CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 17190330.5 which was filed on September 11, 2017 and which is incorporated herein in 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 that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti parallel to this direction.

[0004] The substrate is typically configured to be substantially flat such that height variations across the exposure surface of the substrate are minimised. However, substrates may not be perfectly flat. For example, substrates typically curve at their edge portions (i.e. proximate the perimeter of the substrate). The curvature of the substrate at the edge portion of the substrate may be referred to as "edge roll-off. The focal plane of known projection systems is generally planar, whereas the edge portion of the substrate is curved. This means that the image of the patterning device as projected onto the substrate by the projection system may not be in focus at the edge region of the substrate. This may lead to lithographic errors such as focus errors when performing lithographic exposures.

[0005] It is desirable to provide, for example, an apparatus and method which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.

SUMMARY

[0006] According to a first aspect of the invention, there is provided a lithographic apparatus comprising an illumination system for providing a beam of radiation, a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross- section, a substrate table for holding a substrate, a projection system for projecting the patterned radiation beam onto a target portion of the substrate, and, a wavelength modulator configured to vary a wavelength of the radiation beam in dependence on a position within the cross-section of the radiation beam.

[0007] Varying a wavelength of the radiation beam advantageously enables variation of the shape of the focal plane of the projection system. Rather than being generally planar, curvature may be introduced to the focal plane of the projection system. The shape of the focal plane of the projection system may be varied such that it better matches the shape of a target area of the substrate, thereby reducing focus errors of the lithographic apparatus. For example, substrate edge roll-off effects may be at least partially accounted for by varying the wavelength of the radiation beam such that the shape of the focal plane of the projection system better matches the curvature of the substrate edge. Varying the wavelength of the radiation beam may enable greater control and faster variation of the shape of the focal plane of the projection system compared to adjusting lens elements in the projection system. Different apparatus and methods of varying the wavelength of the radiation beam in order to vary a shape of the focal plane of the projection system are set out below as other aspects of the invention.

[0008] The wavelength modulator may comprise an acousto-optic modulator.

[0009] The acousto-optic modulator may comprise a plurality of transducers configured to generate acoustic waves having different frequencies in different sections of the acousto-optic modulator.

[0010] The lithographic apparatus may further comprise a topography measurement system configured to measure a height map of the substrate and output a signal that is indicative of the height map, a processor configured to receive the signal from the topography measurement system and determine a change in a shape of a focal plane of the projection system and output a signal indicative of the change in shape of the focal plane of the projection system, and, a controller configured to receive the signal from the processor and control the wavelength modulator so as to apply the determined change in shape of the focal plane of the projection system.

[0011] According to a second aspect of the invention, there is provided a lithographic apparatus comprising an illumination system for providing a beam of radiation, a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross- section, a substrate table for holding a substrate, a projection system for projecting the patterned radiation beam onto a target portion of the substrate, a positioning device configured to move the target portion through the radiation beam along a scanning direction, a deflector configured to sweep the radiation beam across the target portion of the substrate in a non-scanning direction, and a wavelength modulator configured to vary a wavelength of the radiation beam in dependence on a position of the radiation beam on the target portion of the substrate.

[0012] The wavelength modulator may comprise an acousto-optic modulator. [0013] The acousto-optic modulator may comprise a plurality of transducers configured to generate acoustic waves having different frequencies in different sections of the acousto-optic modulator.

[0014] The lithographic apparatus may further comprise a topography measurement system configured to measure a height map of the substrate and output a signal that is indicative of the height map, a processor configured to receive the signal from the topography measurement system and determine a change in a focal plane of the projection system and output a signal indicative of the change in the focal plane of the projection system, and a controller configured to receive the signal from the processor and control the wavelength modulator and the deflector so as to apply the determined change in focal plane of the projection system.

[0015] According to a third aspect of the invention, there is provided a lithographic system comprising a radiation source and a lithographic apparatus, the lithographic system comprising an illumination system for providing a beam of radiation, a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section, a substrate table for holding a substrate, a projection system for projecting the patterned radiation beam onto a target portion of the substrate, and a positioning device configured to move the target portion of the substrate through the radiation beam along a scanning direction, wherein the radiation source comprises a laser cavity configured to generate the radiation beam, wherein the lithographic system further comprises a deflector configured to sweep the radiation beam across the target portion of the substrate in a non-scanning direction, and a wavelength modulator configured to vary a wavelength of the radiation beam in dependence on a position of the radiation beam on the target portion of the substrate.

[0016] The wavelength modulator may comprise an acousto-optic modulator.

[0017] The acousto-optic modulator may comprise a plurality of transducers configured to generate acoustic waves having different frequencies in different sections of the acousto-optic modulator.

[0018] The wavelength modulator may comprise a moveable optical element located in the laser cavity.

[0019] The moveable optical element may be deformable.

[0020] The optical element may be a mirror.

[0021] The optical element may be a prism.

[0022] The lithographic system may further comprise a topography measurement system configured to measure a height map of the substrate and output a signal that is indicative of the height map, a processor configured to receive the signal from the topography measurement system and determine a change in a focal plane of the projection system and output a signal indicative of the change in the focal plane of the projection system, and a controller configured to receive the signal from the processor and control the wavelength modulator and the deflector so as to apply the determined change in the focal plane of the projection system.

[0023] According to a fourth aspect of the invention, there is provided a method comprising projecting a patterned beam of radiation onto a substrate, the method comprising providing a substrate, providing a beam of radiation using an illumination system, using a patterning device to impart the radiation beam with a pattern in its cross-section, projecting the patterned radiation beam onto a target portion of the substrate, and varying a wavelength of the radiation beam across the cross-section of the radiation beam.

[0024] Varying the wavelength may comprise using an acousto-optic modulator.

[0025] Using the acousto-optic modulator may comprise generating acoustic waves having different frequencies in different sections of the acousto-optic modulator.

[0026] The method may further comprise measuring a height map of the substrate, determining a change in a focal plane of the projection system, and using the wavelength modulator to apply the determined change in the focal plane of the projection system.

[0027] According to a fifth aspect of the invention, there is provided a method of projecting a patterned beam of radiation onto a substrate, the method comprising providing a substrate, providing a beam of radiation using an illumination system, using a patterning device to impart the radiation beam with a pattern in its cross-section, projecting the patterned radiation beam onto a target portion of the substrate, moving the target portion through the radiation beam along a scanning direction, sweeping the radiation beam across the target portion of the substrate in a non-scanning direction, and varying a wavelength of the radiation beam in dependence on a position of the radiation beam on the target portion of the substrate.

[0028] Varying the wavelength may comprise using an acousto-optic modulator.

[0029] Using the acousto-optic modulator may comprise generating acoustic waves having different frequencies in different sections of the acousto-optic modulator.

[0030] The method may further comprise using a radiation source to generate the radiation beam, the radiation source comprising a laser cavity and an optical element located in the laser cavity, wherein varying the wavelength comprises moving an optical element in the laser cavity.

[0031] Varying the wavelength may further comprise deforming the optical element.

[0032] The method may further comprise measuring a height map of the substrate, determining a change in a focal plane of the projection system, and using the wavelength modulator to apply the determined change in the focal plane of the projection system.

[0033] According to a sixth aspect of the invention, there is provided a computer program comprising computer readable instructions configured to cause a computer to carry out a method according to any of the fifth and sixth aspects of the invention or any of their associated options.

[0034] According to a seventh aspect of the invention, there is provided a computer readable medium carrying a computer program according to the sixth aspect of the invention. According to an eighth aspect of the invention, there is provided a computer apparatus for varying a wavelength of radiation across a target portion of a substrate, the computer apparatus comprising a memory storing processor readable instructions, and a processor arranged to read and execute instructions stored in said memory, wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one the fifth and sixth aspects of the invention or any of their associated options.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 depicts a lithographic apparatus including a wavelength modulator according to an embodiment of the invention;

Figure 2 schematically depicts part of an edge portion of a substrate and a focal plane of a projection system of a known lithographic apparatus

Figure 3 schematically depicts an acousto-optic modulator which may be used as a wavelength modulator in an embodiment of the invention;

Figure 4 schematically depicts a portion of a lithographic apparatus comprising a wavelength modulator according to an embodiment of the invention;

- Figures 5A and 5B schematically depict a portion of a lithographic apparatus comprising a wavelength modulator and a deflector according to an embodiment of the invention;

Figure 6 schematically depicts a portion of a lithographic system comprising a radiation source, a projection system, a wavelength modulator and a deflector according to an embodiment of the invention;

- Figure 7 schematically depicts a portion of a different lithographic system comprising a radiation source, a projection system, a wavelength modulator and a deflector according to an embodiment of the invention; and

Figure 8 schematically depicts a portion of a lithographic system comprising a topography measurement system, a processor, a controller and a wavelength modulator according to an embodiment of the invention.

DETAILED DESCRIPTION

[0036] 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, 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) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. 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.

[0037] The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 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.

[0038] The term "patterning device" used herein should be broadly interpreted as referring to a device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0039] A patterning device may be transmissive or reflective. Examples of patterning device 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; in this manner, the reflected beam is patterned.

[0040] The support structure holds the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support can use mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

[0041] The term "projection system" used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid 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". [0042] The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a "lens".

[0043] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0044] The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0045] Figure 1 schematically depicts a lithographic apparatus including a wavelength modulator la-d according to an embodiment of the invention. The apparatus comprises:

- an illumination system (illuminator) IL to condition a beam PB of radiation (e.g. UV radiation or DUV radiation).

a support structure (e.g. a support structure) MT to support a patterning device (e.g. a mask) MA and connected to first positioning device PM to accurately position the patterning device with respect to item PL;

- a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist coated wafer) W and connected to second positioning device PW for accurately positioning the substrate with respect to item PL;

a projection system (e.g. a refractive projection lens) PL configured to image a pattern imparted to the radiation beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W; and,

a wavelength modulator la-d configured to vary a wavelength of the radiation beam PB.

[0046] 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 reflective mask or programmable mirror array of a type as referred to above).

[0047] The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. [0048] The illuminator IL may comprise adjusting means AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as -outer and -inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation PB, having a desired uniformity and intensity distribution in its cross section.

[0049] The radiation beam PB is incident on the patterning device (e.g. mask) MA, which is held on the support structure MT. Having traversed the patterning device MA, the beam PB passes through the projection system PL, 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), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device MA with respect to the path of the beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning device PM and PW. However, in the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0050] The depicted apparatus can be used in the following preferred modes:

1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam PB is projected onto a target portion C in one go (i.e. a single static exposure). The substrate table WT 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.

2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam PB is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT is determined by the (de-)magnification and image reversal characteristics of the projection system PL. 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.

3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the beam

PB 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 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.

[0051] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0052] In the example of Figure 1 boxes having dashed edges are used to depict example locations of the wavelength modulator la-b within the lithographic apparatus. For example, the wavelength modulator la-d may be located in the radiation source SO, in the illuminator IL, between the projection system PS and the patterning device MA, or between the projection system PS and the substrate W.

[0053] Figure 2 schematically depicts part of an edge portion of a substrate W and a focal plane 3 of a projection system (not shown) of a known lithographic apparatus. The substrate W is configured to be substantially flat such that height variations across the exposure surface of the substrate W are minimised. However, substrates W typically curve at their edge portions (i.e. proximate the perimeter of the substrate W). The curvature of the substrate W at the edge portion of the substrate may be referred to as "edge roll-off. As can be seen, the focal plane 3 of the projection system is planar whereas the edge portion of the substrate W is curved. The image of the reticle (not shown) is not in focus at the edge region of the substrate W, which may lead to lithographic errors when performing lithographic exposures.

[0054] Figure 3 schematically depicts an acousto-optic modulator 10. The acousto-optic modulator 10 may be used as a wavelength modulator in an embodiment of the invention. The acousto- optic modulator 10 comprises a transducer 11, a transparent body 12 and an acoustic absorber 13. The transducer 11 may, for example, comprise a piezoelectric transducer. The transducer 11 is configured to generate acoustic waves 14 within the transparent body 12. The transparent body 12 may, for example, comprise quartz or titanium oxide. The acoustic waves 14 travel through the transparent body 12 and are absorbed by the acoustic absorber 13. The acoustic absorber 13 may, for example, comprise foam and/or rubber. The acoustic waves 14 cause periodic compression and expansion of the transparent body 12 which modulates a refractive index of the transparent body 12. A radiation beam 15 passes through the transparent body 12 of the acousto-optic modulator 10. The modulated refractive index of the transparent body 12 interacts with the radiation beam 15 causing diffraction of the radiation beam 15.

[0055] The way in which the radiation diffracts though the transparent body may be determined at least in part by operation of the transducer 11 (e.g. the frequency and amplitude at which the transducer is being operated) and the angle of incidence 16 of the radiation beam 15 with respect to the acoustic waves 14. Modes of operation of the acousto-optic modulator 10 include the Raman-Nath regime, in which the radiation beam 15 diffracts into multiple diffraction orders, and the Bragg regime, under which the radiation beam 15 diffracts into one diffraction order. In the example of Figure 3, the acousto- optic modulator 10 is operated under the Bragg regime, with the radiation beam 15 diffracting into the first diffraction order 17. Operating the acousto-optic modulator 10 under the Bragg regime may be preferable due to reduced loss of radiation. The Bragg regime may be achieved by, for example, changing an angle of incidence 16 of the radiation beam 15, tilting the acousto-optic modulator 10 with respect to the radiation beam 15 and/or adjusting operation of the transducer 11.

[0056] The wavelength of the radiation beam 15 may be varied by an interaction between the radiation beam 15 and the acoustic waves 14 in the transparent body 12 of the acousto-optic modulator 10. The shifted frequency of the radiation beam 15 may be calculated as follows:

where f _r is the initial frequency of the radiation beam 15, m is the diffraction order of the diffracted radiation beam and f _a is the frequency of the acoustic waves 14 in the transparent body 12 of the acousto-optic modulator 10. Varying the operation frequency of the transducer 11 during the time it takes an acoustic wave 14 to travel through the transparent body 12 enables a varying density pattern to be induced within the transparent body 12. Figure 3 shows an example of a varying acoustic wave 14 frequency inducing a varying density pattern in the transparent body 12. Varying a density of the transparent body 12 varies a refractive index of the transparent body. The refractive index of the transparent body 12 is therefore made to be dependent upon a position within the transparent body. A radiation beam 17 passing through the transparent body 12 experiences a wavelength shift in dependence on a position within the transparent body. The acousto-optic modulator therefore varies a wavelength of the radiation beam 17 in dependence on a position within the cross-section of the radiation beam. For example, quartz carries acoustic waves at a velocity of about 5800 ms ^"1 . An acoustic wave travels through a 130 mm quartz transparent body of an acousto-optic modulator in about 22 μβ. Varying the operation frequency of the transducer within the 22 μβ it takes for the acoustic wave to travel through the quartz body induces a position-dependent shift of the acoustic wavelength within the quartz body. The position-dependent shift of the acoustic wavelength within the quartz body of the acousto-optic transducer may be used to provide a position-dependent shift of the wavelength of a radiation beam travelling through the quartz body. That is, the operating frequency of the transducer may be varied such that different positions within the quartz body correspond with different frequencies of acoustic wave. Different parts of the cross-sectional area of the radiation beam travel through different positions of the quartz body and experience different shifts in wavelength due to interacting with different frequencies of acoustic wave. The acousto-optic modulator may be capable of operating at a frequency of about 3 GHz.

[0057] Variation of the wavelength of the radiation beam in order to change a shape of the focal plane of the projection system may be achieved via two methods. The first method includes varying a wavelength of the radiation beam across the cross-section of the radiation beam. The first method may be achieved using an acousto-optic modulator, as discussed above in relation to Figure 3 and further discussed below in relation to Figure 4. The second method includes varying a wavelength of the entire radiation beam whilst sweeping the radiation beam across a target portion of the substrate in a non- scanning direction. The second method may be achieved using an acousto-optic modulator or a moveable optical element in a radiation source, and by suing a deflector to sweep the radiation beam across the substrate in a non-scanning direction of the lithographic apparatus. The second method is discussed in detail below in relation to Figures 5-7. On comparison, the first method may be thought of as static focal plane shaping (i.e. wavelength shifts applied to the radiation beam do not change as a function of time), whereas the second method may be thought of as dynamic focal plane shaping (i.e. the wavelength shifts applied to the radiation beam change as a function of time).

[0058] When performing the first method (i.e. the "static" focal plane shaping), if the radiation beam consists of pulses of radiation then the acousto-optic modulator may be operated such that a spatial pattern of the acoustic waves induced in the transparent body is substantially the same for each pulse of the radiation beam. Each pulse of radiation in the radiation beam will therefore undergo substantially the same shift in wavelength when travelling through the transparent body. Each pulse may undergo a wavelength shift such that the focal plane of the projection system for each pulse substantially matches the shape of the target area of a substrate. A selection of the amplitude and/or frequency of the acoustic waves generated by the transducer of the acousto-optic modulator may depend at least in part upon a repetition rate of the radiation beam. Alternatively, when performing the second method (i.e. the "dynamic" focal plane shaping), if the radiation beam consists of pulses of radiation then the acousto- optic modulator may be operated such that a first pulse receives the same wavelength shift across its cross-section then a different pulse receives a different shift in wavelength across its cross-section. A deflector may be used to sweep the radiation beam across the target area of the substrate in the non- scanning direction. Different positions across the sweep correspond with different wavelengths of radiation. The position of the focal plane therefore adjusts during the sweep in order to track the shape of the target area of the substrate.

[0059] Varying a wavelength of the entire radiation beam may alter a position of the focal plane of the projection system of the lithographic apparatus. This change in position of the focal plane may be caused by, for example, chromatic aberrations of the projection system. For example, changing the wavelength of the radiation beam by about 60 fm may result in a change in focus at the substrate of about 20 nm. The focal plane of known projection systems may be substantially planar. Varying a wavelength of the radiation beam such that the wavelength of the radiation beam depends on a position within the cross-section of the radiation beam may alter a shape of the focal plane of the projection system. That is, modulating the radiation beam such that different positions within the cross-section of the radiation beam have different wavelengths may induce curvature of the focal plane of the projection system. By operating the transducer of the acousto-optic modulator such that the frequency of the acoustic waves changes over time, the acoustic wave pattern present in the transparent body may be controlled. The acoustic wave pattern may be controlled such that different positions in the transparent body correspond with different shifted frequencies f _s of the radiation beam passing through the transparent body of the acousto-optic modulator. Thus, controlling the frequency of the transducer in the acousto-optic modulator enables control of the position and/or shape of the focal plane of the projection system. The wavelength of the radiation beam may be varied in dependence on a position within the cross-section of the radiation beam such that the shape of the focal plane better matches the shape of the substrate, thereby reducing focus errors.

[0060] Figure 4 schematically depicts a portion 20 of a lithographic apparatus comprising a wavelength modulator 21 according to an embodiment of the invention. In the example of Figure 4, the wavelength modulator 21 comprises an acousto-optic modulator. In the example of Figure 4, the acousto-optic modulator is located between a patterning device MA and a projection system PS of a lithographic apparatus. A radiation beam PB is incident on the patterning device MA. The patterning device is configured to provide the radiation beam PB with a pattern in its cross-section. After being patterned by the patterning device MA, the radiation beam PB is incident on the acousto-optic modulator 21. The acousto-optic modulator 21 is configured to vary a wavelength of the radiation beam in dependence on a position within the cross-section of the radiation beam. That is, the acousto-optic modulator 21 is configured to vary the wavelength of the radiation beam such that different positions within the cross-section of the radiation beam experience different changes in wavelength. The radiation beam PB passes through the acousto-optic modulator 21 and enters the projection system PS of the lithographic apparatus. The projection system PS projects the radiation beam PB onto a substrate W which is located on a substrate table WT of the lithographic apparatus. In the example of Figure 4, a focal plane 24 of the projection system PS is depicted as being separated from the exposure surface of the substrate W in order to clearly show the shape of the focal plane 24 and the shape of the substrate W. In practice, the focal plane 24 of the projection system PS is located on or as close to the exposure surface of the substrate W as possible, e.g. by adjusting a height of the substrate table WT.

[0061] In the example of Figure 4, the acousto-optic modulator 21 comprises six transducers 23a- f. Each transducer 23a-f is configured to generate acoustic waves in different sections of the transparent body of the acousto-optic modulator 21. Providing an acousto-optic modulator 21 having a plurality of transducers 23a-f may enable greater control of the acoustic wave pattern generated within the transparent body of the acousto-optic modulator compared to an acousto-optic modulator having a single transducer. This, in turn, may enable greater control of the wavelength of the radiation beam PB across the cross-section of the radiation beam, thereby enabling greater control of the shape and/or position of the focal plane 24 of the projection system PS. In the example of Figure 4, one of the transducers 23f is being operated at a different time-varying frequency to the other transducers 23a-e. Radiation passing through the segment of the acousto-optic modulator 21 that is under the influence of transducer 23f is given a different wavelength shift compared to radiation passing through the sections of the transparent body that are under the influence of transducers 23a-e. Radiation passing through the segment of the acousto-optic modulator 21 that is under the influence of transducer 23f is therefore in focus after a different optical path length to radiation passing through the sections of the transparent body that are under the influence of transducers 23a-e. The shape of the focal plane 24 of the projection system PS is controlled so as to better match the curvature of the substrate W, thereby improving an accuracy of a lithographic exposure.

[0062] The shape of the focal plane 24 generated by an acousto-optic modulator having a plurality of transducers may comprise a step-like pattern with each step corresponding to a section of the transparent body of the acousto-optic modulator. Increasing the number of sections and associated transducers may decrease the size of the steps in the step-like pattern, and the shape of the generated focal plane 24 may better match the shape of the substrate W. Some interference may occur between acoustic waves generated by different transducers 23a-e. However, these effects may be negligible. Providing a rigid connection between the transducers 23a-e and the transparent body of the acousto- optic modulator 21 may reduce interference effects occurring between acoustic waves generated in different sections of the transparent body. Different sections of the transparent body of the acousto- optic modulator 21 may be acoustically isolated from each other to reduce interference effects occurring between acoustic waves generated by different transducers.

[0063] The radiation beam may have a wavelength of about 193 nm. The edge roll-off effect may, for example, result in a height change of the surface of the substrate of between about 10 nm and about 20 nm. Changing the wavelength of the radiation beam by about 60 fm may change the focal position of the projection system PS by about 20 nm, thereby correcting for a 20 nm height change caused by the substrate edge roll-off effect. Operating an acousto-optic modulator at a frequency of about 500 MHz may enable a 60 fm shift of wavelength of the radiation beam to be achieved. Assuming that the transparent body of the acousto-optic modulator is quartz (which carries acoustic waves at a velocity of about 5800 ms ^"1 ), operating the acousto-optic modulator at a frequency of about 500 MHz may generate an acoustic wave having a wavelength of about 1.16X10 ^"5 m. Assuming that the acousto-optic modulator is operated under the Bragg regime, the diffracted beam exiting the quartz may be about 17 mrad from an optical axis of the lithographic apparatus. A lens (not shown) may be used to collect and re-direct radiation exiting the quartz at a maximum diffraction angle.

[0064] Figures 5A and 5B schematically depict a portion 30 of a lithographic apparatus comprising a wavelength modulator 31 and a deflector 33 according to an embodiment of the invention. In the example of Figures 5 A and 5B the wavelength modulator 31 comprises an acousto-optic modulator having a single transducer (not shown). The acousto-optic modulator 31 may however comprise a plurality of transducers. The acousto-optic modulator 31 is configured to vary the wavelength of a radiation beam 32 such that different points of the cross-section of the radiation beam experience substantially the same change in wavelength. The radiation beam 32 may have been generated by a radiation source (not shown) and conditioned by an illumination system (not shown) before reaching the acousto-optic modulator 31. As was the case in Figure 4, in the examples of Figures 5 A and 5B a focal plane 34 of the projection system PS is depicted as being separated from the exposure surface of the substrate W in order to clearly show the shape of the focal plane 34 and the shape of the substrate W. In practice, the focal plane 34 of the projection system PS is located on or as close to the exposure surface of the substrate W as possible, e.g. by adjusting a height of the substrate table WT. It will be appreciated that the focal plane 34 is two-dimensional entity, and that the focal plane 34 is represented as a one-dimensional line in Figures 5A and 5B for ease of illustration.

[0065] Figure 5A, shows the portion 30 of the projection system at a first time ti during projection of the radiation beam 32. The radiation beam 32 is incident on the acousto-optic modulator 31. At time ti the acousto-optic modulator 31 is operated at a first frequency, thus providing a first shift in wavelength of the radiation beam 32. The radiation beam 32 passes through the acousto-optic modulator 31 and is incident upon a deflector 33. The deflector 33 is configured to sweep the radiation beam 32 across a target portion of a substrate W. In the example of Figure 5, the deflector 33 comprises a rotatable mirror. Changing an angular position 35 of the mirror 33 changes an angle of reflection of the radiation beam 32. In Figure 5A, the mirror 33 is at a first angular position 35 and causes the radiation beam to reflect in a first direction 32a. The radiation beam 32 is then incident upon a patterning device MA. The patterning device MA is configured to impart the radiation beam 32 with a pattern in its cross-section. After becoming patterned by the patterning device MA, the radiation beam 32 enters the projection system PS of the lithographic apparatus. The projection system PS projects the radiation beam 32 onto a substrate W which is located on a substrate table WT of the lithographic apparatus. At time ti the radiation beam 32 is incident on the substrate W proximate an edge of the substrate W. As can be seen from Figure 5, the topography of the substrate W changes proximate the edge of the substrate due to the edge roll-off effect discussed above. The first shift in wavelength provided by the acousto-optic modulator 31 and the first direction 32a provided by the mirror 33 are such that the image of the pattern imparted to the radiation beam 32 by the patterning device MA is in focus at a first position 34a. The first position 34a corresponds with the position of the exposure surface of the substrate W. That is, the shift in wavelength and the direction 32a of the radiation beam 32 are selected such that the shape of the focal plane 34 of the projection system PS substantially matches the shape of the exposure surface of the substrate W. Matching the shape of the focal plane to the shape of the exposure surface of the substrate may reduce lithographic errors such as a focus error when performing a lithographic exposure.

[0066] A positioning device (not shown) is configured to move a target portion of the substrate W through the radiation beam 32 along a scanning direction 36. In the example of Figure 5, the scanning direction is along the y-axis. The angular position 35 of the mirror 33 changes over time such that the radiation beam 32 is swept across a target portion of the substrate W in a non-scanning direction of the lithographic apparatus during movement of the substrate W. In the example of Figure 5, the non- scanning direction is along the x-axis (i.e. perpendicular to the scanning direction). An optical axis 37 of the portion 30 of the lithographic apparatus is located along the z-axis. The deflector 33 is configured to sweep the radiation beam 32 in the non-scanning direction 36 at a greater speed than the speed at which the positioning device moves the substrate W in the scanning direction 36. The positioning device may, for example, be configured to move the substrate W in the scanning direction 36 with a speed of between about 0.1 ms ^"1 , and about 1 ms ^"1 . The deflector 33 may, for example, be configured to sweep the radiation beam 32 in the non-scanning direction with a speed of between about 1 ms ^"1 , and about 100 ms ^"1 . Sweeping the radiation beam in the non-scanning direction may affect a uniformity of the dose of radiation being applied to the substrate W. That is, depending on the repetition rate of the radiation beam and the speed of the sweep, some area of the substrate may receive less radiation than others, which may in turn result in a lithographic error. The repetition rate of the radiation beam may be increased in order to maintain dose uniformity across the substrate when performing the sweep of the radiation beam in the non-scanning direction. For example, the repetition rate of the radiation beam may be increased from between about 6 kHz and about 8 kHz to about 20 kHz. Alternatively, a pulse duration of the radiation beam may be increased in order to maintain dose uniformity across the substrate when performing the sweep of the radiation beam in the non-scanning direction. The time taken to perform one sweep of the radiation beam across the substrate in the non-scanning direction may be selected such that it is substantially the same as the duration of a pulse of the radiation beam.

[0067] The following is a discussion of an example implementation of the invention. The pulse duration of the radiation beam may, for example, be about 200 ns. The repetition rate of the radiation beam may, for example, be about 6 kHz. The width of the exposure slit of the lithographic apparatus may, for example, be about 26 mm. In order to sweep the radiation beam across the exposure slit width within the duration of one laser pulse, the sweeping speed of the radiation beam may be about 1.3X10 ⁵ ms ^"1 . The mirror 33 may be rotated such that the rotation rate of the mirror 33 corresponds with the repetition rate of the radiation beam. For example, when the repetition rate of the radiation beam is about 6 kHz then the mirror 33 may be rotated at a rotation rate of about 6 kHz. The sweep of the radiation beam across the width of the exposure slit may take about 1/800 of a full revolution of the mirror. This equates to about 0.43 degrees of the mirror's rotation. If the optical design of the lithographic were such that a rotation of the mirror through an angle of 4.3 degrees covered the full width of the exposure slit, then the mirror may have to be rotated at a speed of about 60000 revolutions per second. Increasing a pulse duration of the radiation beam may reduce the required speed of rotation of the mirror.

[0068] Figure 5B shows the portion 30 of the projection system at a second time t2 during projection of the radiation beam 32. At time t2 the acousto-optic modulator 31 is operated at a frequency that is different to the frequency used at time ti. The different operating frequency of the acousto-optic modulator at time t2 induces a second shift in the wavelength of the radiation beam 32 that is different to the first shift in wavelength at time ti. The radiation beam 32 passes through the acousto-optic modulator 31 and is incident upon the mirror 33. The mirror 33 is at a second angular position 35 and causes the radiation beam 32 to reflect in a second direction 32b. The second shift in wavelength provided by the acousto-optic modulator 31 and the change in direction 32b of the radiation beam 32 provided by the mirror 33 are such that the image of the pattern imparted to the radiation beam 32 by the patterning device MA is in focus at a second position 34b. The second position 34b corresponds with the position of the exposure surface of the substrate W. The shift in wavelength and the direction 32b of the radiation beam 32 are such that the shape of the focal plane 34 of the projection system PS substantially matches the shape of the exposure surface of the substrate W. As can be seen on comparison between Figure 5A and Figure 5B, the mirror 33 sweeps the radiation beam 32 along the non-scanning direction 38 whilst the operating frequency of the acousto-optic 31 is changed in order to match the shape of the focal plane 34 with the shape of the exposure surface of the substrate W.

[0069] Figure 6 schematically depicts a portion 40 of a lithographic system comprising a radiation source SO, a projection system PS, a wavelength modulator 41 and a deflector 43 according to an embodiment of the invention. The radiation source SO comprises a laser having a laser cavity 44 configured to generate a radiation beam 42. The radiation beam 42 is incident upon a deflector 43. Components such as a beam delivery system (not shown) and an illumination system (not shown) may exist between the radiation source SO and the deflector 43. The deflector 43 is configured to sweep the radiation beam 42 across a target portion of the substrate W in a non-scanning direction 50 of the lithographic system. The radiation beam 42 deflects from the deflector 43 and is incident upon a patterning device MA. The patterning device MA is configured to impart the radiation beam 32 with a pattern in its cross-section. After becoming patterned by the patterning device MA, the radiation beam 32 enters the projection system PS of the lithographic apparatus. The projection system PS projects the radiation beam 32 onto a substrate W which is located on a substrate table WT of the lithographic apparatus. A positioning device (not shown) is configured to move a target portion of the substrate W through the radiation beam 42 along a scanning direction 49. In the example of Figure 6, the scanning direction 49 is along the y-axis. The angular position 47 of the deflector 43 changes over time such that the radiation beam 42 is swept across a target portion of the substrate W in a non-scanning direction 50 of the lithographic apparatus during movement of the substrate W. In the example of Figure 6, the non- scanning direction 50 is along the x-axis (i.e. perpendicular to the scanning direction 49). The deflector 43 is configured to sweep the radiation beam 42 in the non-scanning direction 50 at a greater speed than the speed at which the positioning device moves the substrate W in the scanning direction 49.

[0070] The lithographic system comprises a wavelength modulator 41 configured to vary a wavelength of the radiation beam 42. In the example of Figure 6, the wavelength modulator comprises a moveable optical element 41 located in the laser cavity 44. In the example of Figure 6, the moveable optical element 41 is a mirror. An actuator (not shown) may be configured to move the mirror 41 along a length 45 of the laser cavity 44. The actuator may, for example, comprise a piezoelectric actuator. A partially reflective end mirror 46 (i.e. an output coupler) is located within the laser cavity 44 opposite the moveable mirror 41. The output coupler 46 is configured to transmit some of the radiation generated in the laser cavity 44 so as to form the radiation beam 42 that is provided to the deflector 43. Changing the position of the movable mirror 41 to lengthen and shorten the laser cavity 44 changes a wavelength of the radiation beam 42 generated by the laser cavity 44. [0071] The radiation beam 42 exits the radiation source SO and is incident upon the deflector 43. In the example of Figure 6, the deflector 43 comprises a rotatable mirror. Changing an angular position 47 of the mirror 43 changes an angle of reflection of the radiation beam 42. Figure 6 shows the radiation beam 42 being reflected by the deflector 43 in three different directions 42a-c that correspond with three different times ti-t3 during operation of the lithographic system. At the first time ti the moveable mirror 41 is at a first position 41a within the laser cavity 44 such that the radiation beam 42 has a first wavelength, and the deflector 43 is at a first angular position such that the radiation beam 42 is deflected in a first direction 32a. The radiation beam 42 passes through the projection system PS and is in focus at a first position 48a. At the second time t ₂ the moveable mirror 41 is at a second position 41b within the laser cavity 44 such that the radiation beam 42 has a second wavelength different to the first wavelength, and the deflector 43 is at a second angular position such that the radiation beam 42 is deflected in a second direction 42b that is different to the first direction 42a. The radiation beam 42 passes through the projection system PS and is in focus at a second position 48b. At the third time t ₃ the moveable mirror 41 is at the second position 41b within the laser cavity 44 such that the radiation beam 42 has the second wavelength and the deflector 43 is at a third angular position such that the radiation beam 42 is deflected in a third direction 42c that is different to the first and second directions 42a-b. The radiation beam 42 passes through the projection system PS and is in focus at a third position 48c. As can be seen from Figure 6, between times tu the deflector 43 sweeps the radiation beam 42 along the non-scanning direction 50 whilst the position 41a-b of the movable mirror 41 in the laser cavity 44 is changed in order to match the shape of the focal plane 51 of the projection system PS with the shape of the exposure surface of the substrate W.

[0072] The size of the radiation beam in the sweeping direction (i.e. the non-scanning direction) may at least in part determine a dose uniformity of the lithographic exposure and/or an accuracy to which the focal plane of the projection system may be shaped. A radiation beam having a larger length in the sweeping direction may provide increased dose uniformity compared to a radiation beam having a smaller length in the sweeping direction. However, a radiation beam having a smaller length in the sweeping direction may shape the focal plane of the projection system more accurately than a radiation beam having a larger length in the sweeping direction. The length of the radiation beam in the sweeping direction maybe selected in partial dependence on the desired dose uniformity and focal plane shaping accuracy.

[0073] The moveable mirror 41 may be deformable. That is, the moveable mirror 41 may be bent such that a distance between the moveable mirror 41 and the output coupler 46 depends upon a lateral position within the laser cavity 44. An actuator (not shown) may be configured to controllably deform the moveable mirror 41. The actuator may, for example, comprise a piezoelectric actuator. Deforming the moveable mirror 41 changes a wavelength of the radiation beam 42 generated by the radiation source SO such that the wavelength of the radiation beam 42 depends on a position within the cross-section of the radiation beam 42. Deforming the moveable mirror 41 as well as changing the position of the moveable mirror 41 in the laser cavity 44 may enable finer control of the shape of the focal plane 51 of the projection system PS compared to only changing the position of the moveable mirror 41 within the laser cavity 44.

[0074] Figure 7 schematically depicts a portion 60 of a lithographic system comprising a radiation source SO, a projection system PS, a wavelength modulator 61 and a deflector 63 according to an embodiment of the invention. The radiation source SO comprises a laser cavity 64 configured to generate a radiation beam 62. The radiation beam 62 is incident upon a deflector 63. Components such as a beam delivery system (not shown) and an illumination system (not shown) may exist between the radiation source SO and the deflector 63. The deflector 63 is configured to sweep the radiation beam 62 across a target portion of the substrate W in a non-scanning direction 70 of the lithographic system. The radiation beam 62 deflects from the deflector 63 and is incident upon a patterning device MA. The patterning device MA is configured to impart the radiation beam 62 with a pattern in its cross-section. After becoming patterned by the patterning device MA, the radiation beam 62 enters the projection system PS of the lithographic apparatus. The projection system PS projects the radiation beam 62 onto a substrate W which is located on a substrate table WT of the lithographic apparatus. A positioning device (not shown) is configured to move a target portion of the substrate W through the radiation beam 62 along a scanning direction 69. In the example of Figure 6, the scanning direction 69 is along the y- axis. The angular position 67 of the deflector 63 changes over time such that the radiation beam 62 is swept across a target portion of the substrate W in a non-scanning direction 70 of the lithographic apparatus during movement of the substrate W. In the example of Figure 7, the non-scanning direction 70 is along the x-axis (i.e. perpendicular to the scanning direction 69). The deflector 63 is configured to sweep the radiation beam 62 in the non-scanning direction 70 at a greater speed than the speed at which the positioning device moves the substrate W in the scanning direction 69.

[0075] The lithographic system comprises a wavelength modulator 61 configured to vary a wavelength of the radiation beam 62. In the example of Figure 7, the wavelength modulator comprises a moveable optical element 61 located in the laser cavity 64. In the example of Figure 7, the moveable optical element 61 is a rotatable prism. An actuator (not shown) may be configured to rotate the prism 61 about an axis 65. The actuator may, for example, comprise a piezoelectric actuator. In the example of Figure 7, the axis 65 goes into the page through the prism 61. The axis 65 shown in Figure 7 is intended to demonstrate the axis about which the prism 61 rotates, and does not indicate a solid object, such as a pin, passing through the prism 61. The laser cavity comprises a partially reflective end mirror 66 (i.e. an output coupler), a gain medium 72 and an end mirror 73. The output coupler 66 is configured to transmit some of the radiation generated in the laser cavity 64 so as to form the radiation beam 62 that is provided to the deflector 63. An angular position of the rotatable prism 61 and a wavelength of radiation incident on the rotatable prism 61 determine a refraction angle of radiation travelling though the rotatable prism 61. The angle of refraction of radiation passing through the rotatable prism determines whether the radiation reflects from the end mirror 73 and reaches the gain medium 72 and the output coupler 66, or whether the radiation reflects from the end mirror 73 and away from the rotatable prism 61. Changing the angular position 61a-b of the rotatable prism 61 changes a wavelength of the radiation beam 62 generated by the laser cavity 64.

[0076] The radiation beam 62 exits the radiation source SO and is incident upon the deflector 63. In the example of Figure 7, the deflector 63 comprises a rotatable mirror. Changing an angular position 67 of the mirror 63 changes an angle of reflection of the radiation beam 62. Figure 7 shows the radiation beam 62 being reflected by the deflector 63 into three different directions 62a-c that correspond with three different times ti-t3 during operation of the lithographic system. At the first time ti the rotatable prism 61 is at a first angular position 61a within the laser cavity 64 such that the radiation beam 62 has a first wavelength and the deflector 63 is at a first angular position 67 such that the radiation beam 62 is deflected in a first direction 62a. The radiation beam 62 passes through the projection system PS and is in focus at a first position 68a. At the second time t2 the moveable mirror 61 is at a second angular position 61b within the laser cavity 64 such that the radiation beam 62 has a second wavelength different to the first wavelength, and the deflector 63 is at a second angular position 67 such that the radiation beam 62 is deflected in a second direction 62b that is different to the first direction 62a. The radiation beam 62 passes through the projection system PS and is in focus at a second position 68b. At the third time t3 the moveable mirror 61 is at the second position 61b within the laser cavity 64 such that the radiation beam 62 has the second wavelength and the deflector 63 is at a third angular position 67 such that the radiation beam 62 is deflected in a third direction 62c that is different to the first and second directions 62a-b. The radiation beam 62 passes through the projection system PS and is in focus at a third position 68c. As can be seen from Figure 7, between times tu the deflector 63 sweeps the radiation beam 62 along the non-scanning direction 70 whilst the position 61a-b of the movable mirror 61 in the laser cavity 64 is changed in order to match the shape of the focal plane 71 of the projection system PS with the shape of the exposure surface of the substrate W.

[0077] The rotatable prism 61 may be deformable. That is, the rotatable prism 61 may be bent such that different rays of radiation experience different angles of refraction when travelling through the rotatable prism 61. An actuator (not shown) may be configured to controllably deform the rotatable prism 61. The actuator may, for example, comprise a piezoelectric actuator. Deforming the rotatable prism 61 changes a wavelength of the radiation beam 62 generated by the radiation source SO such that the wavelength of the radiation beam 62 depends on a position within the cross-section of the radiation beam 62. Deforming the rotatable prism 61 as well as changing the angular position of the rotatable prism 61 in the laser cavity 64 may enable finer control of the shape of the focal plane 71 of the projection system PS compared to only changing the angular position of the rotatable prism 61 within the laser cavity 64.

[0078] Any of the embodiments described herein may be used together with a substrate topography measurement system. Figure 8 schematically depicts a portion 80 of a lithographic system comprising a topography measurement system 81, a processor 82, a controller 83 and a wavelength modulator 84 according to an embodiment of the invention. Figure 8 shows a substrate W held on a substrate table WT at two different stages, a measurement stage 87 and an exposure stage 88. In the measurement stage, the topography measurement system 81 is configured to measure a height map of a substrate W and output a signal that is indicative of the height map. For example, the topography measurement system 81 may of the type described in international patent application publication WO 2017/060014 Al. The processor 82 is configured to receive the signal from the topography measurement system 81 and determine a change in a shape of a focal plane of the projection system PS of the lithographic system. The processor 82 determines a change in the shape of the focal plane 85 of the projection system PS such that the focal plane 85 of the projection system PS better matches the topography (i.e. the measured height map) of the substrate W. The processor 82 is further configured to output a signal indicative of the change in shape of the focal plane 85 of the projection system PS. The controller 83 is configured to receive the signal from the processor 82 and control the wavelength modulator 84 so as to apply the determined change in shape of the focal plane 85 of the projection system PS. The projection system PS projects a radiation beam 86 onto the substrate W. In the example of Figure 8, the focal plane 85 of the projection system PS is depicted as being separated from the exposure surface of the substrate W in order to clearly show the shape of the focal plane 85 and the shape of the substrate W. In practice, the focal plane 85 of the projection system PS is located on or as close to the exposure surface of the substrate W as possible, e.g. by adjusting a height of the substrate table WT. It will be appreciated that the focal plane 85 is two-dimensional entity, and that the focal plane 85 is represented as a one-dimensional line in Figure 8 for ease of illustration.

[0079] As discussed above, and with reference to Figure 1, the wavelength modulator may be located in different regions la-d of the lithographic system. When the wavelength modulator is an acousto-optic modulator, the position of the acousto-optic modulator within the lithographic system may affect the dimensions of the acousto-optic modulator and/or how the acousto-optic modulator is operated. This is because the cross-sectional area of the radiation beam varies between different locations within the lithographic system. The dimensions of the transparent body of the acousto-optic modulator depend at least in part upon the operating parameters of the lithographic system with which the acousto-optic modulator is being used. For example, the dimensions of the transparent body of the acousto-optic modulator may be such that the entire cross-sectional area of the radiation beam can pass through the transparent body. The length of the transparent body of the acousto-optic modulator may depend upon a location of the acousto-optic modulator in the lithographic apparatus and a demagnification of the projection system of the lithographic apparatus. For example, the projection system of a lithographic apparatus may provide demagnification by a factor of 4. When the acousto- optic modulator is located proximate the reticle, the transparent body of the acousto-optic modulator may have a length of between about 100 mm and about 400 mm. When the acousto-optic modulator is located proximate the substrate, the transparent body of the acousto-optic modulator may have a length of between about 25 mm and about 100 mm. When the acousto-optic modulator is located proximate a field plane of the illumination system of a lithographic apparatus, the length of the transparent body may depend at least in part upon the optical design of the illumination system. For example, when the acousto-optic modulator is located proximate a field plane of the illumination system of the lithographic apparatus, the transparent body of the acousto-optic modulator may have a length of between about 20 mm and about 110 mm. The possible locations of the acousto-optic modulator within a lithographic apparatus may be limited by an optical design of the lithographic apparatus. For example, it may be preferable to locate the acousto-optic modulator proximate a field plane of the lithographic apparatus rather than a pupil plane of the lithographic apparatus. The locations of field planes within different lithographic apparatus may vary.

[0080] The acousto-optic modulator may comprise any number of transducers. The transducers may have different sizes and/or different shapes. For example, a high concentration of smaller transducers may be located near end regions of the transparent body of the acousto-optic modulator in order to provide greater control of the shape and/or position of end regions of the focal plane of the projection system (e.g. to further reduce edge roll-off focus errors). Whilst use of the invention has primarily been discussed in relation to reducing lithographic focus errors associated with substrate edge roll-off, it will be appreciated that the invention may be used to reduce focus errors in any location of the substrate. For example, a topography of a target portion of the substrate proximate a centre of the substrate may be measured and a wavelength modulator may be used to adjust the focal plane of the projection system so as to better match the topography of the target portion.

[0081] Whilst the deflector has been described and depicted as being a rotatable mirror, it will be appreciated that the deflector may take any form. For example the deflector may comprise a wheel having multiple flat mirrors about the wheel's circumference. Alternatively, the deflector may comprise a moveable prism configured to deflect the radiation beam via refraction. As another alternative, the deflector may comprise a microelectromechanical systems (MEMS) mirror array.

[0082] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. [0083] 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 description is not intended to limit the invention.