 cools the purging fluid in the channel 31 advantageously reduces the dependency of the cooling system 30 on the flow rate of purging fluid 35 provided to the channel 31.

[00086] The cooling system 30 may comprise a mount 38. The mount 38 may be configured to provide a thermally conductive path between the dedicated thermal conductor 34 and the wall 36 of the channel 31. The mount 38 may be formed from a metal such as, for example, steel. The mount 38 may comprise an attachment feature 42 that is configured to enable easy removal and reattachment of the whole or part of the cooling system 30 to the wall 36 of the channel 31. The attachment feature 42 may be configured such that the wall 36 of the channel 31 may be removed from the lithographic apparatus without also having to remove the cooling system 30. The attachment feature 42 may, for example, comprise a blind bore in the wall 36 for receiving a bolt. The mount 38 advantageously enables simple and quick removal and installation of the wall 36 of the channel 31 in the lithographic apparatus, e.g. when the wall 36 of the channel 31 needs to be removed to install or replace or remove a membrane (not shown). The attachment feature 42 is not essential. The mount 38 is not essential (i.e. the dedicated thermal conductors 34 may be in direct contact with the wall 36 of the channel 31.

[00087] The cooling system 30 may comprise a controller 40. The controller 40 may form part of a feedback loop configured to control the temperature of the wall 36 of the channel 31. The controller

40 may, for example, comprise a proportional integral derivative (PID) controller. The cooling system 30 may be provided with a temperature sensor 41. The temperature sensor 41 may, for example, comprise a thermistor or a thermocouple in thermal communication with the wall 36 of the channel 31. The temperature sensor 41 may, for example, have an accuracy of about 0.1 K. The temperature sensor

41 may be configured to measure the temperature of the wall 36 of the channel 31 and output a signal indicative of the temperature of the wall 36 of the channel 31 to the controller 40. The controller 40 may be configured to receive the signal from the temperature sensor 41 and adjust a temperature of the cool region 33 of the heat exchanger 32 (e.g. by adjusting a current supplied to the Peltier device) based on the signal received from the temperature sensor 41.

[00088] A transfer of heat energy to the substrate W due to absorption of radiation during a lithographic exposure may also be partially or fully compensated for when controlling the temperature of the wall 36 of the channel 31. The temperature of the wall 36 of the channel 31 may be controlled such that an overlay error caused by thermal deformation of the substrate W during a lithographic exposure is reduced. The cooling system 30 may be calibrated such that a relationship between the temperature of the wall 36 of the channel 31 and an overlay error of a lithographic exposure that takes place during use of the cooling system 30 is determined. The calibration may comprise a plurality of steps. The first step may include cooling the wall 36 of the channel 31 to a desired temperature. The second step may include carrying out a lithographic exposure of the substrate W. The third step may include processing the substrate W and measuring an overlay error of the lithographic exposure. The three steps may be repeated with the lithographic exposure being carried out at different wall 36 temperatures until a desired reduction in overlay error is achieved.

[00089] The cooling power provided by the cooling system 30 may be calibrated in light of different thermal disturbances such as, for example, different pressure regimes in the first and second chambers 15, 16, movement of movable parts such as the substrate table WT generating heat energy via friction, etc. The calibration may include a plurality of steps. A first step may include cooling the wall 36 of the channel 31 to a desired temperature. A second step may include applying a thermal disturbance, e.g. altering a pressure regime in one or both of the first chamber 15 and the second chamber 16, or moving a movable part such as the substrate table WT. A third step may include measuring a change in temperature of the wall 36 of the channel 31 resulting from the applied thermal disturbance. A fourth step may include adjusting the cooling provided by the cooling system 30 to fully or partially compensate for the measured change in temperature of the wall 36 of the channel 31.

[00090] Figure 4 schematically depicts a cooling system 50 and a channel 51 according to an embodiment of the invention. The cooling system 50 in the example of Figure 4 is configured to cool a portion 52 of the wall 80 of the channel 51. The portion 52 of the wall 80 of the channel 51 that is cooled by the cooling system 50 is represented by a thick line in the example of Figure 4. The cooling system 50 comprises a mount 63 having an attachment feature 62 configured to enable reversible attachment of the cooling system 50 to the wall of 80 of the channel 51. The attachment feature 62 is not essential. The mount 63 is not essential (i.e. the dedicated thermal conductors 61 may be in direct contact with the wall 80 of the channel 51. Purging fluid 56 passes from a source 57 through a conduit 58 and into the channel 51 via a purging fluid inlet 55. The purging fluid inlet 55 extends along a perimeter of the channel 51. The purging fluid inlet 55 has only been shown on one side of the channel 51 depicted in Figure 4 for ease of illustration. Some of the flow of purging fluid in the channel 51 forms a purging fluid curtain 53 that is configured to reduce the amount of contamination reaching the first chamber 15 from the second chamber 16. A cool region 59 of a heat exchanger 60 is in thermal communication with the portion 52 of the wall 80 of the channel 52 via a thermal conductor 61. The wall 80 of the channel 51 may be formed from a thermally conductive material such that a cooling power provided by the heat exchanger 60 via the dedicated thermal conductors 61 and the mount 63 diffuses across the wall 80 of the channel 51. This enables parts of the wall 80 that are not in direct contact with the mount 63 to be cooled by the cooling system 50. The wall 80 of the channel 51 may be formed from a metal, e.g. steel, aluminium and/or titanium. The mount 38 may be formed from a metal such as, for example, steel. The flow of purging fluid 56 passing the cooled portion 52 of the wall 80 of the channel 51 is cooled by the cooled portion of the wall 80 of the channel 51. It may be preferable to cool a lower portion of the wall 80 of the channel 51 (i.e. a portion of the wall of the channel that is located beneath a purging fluid inlet 55 of the wall of the channel). This is because cooling the purging fluid increases a density of the purging fluid, therefore cooling the purging fluid beneath the purging fluid inlet 55 increases the fraction of flow of purging fluid travelling toward the second chamber 16.

[00091] As discussed above, the lithographic apparatus may comprise a cooling apparatus 54 configured to cool one or more regions of the substrate W, e.g. regions that are in the vicinity of an exposure target area of the substrate W. Part of the cooling apparatus 54 may be cooled to cool a flow of purging fluid 56 that is in the vicinity of the cooling apparatus 54. As discussed above, cooling the purging fluid that is beneath the purging fluid inlet of the wall 80 of the channel 51 may increase the fraction of flow of purging fluid travelling toward the second chamber 16 thus reducing the amount of contaminants reaching the first chamber 15 from the second chamber 16. The cooling system 50 may be configured to cool a part of the cooling apparatus 54. For example, the cool region 59 of the heat exchanger 60 may be in thermal communication with a part of the cooling apparatus 54 via a dedicated thermal conductor, such as a heat pipe (not shown).

[00092] If the purging fluid curtain 53 is cooled too much then over-cooling of the substrate W may occur which may result in an overlay error during a lithographic exposure. Over-cooling of the substrate W may be reduced and/or avoided by not cooling a portion of the wall 80 of the channel 51. A portion

64 of the wall 80 of the channel 51 that is located above the purging fluid inlet 55 may be not cooled and may be heated. The lithographic apparatus may further comprise a heating system 68 configured to heat a portion 64 of the wall 80 of the channel 51. For example, a hot region 66 of a heat exchanger

65 may be in thermal communication with the portion 64 of the wall 80 of the channel 51 via a dedicated thermal conductor 67 (e.g. one or more heat pipes). Optionally, a hot side 69 of the heat exchanger 60 of the cooling system 50 may be used to provide the portion 64 of the wall 80 of the channel 51 with heat energy. Heating a portion 64 of the wall 80 of the channel 51 located above the purging fluid inlet 55 may advantageously reduce the amount of contamination reaching the first chamber 15 from the second chamber 16. This is because the purging fluid 56 above the purging fluid inlet 55 will gain heat energy from the heated portion 64 of the wall 80 of the channel 51 and undergo thermal expansion. Thermal expansion of the purging fluid 56 above the purging fluid inlet 55 induces an increased flow of purging fluid 56 towards the second chamber 16. Increasing the fraction of flow of purging fluid travelling towards the second chamber 16 may advantageously reduce the amount of contaminants reaching the first chamber 15 from the second chamber 16.

[00093] Cooling the wall 80 of the channel 51 rather than only cooling the purging fluid 56 before the purging fluid reaches the channel 51 advantageously reduces heat transfer from the purging fluid 56 to the substrate W because the purging fluid 56 is cooled by the wall 80 of the channel 51 and as such, an increase of temperature of the purging fluid 56 when the purging fluid travels from the heat exchanger 49 to the channel 31 is accounted for when the purging fluid 56 is cooled by the wall 80 of the channel 51. The heat exchanger 49 that is configured to pre-cool the purging fluid is optional. Cooling the wall 80 of the channel 51 advantageously increases a mass flow rate of purging fluid 56 flowing towards the second chamber 16 because decreasing the temperature of purging fluid 56 in the channel 51 increases a density of the purging fluid 56 in the channel 51. Cooling the wall 80 of the channel 51 advantageously decreases a diffusion coefficient of the purging fluid for contaminants diffusing from the second chamber 16 to the first chamber 15. This is because although reducing the temperature of the purging fluid 56 increases a density of the purging fluid, a diffusion coefficient of the purging fluid for contaminants scales more strongly with the temperature of the purging fluid than with the density of the purging fluid. Thus, the diffusion coefficient of the purging fluid 56 for contaminants decreases when the temperature of the purging fluid 56 in the fluid curtain 53 is decreased by the wall 80 of the channel 51.

[00094] Cooling the wall 80 of the channel 51 advantageously reduces the dependency of the cooling system 50 on the flow rate of purging fluid 56 provided to the channel 51 because the purging fluid is cooled when the purging fluid reaches the channel 51 rather than the purging fluid only being cooled in a conduit 58 on the way to the channel 51. A greater range of purging fluid flow rates are enabled by the cooling system 50. A membrane (not shown) may be provided in the embodiment of the channel depicted in Figure 4. The lower dependency on the flow rate of the purging fluid 56 allows the channel 51 comprising a cooling system 50 to be suitable for use both when a membrane is present in the channel 51 and when a membrane is not present in the channel 51.

[00095] Cooling the wall 80 of the channel 51 also advantageously reduces the variation of lithographic errors between different lithographic apparatus that are caused by heat transfer from the purging fluid 56 to the substrate W. This is because the purging fluid 56 is cooled by the wall 80 of the channel 51 so the effect of any lithographic apparatus-dependent heat transfer to the purging fluid 56 when the purging fluid travels from the heat exchanger 60 to the channel 51 is reduced. In other words, a thermal history of the purging fluid 56 is less important because the purging fluid 56 is cooled by the wall 80 of the channel 51 once the purging fluid 56 reaches the channel 51. Variation of lithographic errors between different lithographic apparatus may, for example, be a result of manufacturing tolerances causing differences between lithographic apparatus of the same model.

[00096] Cooling the wall 80 of the channel 51 may advantageously reduce the flow rate of purging fluid 56 required to reach a desired level of contaminant suppression. For example, cooling the wall 80 of the channel 51 may reduce the flow rate of purging fluid 56 required to reach a desired level of contaminant suppression by about 10%. Reducing the required flow of purging fluid 56 advantageously reduces the vacuum requirements of the first chamber 15 and/or the second chamber 16, thus reducing operational costs of the lithographic apparatus.

[00097] Figure 5 schematically depicts a view from above an opening of a known channel 70. The wall 81 of the channel 70 comprises a purging fluid inlet 71 that is continuous about a perimeter 72 of the channel 70 such that a flow of purging fluid (not shown) is evenly distributed along the perimeter 72 of the channel 70. However, due to the shape of the perimeter 72 of the channel 70, the flow of purging fluid within the channel 70 may be inhomogeneous. The shape of the opening of the channel 70 may be thought of as a combination of a central portion 73 and two end portions 74a-b at opposing ends of the central portion 73. In the example of Figure 5 the central portion 73 is generally rectangular and the end portions 74a-b are generally semi-circular. In practice, the central portion 73 of the channel 70 may be curved and the channel may therefore have a more kidney-like or banana-like shape than that shown in the example of Figure 5. In general, the central portion 73 and the end portions 74a-b may have different shapes to those shown in the example of Figure 5.

[00098] The central portion 73 of the channel 70 is provided with purging fluid by a first purging fluid inlet section 75 and a second purging fluid inlet section 76. The total length of purging fluid inlet that provides the central portion 73 with purging fluid is equal to the length of the first purging fluid inlet section 75 and the length of the second purging fluid inlet section 76. The first end portion 74a of the channel 70 is provided with purging fluid by a third purging fluid inlet section 77. The total length of purging fluid inlet that provides the first end portion 74a with purging fluid is equal to the length of the third purging fluid inlet section 77. The second end portion 74b of the channel 70 is provided with purging fluid by a fourth purging fluid inlet section 78. The total length of purging fluid inlet that provides the second end portion 74b with purging fluid is equal to the length of the fourth purging fluid inlet section 78.

[00099] It can be seen from Figure 5 that the central portion 73 covers a greater cross-sectional area of the channel 70 than each of the end portions 74a-b. In known channels 70 the flow of purging fluid is evenly distributed across the purging fluid inlet 71. However, different sections of the purging fluid inlet provide the flow of purging fluid to different sized cross-sectional areas 73, 74a-b of the channel 70. That is, a ratio of a size of a cross-sectional area of the channel that a section of the purging fluid inlet provides purging fluid to versus the length of the section of the purging fluid inlet varies around the perimeter 72 of the channel 70. Variation of the channel cross-sectional area to purging fluid inlet length ratio may lead to an inhomogeneous flow of purging fluid within the channel 70. In the example of Figure 5, the ratio of the cross-sectional area of the central portion 73 of the channel 70 to the length 75, 76 of purging fluid inlet 71 that provides purging fluid to the central portion 73 is approximately twice the ratio of the cross-sectional area of an end portion 74a, 74b to a length 77, 78 of purging fluid inlet 71 that provides purging fluid to an end portion 74a, 74b. The difference in channel cross-sectional area to purging fluid inlet length ratio between the different sections of the purging fluid inlet 71 may result in an inhomogeneous flow of purging fluid through the channel 70. An inhomogeneous flow of purging fluid within the channel may not suppress contamination as well as a more homogeneous flow of purging fluid within the channel.

[000100] Figure 6 schematically depicts a view from above an opening of a channel 90 comprising a purging fluid inlet 91 having a flow restrictor 92 according to an embodiment of the invention. The flow restrictor 92 is configured to allow different sections of the purging fluid inlet 91 to provide different flow rates of purging fluid to the channel 90. In the example of Figure 6 the flow restrictor 92 comprises baffles. The baffles 92 are configured to divide the purging fluid inlet 91 into separate sections 93a-d that are capable of providing different flow rates of purging fluid to the channel (e.g. to different cross-sectional areas of the channel 90). First and second sections 93a-b of the purging fluid inlet 91 provide purging fluid to a central area 94a of the channel 90. A third section 93c of the purging fluid inlet 91 provides purging fluid to an end area 94b of the channel 90. A fourth section 93d of the purging fluid inlet 91 provides purging fluid to another end area 94c of the channel 90.

[000101] The baffles 92 separate the purging fluid inlet into different sections 93a-d such that different flow rates of purging fluid may be provided to different cross-sectional areas 94a-c of the channel 90 via the different sections 93a-d of purging fluid inlet 91. The flow rate of purging fluid provided to different sections 93a-d of the purging fluid inlet 91 may vary between different sections 93a-d of the purging fluid inlet 91 to account for the size of the cross-sectional areas 94a-c of the channel 90 that each section 93a-d of the purging fluid inlet 91 provides purging fluid to. For example, the flow rate of purging fluid provided to different sections 93a-d of the purging fluid inlet 91 may depend at least in part upon the ratio of the size of a cross-sectional area 94a-c of the channel 90 to the total length of purging fluid inlet section 93a-d that provides the cross-sectional area of the channel with purging fluid. Each section 93a-d of the purging fluid inlet 91 may, for example, comprise a mass flow controller (not shown). The mass flow controller may be configured to control the flow rate of purging fluid supplied through a section 93a-d of the purging fluid inlet 90.

[000102] In the example of Figure 6, the channel 90 has a kidney-like shape. However, in a similar manner discussed above in relation to Figure 5, the channel 90 may be approximated as having a central portion 94a and two end portions 94b-c. In the example of Figure 6, the central portion 94a and the end portions 94b-c are curved, with the central portion 94a having a smaller radius of curvature than the end portions 94b-c. In this example, the ratio of the cross-sectional area of the central portion 94a of the channel 90 to the total length of the purging fluid inlet sections 93a-b that provide purging fluid to the central portion 94a is approximately twice the ratio of the cross-sectional area of an end portion 94b-c to the length of a purging fluid inlet section 93c-d that provides purging fluid to an end portion 94b-c. In the example of Figure 6, providing first and second sections 93a-b of the purging fluid inlet 91 with twice the flow rate of purging fluid that is provided to the third and fourth sections 93c-d of the purging fluid inlet 91 may reduce an inhomogeneity of purging fluid flow within the channel 90, which may advantageously reduce the amount of contamination reaching the first chamber (not shown) from the second chamber (not shown).

[000103] The channel may have a different cross-sectional shape to those shown in Figure 5 and Figure 6. It will be appreciated by the skilled person that channels having different cross-sectional shapes may be represented as any desired combination of cross-sectional areas. In general, the ratios of the sizes of cross-sectional areas of the channel to the total lengths of the purging fluid inlet sections that provide purging fluid to the cross-sectional areas may be determined for different cross-sectional areas of the channel and the flow rate of purging fluid provided to the different cross-sectional areas via different sections of the purging fluid inlet may be determined therefrom. The flow rate of purging fluid required to be provided to different cross-sectional areas via different sections of the purging fluid inlet in order to reduce an inhomogeneity of flow within the channel may be determined using, for example, a computational fluid dynamics model.

[000104] In the example of Figure 6 baffles 92 are provided to split the purging fluid inlet into four sections 93a-d, with some of the sections receiving different flow rates of purging fluid. In the example of Figure 6, each section of the purging fluid inlet 93a-d is provided with purging fluid via different channels 95a-d such that the flow rate of purging fluid provided to each section of purging fluid inlet 93a-d may vary between different sections of purging fluid inlet 93a-d. A greater or smaller number of baffles 92 may be provided to split the purging fluid inlet 91 into a greater or smaller number of sections 93a-d provided with purging fluid via a greater or smaller number of channels 95a-d. It may be preferable to provide a greater number of baffles 92 to split the purging fluid inlet 91 into a greater number of sections 93a-d such that the sections are smaller and the flow rate provided to different sections 93a-d of the purging fluid inlet 91 may be varied in greater detail about the perimeter of the channel.

[000105] Figure 7 schematically depicts a cross-sectional view of a section of a purging fluid inlet 100 comprising a flow restrictor 101 according to an embodiment of the invention. The direction of the flow of purging fluid is into the page in the example of Figure 7. The flow restrictor 101 is configured to allow different sections 103a-b of the purging fluid inlet 100 to provide different flow rates of purging fluid to the channel (not shown). The flow restrictor 101 comprises a varying purging fluid inlet opening size 102a-b. The first section 103a of the purging fluid inlet 100 has a first opening size 102a and a second section 103b of the purging fluid inlet 100 has a second opening size 102b. The first opening size 102a of the purging fluid inlet 100 is greater than the second opening size 102b of the purging fluid inlet 100 meaning that a greater flow rate of purging fluid may be provided to the channel via the first section 103a of the purging fluid inlet 100 compared to the flow rate of purging fluid provided to the second section 103b of the purging fluid inlet 100.

[000106] The purging fluid inlet 100 may be provided with purging fluid at a uniform, equalized pressure across the length of the purging fluid inlet 100. However, the varying opening size of the purging fluid inlet 100 causes different sections 103a-b of the purging fluid inlet 100 to provide different flow rates of purging fluid to different cross-sectional areas of the channel. In practice, it may be difficult to provide a flow of purging fluid having an equalized pressure across the length of the purging fluid inlet 100 due to a non-zero resistance of flow within the purging fluid inlet. For example, the purging fluid inlet 100 of the wall 104 of the channel may be fed with purging fluid via a conduit that only injects purging fluid into a small section of the purging fluid inlet. The purging fluid may exit the conduit into the purging fluid inlet and then spread out along the purging fluid inlet and flow into the channel. In this example, the pressure of the purging fluid in the purging fluid inlet is larger proximate the conduit and smaller further away from the conduit. A variation of the opening size 102a-b of the purging fluid inlet may be selected in order to improve equalization of the pressure of purging fluid in the purging fluid inlet.

[000107] The flow rate of purging fluid provided to the channel by different sections 103a-b of the purging fluid inlet 100 may depend at least in part upon the ratio of the size of a cross-sectional area of the channel to the total length of purging fluid inlet section 103a-b that provides the cross-sectional area of the channel with purging fluid. For example, the ratio of the size of the cross-sectional area of the channel that is provided with purging fluid by the first section 103a of the purging fluid inlet to the length of the first section 103a of the purging fluid inlet may be approximately twice the ratio of the cross-sectional area of the channel that is provided with purging fluid by the second section 103b of the purging fluid inlet to the length of the second section 103b of the purging fluid inlet. In this example, providing the first section 103a of the purging fluid inlet 100 with approximately twice the opening size 102a of the opening size 102b of the second section 103b of the purging fluid inlet 100 may enable approximately twice the flow rate of purging fluid to be provided to the first sections 103a of purging fluid inlet compared to the second section 103b of purging fluid inlet, thus achieving a more homogeneous flow of purging fluid within the channel. For example, the opening size 102a of the first section of purging fluid inlet 103a may be about 0.5 mm and the opening size 102b of the second section of purging fluid inlet 103b may be about 0.25 mm.

[000108] Baffles may be used in combination with a varying purging fluid inlet opening size. Using a varying purging fluid inlet opening size advantageously allows a more continuous variation of the flow rate of purging fluid about the purging fluid inlet of the channel compared to the use of baffles. For example, the purging fluid inlet opening size may be varied continuously about the perimeter of the channel such that a desired distribution of purging fluid flow rate is achieved about the perimeter of the channel. Reducing the inhomogeneity of the flow of purging fluid within the channel advantageously increases suppression of contaminants reaching the first chamber from the second chamber. Reducing the inhomogeneity of the flow of purging fluid within the channel may allow a lower total flow rate of purging fluid to be used.

[000109] Figure 8 schematically depicts a cross-sectional view from the side of a known channel 110. The known channel 110 comprises a purging fluid inlet 111 that extends along a perimeter of the channel 110. The perimeter of the channel 110 is defined by a wall 116. An injection angle 114 of purging fluid through the purging fluid inlet 111 may be defined as the angle between a line 115 which bisects the purging fluid inlet 111 and an optical axis 113 of the lithographic apparatus. The known channel 110 comprises a substantially perpendicular injection angle 114.

[000110] Figure 9 schematically depicts a cross-sectional view from the side of a channel 120 comprising an angled purging fluid inlet 121 according to an embodiment of the invention. The angled purging fluid inlet 121 extends along a perimeter of the channel 120. An injection angle 122 of purging fluid through the angled purging fluid inlet 121 may be defined as the angle between a line 136 which bisects the angled purging fluid inlet 121 and an optical axis 124 of the lithographic apparatus.

Providing a non-perpendicular injection angle advantageously increases the momentum of purging fluid flow travelling towards the second chamber. Increasing the momentum of purging fluid flow travelling towards the second chamber increases suppression of contamination. Theoretically, the injection angle 122 may match an angle 125 between the wall 126 of the channel 120 and the optical axis 124 of the lithographic apparatus in order to give the purging fluid as much momentum in the direction of the second chamber (not shown) as possible, thus increasing the suppression of contaminants travelling towards the first chamber (not shown). In the example, of Figure 9 the angle 125 between the wall 125 of the channel 120 and the optical axis 124 of the lithographic apparatus (which may be referred to as the channel wall angle) is about 55°, thus the injection angle 122 may preferably be as close to 55° as possible. However, machining limitations when fabricating the wall 126 of the channel 120 comprising an angled purging fluid inlet 121 may limit the extent to which the injection angle 122 substantially matches the channel wall angle 125. For example, the injection angle 122 may be about 43° rather than about 55° due to machining limitations. A difference between the channel wall angle 125 and the injection angle 122 may be between about 1° and about 15°. The injection angle 122 may vary along a perimeter of the channel 120. The wall 126 of the channel 120 and the walls of the angled purging fluid inlet 121 may be formed from a metal, e.g. steel, aluminium and/or titanium.

[000111] Having an angled purging fluid inlet 121 enables the purging fluid to have a greater amount of momentum in the direction of the second chamber (not shown) when the purging fluid enters the channel 120 via the angled purging fluid inlet 121. Increasing the momentum of purging fluid in the direction of the second chamber advantageously increases the fraction of purging fluid flowing towards the second chamber which reduces the amount of amount of contamination reaching the first chamber from the second chamber. However, having an angled purging fluid inlet 121 may decrease an effective suppression length of the fluid curtain in the channel 120 compared with a purging fluid inlet in a known channel (e.g. the purging fluid inlet of the channel depicted in Figure 8) positioned at the same height along the wall 126 of the channel (e.g. about half-way up the wall of the channel). The effective suppression length of the fluid curtain in the channel 120 may be understood as a distance across which purging fluid is flowing towards the second chamber. It may therefore be preferable to increase a height 128 of the angled purging fluid inlet 121 along the wall 126 of the channel 120 such that an effective suppression length of the fluid curtain is maintained or increased. The height 128 of the purging fluid inlet 121 may be defined relative to the position of the substrate table WT as the distance between a lower wall 127 of the purging fluid inlet and an upper surface of the substrate table WT. In known channels (e.g. the channel depicted in Figure 8) the height of the purging fluid inlet may be about 15 mm. The height 128 of the angled purging fluid inlet 121 may be greater than 15 mm, e.g. between about 20 mm and about 30 mm. The height of the angled purging fluid inlet 121 may vary along a perimeter of the channel 120.

[000112] An interior wall 123, 127 of the purging fluid inlet 121 may be curved. The upper interior wall 123 of the purging fluid inlet may be linear and the lower interior wall 127 of the purging fluid inlet 121 may be curved. Alternatively, the upper interior wall 123 of the purging fluid inlet may be curved and the lower interior wall 127 of the purging fluid inlet may be linear. As another alternative, both the upper interior wall 123 of the purging fluid inlet and the lower interior wall 127 of the purging fluid inlet may be curved. An interior wall 123, 127 of the purging fluid inlet 121 may be curved such that there is a smooth transition from the curved interior wall of the purging fluid inlet to the wall 126 of the channel 120. In the example of Figure 9 the lower interior wall 127 of the purging fluid inlet is curved such that there is a smooth transition from the lower interior wall 127 of the purging fluid inlet to the wall 126 of the channel 120. The smooth transition from the lower interior wall 127 of the purging fluid inlet 121 to the channel wall 127 may reduce a separation of the flow of purging fluid into an upward flow towards the first chamber and a downward flow towards the second chamber. That is, a greater fraction of the flow of purging fluid may exit the angled purging fluid inlet 121 and travel towards the second chamber, which may in turn advantageously reduce the amount of contamination reaching the first chamber from the second chamber.

[000113] An interior wall 123, 127 of the purging fluid inlet 121 may be curved such that the purging fluid inlet defines a converging flow path for the purging fluid. In the example of Figure 9 the lower interior wall 127 of the purging fluid inlet 121 is curved such that the purging fluid inlet 121 defines a converging flow path for the purging fluid. That is, a volume through which purging fluid flows decreases when travelling towards the channel 120 via the purging fluid inlet 121. The flow path of the purging fluid may be said to diverge when the purging fluid exits the purging fluid inlet 121 into the channel 120. The converging flow path formed by the one or more curved interior walls 127 of the purging fluid inlet 121 may function similarly to the converging flow path of a de Laval nozzle. The converging flow path of the purging fluid defined by the curved interior wall of the purging fluid inlet may encourage acceleration of the purging fluid (e.g. to supersonic velocities) towards the second chamber, which may in turn advantageously reduce the amount of contamination reaching the first chamber from the second chamber. The interior wall 126, 127 of the purging fluid inlet 121 may be curved such that the opening size 130 of the purging fluid inlet is narrowest at or before an end of the purging fluid inlet. Constructing the angled purging fluid inlet 121 such that the narrowest opening size

130 of the purging fluid inlet is located at or before an end of the purging fluid inlet (rather than at an edge of the purging fluid inlet as shown in Figure 9) may further encourage acceleration of the purging fluid towards the second chamber because the interior wall of the purging fluid inlet that extends beyond the narrowest section of the purging fluid inlet 121 towards the channel 120 is available to define a diverging section of the purging fluid flow path in the purging fluid inlet. Selection of a radius of curvature of the curved interior wall 127 of the purging fluid inlet 121 may be determined at least in part by the size of the channel wall angle 125 and a desired opening size 130 of the purging fluid inlet. A membrane (not shown) may be provided in the embodiment of the channel depicted in Figure 9.

[000114] Figure 10 schematically depicts a perspective view in cross-section of a channel 131 comprising an angled purging fluid inlet 132 according to an embodiment of the invention. The angled purging fluid inlet 132 is supplied with purging fluid via a purging fluid store 133. The purging fluid store 133 extends circumferentially about the channel 131. The purging fluid store 133 may have a volume that is large enough for a pressure of the purging fluid to equalize such that purging fluid is distributed evenly across the angled purging fluid inlet 132. If there is not enough space available in the lithographic apparatus for the purging fluid store 133 to be large enough to equalize the purging fluid pressure within the purging fluid inlet 132 then the opening size of the purging fluid inlet may be varied to enable equalization of the purging fluid pressure within the purging fluid inlet, e.g. as discussed above in relation to Figure 7. A lower interior wall 134 of the angled purging fluid inlet 132 curves to form a smooth transition with the wall 135 of the channel 131. The wall 135 of the channel

131 may be formed from a metal, e.g. steel, aluminium and/or titanium. [000115] Figure 11 schematically depicts the known channel 110 depicted in Figure 8 with a visible flow of purging fluid 140 exiting the purging fluid inlet 111. As can be seen from Figure 11, at least some of the flow of purging fluid 140 recirculates beneath the purging fluid inlet 111. Some of the flow of purging fluid may recirculate in a similar manner above the purging fluid inlet (not shown). Recirculation occurs because the flow of purging fluid separates after leaving the purging fluid inlet 111 and a pressure within the channel 110 increases when travelling down the channel wall 141 beneath the purging fluid inlet 111. That is, a static pressure of the flow of purging fluid at the purging fluid inlet 111 is lower than a stagnant pressure of the flow of purging fluid in the channel 110. Recirculation of purging fluid 140 may lead to contaminants being pushed up along the channel wall 141 towards the first chamber (not shown) by the recirculating flow of purging fluid 140. In the example of Figure 11, the purging fluid inlet 111 is depicted as being located about half-way up the channel wall 141. The purging fluid inlet 111 may be located at a lesser or greater height up the channel wall 141. For example, the purging fluid inlet 111 may be located about a third of the way up the channel wall 141 from the bottom of the channel wall or about a quarter of the way up the channel wall 141 from the bottom of the channel wall.

[000116] Figure 12 schematically depicts a channel 150 comprising multiple purging fluid inlets 151a-b according to an embodiment of the invention. The wall 152 of the channel 150 comprises a first purging fluid inlet 151a which may, for example, be located at substantially the same height along the wall 152 of the channel 150 as the purging fluid inlet shown in Figure 11 (e.g. about halfway along the wall 152 of the channel 150). As was the case for the channel shown in Figure 11, the first purging fluid inlet 151a may be located at a greater or lesser height along the channel wall 152, e.g. about a third of the way along the channel wall 152 from the bottom of the channel wall or about a quarter of the way along the channel wall 152 from the bottom of the channel wall. A second purging fluid inlet 151b is provided closer to the substrate W than the first purging fluid inlet 151a. A first flow of purging fluid 153a is shown exiting the first purging fluid inlet 15 la and a second flow of purging fluid 153b is shown exiting the second purging fluid inlet 151b. The flow rate of purging fluid provided through the second purging fluid inlet 151b may be smaller than the flow rate of purging fluid provided through the first purging fluid inlet 151a. Providing a second purging fluid inlet 151b closer to the substrate W than the first purging fluid inlet 151a provides an area of increased pressure beneath the first purging fluid inlet 151a. The pressure within the channel 150 therefore decreases rather than increases when travelling down the channel wall 152 beneath the second purging fluid inlet 151b. This reversal of pressure gradient causes a reduction of the recirculation of purging fluid 153a-b within the channel 150, which advantageously reduces the amount of contamination travelling up the channel wall 152 from the second chamber to the first chamber. In other words, the flow of purging fluid exiting the second purging fluid inlet 151b defines a velocity vector in the region of the second purging fluid inlet 151b that does not allow significant recirculation to occur in the region of the second purging fluid inlet 151b. [000117] An opening size 154 of the second purging fluid inlet 151b may be between about 0.2 mm and about 5 mm, e.g. about 1 mm. A height 155 of the first purging fluid inlet 151a with respect to the substrate table WT may be between about 10 mm and about 40 mm, e.g. about 20 mm. A height of the second purging fluid inlet 156 with respect to the substrate table WT may be between about 6 mm and about 30 mm, e.g. about 12 mm. The height of the second purging fluid inlet 156 may be between about 25% and about 75% of the height of the first purging fluid inlet 155.

[000118] The flow rate of purging fluid 153b provided through the second purging fluid inlet 151b may depend at least in part upon a flow rate of purging fluid passing from the channel 150 through to the second chamber (not shown) when substantially no flow of purging fluid is provided through the second purging fluid inlet 151b. Preferably, the flow rate of purging fluid provided through the second purging fluid inlet 151b is between about 50% and about 200% of the flow rate of purging fluid that passes from the channel 150 through to the second chamber when substantially no flow of purging fluid is provided through the second purging fluid inlet 151b. In an example, a flow rate of 300 mbarls ^"1 of purging fluid 153a is provided though the first purging fluid inlet 151a when substantially no flow of purging fluid is provided through the second purging fluid inlet 151b and the flow rate passing from the channel through to the second chamber is about 40 mbarls ^"1 . In this example, a flow rate of between about 20 mbarls ^"1 and about 80 mbarls ^"1 of purging fluid is preferably provided through the second purging fluid inlet 151b to reduce recirculation of purging fluid within the channel 150. The wall 152 of the channel 150 may be formed from a metal, e.g. steel, aluminium and/or titanium.

[000119] A controller (not shown) may be provided, the controller being configured to control the flow rate of purging fluid provided through the first purging fluid inlet 151a and the flow rate of purging fluid provided through the second purging fluid inlet 151b. The controller may be configured to control the flow rate of purging fluid provided through the second purging fluid inlet 151b based on a flow rate of purging fluid that passes from the channel 150 through to the second chamber when substantially no flow is provided through the second purging fluid inlet 151b. The controller may be configured to provide a flow rate of purging fluid through the second purging fluid inlet 151b that is between about 50% and about 200% of a flow rate of purging fluid that passes from the channel 150 through to the second chamber when substantially no flow is provided through the second purging fluid inlet 152b.

[000120] The skilled person will appreciate that different features of different embodiments may be combined in any desired combination. For example, the channel may comprise more two or more purging fluid inlets at different heights along the channel wall and one or more of the purging fluid inlets may be angled purging fluid inlets, and/or one or more of the purging fluid inlets may comprise a flow restrictor. As another example, Figure 13 schematically depicts a perspective view of a channel 200 comprising first and second purging fluid inlets 201, 202 and a cooling system 203 according to an embodiment of the invention. The first purging fluid inlet 201 is angled. In the example of Figure 13, the second purging fluid inlet 202 is not angled and is located beneath the first purging fluid inlet 201. Alternatively, the second purging fluid inlet 202 may be angled in a similar manner to the first purging fluid inlet 201. If the second purging fluid inlet 202 is angled then it may be preferable to locate the second purging fluid inlet 202 at a greater height along the wall 205 of the channel 200 than would be the case if the second purging fluid inlet 202 was not angled. An angled second purging fluid inlet is not required to provide an area of increased pressure beneath the first purging fluid inlet 201. The channel 200 shown in Figure 13 is part of a lithographic apparatus that also comprises a cooling apparatus 204. Parts of the cooling apparatus 204 may be cooled as discussed above in relation to Figure 4.

[000121] The purging fluid may be Hydrogen gas. Other purging fluids may be used. For example, the purging fluid may comprise Nitrogen gas, Helium gas and/or Argon gas, or any combination thereof. The channel has been depicted as having a generally conical cross-section. However, the channel may take other forms. For example, the channel may have a generally rectangular cross-section or a generally square cross-section. The cooling system may comprise one or more heat pipes. The heat pipes may be connected in series or in parallel.

[000122] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[000123] Although Figure 1 depicts the radiation source SO as a laser produced plasma LPP source, any suitable source may be used to generate EUV radiation. For example, EUV emitting plasma may be produced by using an electrical discharge to convert fuel (e.g. tin) to a plasma state. A radiation source of this type may be referred to as a discharge produced plasma (DPP) source. The electrical discharge may be generated by a power supply which may form part of the radiation source or may be a separate entity that is connected via an electrical connection to the radiation source SO.

[000124] 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.

[000125] 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.