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Title:
METHOD FOR PROVIDING A PULSED RADIATION BEAM
Document Type and Number:
WIPO Patent Application WO/2020/212000
Kind Code:
A1
Abstract:
The present invention provides a method of providing a pulsed radiation beam for exposing a first object and a second object in a lithographic apparatus. The method comprising providing the pulsed radiation beam at a first frequency for exposing the first object and providing the pulsed radiation beam at a second frequency for exposing the second object, wherein the first frequency is lower than the second frequency.

Inventors:
DIKKEN DIRK JAN (NL)
WRICKE SANDRO (NL)
BECKERS JOHAN (NL)
Application Number:
PCT/EP2020/055128
Publication Date:
October 22, 2020
Filing Date:
February 27, 2020
Export Citation:
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Assignee:
ASML NETHERLANDS BV (NL)
International Classes:
G03F7/20; G03F9/00
Foreign References:
US20050122498A12005-06-09
US20150355538A12015-12-10
US20030098959A12003-05-29
Other References:
"Research Disclosure", RESEARCH DISCLOSURE, KENNETH MASON PUBLICATIONS, HAMPSHIRE, UK, GB, vol. 594, no. 42, 1 October 2013 (2013-10-01), pages 6, XP007142594, ISSN: 0374-4353
Attorney, Agent or Firm:
VERHOEVEN, Johannes Theodorus Maria (NL)
Download PDF:
Claims:
CLAIMS

1. A method of providing a pulsed radiation beam for exposing a first object and a second object in a lithographic apparatus, the method comprising:

providing the pulsed radiation beam at a first pulse frequency for exposing the first object; and

providing the pulsed radiation beam at a second pulse frequency for exposing the second object, wherein the first pulse frequency is lower than the second pulse frequency.

2. The method of claim 1, wherein the pulsed radiation beam at the first pulse frequency has a first intensity and the pulsed radiation beam at the second pulse frequency has a second intensity, and the first intensity is greater than the second intensity.

3. The method of claim 2, wherein a change of intensity of the pulsed radiation beam between the first intensity and the second intensity is inversely proportional to a change in pulse frequency between the first pulse frequency and the second pulse frequency.

4. The method of any one of the preceding claims, wherein the pulsed radiation beam having the first pulse frequency has a first pulse length and the pulsed radiation beam having the second pulse frequency has a second pulse length, and the first pulse length is longer than the second pulse length.

5. The method of claim 4, wherein a change of pulse length of the pulsed radiation beam between the first pulse length and the second pulse length is inversely proportional to a change in frequency between the first pulse frequency and the second pulse frequency.

6. The method of any one of the previous claims, wherein the second pulse frequency is approximately 6 kHz, and wherein the first pulse frequency is less than or equal to approximately 5 kHz.

7. The method of any one of the preceding claims, wherein the first pulse frequency is approximately less than or equal to 40% of second frequency.

8. The method of any one of the preceding claims, wherein the first object comprises chromium and/or chromium oxide.

9. The method of any one of the preceding claims, wherein the first object is a sensor for determining the position of the second object, and/or for determining the alignment of the second object with respect to a patterning device for patterning the pulsed radiation beam. 10. The method of any one of the preceding claims, wherein the second object is a substrate.

11. A method of exposing a first object and a second object in a lithographic apparatus by a pulsed radiation beam, the method comprising the method of providing a pulsed radiation beam of any one of claims 1 to 10.

12. A radiation source apparatus configured to generate a pulsed radiation beam for a lithographic apparatus, wherein the radiation source apparatus is configured to carry out the method of providing a pulsed radiation beam of any one of claims 1 to 10. 13. A computer program comprising computer readable instructions which, when run on a suitable computer controlled system, cause the computer controlled system to implement the method of any one of claims 1 to 11.

14. A lithographic apparatus configured to carry out the method of any one of claims 1 to 11.

Description:
METHOD FOR PROVIDING A PULSED RADIATION BEAM

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application claims priority of EP application no. 19170081.4, which was filed on 18 April 2019 and which is incorporated herein in its entirety.

FIELD OF THE INVENTION

[002] The present invention relates to a method of providing a radiation beam for exposing a first object and a second object, a computer program, a source apparatus and a lithographic apparatus.

BACKGROUND

[003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[004] There is a continuing desire to manufacture devices, e.g. integrated circuits, with ever smaller features. Integrated circuits and other microscale devices are often manufactured using optical lithography, but other manufacturing techniques, such as imprint lithography, e-beam lithography and nano-scale self-assembly are known.

[005] During manufacturing, the device is irradiated, i.e. exposed to radiation. It is important to ensure that the irradiation process is as accurate as possible. One of the issues with making the irradiation processes as accurate as possible is ensuring that the device to be irradiated is in the correct position. In order to control the position of the device, at least one sensor is provided to measure the location and/or alignment of the substrate to ensure that the substrate is positioned correctly with respect to the radiation used to expose the substrate.

[006] Some known sensors generally degrade over time as they are exposed to radiation. When a sensor has degraded by a certain amount, it can no longer be accurately used to measure the position of the substrate in the lithographic apparatus as desired. For example, a sensor may have a diffraction grating on a top surface which is used for measurement. When the diffraction grating is too degraded, the sensor may no longer accurately make measurements. Thus, the degraded sensor may be replaced by a new sensor.

[007] These sensors are generally expensive, and replacement of such a sensor can take up to approximately 30 hours which means that the lithographic apparatus is not available to manufacture devices in this time. Decreasing the rate of degradation of these sensors would be highly beneficial in reducing the frequency of sensor replacements in a lithographic apparatus and thus, reducing down time of a lithographic apparatus. It is preferable that the rate of degradation can be reduced whilst not affecting the performance of the sensor.

[008] Thus, it is desirable to reduce degradation of these sensors, and preferably, find a way of using the sensor in which the sensor can still perform as desired, but in which the sensor is replaced less frequency.

SUMMARY

[009] It is desirable to provide a way of increasing the length of time that known sensors can be used to measure the position of a substrate.

[0010] In the present invention, there is provided a method of providing a pulsed radiation beam for exposing a first object and a second object in a lithographic apparatus, the method comprising:

providing the pulsed radiation beam at a first pulse frequency for exposing the first object; and providing the pulsed radiation beam at a second pulse frequency for exposing the second object, wherein the first pulse frequency is lower than the second pulse frequency. The present invention further provides a radiation source apparatus configured to generate a pulsed radiation beam, a computer program comprising computer readable instructions and a lithographic apparatus.

[0011] Further embodiments, features and advantages to the present inventions, as well the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING/FIGURE

[0012] Figure 1 schematically depicts a lithographic apparatus. The accompanying drawing, which is incorporated herein and form part of the specification, illustrate one or more embodiments of the present invention and, together with the description, further serve to explain the principals of the invention and to enable a person skilled in the pertinent art to make and use the invention.

[0013] One or more embodiments of the present invention will now be described with reference to the accompanying drawing. The drawing provides an indication of certain features included in some embodiments of the invention. However, the drawing is not to scale. Examples of the size and range of sizes of certain features are described in the description below. DETAILED DESCRIPTION

[0014] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.

[0015] The embodiment(s) described, and references in the specification to“one embodiment”,“an embodiment”,“an example embodiment”, etc., indicate that the embodiment(s) described can include a particular feature, structure, or characteristic, but every embodiment cannot necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0016] Figure 1 schematically depicts a lithographic apparatus. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or any other suitable radiation), a patterning device support or support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device in accordance with certain parameters. The lithographic apparatus also includes a substrate table (e.g. a wafer table) WT or “substrate support” constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate in accordance with certain parameters. The substrate support may comprise a substrate table WT (otherwise referred to as a chuck) on which a substrate holder is supported. The substrate holder may be configured to support the substrate W. The apparatus further includes a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. including one or more dies) of the substrate W.

[0017] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, to direct, shape, or control radiation.

[0018] The patterning device support holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device MA is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS. Any use of the terms“reticle” or“mask” herein may be considered synonymous with the more general term“patterning device”. [0019] The term“patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam B may not exactly correspond to the desired pattern in the target portion of the substrate W, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam B will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0020] The patterning device MA may be transmissive or reflective. Examples of patterning devices MA include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0021] The term“projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0022] As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0023] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or“substrate supports” (and/or two or more mask tables or“mask supports”). In such“multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

[0024] The lithographic apparatus may also be of a type wherein at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system PS and the substrate W. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the patterning device (e.g. mask) MA and the projection system PS. Immersion techniques can be used to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that a liquid is located between the projection system PS and the substrate W during exposure. [0025] Referring to figure 1, the illuminator IL receives a radiation beam B from a radiation source SO. The radiation source SO and the lithographic apparatus may be separate entities, for example when the radiation source SO is an excimer laser. In such cases, the radiation source SO is not considered to form part of the lithographic apparatus and the radiation beam B is passed from the radiation source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0026] The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam B. Generally, at least the outer and or inner radial extent

(commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.

[0027] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the mask support structure (e.g., mask table) MT, and is patterned by the patterning device MA. Having traversed the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the patterning device support (e.g. mask table) MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioning device PM. Similarly, movement of the substrate table WT or“substrate support” may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the patterning device support (e.g. mask table) MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device (e.g. mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the patterning device alignment marks may be located between the dies. [0028] The lithographic apparatus may additionally comprise sensor S used for measuring alignment of the substrate W and/or lens aberration within the lithographic apparatus.

[0029] The depicted apparatus could be used in at least one of the following modes:

a. In step mode, the patterning device support (e.g. mask table) MT or“mask support” and the substrate table WT or“substrate support” are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT or“substrate support” is then shifted in the X and or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

b. In scan mode, the patterning device support (e.g. mask table) MT or“mask support” and the substrate table WT or“substrate support” are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT or“substrate support” relative to the patterning device support (e.g. mask table) MT or“mask support” may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

c. In another mode, the patterning device (e.g. mask table) MT or“mask support” is kept essentially stationary holding a programmable patterning device, and the substrate table WT or “substrate support” is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or“substrate support” or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0030] Combinations and or variations on the above described modes of use or entirely different modes of use may also be employed.

[0031] As described above, it can be beneficial to reduce degradation of certain objects. The sensors may function by use of a top-coating formed of at least one layer comprising chrome. Such sensors may be used to measure each substrate. However, the lifetime of these known sensors may be reduced due to degradation of the at least one layer formed using chrome. Generally, these sensors degrade such that they need to be replaced with a different sensor. As described, swapping such a sensor takes time in which the lithographic apparatus cannot be used. It is beneficial to reduce the degradation of these sensors such that the sensor lifetime is longer and the downtime of the lithographic apparatus is less frequent. [0032] The lifetime of known sensors (i.e. the length of time before the sensor needs to be replaced) are generally a few years. These sensor are expensive and take a significant time to replace within a lithographic apparatus. Increasing the lifetime of the sensor will improve availability of the lithographic apparatus and reduce costs.

[0033] Degradation of such sensors generally seems to occur on layers comprising chrome which is exposed to radiation. It is noted that the radiation is provided by radiation beam B which is pulsed.

In other words, the radiation beam B is not a continuous beam of radiation, but rather short bursts (i.e. pulses) of radiation. The pulse frequency is the number of pulses of the radiation beam in a given unit of time. The pulse frequency may otherwise be referred to as a laser repetition rate, or radiation beam repetition rate. Tests carried out in relation to the present invention surprisingly indicated that the degradation of the known sensors is not driven by peak-intensity of pulses of the radiation beam B but rather by the number of pulses of the radiation beam B. In other words, it was determined that the rate of degradation of the sensor depended on the number of radiation pulses rather than the peak intensity of the radiation beam B. Thus, the lifetime of the sensor can be extended by reducing the number of radiation beam B pulses received by the sensor.

[0034] Thus, it can be beneficial to control characteristics of the pulsed radiation beam B, and in particular the frequency of the pulses of the pulsed radiation beam B, to change the effect of the pulsed radiation beam B on an object. Meanwhile, it is desirable that the pulsed radiation beam B still provides a certain amount of radiation to an object, such as a substrate, in a given time period. Thus, it is advantageous to provide the pulsed radiation beam B having adjustable characteristics depending on the object to be exposed. For example, a pulsed radiation beam B having a higher pulse frequency may be used for certain objects such as a substrate, and having a lower pulse frequency for other objects which may be degraded by the radiation, such as a sensor.

[0035] The present invention can be applied to a number of different situations. In particular, the present invention may be particularly useful when applied to sensors used for measuring alignment and/or lens aberration within a lithographic apparatus. However, it is understood that the invention may be applied to any objects which are exposed to a pulsed radiation beam B, e.g. in a lithographic apparatus, wherein characteristics of the radiation, such as frequency, have different effects on the objects.

[0036] In an embodiment, a method of providing a pulsed radiation beam B for exposing a first object and a second object in a lithographic apparatus is provided. The method comprises providing the pulsed radiation beam B at a first pulse frequency for exposing the first object and providing the radiation beam B at a second pulse frequency for exposing the second object. The first pulse frequency is lower than the second pulse frequency. Controlling the pulsed radiation beam B in this way means that the first and second objects can be exposed having preferred pulse frequencies.

[0037] The present invention uses a pulsed radiation beam B with a lower (first) pulse frequency than a second pulse frequency. Altering the pulse frequency of the radiation beam B allows a pulsed radiation beam B to be provided having different characteristics to be provided for use with different objects. This is beneficial in that the pulsed radiation beam B can be provided which reduces degradation of an object whilst providing a higher frequency pulsed radiation beam B to other objects which are not degraded by the pulsed radiation beam B in the same way.

[0038] The pulsed radiation beam B may be provided at the first pulse frequency before being changed to the second pulse frequency. Alternatively, the pulsed radiation beam B may be provided at the second pulse frequency before being changed to the first pulse frequency. In other words, there are advantages relating specifically to providing the pulsed radiation beam B for exposing objects in a lithographic apparatus at a first pulse frequency, and a second pulse frequency higher than the first pulse frequency. The order in which the frequencies are changed, i.e. from first to second or from second to first may have further advantages in relation to how the pulsed radiation beam B is to be used at the first and second pulse frequency.

[0039] The way the pulse frequency is changed will depend on how the pulsed radiation beam B is provided. Various different methods for providing a pulsed radiation beam B for use in a lithographic apparatus are known. Thus, any known apparatus may be controlled as appropriate.

[0040] The frequency of the pulsed radiation beam B may be altered in a radiation source apparatus. The radiation source apparatus may be as described in relation to the radiation source SO referred to above. The radiation source SO may comprise a processor SP used to control the pulses of the pulsed radiation beam B. As described above, the radiation source SO may be separate from, or part of, the lithographic apparatus. If the radiation source SO is part of the lithographic apparatus, the processor SP in the radiation source SO and/or a processor LAP located elsewhere in the lithographic apparatus may be used to control the pulse frequency of the pulsed radiation beam B. The processor SP or LAP may comprise software which electronically controls the radiation source SO and thus, the pulse frequency of the pulsed radiation beam B.

[0041] For example only, the radiation source SO may be a separate device from the lithographic apparatus. The separate device may be an excimer laser. In this case, the excimer laser may use a device which can be electromagnetically triggered. For example, a wave-plate, such as a Pockel cell trigger may be used. In this example, the Pockels cell may receive a current and change

birefringence, and combined with a polarizer, the Pockels cell triggers the release of a pulse from an optical cavity which can be controlled. Thus the pulse frequency of the pulsed radiation beam may be triggered and controlled. Alternatively, the separate device may be an EUV laser. In this case, generation of the radiation beam B may be from droplets of fuel in the laser. Thus, the radiation source SO may alter the pulse frequency of the pulsed radiation beam B by reducing or increasing the rate of release of the droplets of fuel as appropriate and controlling the firing of a laser, which may be referred to as a pump laser, at the droplets.

[0042] As described, using a lower pulse frequency for the pulsed radiation beam B may be particularly useful for objects which have been determined to degrade at higher pulse frequencies. In particular, the first object may comprise a metal, or metal oxide. More specifically, the first object may comprise chromium and/or chromium oxide. The first object may comprise other materials which also degrade quicker at higher pulse frequencies. In other words, the embodiments could be applied to a range of objects which have functionality which may be degraded by the pulsed radiation beam B.

[0043] The first object may be a sensor, for example, a sensor for determining the position of the second object, and or for determining the alignment of the second object with respect to a patterning device for patterning the pulsed radiation beam, e.g. the patterning device MA described above. This is for example only, and the sensor may determine alignment of the second object with respect to any appropriate component, for example, a component of the lithographic apparatus. In an example, the first object could be the same as the position sensor IF and/or the sensor S.

[0044] Additionally or alternatively, the second object may be a substrate. The substrate W may be as described above, for example, the substrate W may be a silicon wafer. Generally, the second object may be any object which functions better at a higher pulse frequency and does not degrade due to the pulsed radiation beam B in the same way as the first object, or at least, at a slower rate than the first object.

[0045] The method may comprise exposing the first object using the pulsed radiation beam B with the first pulse frequency. Additionally or alternatively, the method may comprise exposing the second objection using the pulsed radiation beam B with the second pulse frequency. Thus, the method may comprise the steps of exposing the first and/or second object using the pulsed radiation beam B having the appropriate pulse frequency. Thus, the first and/or second object can be exposed to a pulsed radiation beam B having a pulse frequency which has been set as appropriate. For example, the pulse frequency of the pulsed radiation beam B used for the sensor may be reduced compared to the pulse frequency of the pulsed radiation beam B used for the substrate. The steps of exposing the first and or second object may comprise using a part of the lithographic apparatus (e.g. a substrate table WT) to support the first and or second object respectively and directing the pulsed radiation beam B to the surface of the first and/or second object respectively. The same part of the lithographic apparatus may be used to support the first and second object. Alternatively, different components of the lithographic apparatus may be used to support the first and or second objects.

[0046] For example only, the pulse frequency of a radiation beam previously used for a lithographic apparatus may have been set to 6 kHz during exposure of a first object (such as a sensor) and during exposure of a second object (such as a substrate). In the present embodiment, the lifetime of the sensor may be extended by reducing the first pulse frequency from 6 kHz to 5 kHz (or lower) for exposing the sensor but using the pulsed radiation beam B at a frequency of 6 kHz for exposing the substrate. Other characteristics of the pulsed radiation beam B may be scaled according to the change in frequency (as described below). The lifetime of the sensor described may increase depending on the ratio between the newly chosen frequency when exposing the sensor and the old setting of 6 kHz. [0047] Ideally, the first pulse frequency is significantly lower than the second pulse frequency. For example, the first pulse frequency may be approximately less than or equal to 90% of second pulse frequency, or preferably less than or equal to 80%, or preferably, less than or equal to 70%, or preferably less than or equal to 60%, or preferably, less than or equal to 50%, or preferably less than or equal to 40%. The greater the reduction in pulse frequency (i.e. the lower the percentage), the greater the effect of reducing degradation whilst still providing the pulsed radiation beam B with preferred characteristics for exposure to the second object.

[0048] The first pulse frequency is less than the second pulse frequency. The first pulse frequency may be any value lower than the second pulse frequency. Preferably, as described above, the difference between the first pulse frequency and the second pulse frequency is greater, rather than smaller. The first pulse frequency may be less than or equal to approximately 5 kHz, or preferably less than or equal to approximately 4 kHz, or more preferably less than or equal to approximately 3 kHz. The first pulse frequency may be higher than 5 kHz. These values/ranges may be considered independently or in combination with the percentages above. Thus, the first pulsed frequency may have a value/range defined here, and may be less than or equal to the second pulsed frequency based on any of the above percentages.

[0049] The second pulse frequency may be approximately 6 kHz. The second pulse frequency may be less than approximately 6 kHz, or preferably less than or equal to approximately 5 kHz, or more preferably less than or equal to approximately 4 kHz. The second pulse frequency may be higher than 6 kHz. These values/ranges may be considered independently or in combination with the percentages above. Thus, the second pulsed frequency may have a value/range defined here, and the first pulsed frequency may be less than or equal to the second pulsed frequency based on any of the above percentages. Additionally or alternatively, the second pulsed frequency may have a value/range defined here and the first pulsed frequency may have a value/range define above, as long as the first pulse frequency is lower than the second pulse frequency. Generally, the second pulse frequency is preferably as high as possible, depending on the second object to be exposed. If for example the second object is the substrate, it is advantageous to have the second pulsed frequency as high as possible because this can improve the throughput of production of the substrates.

[0050] The performance of an object may be affected by the amount of radiation received from a pulsed radiation beam B. The object may function based on receiving a certain amount of radiation. Therefore, it may be preferable to provide a certain amount of radiation for an object to function as desired whilst reducing degradation of the object . Although the pulse frequency of the pulse radiation beam B for the first object may be lower than the pulse second frequency, other characteristics of the pulsed radiation beam B may be controlled to increase the amount of radiation provided in a given time period such that a desired amount of radiation is provided to the first object. Thus, the dose of radiation may be preserved, even when the frequency is reduced, by altering other characteristics in addition to the frequency. Such characteristics may include the intensity, pulse fluence and/or the pulse length. The pulse intensity is the peak power of the radiation beam per unit area. Reference to the intensity may be interchangeable with pulse fluence. The pulse fluence is the pulse energy per unit area. Thus, the radiation beam may be controlled as described by using and measuring pulse fluence or intensity interchangeably.

[0051] The method may comprise altering the intensity of the pulsed radiation beam B between a first intensity and a second intensity, or vice versa. Thus, the pulsed radiation beam B at the first pulse frequency may have the first intensity and the pulsed radiation beam B at the second pulse frequency may have the second intensity. The first intensity may be greater than the second intensity. The change from the first intensity to the second intensity may be inverse to a change from the first pulse frequency to the second pulse frequency. Thus, as the pulse frequency reduces, the intensity may increase.

[0052] This may be particularly advantageous for providing a desired amount of radiation to an object, even when the pulse frequency is reduced. Thus, the intensity of the pulsed radiation beam B can be increased when the pulsed radiation beam B is at a lower pulse frequency such that the overall amount of radiation received by an object (over a given time period) is the same as it would be if the intensity were lower, but the pulse frequency were higher (over the same given time period). In this way, the reduction in radiation received by the object may not be proportional to the pulsed frequency reduction.

[0053] Preferably, the change in intensity of the pulsed radiation beam B between the first intensity and the second intensity is inversely proportional to a change in pulse frequency between the first pulse frequency and the pulse second frequency. Thus, not only does the intensity increase as the pulse frequency reduces, but the change of the intensity is scaled by the inverse of the change in pulse frequency. This means that the amount of radiation received by the first object and the second object in a given time period may be substantially the same, even if the pulse frequency is reduced for one object compared to the other. The intensity may be selected as appropriate.

[0054] In general, performance of the types of sensor described is strongly dependant on the total number of photons it receives. Altering other characteristics of the pulsed radiation beam B, such as the intensity (and or pulse length as described below) means that the performance of the sensor can be at least maintained, as well as increasing the lifetime of the sensor. In particular, the performance of the sensor could be maintained and the lifetime doubled if the pulsed radiation beam B provided for exposing the sensor (i.e. the pulsed radiation beam B at the first pulse frequency and intensity) has half the pulse frequency and double the intensity of a pulsed radiation beam B that would otherwise be used (i.e. the radiation beam B at the second pulse frequency and intensity).

[0055] The pulsed radiation beam B intensity may be controlled/altered in a variety of ways. The intensity of the pulsed radiation beam B may be controlled by the radiation source apparatus SO, e.g. which may be controlled by the source processor SP. Additionally or alternatively, the intensity may be controlled by a device in the lithographic apparatus. For example, the adjuster AD may alter the intensity of the pulsed radiation beam B. In further detail, in a lithographic machine the pulsed radiation beam may be manipulated, and possibly optimized, in the illumination system IL. In this illumination system IL properties of the pulsed radiation beam B may be changed, including changing the intensity or particularly, the peak intensity. This might be done by using a material which is partially transparent, and wherein part of the radiation beam B is reflected. By changing the angle with respect to the incoming radiation beam, the ratio between transmitted light and reflected light is changed. Another way of doing this would be by using a polarizing beam splitter and a half wave plate. If the radiation beam is polarized, the polarization angle of the radiation beam traveling through the half-wave plate is rotated depending on the respective angle of the half-wave plate. After passing through the half-wave plate, the radiation beam travels through a polarizer beam splitter, depending on the angle of the polarized radiation, a ratio of the radiation is transmitted and reflected. This ratio is changed by rotating the half-wave plate. Another way of doing this would be to use at least two lasers, which can be placed in parallel, and pulses from the at least two lasers can be combined in a beam path to form the pulsed radiation beam B. Any presently known technologies, or those yet to be developed, may be used to control intensity.

[0056] Additionally or alternatively, other characteristics of the pulsed radiation beam B may be altered. For example, the method may comprise altering the pulse length of the pulsed radiation beam B between a first pulse length and a second pulse length, or vice versa. The pulsed radiation beam B at the first pulse frequency may have the first pulse length and the pulsed radiation beam B at the second pulse frequency may have the second pulse length. The first pulse length may be greater than the second pulse length.

[0057] Altering another characteristic of the pulsed radiation beam B as described (e.g. altering the intensity and/or pulse length as described) means that the lifetime of the sensors described can be increased due to the reduction of the number of pulses used during exposing the sensors without detrimentally affecting the performance of the sensor. With this strategy, the lifetime of the sensor may be increased, for example by a factor of 2 if the pulse frequency is halved without affecting sensor accuracy.

[0058] Altering the pulse length may be particularly advantageous in providing a desired amount of radiation to an object, even when the pulse frequency is reduced as indicated above. The pulse length of the pulsed radiation beam B can be increased when the pulsed radiation beam B is at a lower pulse frequency such that the overall amount of radiation received by an object (over a given time period) is the same as it would be if the pulse length were shorter, but the pulse frequency were higher (over the same given time period). In this way, the reduction in radiation received by the object may not be proportional to the pulse frequency reduction.

[0059] Preferably, the change in pulse length of the pulsed radiation beam B between the first pulse length and the second pulse length is inversely proportional to the change in pulse frequency between the first pulse frequency and the pulse second frequency. Thus, not only does the pulse length increase as the pulse frequency reduces, but the change of the pulse length is scaled by the inverse of the change in pulse frequency. This means that the amount of radiation received by the first object and the second object in a given time period may be substantially the same, even if the pulse frequency is reduced for one object compared to the other. The pulse length may be selected as appropriate.

[0060] As described above, the performance of the sensor could be maintained and the lifetime doubled if the pulsed radiation beam B provided for exposing the sensor (i.e. the pulsed radiation beam B at the first frequency and pulse length) has half the pulse frequency and double the pulse length of a pulsed radiation beam B that would otherwise be used (i.e. the pulsed radiation beam B at the second pulse frequency and pulse length).

[0061] The pulse length of the pulsed radiation beam B may be controlled/altered in a variety of ways. The pulse length of the pulsed radiation beam B may be controlled by the radiation source apparatus SO, e.g. which may be controlled by the source processor SP. For example, the pulse length of the pulsed radiation beam B may be controlled by using multiple sources devices in a radiation source apparatus SO. Thus, the pulses from the multiple source devices could overlap to increase pulse length. Additionally or alternatively, the pulse length may be controlled by a device in the lithographic apparatus. For example, the adjuster AD may alter the pulse length of the pulsed radiation beam B. Additionally or alternatively, a pulse stretcher can be used. This is a device which makes copies of the pulse and gives a slight delay to each copy, thus generating a train of pulses. The subsequent pulses generally have reduced peak intensity. Any known technology, or those yet to be developed, may be used to control pulse length.

[0062] A computer program comprising computer readable instructions which, when run on a suitable computer controlled system, causes the computer controlled system to implement the method described above in any embodiment or variation. For example, a computer program may be provided for use in any of the processors described.

[0063] The radiation source apparatus may be configured to generate a pulsed radiation beam B for a lithographic apparatus. The radiation source apparatus may be configured to carry out the method described above in any embodiment or variation. The radiation source apparatus may be the same as the radiation source SO described above. The radiation source apparatus may be the a part of the lithographic apparatus (e.g. as a mercury lamp), or not part of the lithographic apparatus (e.g. an excimer laser) as described above. The radiation source apparatus may comprise the processor SP configured to control the pulsed radiation beam B in accordance with the method described above in any embodiment or variation.

[0064] The lithographic apparatus may be configured to carry out the method described above in any embodiment or variation. Thus, the lithographic apparatus may have at least some of the components described above and shown in figure 1 and may be configured to provide the pulsed radiation beam B and/or expose the first and/or second object to the pulsed radiation beam B as described. [0065] 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, such as 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. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms“wafer” or“die” herein may be considered as synonymous with the more general terms“substrate” or“target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tool Is. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0066] 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 may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0067] The terms“radiation” and“beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193,

157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5- 20 nm), as well as particle beams, such as ion beams or electron beams.

[0068] The term“lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0069] 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.

[0070] 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.