Interferometer system, wavefront analysis system, projection system, lithographic apparatus and method for analyzing wavefront of light beam of heterodyne interferometer system

Abstract

An interferometer system includes a light source providing a light beam; an optical system for splitting the light beam into a measurement beam and a reference beam, the measurement beam and the reference beam each having a different wavelength, and the optical system being arranged to direct the measurement beam to the reflective measurement surface, direct the reference beam to the reflective reference surface, and recombine the reflective measurement beam and the reflective reference beam to provide a reflective light beam. A reference detector receives the light beam and provides a reference detector signal and / or a measurement detector receives the reflected light beam and provides a measurement detector signal. A time-of-flight camera receives the reflected light beam and a demodulated signal based on the reference detector signal or the measurement detector signal, and provides a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam demodulated with the demodulated signal. The interferometer system analyzes the wavefront difference based on the camera signal.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from European application 23158336.0, filed on February 23, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to an interferometer system and a wavefront analysis system for analyzing the wavefront difference of a reflected light beam of a heterodyne interferometer system (interferometer system). The present invention also relates to a projection system for an optical lithography system including such an interferometer system and / or a lithography apparatus including such an interferometer system, and a method for analyzing the wavefront difference of a reflected light beam of a heterodyne interferometer system. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate, typically to a target portion of the substrate. For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). In this case, a patterning device (alternatively, a mask or reticle) can be used to generate the circuit pattern to be formed on the individual layers of the IC. The pattern can be transferred to a target portion (e.g., a portion comprising one or more dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will include a network of adjacent target portions that are patterned continuously. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing the entire pattern to the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern with a radiation beam in a given direction (the "scanning" direction) while simultaneously scanning the substrate parallel or antiparallel to that direction. A pattern can also be transferred from a patterning device to a substrate by imprinting the pattern onto the substrate.

[0005] In embodiments of lithographic apparatus, interferometer systems are used to determine the position of movable objects with high precision. Examples of these movable objects are substrate supports and optical elements, such as mirrors in projection optics. Interferometer systems can also be used to accurately determine the path length to fixed objects, such as in wavelength trackers.

[0006] The interferometer system may include a light source device, an optical system, and a measurement detector. The light source device is arranged to provide a light beam that is directed to the optical system. The optical system is arranged to separate the light beam into a measurement beam and a reference beam, direct the measurement beam along a measurement path to a reflective measurement surface, and direct the reference beam along a reference path to a reflective reference surface. After the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface, the optical system may recombine the measurement beam and the reference beam to provide a reflected light beam. The measurement detector is arranged to receive the reflected light beam and provide a measurement detector signal. The measurement detector signal represents the position of the reflective measurement surface. A processing device may be provided to determine the position of the reflective measurement surface based on the measurement detector signal.

[0007] Interferometer systems can have measurement errors that depend on the tilt of the reflective measurement surface. These tilt-related errors are typically related to the wavefront quality of the reflected beam, specifically the wavefront quality of the reflected measurement beam relative to the reflected reference beam. Other factors can also affect the wavefront quality of the reflected beam, such as alignment of the optical components of the optical system, manufacturing tolerances, and fiber noise.

[0008] A Shack-Hartman sensor can be provided to measure the gradient of the wavefront using a segmented microlens array and by integrating the gradient into a wavefront map. However, a Shack-Hartmann sensor can only measure the wavefront gradient, not the phase shift of the beam. Summary of the Invention

[0009] An object of the present invention is to provide an improved system for analyzing (e.g., characterizing) the wavefronts of a reflected measurement beam and a reflected reference beam of an interferometer optical device. In particular, an object of the present invention is to provide an interferometer system that can be used to analyze the wavefront difference of a reflected measurement beam and a reflected reference beam of a heterodyne interferometer system to determine the wavefront deformation for use in diagnosing and / or calibrating the interferometer system.

[0010] Another object is to provide an improved method for analysing the wavefront difference of a reflected light beam of a heterodyne interferometer system, or at least to provide an alternative solution.

[0011] According to one aspect of the present invention, there is provided an interferometer system, the interferometer system comprising:

[0012] a light source device arranged to provide a light beam;

[0013] an optical system arranged to split the optical beam into a measuring beam and a reference beam, the measuring beam having a first wavelength and the reference beam having a second wavelength, wherein the first wavelength and the second wavelength are different, wherein the optical system is arranged to direct the measuring beam along a measurement path to the reflective measurement surface, direct the reference beam along a reference path to the reflective reference surface, and after the measuring beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface, recombining the reflected measuring beam and the reflected reference beam to provide a reflected optical beam,

[0014] a reference detector arranged to receive the light beam to provide a reference detector signal and / or a measurement detector arranged to receive the reflected light beam to provide a measurement detector signal, and

[0015] a time-of-flight camera arranged to receive the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal, and to provide a camera signal representing a wavefront difference between a reflected measurement beam and a reflected reference beam of the reflected light beam demodulated with the demodulation signal,

[0016] Therein the interferometer system is arranged to analyze the wavefront difference based on the camera signal.

[0017] According to one aspect of the present invention, a wavefront analysis system for analyzing a wavefront difference of a reflected light beam of a heterodyne interferometer system is provided, wherein the interferometer system provides a reflected light beam and a reference detector signal and / or a measurement detector signal, wherein the reflected light beam includes a reflected measurement beam and a reflected reference beam, the reflected measurement beam has a first wavelength and the reflected reference beam has a second wavelength, the first wavelength and the second wavelength being different, and the wavefront analysis system includes:

[0018] a time-of-flight camera arranged to receive the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal, and to provide a camera signal representing a wavefront difference between a reflected measurement beam and a reflected reference beam of the reflected light beam demodulated with the demodulation signal, and

[0019] A processing device for analyzing wavefront errors based on camera signals.

[0020] According to one aspect of the present invention, a projection system for optical lithography comprising an interferometer system and / or a lithography apparatus comprising an interferometer system is provided.

[0021] According to one aspect of the present invention, there is provided a method for analyzing the wavefront difference of a reflected light beam of a heterodyne interferometer system, the method comprising the following steps:

[0022] Provide a light beam;

[0023] splitting the light beam into a measurement beam having a first wavelength and a reference beam having a second wavelength, the first wavelength and the second wavelength being different;

[0024] directing a measurement beam along a measurement path toward a reflective measurement surface on the object of interest;

[0025] directing a reference beam along a reference path toward a reflective reference surface on a reference object;

[0026] recombining the reflected measurement beam and the reflected reference beam after the measurement beam reflects off the reflective measurement surface and the reference beam reflects off the reflective reference surface to provide a reflected light beam;

[0027] receiving the light beam at a reference detector to provide a reference detector signal and / or receiving the reflected light beam at a measurement detector to provide a measurement detector signal,

[0028] receiving the reflected light beam and a demodulated signal based on the reference detector signal or the measured detector signal at a time-of-flight camera;

[0029] measuring a camera signal representing a wavefront difference between a reflected measurement beam and a reflected reference beam of the reflected light beam demodulated with the demodulation signal; and

[0030] The camera signal is analyzed to analyze the wavefront difference. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which corresponding reference numerals indicate corresponding parts, and in which:

[0032] - Figure 1 A photolithographic apparatus is schematically depicted;

[0033] - Figure 2 An embodiment of an interferometer system according to the present invention is shown;

[0034] - Figure 3 shows a first alternative embodiment of an interferometer system according to the present invention; and

[0035] - Figure 4 A second alternative embodiment of the interferometer system according to the invention is shown. DETAILED DESCRIPTION

[0036] Figure 1 A lithographic apparatus according to an embodiment of the invention is schematically depicted. The apparatus comprises an illumination system IL, a support structure MT, a substrate table WT and a projection system PS.

[0037] The illumination system IL is configured to condition the radiation beam B. The support structure MT (e.g., a mask table) is configured to support the patterning device MA (e.g., a mask) and is connected to a first positioner PM that is configured to accurately position the patterning device according to certain parameters. The substrate table WT (e.g., a wafer stage) is configured to hold a substrate W (e.g., a resist-coated wafer) W and is connected to a second positioner PW that is configured to accurately position the substrate according to certain parameters. The projection system PS is configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0038] The illumination system IL may include various types of optical components for directing, shaping or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof.

[0039] As used herein, the term "radiation beam" encompasses all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm-20 nm), as well as particle beams, such as ion beams or electron beams.

[0040] The support structure MT supports (i.e., carries) the weight of the patterning device MA. The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatus, and other conditions (such as, for example, whether the patterning device MA is held in a vacuum environment). The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be, for example, a frame or a worktable that can be fixed or movable as needed. The support structure MT can ensure that the patterning device MA is in a desired position, for example, relative to the projection system PS.

[0041] The term “patterning device” as used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to produce a pattern in a target portion C of the substrate W. It should be noted that the pattern imparted to the radiation beam B may not exactly correspond to the desired pattern in the target portion C of the substrate W, for example if the pattern includes phase-shifting features or so-called assist features. Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device (such as an integrated circuit) being produced in the target portion C.

[0042] The patterning device MA can be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in photolithography and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. One 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 B in different directions. The tilted mirrors produce a pattern in the radiation beam B, which is reflected by the mirror matrix.

[0043] The term "projection system" as used herein should be broadly interpreted to cover any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, depending on the exposure radiation used, or other factors such as the use of immersion liquid or the use of a vacuum.

[0044] As shown, the device is transmissive (eg, employing a transmissive mask). Alternatively, the device may be reflective (eg, employing a programmable mirror array of the type described above, or employing a reflective mask).

[0045] The lithographic apparatus may be of a type having two (dual-stage) or more substrate tables WT (and / or two or more mask tables). In such a "multi-stage" machine, additional worktables may be used in parallel, or preparatory steps may be performed on one or more worktables while exposure is performed using one or more other worktables. In addition to the one or more substrate tables WT, the lithographic apparatus may also have a measurement table that is arranged at a position where the substrate table WT is away from a position below the projection system PS. Instead of supporting the substrate W, the measurement table may be provided with a sensor for measuring characteristics of the lithographic apparatus. For example, the projection system may project an image onto a sensor on the measurement table to determine image quality.

[0046] The lithographic apparatus may also be of a type in which at least a portion of the substrate W may be covered with a liquid having a relatively high refractive index (e.g., water) to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, such as between the patterning device MA and the projection system PS. Immersion techniques are well known in the art and are used to increase the numerical aperture of the projection system. The term "immersion" as used herein does not imply that structures such as the substrate W are necessarily immersed in the liquid, but only that the liquid is located between the projection system PS and the substrate W during exposure.

[0047] refer to Figure 1, the illumination system IL receives a radiation beam B from a radiation source SO. For example, when the radiation source SO is an excimer laser, the radiation source SO and the lithographic apparatus may be separate entities. In this case, the source is not considered to form part of the lithographic apparatus, and the radiation beam B is transferred from the radiation source SO to the illumination system IL with the aid of a beam delivery system BD comprising, for example, suitable guide mirrors and / or a beam expander. In other cases, such as when the radiation source SO is a mercury lamp, the radiation source SO may be an integral part of the lithographic apparatus. The radiation source SO and the illuminator IL, together with the beam delivery system BD (if necessary), may be referred to as a radiation system.

[0048] The illumination system IL may include an adjuster AD for adjusting the angular intensity distribution of the radiation beam B. Typically, at least the outer and / or inner radial extent of the intensity distribution in a pupil plane of the illumination system (commonly referred to as outer σ and inner σ, respectively) may be adjusted. Furthermore, the illumination system IL may include various other components, such as an integrator IN and a condenser CO. The illumination system IL may be used to condition the radiation beam B so that it has a desired uniformity and intensity distribution in its cross-section.

[0049] The radiation beam B is incident on the patterning device MT, which is fixed to the support structure MT, and is patterned by the patterning device MA. After passing through the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the help of a second positioner PW and a position sensor IF (e.g., an interferometer, a linear encoder, or a capacitive sensor), the substrate table WT can be precisely moved, for example, to position a different target portion C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor ( Figure 1The support structure MT may be connected to the short-stroke actuator only, or may be fixed. The support structure MT may be arranged to move relative to the path of the radiation beam B, for example after mechanical retrieval from a mask library, or during a scan. Typically, movement of the support structure MT may be achieved with the aid of a long-stroke module and a short-stroke module, which form part of the first positioner PM. The long-stroke module may provide coarse positioning of the short-stroke module over a larger range of movement. The short-stroke module may provide fine positioning of the support structure MT relative to the long-stroke module over a smaller range of movement. Similarly, movement of the substrate table WT may be achieved using a long-stroke module and a short-stroke module, which form part of the second positioner PW. The long-stroke module may provide coarse positioning of the short-stroke module over a larger range of movement. The short-stroke module may provide fine positioning of the substrate table WT relative to the long-stroke module over a smaller range of movement. 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 mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the patterning device MA and the substrate W. Although substrate alignment marks P1, P2 are shown as occupying dedicated target portions, they can be located in the spaces between target portions C (these are referred to as scribe street alignment marks). Similarly, mask alignment marks M1, M2 can be located between dies where more than one die is provided on patterning tool MA.

[0050] The depicted device can be used in at least one of the following modes:

[0051] In a first mode, so-called step mode, the support structure MT and the substrate table WT are kept substantially stationary, and an entire pattern applied to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then moved 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 that can be imaged in a single static exposure.

[0052] In a second mode (the so-called scan mode), the support structure MT and substrate table WT are scanned synchronously while a pattern applied to the radiation beam B 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 can be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width of the target portion in a single dynamic exposure (in the non-scanning direction), while the length of the scanning motion determines the height of the target portion (in the scanning direction).

[0053] In a third mode, the support structure MT remains substantially stationary so as to hold the programmable patterning device, and the substrate table WT is moved or scanned while a pattern applied to the radiation beam B 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 or between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography utilizing a programmable patterning device, such as a programmable mirror array of the type described above.

[0054] Combinations and / or variations of the above-described modes of use, or entirely different modes of use, may also be employed.

[0055] Figure 2 An interferometer system 100 according to an embodiment of the present invention is depicted. The interferometer system 100 is arranged to measure a change in position of a movable object 200. The movable object 200 is Figure 1 FIG2 is a diagram illustrating a portion of a lithographic apparatus shown in FIG2 . The interferometer system 100 can be used, for example, to measure the position of a mirror or lens element of a projection system PS, a patterning device support MT, or a substrate support WT. The movable object 200 includes a reflective measurement surface 201. The position change of the movable object 200 is determined relative to a reference object 300 having a reference reflective surface 301. When the starting position of the movable object 200 is known, the actual absolute position of the movable object 200 can be determined based on the starting position and the measured position change.

[0056] The interferometer system 100 includes a light source device 101 for providing a light beam 102. The light source device 101 includes a light source 103 (eg, a stabilized laser source), a first polarization and frequency shift device 104, a second polarization and frequency shift device 105, and a Rochon prism 106.

[0057] Interferometer system 100 is a heterodyne interferometer system. Light originating from light source 103 is split into a first beam portion and a second beam portion. The first beam portion has a first polarization and a first wavelength in first polarization and frequency shift device 104. The second beam portion has a second polarization and a second wavelength in second polarization and frequency shift device 105. The first and second polarizations are orthogonal to each other. The first and second wavelengths are different. The difference between the first frequency of the first beam portion and the second frequency of the second beam portion can be in the range of 0.5 MHz to 50 MHz, for example, in the range of 5 MHz to 20 MHz. The first beam portion is intended to form a measurement beam, and the second beam portion is intended to form a reference beam.

[0058] The first polarization and frequency shifting device 104 and the second polarization and frequency shifting device 105 can each include a separate polarization unit and a frequency shifting unit. The frequency shifting unit can include, for example, a photoacoustic modulator. The first beam portion and the second beam portion are recombined in a Rochon prism 106. Any other suitable optical component besides the Rochon prism 106 can also be used to recombine the first beam portion and the second beam portion.

[0059] In practice, one of the first wavelength of the first beam portion or the second wavelength of the second beam portion may be identical to the wavelength of the light provided by the light source 103, while the other of the first wavelength or the second wavelength is shifted by the respective polarization and frequency shifting device 104, 105. Obviously, no means for frequency shifting is required for one of the first wavelength or the second wavelength which is not shifted.

[0060] As an alternative to this configuration, a free-space Zeeman splitting laser can be used. Such a free-space Zeeman splitting laser can provide a beam having a first beam portion and a second beam portion, the first beam portion and the second beam portion having orthogonal polarizations and different wavelengths.

[0061] Thus, the light source system 101 provides a light beam 102 having a first beam portion and a second beam portion, the first beam portion and the second beam portion having orthogonal polarizations and different wavelengths.

[0062] The light beam 102 is directed to an optical system comprising a polarising beam splitter 107. The polarising beam splitter 107 is arranged to separate the first beam portion and the second beam portion to provide a measurement beam based on the first beam portion and a reference beam based on the second beam portion.

[0063] The measurement beam is directed along a measurement path 205 towards a reflective measurement surface 201 on the object 200. The reference beam is directed along a reference path 305 towards a reflective reference surface 301 on the reference object 300.

[0064] After reflection of the measurement beam on reflective measurement surface 201 and reflection of the reference beam on reflective reference surface 301, the measurement beam and the reference beam are recombined at polarization beam splitter 107 to form reflected beam 108. Reflected beam 108 is directed to measurement detector 109, such as an avalanche photodiode. At measurement detector 109, a measurement detector signal based on reflected beam 108 is measured.

[0065] The measured detector signals can be directed to the processing device 110. Based on the measured detector signals, the relative movement of the movable object 200, i.e., the change in the path length Lx, can be determined with high accuracy. The movement of the movable object 200 causes a phase shift in the phase signal. Based on these phase shifts in the phase signal, the processing device 110 can determine the relative displacement of the movable object 200 relative to the reference object 300. When the starting position of the movable object 200 is known, the position of the movable object 200 can be determined.

[0066] A portion of the light beam 102 from the light source device 101 is directed by a semi-transparent mirror 111 to a reference detector 112, such as an avalanche photodiode. This portion of the light beam 102 does not interact with either the reflective measurement surface 201 or the reflective reference surface 103. At the reference detector 112, a reference detector signal based on the light beam 102 is measured. This reference detector signal can be directed to the processing device 110 for further processing. This reference detector signal can, for example, be used as a reference signal for the first wavelength and the second wavelength to improve the measurement accuracy of the interferometer system, because it represents the light beam 102 directed to the optical system of the interferometer system 110, in particular the polarization beam splitter 107. The reference detector 112 provides a reference detector signal representing the light beam 102.

[0067] Figure 2 The interferometer system 100 may have measurement errors that depend on the tilt of the reflective measurement surface 201. These tilt-related errors are generally related to the wavefront quality of the reflected beam 108, for example, the wavefront of the measurement beam relative to the reference beam. Furthermore, other factors may also affect the quality of the wavefront difference of the reflected beam 108, such as alignment of the optical components of the optical system, manufacturing tolerances, and fiber noise.

[0068] It is desirable to obtain knowledge about the wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam 108. In order to analyze the wavefront difference of the reflected light beam 108, the interferometer system 100 is provided with a time-of-flight camera 115 arranged to receive the reflected light beam 108. In order to direct the reflected light beam 108 toward the time-of-flight camera 115, the optical system of the interferometer system 100 includes a semi-transparent mirror 109, which splits the reflected light beam 108 into a first portion, which is directed to the measurement detector 109, and a second portion, which is directed to the time-of-flight camera 115. A time-of-flight camera is a digital camera capable of providing a depth value at each pixel of the camera.

[0069] Note that the measurement detector 109 and the time-of-flight camera 115 may include at least one polarizer to generate interference between the reflected measurement beam and the reflected reference beam of orthogonal polarizations. Correspondingly, the reference detector 112 may include at least one polarizer to generate interference between the measurement beam and the reference beam of orthogonal polarizations.

[0070] In order to analyze the wavefront difference between the measurement beam and the reference beam of the reflected light beam 108, the reflected light beam is demodulated using a demodulation signal. Figure 2 In the embodiment of FIG. 5 , the measurement detector signal provided by the measurement detector 109 is directed to the time-of-flight camera 115 as a demodulation signal for demodulating the reflected light beam 108 .

[0071] By demodulating the reflected light beam 109 using the measurement detector signal, the wavefront difference of the reflected light beam 108, in particular the wavefront difference of the reflected measurement beam relative to the reflected measurement beam, can be determined. In an alternative embodiment, the reference detector signal can be used as a demodulation signal to demodulate the reflected light beam 108 received by the time-of-flight camera 115.

[0072] Time-of-flight camera 115 provides a camera signal representing the wavefront difference between the wavefront of the reflected measurement beam and the wavefront of the reflected reference beam of the reflected light beam demodulated using the demodulation signal to processing device 110 for further processing of the camera signal. The processing device for processing the camera signal may also be another processing device other than processing device 110 for processing measurement detector signals and reference detector signals received directly from the measurement detector and reference detector.

[0073] The camera signal includes information about the wavefront difference of the reflected light beam 108. This information can be determined by analyzing the camera signal. For example, the processing device can be arranged to upwrap the camera signal representing the wavefront difference of the reflected light beam. By upwrapping the camera signal, a spatial representation of the wavefront can be determined. This spatial representation facilitates analysis of the wavefront quality.

[0074] The processing device 110 can be arranged to determine a wavefront deformation of the wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam, in particular a wavefront deformation of the reflected measurement beam of the reflected light beam relative to the reflected reference beam of the reflected light beam. The deformation of the wavefront can be, for example, the result of air disturbances and / or position / tilt-related deformations that are the result of alignment and / or manufacturing tolerances of optical elements of the optical system, such as defects in the reflection measurement surface 201, which can, for example, produce ghost reflections.

[0075] By analyzing the wavefront deformation of the reflected beam 108, the measurement quality and robustness of the interferometer system 100 can be diagnosed. Wavefront analysis can be added to existing interferometer systems 100 to diagnose the performance of these interferometer systems 100. Wavefront analysis can also be used in new interferometer setups to determine product quality relative to the wavefront difference of the reflected beam 108.

[0076] The wavefront analysis system may be integrated into the interferometer system 100 or may be provided as a separate device that can be easily used to diagnose the wavefront characteristics of different interferometer systems 100 .

[0077] The processing device 110 may be arranged to calculate corrections and / or compensations to correct and / or compensate for wavefront deformations of the wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam. Such corrections and / or compensations may be used to calibrate the interferometer system 100 so that undesirable wavefront deformations of the wavefront of the reflected measurement beam and / or the reference beam may be corrected and / or compensated by the interferometer system 100, for example by software correction of the measurement results acquired by the measurement detector 109 and / or the measurement results acquired by the time-of-flight camera 115. The corrections may include subtracting a pre-calibrated reference.

[0078] The wavefront of either the measurement beam or the reference beam can be pre-calibrated so that the wavefront of that beam is known. The time-of-flight camera 115 can then be used to determine the absolute wavefront of the other of the measurement beam or the reference beam. For example, when the wavefront of the reference beam is pre-calibrated, the time-of-flight camera 115 can be used to determine the absolute wavefront of the measurement beam. Before mixing one of the measurement beam or the reference beam at the wavefront difference sensor 115, it can be pre-calibrated using a perfect or known reference wavefront, or using another type of absolute wavefront measurement device, such as a Shack-Hartman sensor. The sensor used for absolute measurement should then be used for either the measurement or reference beam.

[0079] Furthermore, only one of the first and second beam portions can be directed to the optical system of interferometer 100, while the other can be directed directly to time-of-flight camera 115. For example, the first beam portion intended for forming the measurement beam can be directed to polarizing beam splitter 107 to follow measurement path 205, while the second beam portion intended for forming the reference beam is not directed to polarizing beam splitter 107 but instead is directed directly to time-of-flight camera 115. The reflected measurement beam and the second beam portion can be combined before detection at time-of-flight camera 115. The second beam portion can then be a fully collimated beam originating from a high-quality or pre-calibrated collimator, or it can be emitted from the end of an optical fiber tip without the need for optics to emerge as a perfectly spherical wavefront with a known wavefront reference. It should be noted that the first and second beam portions can be reversed, for example, with the second beam portion directed to the optical system of interferometer 100 and the first beam portion directed directly to time-of-flight camera 115 to analyze the wavefront quality of the reference beam's wavefront.

[0080] Figure 3A first alternative embodiment of an interferometer system 100 including a wavefront analysis system is shown. In this embodiment, both the reference detector signal and the measurement signal can be used as demodulation signals. In this embodiment, the wavefront analysis is provided as a separate wavefront analysis system 400, which includes a time-of-flight camera 115, a demodulation signal selection device 116, and a separate processing device 117.

[0081] The wavefront analysis system 400 is provided with the reflected light beam 108 via the semi-transparent mirror 113, and is provided with a reference detector signal from the reference detector 112 and a measurement detector signal from the measurement detector 113 to allow selection of one of the reference detector signal or the measurement signal as the demodulation signal. As an alternative to the transparent mirror 113, a polarization beam splitter, or a combination of a wave plate and a polarization beam splitter, can be provided, which are aligned so that the reflected measurement beam and the reflected reference beam interfere. The wavefront analysis system 400 can then be implemented without causing additional signal loss to the detector 109. The measurement detector 109 and the time-of-flight camera 115 will then detect heterodyne signals that are 180° out of phase.

[0082] The demodulated signal is provided to the time-of-flight camera 115 by a demodulated signal selection device 116. The demodulated signal selection device 116 is connected to the reference detector 112 to receive the reference detector signal and to the measurement detector 109 to receive the measurement detector signal. The demodulated signal selection device 116 is arranged to selectively direct a selected one of the reference detector signal and the measurement detector signal to the time-of-flight camera 115 as the demodulated signal to demodulate the reflected light beam 108.

[0083] Demodulating the reflected light beam using the reference detector signal as a demodulation signal yields a camera signal representing the wavefront difference between the measurement beam of the reflected light beam and the reference beam, including the displacement of the reflective measurement surface 201. Demodulating the reflected light beam using the measurement detector signal as a demodulation signal yields a camera signal representing the wavefront of the reflected light beam, without the displacement of the reflective measurement surface 201.

[0084] The demodulation signal selection device 116 can be directly controlled by the processing device 117 of the wavefront analysis system 400 to select a desired one of the reference detector signal and the measurement signal as the demodulation signal, such as Figure 3 In an alternative embodiment, the demodulated signal selection device 116 may be controlled by another device (eg, the processing device 110).

[0085] Figure 4 A second alternative embodiment of an interferometer system 100 including a wavefront analysis system is shown. In this embodiment, both the reference detector signal and the measurement signal can be used as demodulation signals. Figure 3In an embodiment of the present invention, the demodulation signal selection device 116 is arranged to selectively direct the reference detector signal or the measurement detector signal to the time-of-flight camera 115. The demodulation signal selection device 116 and the time-of-flight camera 115 are integrated in an interferometer system in this embodiment, but may also be configured as Figure 3 Shown as a separate system.

[0086] The demodulation signal selection device 116 may be directly controlled to select a desired one of the reference detector signal and the measurement signal as the demodulation signal, or the demodulation signal selection device 116 may be controlled by another device, e.g. Figure 4 The processing device 110 is shown by the dotted arrow.

[0087] exist Figure 4 In the embodiment of FIG. 1 , the reference detector signal measured at the reference detector 112 and the measurement detector signal measured at the measurement detector 109 are not directly directed to the processing device 110. In this embodiment, the time-of-flight camera 115 is used not only to analyze the wavefront of the reflected light beam 108, in particular the wavefront of the reflected measurement beam relative to the wavefront of the reflected reference beam, but also to determine the displacement of the reflecting measurement surface 201 (i.e., the movable object 200).

[0088] The reference detector signal is used as the demodulation signal to determine the displacement of the reflective measurement surface 201. If it is desired to continuously measure the displacement of the reflective measurement surface 201 and if there is no displacement, and there is no need to analyze the wavefront of the reflected light beam 108, the demodulation signal selection device 116 can be removed and the reference detector signal of the reference detector 112 can be directed directly to the time-of-flight camera 115 as the demodulation signal.

[0089] In the above, an interferometer system has been described in which a time-of-flight camera is arranged to receive a reflected light beam and a demodulation signal to analyze the wavefront difference between the reflected measurement beam and the reflected reference beam. The demodulation signal is based on the reference detector signal or the measurement detector signal. In alternative embodiments, any signal having a wavelength that is the same as or close to the wavelength of the measurement beam or the reference beam, or any signal having a wavelength between the wavelength of the measurement beam and the wavelength of the reference beam, such as a frequency division between the wavelength of the measurement beam and the wavelength of the reference beam, can be used. The demodulation signal can be measured by the measurement detector or the reference detector, or can be obtained, for example, from a driver signal used in the light source device to introduce a wavelength difference between the first beam portion and the second beam portion.

[0090] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it will be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film heads, and the like. One skilled in the art will understand that in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion," respectively. The substrates referred to herein may be processed before or after exposure, such as in a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, a substrate may be processed multiple times, such as to manufacture a multi-layer IC, and thus the term substrate as used herein may also refer to a substrate that already includes multiple processing layers.

[0091] Although specific reference may be made above to the use of embodiments of the present invention in the context of optical lithography, it should be understood that the present invention may be used in other applications, such as imprint lithography, and is not limited to optical lithography, where the context permits. In imprint lithography, the topography in the patterning device defines the pattern created on the substrate. The topography of the patterning device can be pressed into a resist layer supplied to the substrate, and the resist is then cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device is removed from the resist to leave a pattern therein.

[0092] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in ways other than those described. For example, the present invention may take the form of a computer program comprising one or more machine-readable instruction sequences describing the above-described methods, or may take the form of a data storage medium (e.g., a semiconductor memory, a magnetic disk, or an optical disk) having such a computer program stored therein.

[0093] The above description is intended to be illustrative and not limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the invention described without departing from the scope of the following claims. Other aspects of the invention are described in the following numbered clauses.

[0094] 1. An interferometer system comprising:

[0095] a light source device arranged to provide a light beam;

[0096] an optical system arranged to split the light beam into a measurement beam and a reference beam, the measurement beam having a first wavelength and the reference beam having a second wavelength, wherein the first wavelength and the second wavelength are different, wherein the optical system is arranged to direct the measurement beam along a measurement path to a reflective measurement surface, direct the reference beam along a reference path to a reflective reference surface, and recombining the reflected measurement beam and the reflected reference beam after the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface to provide a reflected light beam,

[0097] a reference detector arranged to receive the light beam to provide a reference detector signal and / or a measurement detector arranged to receive the reflected light beam to provide a measurement detector signal, and

[0098] a time-of-flight camera arranged to receive the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal, and to provide a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal,

[0099] Wherein the interferometer system is arranged to analyse the wavefront difference based on the camera signal.

[0100] 2. An interferometer system according to claim 1, wherein the reference detector is arranged to receive the light beam to provide a reference detector signal, and the measurement detector is arranged to receive the reflected light beam to provide a measurement detector signal, wherein the demodulated signal is a selected one of the reference detector signal and the measurement detector signal.

[0101] 3. An interferometer system according to claim 2, wherein the interferometer system includes a demodulation signal selection device, which is connected to the reference detector to receive the reference detector signal and is connected to the measurement detector to receive the measurement detector signal, wherein the demodulation signal selection device is arranged to selectively direct one of the reference detector signal and the measurement detector signal as the demodulation signal to the time-of-flight camera to demodulate the reflected light beam.

[0102] 4. The interferometer system of clause 1, wherein the interferometer system comprises a processing device for analyzing the wavefront difference of the reflected light beam.

[0103] 5. An interferometer system according to clause 4, wherein the processing device is arranged to unfold the camera signal representing the wavefront difference of the reflected light beam.

[0104] 6. Interferometer system according to clause 4, wherein the processing device is arranged to determine a wavefront deformation of a wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam.

[0105] 7. An interferometer system according to clause 6, wherein the processing device is arranged to calculate corrections and / or compensations to correct and / or compensate for wavefront deformations of the wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam.

[0106] 8. An interferometer system according to claim 7, wherein the light beam comprises a first beam portion having a first polarization and a second beam portion having a second polarization, wherein the first beam portion has a different wavelength than the second beam portion, wherein the first beam portion is intended to form the measurement beam and the second beam portion is intended to form the reference beam.

[0107] 9. An interferometer system according to claim 8, wherein the optical system includes a polarization beam splitter, which is arranged to split the light beam into the first beam portion and the second beam portion to provide the measurement beam and the reference beam, and after the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface, the measurement beam and the reference beam are recombined to provide the reflected light beam.

[0108] 10. The interferometer system of any of clauses 1 to 9, wherein the movable object is a substrate support of a lithographic apparatus, a patterning device support, or an optical element of a projection system.

[0109] 11. A wavefront analysis system for analyzing a wavefront difference of a reflected light beam of a heterodyne interferometer system, the interferometer system providing a reflected light beam and a reference detector signal and / or a measurement detector signal, wherein the reflected light beam comprises a reflected measurement beam and a reflected reference beam, the reflected measurement beam having a first wavelength and the reflected reference beam having a second wavelength, the first wavelength and the second wavelength being different, the wavefront analysis system comprising:

[0110] a time-of-flight camera arranged to receive the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal, and to provide a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal, and

[0111] A processing device is configured to analyze the wavefront difference based on the camera signal.

[0112] 12. A wavefront analysis system according to claim 11, wherein the wavefront analysis system includes a demodulation signal selection device, which is connected to a reference detector to receive the reference detector signal and to a measurement detector to receive the measurement detector signal, wherein the demodulation signal selection device is arranged to selectively direct one of the reference detector signal and the measurement detector signal as the demodulation signal to the time-of-flight camera to demodulate the reflected light beam.

[0113] 13. A projection system for optical lithography comprising an interferometer system according to any of clauses 1 to 10.

[0114] 14. A lithographic apparatus comprising an interferometer system according to any of clauses 1 to 10.

[0115] 15. A method for analyzing the wavefront difference of a reflected light beam of a heterodyne interferometer system, comprising the following steps:

[0116] Provide a light beam;

[0117] Splitting the light beam into a measurement beam and a reference beam, the measurement beam having a first wavelength and the reference beam having a second wavelength, the first wavelength and the second wavelength being different;

[0118] directing the measurement beam along a measurement path towards a reflective measurement surface on an object of interest;

[0119] directing the reference beam along a reference path toward a reflective reference surface on a reference object;

[0120] after the measurement beam reflects off the reflective measurement surface and the reference beam reflects off the reflective reference surface, recombining the reflected measurement beam and the reflected reference beam to provide a reflected light beam;

[0121] receiving the light beam at a reference detector to provide a reference detector signal and / or receiving the reflected light beam at a measurement detector to provide a measurement detector signal,

[0122] receiving the reflected light beam and a demodulated signal based on the reference detector signal or the measurement detector signal at a time-of-flight camera;

[0123] measuring a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated using the demodulation signal; and

[0124] The camera signal is analyzed to analyze the wavefront difference.

[0125] 16. The method according to clause 13, comprising the steps of:

[0126] receiving the light beam at the reference detector to provide the reference detector signal,

[0127] receiving the reflected light beam at the measurement detector to provide the measurement detector signal, and

[0128] One of the reference detector signal and the measurement detector signal is selected as the demodulated signal.

[0129] 17. The method according to clause 15, comprising the steps of:

[0130] selectively directing the reference detector signal or the measurement detector signal as the demodulation signal to the time-of-flight camera to demodulate the reflected light beam,

[0131] wherein demodulation of the reflected light beam using the reference detector signal as a demodulation signal results in a camera signal representing a wavefront difference of the reflected light beam including a displacement of the reflective measurement surface, and

[0132] Wherein demodulation of the reflected light beam using the measurement detector signal as a demodulation signal results in a camera signal representing the wavefront difference of the reflected light beam without a displacement of the reflective measurement surface.

[0133] 18. The method of clause 15, wherein analyzing the camera signal comprises unfolding the camera signal representative of the wavefront difference of the reflected light beam.

[0134] 19. The method of clause 15, wherein analyzing the camera signal comprises determining a wavefront deformation of a wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam.

[0135] 20. The method according to clause 19, wherein analyzing the camera signal comprises calculating corrections and / or compensations to correct and / or compensate for wavefront deformations of the wavefronts of the measurement beam and / or the reference beam of the reflected light beam.

Claims

1. An interferometer system comprising: a light source device arranged to provide a light beam; an optical system arranged to split the light beam into a measurement beam and a reference beam, the measurement beam having a first wavelength and the reference beam having a second wavelength, wherein the first wavelength and the second wavelength are different, wherein the optical system is arranged to direct the measurement beam along a measurement path to a reflective measurement surface, direct the reference beam along a reference path to a reflective reference surface, and recombining the reflected measurement beam and the reflected reference beam after the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface to provide a reflected light beam, a reference detector arranged to receive the light beam to provide a reference detector signal and / or a measurement detector arranged to receive the reflected light beam to provide a measurement detector signal, and a time-of-flight camera arranged to receive the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal, and to provide a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal, Wherein the interferometer system is arranged to analyse the wavefront difference based on the camera signal.

2. The interferometer system of claim 1 , wherein the reference detector is arranged to receive the light beam to provide a reference detector signal, and the measurement detector is arranged to receive the reflected light beam to provide a measurement detector signal, wherein the demodulated signal is a selected one of the reference detector signal and the measurement detector signal.

3. The interferometer system of claim 2 , wherein the interferometer system comprises a demodulation signal selection device connected to the reference detector to receive the reference detector signal and to the measurement detector to receive the measurement detector signal, wherein the demodulation signal selection device is arranged to selectively direct one of the reference detector signal and the measurement detector signal as the demodulation signal to the time-of-flight camera to demodulate the reflected light beam.

4. The interferometer system of claim 1, wherein the interferometer system comprises a processing device for analyzing the wavefront difference of the reflected light beam.

5. An interferometer system according to claim 4, wherein the processing device is arranged to unfold the camera signal representing the wavefront difference of the reflected light beam.

6. Interferometer system according to claim 4, wherein the processing device is arranged to determine a wavefront deformation of a wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam.

7. A wavefront analysis system for analyzing a wavefront difference of a reflected light beam of a heterodyne interferometer system, the interferometer system providing a reflected light beam and a reference detector signal and / or a measurement detector signal, wherein the reflected light beam comprises a reflected measurement beam and a reflected reference beam, the reflected measurement beam having a first wavelength and the reflected reference beam having a second wavelength, the first wavelength and the second wavelength being different, the wavefront analysis system comprising: a time-of-flight camera arranged to receive the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal, and to provide a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal, and A processing device is configured to analyze the wavefront difference based on the camera signal.

8. A wavefront analysis system according to claim 7, wherein the wavefront analysis system includes a demodulation signal selection device, which is connected to a reference detector to receive the reference detector signal and is connected to a measurement detector to receive the measurement detector signal, wherein the demodulation signal selection device is arranged to selectively guide one of the reference detector signal and the measurement detector signal as the demodulation signal to the time-of-flight camera to demodulate the reflected light beam.

9. A projection system for optical lithography comprising the interferometer system according to any one of claims 1 to 6.

10. A lithographic apparatus comprising the interferometer system according to any one of claims 1 to 6.

11. A method for analyzing the wavefront difference of a reflected light beam of a heterodyne interferometer system, comprising the following steps: Provide a light beam; Splitting the light beam into a measurement beam and a reference beam, the measurement beam having a first wavelength and the reference beam having a second wavelength, the first wavelength and the second wavelength being different; directing the measurement beam along a measurement path towards a reflective measurement surface on an object of interest; directing the reference beam along a reference path toward a reflective reference surface on a reference object; after the measurement beam reflects off the reflective measurement surface and the reference beam reflects off the reflective reference surface, recombining the reflected measurement beam and the reflected reference beam to provide a reflected light beam; receiving the light beam at a reference detector to provide a reference detector signal and / or receiving the reflected light beam at a measurement detector to provide a measurement detector signal, receiving the reflected light beam and a demodulated signal based on the reference detector signal or the measurement detector signal at a time-of-flight camera; measuring a camera signal representing a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated using the demodulation signal; as well as The camera signal is analyzed to analyze the wavefront difference.

12. The method according to claim 11, comprising the steps of: receiving the light beam at the reference detector to provide the reference detector signal, receiving the reflected light beam at the measurement detector to provide the measurement detector signal, and One of the reference detector signal and the measurement detector signal is selected as the demodulated signal.

13. The method according to claim 11, comprising the steps of: selectively directing the reference detector signal or the measurement detector signal as the demodulation signal to the time-of-flight camera to demodulate the reflected light beam, wherein demodulation of the reflected light beam using the reference detector signal as a demodulation signal obtains a camera signal representing a wavefront difference of the reflected light beam with a displacement of the reflective measurement surface, and Wherein demodulation of the reflected light beam using the measurement detector signal as a demodulation signal results in a camera signal representing the wavefront difference of the reflected light beam without a displacement of the reflective measurement surface.

14. The method of claim 11 , wherein analyzing the camera signal comprises: The camera signal representing the wavefront difference of the reflected light beam is expanded.

15. The method of claim 11, wherein analyzing the camera signal comprises: A wavefront deformation of a wavefront of the reflected measurement beam and / or the reflected reference beam of the reflected light beam is determined.

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