Photolithography apparatus and method

Abstract

A lithographic apparatus comprises an illumination system for providing a radiation beam, a support structure for supporting a pattern forming device for applying a pattern to the radiation beam in a cross-section thereof, 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 the wavelength of the radiation beam across the cross-section of the radiation beam.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 17190330.5 filed on September 11, 2017, and which is incorporated herein by reference in its entirety. Technical Field

[0003] The invention relates to a photolithography device and a method for manufacturing the device. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a target portion of a substrate. A lithographic apparatus can be used, for example, in the manufacture of an integrated circuit (IC). In this case, a pattern forming device, alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC, and the pattern can be imaged onto a target portion (e.g., a portion including one or more bare chips) having a radiation-sensitive material (resist) layer on a substrate (e.g., a silicon wafer). In general, a single substrate will contain a network of adjacent target portions that are continuously exposed. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing the entire pattern to each target portion in one pass, and so-called scanners, in which a pattern is scanned by a beam of light in a given direction (the "scanning" direction), while scanning the substrate parallel or antiparallel to the direction, and irradiating each target portion.

[0005] The substrate is typically configured to be substantially flat so that the height variation across the exposure surface of the substrate is minimized. However, the substrate may not be completely flat. For example, substrates are typically curved at their edge portions (i.e., close to 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 the known projection system is typically planar, while the edge portion of the substrate is curved. This means that the image of the pattern forming device projected onto the substrate by the projection system may be out of focus at the edge region of the substrate. This can cause lithography errors such as focus errors when performing lithographic exposure.

[0006] It would be desirable, for example, to provide an apparatus and method that mitigates or eliminates one or more problems of the prior art, whether identified herein or elsewhere. Summary of the invention

[0007] According to a first aspect of the present invention, there is provided a lithography apparatus comprising an irradiation system for providing a radiation beam, a support structure for supporting a pattern forming device for applying a pattern to the radiation beam in a cross-section thereof, 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 change the wavelength of the radiation beam according to the position within the cross-section of the radiation beam.

[0008] Changing the wavelength of the radiation beam advantageously enables the shape of the focal plane of the projection system to be changed. Instead of being generally planar, a curvature can be introduced into the focal plane of the projection system. The shape of the focal plane of the projection system can be varied so that it better matches the shape of the target area of ​​the substrate, thereby reducing the focus error of the lithographic apparatus. For example, substrate edge roll-off effects can be at least partially accounted for by changing the wavelength of the radiation beam so that the shape of the focal plane of the projection system better matches the curvature of the substrate edge. Changing the wavelength of the radiation beam can enable the shape of the focal plane of the projection system to be more controlled and changed more quickly than adjusting the lens elements in the projection system. Different devices and methods for changing the wavelength of the radiation beam in order to change the shape of the focal plane of the projection system are given below as other aspects of the present invention.

[0009] The wavelength modulator may include an acousto-optic modulator.

[0010] The AOM may include a plurality of transducers configured to generate acoustic waves having different frequencies in different segments of the AOM.

[0011] The lithography apparatus may further include a topography measurement system configured to measure a height map of the substrate and output a signal indicative of the height map, a processor configured to receive a signal from the topography measurement system and determine a shape change of the focal plane of the projection system and output a signal indicative of the shape change of the focal plane of the projection system, and a controller configured to receive a signal from the processor and control the wavelength modulator to apply the determined shape change of the focal plane of the projection system.

[0012] According to a second aspect of the present invention, there is provided a lithography apparatus comprising an irradiation system for providing a radiation beam, a support structure for supporting a pattern forming device for applying a pattern to the radiation beam 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 in a scanning direction, a deflector configured to scan the radiation beam across the target portion of the substrate in a non-scanning direction, and a wavelength modulator configured to change the wavelength of the radiation beam depending on the position of the radiation beam on the target portion of the substrate.

[0013] The wavelength modulator may include an acousto-optic modulator.

[0014] The AOM may include a plurality of transducers configured to generate acoustic waves having different frequencies in different segments of the AOM.

[0015] The lithography apparatus may further include a topography measurement system configured to measure a height map of the substrate and output a signal indicative of the height map, a processor configured to receive signals 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 signals from the processor and control the wavelength modulator and the deflector to apply the determined change in the focal plane of the projection system.

[0016] According to a third aspect of the present invention, a lithography system is provided, which includes a radiation source and a lithography device, the lithography system including an irradiation system for providing a radiation beam; a support structure for supporting a pattern forming device, the pattern forming device being used to apply a pattern to the radiation beam in a cross-section thereof; 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 includes a laser cavity configured to generate the radiation beam, wherein the lithography system further includes a deflector configured to scan the radiation beam across the target portion of the substrate in a non-scanning direction, and a wavelength modulator configured to change the wavelength of the radiation beam depending on the position of the radiation beam on the target portion of the substrate.

[0017] The wavelength modulator may include an acousto-optic modulator.

[0018] The AOM may include a plurality of transducers configured to generate acoustic waves having different frequencies in different segments of the AOM.

[0019] The wavelength modulator may include a movable optical element located in the laser cavity.

[0020] The movable optical element may be deformable.

[0021] The optical element may be a mirror.

[0022] The optical element may be a prism.

[0023] The lithography system may further include a topography measurement system configured to measure a height map of the substrate and output a signal indicative of the height map, a processor configured to receive signals from the topography measurement system and determine a focal plane change of the projection system and output a signal indicative of the focal plane change of the projection system, and a controller configured to receive signals from the processor and control the wavelength modulator and the deflector to apply the determined focal plane change of the projection system.

[0024] According to a fourth aspect of the present invention, a method is provided, comprising projecting a patterned radiation beam onto a substrate, the method comprising providing a substrate, providing a radiation beam using an irradiation system, applying a pattern to the radiation beam in a cross-section thereof using a pattern forming device, projecting the patterned radiation beam onto a target portion of the substrate, and changing the wavelength of the radiation beam across the cross-section of the radiation beam.

[0025] Changing the wavelength may include using an acousto-optic modulator.

[0026] Using the AOM may include generating acoustic waves having different frequencies in different segments of the AOM.

[0027] The method may further comprise measuring a height map of the substrate, determining a focal plane variation of the projection system, and applying the determined focal plane variation of the projection system using the wavelength modulator.

[0028] According to a fifth aspect of the present invention, there is provided a method for projecting a patterned radiation beam onto a substrate, the method comprising providing a substrate, providing a radiation beam using an irradiation system, applying a pattern to the radiation beam in a cross-section using a pattern forming device, projecting the patterned radiation beam onto a target portion of the substrate so that the target portion moves through the radiation beam in a scanning direction, scanning the radiation beam across the target portion of the substrate in a non-scanning direction, and changing the wavelength of the radiation beam depending on the position of the radiation beam on the target portion of the substrate.

[0029] Changing the wavelength may include using an acousto-optic modulator.

[0030] Using the AOM may include generating acoustic waves having different frequencies in different segments of the AOM.

[0031] The method may further include generating the radiation beam using a radiation source comprising a laser cavity and an optical element located in the laser cavity, wherein changing the wavelength comprises moving the optical element in the laser cavity.

[0032] Changing the wavelength may further include deforming the optical element.

[0033] The method may further comprise measuring a height map of the substrate, determining a focal plane variation of the projection system, and applying the determined focal plane variation of the projection system using the wavelength modulator.

[0034] According to a sixth aspect of the present invention, there is provided a computer program comprising computer readable instructions configured to cause a computer to implement a method according to any one of the fifth and sixth aspects of the present invention or any associated options thereof.

[0035] According to a seventh aspect of the present invention, there is provided a computer readable medium carrying a computer program according to the sixth aspect of the present invention.

[0036] According to an eighth aspect of the present invention, there is provided a computer device for changing the wavelength of radiation across a target portion of a substrate, the computer device comprising a memory storing processor-readable instructions, and a processor configured to read and execute the instructions stored in the memory, wherein the processor-readable instructions include instructions arranged to control the computer to implement a method according to any one of the fifth and sixth aspects of the present invention or any associated options thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference numerals indicate corresponding parts and in which:

[0038] - Figure 1 depicts a lithographic apparatus including a wavelength modulator according to an embodiment of the present invention;

[0039] - Figure 2 schematically depicts an edge portion of a substrate and a portion of a focal plane of a projection system of a known lithographic apparatus;

[0040] - Figure 3 schematically depicts an acousto-optic modulator that may be used as a wavelength modulator in embodiments of the present invention;

[0041] - Figure 4 schematically depicts a portion of a lithographic apparatus including a wavelength modulator according to an embodiment of the present invention;

[0042] - Figure 5A and Figure 5B schematically depicts a portion of a lithographic apparatus including a wavelength modulator and a deflector according to an embodiment of the present invention;

[0043] - Figure 6 schematically depicts a portion of a lithography system including a radiation source, a projection system, a wavelength modulator and a deflector according to an embodiment of the present invention;

[0044] - Figure 7schematically depicts a portion of a different lithography system including a radiation source, a projection system, a wavelength modulator and a deflector according to an embodiment of the present invention; and

[0045] - Figure 8 A portion of a lithography system including a topography measurement system, a processor, a controller, and a wavelength modulator according to an embodiment of the present invention is schematically depicted. DETAILED DESCRIPTION

[0046] Although specific reference is made herein to the use of lithographic apparatus in IC manufacturing, 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 heads, and the like. Those skilled in the art will appreciate that in the context of such alternative applications, the use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed before or after exposure, for example in a track (a tool that typically applies a resist layer to a substrate and develops the exposed resist) or a measurement or inspection tool. In addition, a substrate may be processed more than once, for example to form a multi-layer IC, so that the terms used herein may also refer to a substrate that already contains multiple processed layers.

[0047] As used herein, the terms "radiation" and "beam" 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 ultraviolet (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.

[0048] The term "patterning device" as used herein should be broadly interpreted as referring to a device that can be used to impart a pattern to a radiation beam in its cross-section, such as to form a pattern in a target portion of a 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. Typically, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device, such as an integrated circuit, that is created in the target portion.

[0049] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and may 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 to reflect an incoming radiation beam in different directions; in this way, the reflected beam is patterned.

[0050] The support structure holds the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether the patterning device is held in a vacuum environment. The support may use mechanical clamping, vacuum or other clamping techniques, such as electrostatic clamping under vacuum conditions. The support structure may be a frame or table, which may be fixed or movable, for example, as required, and which may ensure that the patterning device is in a desired position, for example, with respect to a projection system. Any use of the term "reticle" or "mask" herein may be considered to be synonymous with the more general term "patterning device".

[0051] The term "projection system" as used herein should be interpreted broadly as covering various types of projection systems, including refractive optical systems, reflective optical systems and catadioptric optical systems, as appropriate, for example, for the exposure radiation used, or for other factors such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered a synonym for the more general term "projection system".

[0052] The illumination system may also include various types of optical components for directing, shaping or controlling the radiation beam, including refractive, reflective and catadioptric optical components, and such components may also be referred to below, collectively or individually, as "lenses."

[0053] 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 a "multi-stage" machine, 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.

[0054] The lithographic apparatus may also be of a type in which the substrate is immersed in a liquid having a relatively high refractive index, such as water, so as to fill the space between the final element of the projection system and the substrate. Immersion techniques are well known in the art to increase the numerical aperture of a projection system.

[0055] Figure 1 A lithographic apparatus including wavelength modulators 1a-d according to an embodiment of the present invention is schematically depicted. The apparatus comprises:

[0056] - an illumination system (illuminator) IL for conditioning a radiation beam PB (eg UV radiation or DUV radiation);

[0057] a support structure (eg support structure) MT for supporting a patterning device (eg mask) MA and connected to a first positioning device PM for accurately positioning the patterning device with respect to the object PL;

[0058] a substrate table (e.g. wafer table) WT for holding a substrate (e.g. resist-coated wafer) W and connected to a second positioning device PW in order to accurately position the substrate with respect to the object PL;

[0059] a projection system (e.g. a refractive projection lens) PL configured to image the pattern applied to the radiation beam PB by the patterning device MA onto a target portion C of the substrate W (e.g. comprising one or more dies); and

[0060] - A wavelength modulator 1a-d configured to change the wavelength of the radiation beam PB.

[0061] As depicted herein, the device is of the transmissive type (eg, employing a transmissive mask). Alternatively, the device may be of the reflective type (eg, employing a reflective mask or a programmable mirror array of the type mentioned above).

[0062] The illuminator IL receives a radiation beam 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 a case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is delivered from the source SO to the illuminator IL by means of a beam delivery system BD, which for example comprises appropriate directional mirrors and / or a beam expander. In other cases, the source may be an 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 where necessary - may be referred to as a radiation system.

[0063] The illuminator IL may include an adjustment device AM for adjusting the angular intensity distribution of the beam. Typically, at least the outer and / or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. Furthermore, the illuminator IL typically includes various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned radiation beam PB having a desired uniformity and intensity distribution in its cross-section.

[0064] The radiation beam PB is incident on a patterning device (e.g. a mask) MA which is held on the support structure MT. After passing through 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 a second positioning device PW and a position sensor IF (e.g. an interferometer device), the substrate table WT can be accurately moved, for example, to position a different target portion C in the path of the beam PB. Similarly, the first positioning device PM and another position sensor (which is not in the substrate) can be used to position the substrate table WT. Figure 1The support structure MT (as explicitly depicted in Figure 1) can be used to accurately position the patterning device MA with respect to the path of the beam PB, for example after mechanical retrieval from a mask library or during scanning. Typically, movement of the stages MT and WT will be achieved with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning devices PM and PW. However, in the case of a stepper (as opposed to a scanner), the support structure MT may be connected to the short-stroke actuator only, or may be fixed. The patterning device MA and the substrate W may be aligned using the patterning device alignment masks M1, M2 and the substrate alignment masks P1, P2.

[0065] The depicted device can be used in the following preferred modes:

[0066] - In step mode, the support structure MT and substrate table WT may remain substantially stationary while the entire pattern applied to the beam PB is projected onto a target portion C in one pass (i.e. a single static exposure). The substrate table WT is subsequently displaced 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.

[0067] 2. In scan mode, the support structure MT and substrate table WT are scanned synchronously while a pattern applied to beam PB is projected onto a target portion C (i.e. a single dynamic exposure). The speed 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 of the target portion (in the non-scanning direction) in a single dynamic exposure, while the length of the scanning movement determines the height of the target portion (in the scanning direction).

[0068] 3. In another mode, the support structure MT remains essentially stationary while the programmable patterning device is held, and the substrate table WT is moved or scanned while the pattern applied to the beam PB is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as required after each movement of the substrate table WT during a scan or between consecutive radiation pulses. This mode of operation can be readily applied to maskless lithography using a programmable patterning device, such as a programmable mirror array of the type mentioned above.

[0069] Combinations and / or variations of the above-described modes of use or entirely different modes of use may also be employed.

[0070] exist Figure 1In the example of FIG, a box with a dashed edge is used to depict example locations of wavelength modulators 1a-b within a lithographic apparatus. For example, wavelength modulators 1a-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.

[0071] Figure 2 The edge portion of a substrate W and a portion of a focal plane 3 of a projection system (not shown) of a known lithographic apparatus are schematically depicted. The substrate W is configured to be substantially flat so that the height variation across the exposure surface of the substrate W is minimized. However, substrates W are typically curved at their edge portions (i.e., close to 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, while the edge portion of the substrate is curved. The image of the mask (not shown) is not in focus at the edge region of the substrate W, which may cause lithographic errors when performing lithographic exposure.

[0072] Figure 3 An acousto-optic modulator 10 is schematically depicted. The acousto-optic modulator 10 may be used as a wavelength modulator in an embodiment of the present invention. The acousto-optic modulator 10 comprises a transducer 11, a transparent body 12 and an absorber 13. The transducer 11 may, for example, comprise a piezoelectric transducer. The transducer 11 is configured to generate an acoustic wave 14 within the transparent body 12. The transparent body 12 may, for example, comprise quartz or titanium oxide. The acoustic wave 14 travels through the transparent body 12 and is absorbed by the absorber 13. The absorber 13 may, for example, comprise foam and / or rubber. The acoustic wave 14 causes a periodic compression and expansion of the transparent body 12, which modulates the 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, resulting in diffraction of the radiation beam 15.

[0073] The manner in which radiation is diffracted by the transparent body can be determined at least in part by the operation of the transducer 11 (e.g., the frequency and amplitude at which the transducer is operated) and the angle of incidence 16 of the radiation beam 15 with respect to the acoustic wave 14. Modes of operation of the AOM 10 include the Raman-Nath mechanism, in which the radiation beam 15 is diffracted into multiple diffraction orders, and the Bragg mechanism, in which the radiation beam 15 is diffracted into one diffraction order. Figure 3 In the example of FIG. 1 , the AOM 10 operates in a Bragg mechanism, wherein the radiation beam 15 is diffracted into a first diffraction order 17. Operating the AOM 10 in a Bragg mechanism may be preferred due to reduced radiation losses. The Bragg mechanism may be implemented, for example, by changing the incident angle 16 of the radiation beam 15, tilting the AOM 10 with respect to the radiation beam 15, and / or adjusting the operation of the transducer 11.

[0074] The wavelength of the radiation beam 15 may be varied by the interaction between the radiation beam 15 and the acoustic wave 14 in the transparent body 12 of the AOM 10. The shift frequency of the radiation beam 15 may be calculated as follows:

[0075] f s =f r +(m×f a )

[0076] 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 wave 14 in the transparent body 12 of the AOM 10. Changing its operating frequency during the time that the transducer 11 causes the acoustic wave 14 to travel through the transparent body 12 enables inducing a varying density pattern within the transparent body 12. Figure 3 An example is shown of a changing frequency of an acoustic wave 14 inducing a changing density pattern in a transparent body 12. Changing the density of the transparent body 12 changes the refractive index of the transparent body. The refractive index of the transparent body 12 is thus made dependent on the position within the transparent body. A radiation beam 17 passing through the transparent body 12 experiences a wavelength shift that depends on the position within the transparent body. The acousto-optic modulator thus changes the wavelength of the radiation beam 17 depending on the position within the cross section of the radiation beam. For example, the carrier speed of quartz is approximately 5800 ms -1 The acoustic wave travels through the 130 mm quartz transparent body of the AOM in approximately 22 μs. Changing the operating frequency of the transducer induces a position-dependent shift of the wavelength of the acoustic wave within the quartz body within the 22 μs of the acoustic wave traveling through the quartz body. The position-dependent shift of the wavelength of the acoustic wave within the quartz body of the AOM transducer can be used to provide a position-dependent shift of the wavelength of the radiation beam passing through the quartz body. That is, the operating frequency of the transducer can be changed so that different locations within the quartz body correspond to different frequencies of the acoustic wave. Different portions of the cross-sectional area of ​​the radiation beam pass through different locations of the quartz body and experience different wavelength shifts due to interaction with the different frequencies of the acoustic wave. The AOM can be capable of operating at a frequency of approximately 3 GHz.

[0077] The change in wavelength of the radiation beam in order to change the shape of the focal plane of the projection system can be achieved via two methods. The first method involves changing the wavelength of the radiation beam across the cross section of the radiation beam. This first method can be used as described above with respect to Figure 3 The discussion and the following Figure 4The second method comprises changing the wavelength of the entire radiation beam while scanning the radiation beam across a target portion of the substrate in a non-scanning direction. The second method can be implemented using an acousto-optic modulator or a movable optical element in the radiation source, and by scanning the radiation beam across the substrate in a non-scanning direction of the lithographic apparatus using a deflector. The second method is discussed in detail below with respect to Figures 5-7. In contrast, the first method can be considered as static focal plane shaping (i.e., the wavelength shift applied to the radiation beam does not vary as a function of time), while the second method can be considered as dynamic focal plane shaping (i.e., the wavelength shift applied to the radiation beam varies as a function of time).

[0078] When performing the first approach (i.e., "static" focal plane shaping), if the radiation beam consists of radiation pulses, the AOM may be operated such that the spatial pattern of acoustic waves induced in the transparent body is substantially the same for each pulse of the radiation beam. Each radiation pulse in the radiation beam will therefore experience substantially the same wavelength shift when passing through the transparent body. Each pulse may experience a wavelength shift such that the focal plane of the projection system for each pulse substantially matches the shape of the target area of ​​the substrate. The selection of the amplitude and / or frequency of the acoustic waves generated by the transducer of the AOM may depend at least in part on the repetition rate of the radiation beam. Alternatively, when performing the second approach (i.e., "dynamic" focal plane shaping), if the radiation beam consists of radiation pulses, the AOM may be operated such that a first pulse receives the same wavelength shift across its cross section and subsequent different pulses receive different wavelength shifts across their cross sections. A deflector may be used to sweep the radiation beam across the target area of ​​the substrate in a non-scanning direction. Different positions across the sweep correspond to different wavelengths of radiation. The position of the focal plane is therefore adjusted during the sweep so as to track the shape of the target area of ​​the substrate.

[0079] Changing the wavelength of the entire radiation beam can change the position of the focal plane of the projection system of the lithographic apparatus. Such a change in the position of the focal plane can be caused by, for example, chromatic aberration of the projection system. For example, changing the wavelength of the radiation beam by approximately 60 fm can cause the focus at the substrate to change by approximately 20 nm. It is known that the focal plane of the projection system can be substantially planar. Changing the wavelength of the radiation beam so that the wavelength of the radiation beam depends on the position within the cross-section of the radiation beam can change the shape of the focal plane of the projection system. In other words, modulating the radiation beam so that different positions within the cross-section of the radiation beam have different wavelengths can induce bending of the focal plane of the projection system. By operating the transducer of the acousto-optic modulator so that the frequency of the sound wave varies with time, the sound wave pattern that appears in the transparent body can be controlled. The sound wave pattern can be controlled so that the positions in the transparent body are different from the different offset frequencies f of the radiation beam passing through the transparent body of the acousto-optic modulator. sThus, controlling the frequency of the transducer in the AOM enables the position and / or shape of the focal plane of the projection system to be controlled. The wavelength of the radiation beam may be varied depending on the position within the cross section of the radiation beam, so that the shape of the focal plane better matches the shape of the substrate, thereby reducing focus errors.

[0080] Figure 4 A portion 20 of a lithographic apparatus including a wavelength modulator 21 according to an embodiment of the invention is schematically depicted. Figure 4 In the example of , the wavelength modulator 21 comprises an acousto-optic modulator. Figure 4 In the example of , the AOM is located between the pattern forming device MA and the projection system PS of the lithographic apparatus. The radiation beam PB is incident on the pattern forming device MA. The pattern forming device is configured to provide the radiation beam PB with a pattern in its cross section. After being patterned by the pattern forming device MA, the radiation beam PB is incident on the AOM 21. The AOM 21 is configured to change the wavelength of the radiation beam depending on the position within the cross section of the radiation beam. In other words, the AOM 21 is configured to change the wavelength of the radiation beam so that different positions within the cross section of the radiation beam experience different side length changes. The radiation beam PB passes through the AOM 21 and enters the projection system PS of the lithographic apparatus. The projection system PS projects the radiation beam PB onto a substrate W located on a substrate table WT of the lithographic apparatus. In Figure 4 In the example of , the 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 illustrate the shapes 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 the exposure surface of the substrate W or as close thereto as possible, for example by adjusting the height of the substrate table WT.

[0081] exist Figure 4 In the example of , the AOM 21 comprises six transducers 23a-f. Each transducer 23a-f is configured to generate acoustic waves in a different segment of the transparent body of the AOM 21. Providing an AOM with multiple transducers 23a-f may enable greater control over the acoustic wave pattern generated within the transparent body of the AOM compared to an AOM with a single transducer. This in turn may enable greater control over the wavelength of the radiation beam PB across the cross-section of the radiation beam, thereby enabling greater control over the shape and / or position of the focal plane of the projection system PS. Figure 4In the example of FIG. 1 , one of the transducers 23f operates at a different time-varying frequency than the other transducers 23a-e. Radiation passing through a segment of the AOM 21 under the influence of transducer 23f is given a different wavelength shift than radiation passing through a segment of the AOM 21 under the influence of transducers 23a-e. Radiation passing through a segment of the AOM 21 under the influence of transducers 23f is therefore focused after a different optical path than radiation passing through a segment of the AOM 21 under the influence of transducers 23a-e. The shape of the focal plane 24 of the projection system PS is controlled to better match the curvature of the substrate W, thereby improving the accuracy of the lithographic exposure.

[0082] The shape of the focal plane 24 generated by the AOM with multiple transducers may include a stepped pattern, where each step corresponds to a segment of the transparent body of the AOM. Increasing the number of segments and associated transducers may reduce the size of the steps in the stepped pattern, and the shape of the generated focal plane 24 may better match the shape of the substrate W. Some interference may occur between the acoustic waves generated by the different transducers 23a-e. However, these effects may be ignored. Providing a rigid connection between the transducers 23a-e and the transparent body of the AOM 21 may reduce interference effects occurring between the acoustic waves generated in the different segments of the transparent body. The different segments of the transparent body of the AOM 21 may be acoustically isolated from each other to reduce interference effects occurring between the acoustic waves generated by the different transducers.

[0083] The radiation beam may have a wavelength of about 193 nm. Edge roll-off effects may, for example, result in a height variation of between about 10 nm and about 20 nm at the substrate surface. Varying the wavelength of the radiation beam by about 60 fm may result in a focus position variation of the projection system PS by about 20 nm, thereby correcting for the 20 nm height variation caused by the roll-off effect at the substrate edge. Operating the AOM at a frequency of about 500 MHz may enable a 60 fm wavelength shift of the radiation beam. Assuming the transparent body of the AOM is quartz (with a carrier speed of about 5800 ms -1 If the acoustic wave is about 1.16×10 -5 Assuming the AOM operates in the Bragg mechanism, the diffracted beam leaving the quartz can be about 17 mrad from the optical axis of the lithographic apparatus. A lens (not shown) can be used to collect and redirect radiation leaving the quartz at the maximum diffraction angle.

[0084] Figure 5A and 5B A portion 30 of a lithographic apparatus including a wavelength modulator 31 and a deflector 33 according to an embodiment of the present invention is schematically depicted. Figure 5A and 5B In the example of , the wavelength modulator 31 comprises an AOM having a single transducer (not shown). However, the AOM 31 may comprise a plurality of transducers. The AOM 31 is configured to change the wavelength of the radiation beam 32 such that different points of the cross section of the radiation beam experience substantially the same wavelength change. 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 AOM 31. Figure 4 In the situation Figure 5A and 5B In the example of FIG. 2 , the 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 illustrate the shape of the focal plane 24 and the shape of the substrate W. In practice, by, for example, adjusting the height of the substrate table WT, the focal plane 34 of the projection system PS is located on the exposure surface of the substrate W or as close thereto as possible. It will be appreciated that the focal plane 34 is a two-dimensional entity and, for ease of illustration, is shown in FIG. Figure 5A and 5B is represented as a one-dimensional line.

[0085] Figure 5A A portion 30 of a projection system is shown at a first time t1 during projection of a radiation beam 32. The radiation beam 32 is incident on an AOM 31. At time t1, the AOM 31 is operated at a first frequency, thereby providing a first wavelength shift of the radiation beam 32. The radiation beam 32 passes through the AOM 31 and is incident on a deflector 33. The deflector 33 is configured to scan the radiation beam 32 across a target portion of a substrate W. In the example of FIG. 5 , the deflector 33 comprises a rotatable mirror. Changing the angular position 35 of the mirror 33 causes the angle of reflection of the radiation beam 32 to change. Figure 5A5 , the reflector 33 is in a first angular position 35 and causes the radiation beam to reflect in a first direction 32a. The radiation beam 32 is then incident on the pattern forming device MA. The pattern forming device MA is configured to apply a pattern to the radiation beam 32 in a cross section. After being patterned by the pattern forming device MA, the radiation beam 32 enters the projection system PS of the lithography apparatus. The projection system PS projects the radiation beam 32 onto a substrate W located on a substrate table WT of the lithography apparatus. At time t1, the radiation beam 32 is incident on the substrate W close to the edge of the substrate W. As can be seen from FIG. 5 , the morphology of the substrate W changes near the edge of the substrate due to the edge roll-off effect discussed above. The first wavelength shift provided by the acousto-optic modulator 31 and the first direction 32a provided by the reflector 33 cause the image of the pattern applied to the radiation beam 32 by the pattern forming device MA to be focused at a first position 34a. The first position 34a corresponds to the position of the exposure surface of the substrate W. That is, the wavelength offset and direction 32a of the radiation beam 32 are selected so 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 can reduce lithographic errors such as focus errors when performing lithographic exposures.

[0086] 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 FIG. 5 , the scanning direction is along the Y-axis. The angular position 35 of the mirror 33 varies with time so that the radiation beam 32 is scanned across the 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 FIG. 5 , the non-scanning direction is along the X-axis (i.e., perpendicular to the scanning direction). The optical axis 37 of the portion 30 of the lithographic apparatus is positioned along the z-axis. The deflector 33 is configured to scan the radiation beam 32 in the non-scanning direction at a speed greater than the speed at which the positioning device moves the substrate W in the scanning direction 36. For example, the positioning device can be configured to utilize approximately 0.1 ms. -1 and about 1ms -1 The deflector 33 may be configured to move the substrate W in the scanning direction 36 at a speed between about 1 ms and about 2 ms. -1 and about 100ms -1The radiation beam 32 is scanned in the non-scanning direction at a speed between 0.1 kHz and 0.2 kHz. Sweeping the radiation beam in the non-scanning direction may affect the consistency of the radiation dose applied to the substrate W. That is, depending on the repetition rate of the radiation beam and the speed of the sweep, some areas of the substrate may receive less radiation than other areas, which in turn may cause lithography errors. The repetition rate of the radiation beam can be increased so as to maintain dose consistency across the substrate when the radiation beam is scanned in the non-scanning direction. For example, the repetition rate of the radiation beam can be increased from between about 6 kHz and about 8 kHz to about 20 kHz. Alternatively, the pulse duration of the radiation beam can be increased so as to maintain dose consistency across the substrate when the radiation beam is scanned in the non-scanning direction. The time used to perform one sweep of the radiation beam across the substrate in the non-scanning direction can be selected so that it is substantially the same as the duration of the pulse of the radiation beam.

[0087] The following is a discussion of an example embodiment of the present invention. The pulse duration of the radiation beam may be, for example, about 200 ns. The repetition rate of the radiation beam may be, for example, about 6 kHz. The width of the exposure slit of the lithographic apparatus may be, for example, about 26 mm. In order to sweep the radiation beam across the exposure slit width within the duration of one laser pulse, the sweep speed of the radiation beam may be about 1.3×10 5 m -1 . The reflector 33 can be rotated so that the rotation rate of the reflector 33 corresponds to the repetition rate of the radiation beam. For example, when the repetition rate of the radiation beam is about 6 kHz, the reflector 33 can 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 up about 1 / 800 of a full rotation of the reflector. This is equivalent to a mirror rotation of about 0.43 degrees. If the optical design of the lithography is such that the reflector covers the entire width of the exposure slit through a rotation of an angle of 4.3 degrees, the reflector may have to be rotated at a speed of about 60,000 revolutions per second. Increasing the pulse duration of the radiation beam can reduce the required mirror rotation speed.

[0088] Figure 5BPart 30 of the projection system is shown at a second time t2 during projection of the radiation beam 32. At time t2, the AOM 31 is operated at a frequency different from the frequency used at time t1. The different operating frequency of the AOM at time t2 induces a second wavelength shift of the radiation beam 32 that is different from the first wavelength shift at time t1. The radiation beam 32 passes through the AOM 31 and is incident on the mirror 33. The mirror 33 is in a second angular position 35 and causes the radiation beam 32 to be reflected in a second direction 32b. The second wavelength shift provided by the AOM 31 and the change in direction 36b of the radiation beam 32 provided by the mirror 33 cause the image of the pattern applied to the radiation beam 32 by the pattern forming device MA to be focused at a second position 34b. The second position 34b corresponds to the position of the exposure surface of the substrate W. The wavelength shift and direction 32b of the radiation beam 32 cause the shape of the focal plane 34 of the projection system to substantially match the shape of the exposure surface of the substrate W. As can be seen in FIG. Figure 5A and Figure 5B As can be seen from the comparison between the two, the mirror 33 scans the radiation beam 32 along the non-scanning direction 38 while the operating frequency of the acousto-optic device 31 is changed, so as to match the shape of the focal plane 34 with the shape of the exposure surface of the substrate W.

[0089] Figure 6 A portion 40 of a lithography system including a radiation source SO, a projection system PS, a wavelength modulator 41 and a deflector 43 according to an embodiment of the present invention is schematically depicted. 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 on the deflector 43. Components such as a beam transport system (not shown) and an illumination system (not shown) may be present between the radiation source SO and the deflector 43. The deflector 43 is configured to scan the radiation beam 42 across a target portion of the substrate W in a non-scanning direction 50 of the lithography system. The radiation beam 42 is deflected from the deflector 43 and is incident on a patterning device MA. The patterning device MA is configured to apply a pattern to the radiation beam 32 in a cross section thereof. After being patterned by the patterning device MA, the radiation beam 32 enters the projection system PS of the lithography apparatus. The projection system PS projects the radiation beam 32 onto a substrate W located on a substrate table WT of the lithography apparatus. A positioning device (not shown) is configured to move a target portion of the substrate W through the radiation beam 42 along a scan direction 49. Figure 6 In the example of , the scanning direction 49 is along the y-axis. The angular position 47 of the deflector 43 varies with time so that the radiation beam 42 is scanned across the target portion of the substrate W in a non-scanning direction 50 of the lithographic apparatus during movement of the substrate W. Figure 6In the example of , the non-scanning direction 50 is along the x-axis (ie perpendicular to the scanning direction 49 ). The deflector 43 is configured to scan the radiation beam 42 in the non-scanning direction 50 at a speed greater than the speed at which the positioning device moves the substrate W in the scanning direction 49 .

[0090] The lithography system comprises a wavelength modulator 41 configured to change the wavelength of a radiation beam 42. Figure 6 In the example of , the wavelength modulator comprises a movable optical element 41 located in a laser cavity 44. Figure 6 In the example of , the movable optical element 41 is a mirror. An actuator (not shown) can be configured to move the mirror 41 along the length 45 of the laser cavity 44. The actuator can include, for example, a piezoelectric actuator. A partially reflective end mirror 46 (i.e., an output coupler) is located within the laser cavity 44 opposite to the movable mirror 41. The output coupler 46 is configured to transmit some of the radiation generated in the laser cavity 44, thereby forming a 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 the wavelength of the radiation beam 42 generated by the laser cavity 44.

[0091] The radiation beam 42 leaves the radiation source SO and is incident on the deflector 43. Figure 6 In the example of FIG. 4 , the deflector 43 comprises a rotatable mirror. Changing the angular position 47 of the mirror 43 changes the reflection angle of the radiation beam 42 . Figure 6 A radiation beam 42 is shown reflected by a deflector 43 along three different directions 42a-c corresponding to three different times t1-t3 during operation of the lithography system. At a first time t1, the movable mirror 41 is in a first position 41a within the laser cavity 44 so that the radiation beam 42 has a first wavelength, and the deflector 43 is in a first angular position so that the radiation beam 42 is deflected in a first direction 32a. The radiation beam 42 passes through the projection system PS and is focused at a first position 48a. At a second time t2, the movable mirror 41 is in a second position 41b within the laser cavity 44 so that the radiation beam 42 has a second wavelength different from the first wavelength, and the deflector 43 is in a second angular position so that the radiation beam 42 is deflected in a second direction 42b different from the first direction 42a. The radiation beam 42 passes through the projection system PS and is focused at a second position 48b. At a third time t3, the movable mirror 41 is in a second position 41b within the laser cavity 44 so that the radiation beam 42 has a second wavelength, and the deflector 43 is in a third angular position so that the radiation beam 42 is deflected in a third direction 42c different from the first and second directions 42a-b. The radiation beam 42 passes through the projection system PS and is focused at a third position 48c. Figure 6It can be seen that between times t1-t3, while the position 41a-b of the movable mirror 41 in the laser cavity 44 changes, the deflector 43 scans the radiation beam 42 along the non-scanning direction 50 so that the shape of the focal plane 51 of the projection system PS matches the shape of the exposure surface of the substrate W.

[0092] The size of the radiation beam in the scan direction (i.e., non-scanning direction) may at least partially determine the dose uniformity of the lithographic exposure and / or the accuracy with which the focal plane of the projection system may be shaped. A radiation beam having a greater length in the scan direction may provide improved dose uniformity compared to a radiation beam having a smaller length in the scan direction. However, a radiation beam having a smaller length in the scan direction may more accurately shape the focal plane of the projection system than a radiation beam having a greater length in the scan direction. The length of the radiation beam in the scan direction may be selected based in part on the desired dose uniformity and focal plane shaping accuracy.

[0093] The movable mirror 41 may be deformable. That is, the movable mirror 41 may be bent so that the distance between the movable mirror 41 and the output coupler 46 depends on the lateral position within the laser cavity 44. An actuator (not shown) may be configured to deform the movable mirror 41 in a controllable manner. For example, the actuator may include a piezoelectric actuator. Deforming the movable mirror 41 changes the wavelength of the radiation beam 42 generated by the radiation source SO so that the wavelength of the radiation beam 42 depends on the position within the cross-section of the radiation beam 42. Deforming the movable mirror 41 and changing the position of the movable mirror 41 in the laser cavity 44 may enable finer control over the shape of the focal plane 51 of the projection system PS than simply changing the position of the movable mirror 41 in the laser cavity 44.

[0094] Figure 7A portion 60 of a lithography system including a radiation source SO, a projection system PS, a wavelength modulator 61 and a deflector 63 according to an embodiment of the present invention is schematically depicted. The radiation source SO comprises a laser having a laser cavity 64 configured to generate a radiation beam 62. The radiation beam 62 is incident on the deflector 63. Components such as a beam transport system (not shown) and an illumination system (not shown) may be present between the radiation beam SO and the deflector 63. The deflector 63 is configured to scan the radiation beam 62 across a target portion of the substrate W in a non-scanning direction 70 of the lithography system. The radiation beam 62 is deflected from the deflector 63 and is incident on a patterning device MA. The patterning device MA is configured to apply a pattern to the radiation beam 62 in a cross section thereof. After being patterned by the patterning device MA, the radiation beam 62 enters the projection system PS of the lithography apparatus. The projection system PS projects the radiation beam 32 onto a substrate W located on a substrate table WT of the lithography apparatus. A positioning device (not shown) is configured to move a target portion of the substrate W through the radiation beam 62 along a scan direction 69. Figure 6 In the example of , the scanning direction 69 is along the y-axis. The angular position 67 of the deflector 63 varies with time so that the radiation beam 62 is scanned across the target portion of the substrate W in a non-scanning direction 70 of the lithographic apparatus during movement of the substrate W. Figure 7 In the example of FIG. 5 , the non-scanning direction 70 is along the x-axis (ie perpendicular to the scanning direction 69 ). The deflector 63 is configured to scan the radiation beam 62 in the non-scanning direction 70 at a speed greater than the speed at which the positioning device moves the substrate W in the scanning direction 69 .

[0095] The lithography system comprises a wavelength modulator 61 configured to change the wavelength of a radiation beam 62. Figure 7 In the example of , the wavelength modulator comprises a movable optical element 61 located in a laser cavity 64. Figure 7 In the example of , the movable optical element 61 is a rotatable prism. An actuator (not shown) may be configured to rotate the prism 61 around an axis 65. The actuator may include a piezoelectric actuator, for example. Figure 7 In the example of FIG. 6 , axis 65 enters the page through prism 61 . Figure 7The axis 65 shown is intended to illustrate the axis about which the prism 61 rotates, and is not intended to indicate a solid object such as a needle passing through the prism 61. The laser cavity includes a partially reflecting end mirror 66 (i.e., 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 to form a radiation beam 62 that is provided to the deflector 63. The angular position of the rotatable prism 61 and the wavelength of the radiation incident on the rotatable prism 61 determine the refraction angle of the radiation traveling through the rotatable prism 61. The refraction angle of the radiation passing through the rotatable prism determines whether the radiation is reflected from the end mirror 73 and reaches the gain medium 72 and the output coupler 66, or whether the radiation is reflected from the end mirror 73 and away from the rotatable prism 61. Changing the angular position 61a-b of the rotatable prism 61 changes the wavelength of the radiation beam 62 generated by the laser cavity 64.

[0096] The radiation beam 62 leaves the radiation source SO and is incident on the deflector 63. Figure 7 In the example of FIG. 6 , the deflector 63 comprises a rotatable mirror. Changing the angular position 67 of the mirror 63 changes the reflection angle of the radiation beam 62 . Figure 7 A radiation beam 62 is shown reflected by a deflector 63 in three different directions 62a-c corresponding to three different times t1-t3 during operation of the lithography system. At a first time t1, the movable mirror 61 is in a first angular position 61a within the laser cavity 64 so that the radiation beam 62 has a first wavelength, and the deflector 63 is in a first angular position 67 such that the radiation beam 62 is deflected in the first direction 62a. The radiation beam 62 passes through the projection system PS and is focused at a first position 48a. At a second time t2, the movable mirror 61 is in a second angular position 61b within the laser cavity 64 so that the radiation beam 62 has a second wavelength different from the first wavelength, and the deflector 63 is in a second angular position 67 such that the radiation beam 62 is deflected in a second direction 62b different from the first direction 62a. The radiation beam 62 passes through the projection system PS and is focused at a second position 68b. At a third time t3, the movable mirror 61 is in a second position 61b within the laser cavity 64 such that the radiation beam 62 has a second wavelength, and the deflector 63 is in a third angular position 67 such that the radiation beam 62 is deflected in a third direction 62c different from the first and second directions 62a-b. The radiation beam 62 passes through the projection system PS and is focused at a third position 68c. Figure 7 It can be seen that between times t1-t3, while the position 61a-b of the movable mirror 61 in the laser cavity 64 changes, the deflector 63 scans the radiation beam 62 along the non-scanning direction 70 so that the shape of the focal plane 71 of the projection system PS matches the shape of the exposure surface of the substrate W.

[0097] The rotatable prism 61 may be deformable. That is, the rotatable prism 61 may be bent so that different rays of radiation experience different angles of refraction when passing through the rotatable prism 61. An actuator (not shown) may be configured to deform the rotatable prism 61 in a controllable manner. For example, the actuator may include a piezoelectric actuator. Deforming the rotatable prism 61 changes the wavelength of the radiation beam 62 generated by the radiation source SO, so that the wavelength of the radiation beam 62 depends on the position within the cross-section of the radiation beam 62. Deforming the rotatable prism 61 and changing the angular position of the rotatable prism 61 in the laser cavity 44 can enable more precise control over the shape of the focal plane 71 of the projection system PS than simply changing the angular position of the rotatable prism 61 in the laser cavity 44.

[0098] Any of the embodiments described herein may be used in conjunction with a substrate topography measurement system. Figure 8 A portion 80 of a lithography system including a topography measurement system 81 , a processor 82 , a controller 83 and a wavelength modulator 84 according to an embodiment of the present invention is schematically depicted. Figure 8 A substrate W is shown being held on a substrate table WT at two different stages - a measurement stage 87 and an exposure stage 88. In the measurement stage, a topography measurement system 81 is configured to measure a height map of the substrate W and output a signal indicative of the height map. For example, the topography measurement system 81 may be of the type described in international patent application publication WO 2017 / 060014 A1. The processor 82 is configured to receive signals from the topography measurement system 81 and determine a change in the shape of a focal plane of a projection system PS of the lithography system. The processor 82 determines a change in the shape of a focal plane 85 of the projection system PS so that the focal plane 85 of the projection system PS better matches the topography of the substrate W (i.e., the measured height map). The processor 82 is further configured to output a signal indicative of a change in the shape of the focal plane 85 of the projection system PS. The controller 83 is configured to receive signals 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. At Figure 8 In the example of FIG. 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 illustrate 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 the exposure surface of the substrate W or as close thereto as possible, for example by adjusting the height of the substrate table WT. It will be appreciated that the focal plane 85 is a two-dimensional entity and for ease of illustration, the focal plane 85 is shown in FIG. Figure 8 is represented as a one-dimensional line.

[0099] As above and references Figure 1As discussed, the wavelength modulator may be located in different regions 1a-d of the lithography system. When the wavelength modulator is an AOM, the location of the AOM within the lithography system may affect the size of the AOM and / or how the AOM is operated. This is because the cross-sectional area of ​​the radiation beam varies between different locations within the lithography system. The size of the transparent body of the AOM depends at least in part on the operating parameters of the lithography system with which the AOM is used. For example, the size of the transparent body of the AOM may be such that the entire cross-sectional area of ​​the radiation beam can pass through the transparent body. The size of the transparent body of the AOM may depend on the location of the AOM in the lithography apparatus and the demagnification of the projection system of the lithography apparatus. For example, the projection system of the lithography system may provide a demagnification of a factor of 4. When the AOM is positioned close to the reticle, the transparent body of the AOM may have a length between about 100 mm and about 400 mm. When the AOM is positioned close to the substrate, the transparent body of the AOM may have a length between about 25 mm and about 100 mm. When the AOM is positioned close to the field plane of an illumination system of a lithographic apparatus, the length of the transparent body may depend at least in part on the optical design of the illumination system. For example, when the AOM is positioned close to the field plane of an illumination system of a lithographic apparatus, the transparent body of the AOM may have a length between about 20 mm and about 110 mm. The possible positions of the AOM within a lithographic apparatus may be limited by the optical design of the lithographic apparatus. For example, it may be preferred to position the AOM close to the field plane of a lithographic apparatus rather than the pupil plane of the lithographic apparatus. The position of the field plane may vary in different lithographic apparatuses.

[0100] The AOM may include any number of transducers. The transducers may be of different sizes and / or different shapes. For example, a high concentration of smaller transducers may be positioned near an end region of a transparent body of the AOM to provide greater control over the shape and / or position of the end region of the focal plane of the projection system (e.g., to further reduce edge roll-off focus errors). Although the use of the present invention has been discussed primarily with respect to reducing lithography focus errors associated with substrate edge roll-off, it will be appreciated that the present invention may be used to reduce focus errors at any location on the substrate. For example, the topography of a target portion of the substrate near the center of the substrate may be measured, and a wavelength modulator may be used to adjust the focal plane of the projection system to better match the topography of the target portion.

[0101] Although the deflector has been described and depicted as a rotatable mirror, it will be appreciated that the deflector may take any form. For example, the deflector may include a wheel having a plurality of planar mirrors around the circumference of the wheel. Alternatively, the deflector may include a movable prism configured to deflect the radiation beam via refraction. As another alternative, the deflector may include a micro-electromechanical system (MEMS) mirror array.

[0102] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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 that can be read by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM), a random access memory (RAM), a disk storage medium, an optical storage medium, a flash memory device; electrical, optical, acoustic or other forms of propagation signals (e.g., carrier waves, infrared signals, digital signals, etc.), etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are only for convenience and such actions are actually derived from computing devices, processors, controllers or other devices that execute the firmware, software, routines, instructions, etc.

[0103] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. This description is not intended to limit the invention.

Claims

1. A lithography system, comprising: a radiation source, wherein the radiation source comprises a laser cavity configured to generate a radiation beam; an irradiation system for providing said radiation beam; a support structure for supporting a patterning device for imparting a pattern to the radiation beam in its cross-section; a substrate table for holding a substrate; a projection system for projecting a patterned radiation beam onto a target portion of the substrate; as well as A wavelength modulator operative to change the wavelength according to at least one of: a position within a cross-section of the patterned radiation beam; and The position of the patterned radiation beam on the target portion of the substrate.

2. The lithography system of claim 1, wherein the wavelength modulator comprises an acousto-optic modulator.

3. The lithography system of claim 2, wherein the AOM comprises a plurality of transducers configured to generate acoustic waves having different frequencies in different segments of the AOM.

4. The lithography system according to any one of claims 1 to 3, wherein the lithography system further comprises: a topography measurement system configured to measure a height map of the substrate and output a signal indicative of the height map; a processor configured to receive the signal from the topography measurement system and determine a change in shape of a focal plane of the projection system, and to output a signal indicative of the change in shape of the focal plane of the projection system; and A controller is configured to receive the signal from the processor and control the wavelength modulator so as to apply the determined shape change of the focal plane of the projection system.

5. The lithography system of claim 1, wherein the wavelength modulator comprises a movable optical element located in the laser cavity.

6. The lithography system of claim 5, wherein the movable optical element is deformable.

7. The lithography system of claim 5 or 6, wherein the optical element is a mirror.

8. The lithography system of claim 5 or 6, wherein the movable optical element is a prism.

9. The lithography system according to any one of claims 1 to 8, wherein the lithography system further comprises: A deflector is configured to scan the patterned radiation beam across the target portion of the substrate in a non-scanning direction.

10. A method of projecting a patterned radiation beam onto a substrate, the method comprising: providing the substrate; providing a radiation beam using an irradiation system; imparting a pattern to the radiation beam in its cross-section using a patterning device; projecting the patterned radiation beam onto a target portion of the substrate; and The wavelength of the radiation beam is varied according to at least one of: a position within the cross-section of the patterned radiation beam; and The position of the patterned radiation beam on the target portion of the substrate.

11. The method of claim 10, wherein changing the wavelength comprises: Use an acousto-optic modulator.

12. The method of claim 11, wherein using the AOM comprises: Acoustic waves with different frequencies are generated in different segments of the AOM.

13. The method according to claim 11 or 12, wherein the method further comprises: measuring a height map of the substrate; Determining focal plane changes of projection systems; and The determined focal plane change of the projection system is applied using the acousto-optic modulator.

14. The method according to claim 10, wherein the method further comprises: The radiation beam is generated using a radiation source comprising a laser cavity and an optical element located in the laser cavity, wherein changing the wavelength comprises moving the optical element in the laser cavity.

15. The method of claim 14, wherein changing the wavelength further comprises: The optical element in the laser cavity is deformed.

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