EP4670005A1

EP4670005A1 - INTERFEROMETER SYSTEM, WAVE FRONT ANALYSIS SYSTEM, PROJECTION SYSTEM, LITHOGRAPHIC DEVICE AND METHOD FOR ANALYSIS OF A WAVE FRONT OF A LIGHT BEAM OF A HETERODYN INTERFEROMETER SYSTEM

Info

Publication number: EP4670005A1
Application number: EP24701223.0A
Authority: EP
Inventors: Maarten Jozef JANSEN
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-02-23
Filing date: 2024-01-22
Publication date: 2025-12-31
Also published as: JP2026509052A; WO2024175282A1; CN120752581A

Abstract

An interferometer system comprises a light source providing a light beam; an optical system to split the light beam into measurement and reference beams each having different wavelengths, and arranged to guide the measurement beam to a reflective measurement surface, guide the reference beam to a reflective reference surface, and recombine the reflected measurement and reference beams to provide a reflected 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 demodulation signal based on the reference detector signal or the measurement detector signal and provides a camera signal representative of a wavefront difference between the reflected measurement and reference beams demodulated with the demodulation signal. The interferometer system analyzes the wavefront difference on the basis of the camera signal.

Description

INTERFEROMETER SYSTEM, WAVEFRONT ANALYSIS SYSTEM, PROJECTION SYSTEM, LITHOGRAPHIC APPARATUS AND METHOD TO ANALYZE A WAVEFRONT OF A LIGHT BEAM OF AN HETERODYNE INTERFEROMETER SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The application claims priority of EP application 23158336.0 which was filed on 23 February, 2023 and which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a interferometer system and a wavefront analysis system to analyze a wavefront difference of a reflected light beam of an heterodyne interferometer system an interferometer system. The invention further relates to a projection system for optical lithography systems and/or lithographic apparatus comprising such interferometer system and a method to analyze a wavefront difference of a reflected light beam of an heterodyne interferometer system.

BACKGROUND ART

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] In embodiments of a lithographic apparatus, interferometer systems are used to determine the position of movable objects with high accuracy. Examples of these movable objects are the substrate support and optical elements, for example mirrors of projection optics. Interferometer systems may also be used to accurately determine a path length to a fixed object, for example in a wavelength tracker.

[0005] An interferometer system may comprise a light source device, an optical system and a measurement detector. The light source device is arranged to provide a light beam that is guided to the optical system. The optical system is arranged to split the light beam into a measurement beam and a reference beam, to guide the measurement beam along a measurement path to a reflective measurement surface and to guide 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 with the reference beam to provide a reflected light beam. The measurement detector is arranged to receive the reflected light beam to provide a measurement detector signal. This measurement detector signal is representative for a position of the reflective measurement surface. A processing device may be provided to determine the position of the reflective measurement surface on the basis of the measurement detector signal.

[0006] Interferometer systems may have measurement errors that depend on tilt of the reflective measurement surface. These tilt dependent errors are often related to the wavefront quality of the reflected light beam, in particular the wavefront quality of the reflected measurement beam relative to the reflected reference beam. Also other causes may affect the quality of the wavefront of the reflected light beam, such as alignment of optical elements of the optical system, manufacturing tolerances and fiber noise.

[0007] A Shack Hartman sensor may be provided to measure the gradient of a wavefront using segmented microlens arrays and by integrating the gradient into a wavefront map. A Shack Hartmann sensor however can only measure the wavefront gradient and not the phase offset of a beam.

SUMMARY OF THE INVENTION

[0008] It is an aim of the invention to provide an improved system to analyze, e.g. characterize the wavefront of a reflected measurement beam and reference beam of interferometer optics. In particular, it is an aim of the invention to provide an interferometer system that can be used to analyze a wavefront difference of a reflected measurement beam and reference beam of a heterodyne interferometer system to determine wavefront deformation for diagnosis and/or calibration of the interferometer system.

[0009] It is a further aim to provide an improved method to analyze a wavefront difference of a reflected light beam of an heterodyne interferometer system, or at least to provide an alternative.

[0010] According to an aspect of the invention, there is provided 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 second wavelength are different, wherein the optical system is arranged to guide the measurement beam along a measurement path to a reflective measurement surface, to guide the reference beam along a reference path to a reflective reference surface, and to recombine the reflected measurement beam with the reflected reference beam to provide a reflected light beam after the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface, 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 representative for 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 analyze the wavefront difference on the basis of the camera signal.

[0011] According to an aspect of the invention, there is provided a wavefront analysis system to analyze a wavefront difference of a reflected light beam of an 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 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 representative for 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 to analyze the wavefront difference on the basis of the camera signal.

[0012] According to an aspect of the invention, there is provided a projection system for optical lithography and/or a lithographic apparatus comprising the interferometer system.

[0013] According to an aspect of the invention, there is provided a method to analyze a wavefront difference of a reflected light beam of an heterodyne interferometer system, comprising the steps of: providing a light beam; splitting the light beam in a measurement beam having a first wavelength and a reference beam having a second wavelength, the first wavelength and second wavelength being different; guiding the measurement beam along a measurement path towards a reflective measurement surface on an object of interest; guiding the reference beam along a reference path towards a reflective reference surface on a reference object; recombining the reflected measurement beam and the reflected reference beam to provide a reflected light beam after reflection of the measurement beam on the reflective measurement surface and reflection of the reference beam on the reflective reference surface; receiving the light beam at a reference detector to provide a reference detector signal and/or receiving the reflected light beam at measurement detector to provide a measurement detector signal, receiving the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal at a time-of-flight camera; measuring a camera signal representative for a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal; and analyzing the camera signal to analyze the wavefront difference.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 depicts schematically a lithographic apparatus;

Figure 2 shows an embodiment of an interferometer system according to the invention;

Figure 3 shows a first alternative embodiment of an interferometer system according to the invention; and

Figure 4 shows a second alternative embodiment of an interferometer system according to the invention.

DETAILED DESCRIPTION

[0015] Figure 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises an illumination system IL, a support structure MT, a substrate table WT and a projection system PS.

[0016] The illumination system IL is configured to condition a radiation beam B. The support structure MT (e.g. a mask table) is constructed to support a patterning device MA (e.g. a mask) and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters. The substrate table WT (e.g. a wafer table) is constructed to hold a substrate W (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters. The projection system PS is configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0017] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0018] The term “radiation beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0019] The support structure MT supports, i.e. bears 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 or not 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 may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0020] The term “patterning device” 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 create 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. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

[0021] The patterning device MA 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 include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam B in different directions. The tilted mirrors impart a pattern in a radiation beam B which is-reflected by the mirror matrix.

[0022] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum.

[0023] As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0024] 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 “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. In addition to one or more substrate tables WT, the lithographic apparatus may have a measurement stage that is arranged to be at a position beneath the projection system PS when the substrate table WT is away from that position. Instead of supporting a substrate W, the measurement stage may be provided with sensors to measure properties of the lithographic apparatus. For example, the projection system may project an image on a sensor on the measurement stage to determine an image quality.

[0025] The lithographic apparatus may also be of a type wherein at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate.-An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the patterning device MA and the projection system PS. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate W, must be submerged in liquid, but rather only means that liquid is located between the projection system PS and the substrate W during exposure.

[0026] Referring to figure 1, the illumination system IL receives a radiation beam B from a radiation source SO. The radiation source SO and the lithographic apparatus may be separate entities, for example when the radiation source SO is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam B is passed from the radiation source SO to the illumination system IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the radiation source SO may be an integral part of the lithographic apparatus, for example when the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0027] The illumination system IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam B. Generally, at least the outer and/or inner radial extent (commonly referred to as G-outcr and G-inncr, respectively) of the intensity distribution in a pupil plane of the illumination system can be adjusted. In addition, the illumination system IL may comprise 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, to have a desired uniformity and intensity distribution in its cross-section.

[0028] The radiation beam B is incident on the patterning device MT, which is held on the support structure MT, and is patterned by the patterning device MA. Having traversed 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 aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may be realized 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 large range of movement. The short-stroke module may provide fine positioning of the support structure MT relative to the long-stroke module over a small range of movement. Similarly, movement of the substrate table WT may be realized 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 large range of movement. The short-stroke module may provide fine positioning of the substrate table WT relative to the long- stroke module over a small range of movement. In the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions C (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device MA, the mask alignment marks Ml, M2 may be located between the dies.

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

[0030] In a first mode, the so-called step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0031] In a second mode, the so-called scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. [0032] In a third mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0033] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0034] Figure 2 depicts an interferometer system 100 according to an embodiment of the invention. The interferometer system 100 is arranged to measure a change in position of a movable object 200. The movable object 200 is a part of the lithographic apparatus shown in Figure 1. The interferometer system 100 may for example be used to measure a position of a mirror or lens element of the projection system PS, the patterning device support MT or the substrate support WT. The movable object 200 comprises a reflective measurement surface 201. The change in position of the movable object 200 is determined with respect to a reference object 300 having a reference reflective surface 301. When a start position of the movable object 200 is known, the actual absolute position of the movable object 200 can be determined based on the start position and the measured change in position.

[0035] The interferometer system 100 comprises a light source device 101 to provide a light beam 102. The light source device 101 comprises a light source 103, for example 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.

[0036] The interferometer system 100 is a heterodyne interferometer system. Light originating from the light source 103 is split into a first light beam part and a second light beam part. The first light beam part is provided in the first polarization and frequency shift device 104 with a first polarization and a first wavelength. The second light beam part is provided in the second polarization and frequency shift device 105 with a second polarization and a second wavelength. The first polarization and second polarization are orthogonal to each other. The first wavelength and the second wavelength are different. The difference between a first frequency of the first light beam part and a second frequency of the second light beam part may in the range of 0.5 - 50 MHz, for example in the range of 5 - 20 MHz. The first light beam part is intended to form a measurement beam and the second light beam part is intended to form a reference beam.

[0037] The first polarization and frequency shift device 104 and the second polarization and frequency shift device 105 may each comprise a separate polarizing unit and a frequency shift unit. The frequency shift unit for example comprises an opto-acoustic modulator. The first light beam part and the second light beam part are recombined in a Rochon prism 106. Any other suitable optical component than a Rochon prism 106 may also be used to recombine the first light beam part and the second light beam part.

[0038] In practice, one of the first wavelength of the first light beam part or the second wavelength of the second light beam part may be the same as the wavelength of the light provided by the light source 103, while the other of the first wavelength or second wavelength is shifted by the respective polarization and frequency shift device 104, 105. It will be clear that for the one of the first or second wavelength that is not shifted, also no device for frequency shift is required.

[0039] As an alternative for this configuration a freespace Zeeman-split laser may be applied. Such a freespace Zeeman-split laser may provide a light beam having a first light beam part and a second light beam part, the first beam part and the second beam part having orthogonal polarizations and different wavelengths. [0040] The light source system 101 thus provides a light beam 102 having the first light beam part and the second light beam part, the first beam part and the second beam part having orthogonal polarizations and different wavelengths.

[0041] The light beam 102 is guided to an optical system comprising a polarizing beam splitter 107. The polarizing beam splitter 107 is arranged to split the first light beam part and the second light beam part to provide a measurement beam based on the first light beam part and a reference beam based on the second light beam part.

[0042] The measurement beam is guided along a measurement path 205 towards the reflective measurement surface 201 on the object 200. The reference beam is guided along a reference path 305 towards the reflective reference surface 301 on the reference object 300.

[0043] After reflection of the measurement beam on the reflective measurement surface 201 and the reference beam on the reflective reference surface 301, the measurement beam and the reference beam are recombined at the polarizing beam splitter 107 into a reflected light beam 108. The reflected light beam 108 is guided to a measurement detector 109, for example an avalanche photo diode. At the measurement detector 109 a measurement detector signal based on the reflected light beam 108 is measured.

[0044] The measurement detector signal may be guided to a processing device 110. On the basis of the measurement detector signal a relative movement of the movable object 200, i.e. a change in path length Lx, can be determined with high accuracy. A movement of the movable object 200 causes a phase shift in the phase signal. On the basis of these phase shifts in the phase signal, the processing device 110 is capable to determine the relative displacements of the movable object 200 with respect to the reference object 300. When a start position of the movable object 200 is known, the position of the movable object 200 can be determined.

[0045] A part of the light beam 102 of the light source device 101 is directed by a semi-transparent mirror 111 to a reference detector 112, for example an avalanche photo diode. This part of the light beam 102 has not interacted with any of the reflective measurement surface 201 and 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 may be guided to the processing device 110 for further processing. This reference detector signal can for example be used as a reference signal for the first and second wavelengths to improve the measurement accuracy of the interferometer system as it is representative for the light beam 102 that is guided towards the optical system of the interferometer system 110, in particular the polarizing beam splitter 107. The reference detector 112 provides a reference detector signal representative for the light beam 102.

[0046] The interferometer system 100 of Figure 2 may have measurement errors that depend on tilt of the reflective measurement surface 201. These tilt dependent errors are often related to a wavefront quality of the reflected light beam 108, for example the wavefront of the measurement beam relative to the reference beam. Also other causes may affect the quality of a wavefront difference of the reflected light beam 108, such as alignment of optical elements of the optical system, manufacturing tolerances and fiber noise.

[0047] It is desirable to obtain knowledge on a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam 108. 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. To guide the reflected light beam 108 towards the time-of-flight camera 115, the optical system of the interferometer system 100 comprises a semi-transparent mirror 109 that splits the reflected light beam 108 in a first part that is guided to the measurement detector 109 and a second part that is guided to the time-of-flight camera 115. A time-of-flight camera is a digital camera that is capable to provide a depth value at each pixel of the camera.

[0048] It is noted that the measurement detector 109 and the time-of-flight camera 115 may comprise at least one polarizer in order to create interference between the orthogonally polarized reflected measurement beam and reflected reference beam. Correspondingly, the reference detector 112 may comprise at least one polarizer in order to create interference between the orthogonally polarized measurement beam and reference beam.

[0049] To analyze a wavefront difference between the measurement beam and the reference beam of the reflected light beam 108, the reflected light beam is demodulated with a demodulation signal. In the embodiment of Figure 2, the measurement detector signal provided by the measurement detector 109 is guided to the time-of-flight camera 115 as the demodulation signal for demodulation of the reflected light beam 108.

[0050] By demodulation of the reflected light beam 109 using the measurement detector signal, the wavefront difference of the reflected light beam 108, in particular the reflected measurement beam with respect to the reflected measurement beam may be determined. In an alternative embodiment, the reference detector signal may be used as the demodulation signal to demodulate the reflected light beam 108 as received by the time-of-flight camera 115.

[0051] The time-of-flight camera 115 provides a camera signal representative for a wavefront difference between the wavefronts of the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal to the processing device 110 for further processing of the camera signal. This processing device for processing the camera signal may also be another processing device than the processing device 110 used to process the measurement detector signal and the reference detector signal directly received from the measurement detector and the reference detector.

[0052] The camera signal comprises information of the wavefront difference of the reflected light beam 108. By analyzing the camera signal this information may be determined. The processing device may for example be arranged to unwrap the camera signal representative for the wavefront difference of the reflected light beam. By unwrapping the camera signal, a spatial representation of the wavefront may be determined. This spatial representation facilitates analysis of the wavefront quality.

[0053] The processing device 110 may be arranged to determine wavefront deformation of the wavefronts of the reflected measurement beam and/or the reflected reference beam of the reflected light beam, in particular the wavefront deformation of the reflected measurement beam of the reflected light beam with respect to the reflected reference beam of the reflected light beam. The deformations of the wavefront may for example be the result of air perturbations and/or position/tilt dependent deformations as the result of alignment of optic elements of the optical system and/or manufacturing tolerances, such as imperfections in the reflective measurement surface 201 that for example may create ghost reflections.

[0054] By analysis of the wavefront deformation of the reflected light beam 108, a diagnosis of the measurement quality and robustness of the interferometer system 100 may be made. The wavefront analysis may be added in existing interferometer systems 100 to diagnose the performance of these interferometer systems 100. It is also possible to use the wavefront analysis in a new interferometer setup to determine the product quality with respect to the wavefront difference of the reflected light beam 108.

[0055] The wavefront analysis system may be integrated in the interferometer system 100 or be provided as a separate device that can easily be used to diagnose wavefront characteristics of different interferometers systems 100.

[0056] The processing device 110 may be arranged to calculate a correction and/or compensation to correct and/or compensate wavefront deformation of the wavefronts of the reflected measurement beam and/or the reflected reference beam of the reflected light beam. Such correction and/or compensation can be used to calibrate the interferometer system 100 such that undesired wavefront deformation of the wavefronts of the reflected measurement beam and/or reference beam can be corrected and/or compensated by the interferometer system 100, for example by software correction of the measurement results obtained by the measurement detector 109 and/or the measurement results obtained by the time- of-flight camera 115. The correction may include subtracting a pre-calibrated reference.

[0057] The wavefront of the measurement beam or the reference beam, may be pre-calibrated such that the wavefront of this 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 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. Pre-calibration may be done using a perfect or known reference wave front for one of the measurement beam or the reference beam prior to mixing it at the wavefront difference sensor 115 or by using another type of absolute wavefront measurement device like a Shack-Hartman sensor. This sensor used for absolute measurement should then be used either for the measurement or for the reference beam.

[0058] Further, it is possible to guide only one of the first light beam part and the second light beam part to the optical system of the interferometer 100 and the other of the first light beam part and the second light beam part directly to the time-of-flight camera 115. For example, the first light beam part that is intended to form the measurement beam may be guided to the polarizing beam splitter 107 to follow the measurement path 205, while the second light beam part that is intended to form the reference beam is not guided to the polarizing beam splitter 107, but directly to the time-of-flight camera 115. The reflected measurement beam and the second light beam part may be mixed just prior to being detecting at the time-of-flight camera 115. The second light beam part could then be a perfectly collimated beam originating from a high quality or pre-calibrated collimator, or the second light beam part could emerge from the end of a fiber tip without optics to emerge as a perfectly spherical wavefront as a known wavefront reference. It is noted that the first light beam part and the second light beam part may be reversed, e.g. the second light beam part is guided to the optical system of the interferometer 100 and the first light beam part is guided directly to the time-of-flight camera 115 to analyze the wavefront quality of the wavefront of the reference beam.

[0059] Figure 3 shows a first alternative embodiment of an interferometer system 100 comprising a wavefront analysis system. In this embodiment, both the reference detector signal and the measurement signal can be used as the demodulation signal. The wavefront analysis is, in this embodiment, provided as a separate wavefront analysis system 400 comprising the time-of-flight camera 115, a demodulation signal selection device 116, and a separate processing device 117.

[0060] The wavefront analysis system 400 is provided with the reflected light beam 108 via semitransparent mirror 113, and with the reference detector signal from the reference detector 112 and the measurement detector signal from the measurement detector 113 to allow selection of one of the reference detector signal and the measurement signal as the demodulation signal. As an alternative for the transparent mirror 113 a polarizing beam splitter or a combination of a waveplate and a polarizing beam splitter may be provided that are aligned such that the reflected measurement beam and the reflected reference beam are made to interfere. Then the wavefront analysis system 400 can be implemented without additional loss of signal to the detector 109. The measurement detector 109 and the time-of-flight camera 115 would then detect a heterodyne signal that is 180° out-of-phase.

[0061] The demodulation signal is provided to the time-of-flight camera 115 by the demodulation signal selection device 116. The demodulation signal selection device 116 is connected to the reference detector 112 to receive the reference detector signal and connected to the measurement detector 109 to receive the measurement detector signal. The demodulation signal selection device 116 is arranged to selectively guide a selected one of the reference detector signal and the measurement detector signal as the demodulation signal to the time-of-flight camera 115 for demodulation of the reflected light beam 108.

[0062] Demodulation of the reflected light beam using the reference detector signal as demodulation signal results in a camera signal representative for a wavefront difference between the measurement beam and the reference beam of the reflected light beam including displacement of the reflective measurement surface 201. Demodulation of the reflected light beam using the measurement detector signal as demodulation signal results in a camera signal representative for the wavefront of the reflected light beam without displacement of the reflective measurement surface 201.

[0063] The demodulation signal selection device 116 may be directly controlled by the processing device 117 of the wavefront analysis system 400 to select the desired one of the reference detector signal and the measurement signal as the demodulation signal, as indicated by the dashed arrow in Figure 3. In an alternative embodiment the demodulation signal selection device 116 may be controlled by another device, for example the processing device 110.

[0064] Figure 4 shows a second alternative embodiment of an interferometer system 100 comprising a wavefront analysis system. In this embodiment, also both the reference detector signal and the measurement signal can be used as the demodulation signal. Corresponding to the embodiment of Figure 3, the demodulation signal selection device 116 is arranged to selectively guide 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 in this embodiment integrated in the interferometer system, but could also be provided as a separate system as shown in Figure 3.

[0065] The demodulation signal selection device 116 may be directly controlled to select the 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, for example the processing device 110 as indicated by the dashed arrow in Figure 4.

[0066] In the embodiment of Figure 4, the reference detector signal measured at the reference detector 112 and the measurement detector signal measured at the measurement detector 109 are not directly guided to the processing device 110. In this embodiment, the time-of-flight camera 115 is not only used for the analysis of the wavefronts of the reflected light beam 108, in particular the wavefront of the reflected measurement beam with respect to the wavefront of the reflected reference beam, but also for the determination of the displacement of the reflective measurement surface 201, i.e. the movable object 200.

[0067] To determine displacement of the reflective measurement surface 201, the reference detector signal is used as demodulation signal. If it desired to continuously measure the displacement of the reflective measurement surface 201 and there is no need to analyze the wavefronts of the reflected light beam 108 without displacement, the demodulation signal selection device 116 may be removed and the reference detector signal of the reference detector 112 may be directly guided to the time-of-flight camera 115 as the demodulation signal.

[0068] Hereinabove, 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 a 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 identical or close to the wavelength of the measurement beam or the wavelength of the reference beam or a wavelength between the wavelength of the measurement beam and the wavelength of the reference beam, for example the split-frequency between the wavelength of the measurement beam and the wavelength of the reference beam may be used. The demodulation signal may be measured by the measurement detector or the reference detector or it may for example be taken from a driver signal that is used in the light source device to introduce the wavelength difference between the first light beam part and the second light beam part.

[0069] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0070] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0071] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

[0071] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. Other aspects of the invention are set-out as in the following numbered clauses.

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 second wavelength are different, wherein the optical system is arranged to guide the measurement beam along a measurement path to a reflective measurement surface, to guide the reference beam along a reference path to a reflective reference surface, and to recombine the reflected measurement beam with the reflected reference beam to provide a reflected light beam after the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface, 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 representative for 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 analyze the wavefront difference on the basis of the camera signal.

2. The interferometer system of clause 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 demodulation signal is a selected one of the reference detector signal and the measurement detector signal.

3. The interferometer system of clause 2, wherein the interferometer system comprises a demodulation signal selection device connected to the reference detector to receive the reference detector signal and connected to the measurement detector to receive the measurement detector signal, wherein the modulation 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 for demodulation of the reflected light beam.

4. The interferometer system of clause 1, wherein the interferometer system comprises a processing device to analyze the wavefront difference of the reflected light beam.

5. The interferometer system of clause 4, wherein the processing device is arranged to unwrap the camera signal representative for the wavefront difference of the reflected light beam.

6. The interferometer system of clause 4, wherein the processing device is arranged to determine wavefront deformation of the wavefronts of the reflected measurement beam and/or the reflected reference beam of the reflected light beam.

7. The interferometer system of clause 6, wherein the processing device is arranged to calculate a correction and/or compensation to correct and/or compensate wavefront deformation of the wavefronts of the reflected measurement beam and/or the reflected reference beam of the reflected light beam.

8. The interferometer system of clause 7, wherein the light beam comprises a first light beam part having a first polarization and a second light beam part having a second polarization, wherein the first light beam part has a different wavelength than the second light beam part, wherein the first light beam part is intended to form the measurement beam and the second light beam part is intended to form the reference beam.

9. The interferometer system of clause 8, wherein the optical system comprises a polarizing beam splitter arranged to split the light beam into the first light beam part and the second light beam part to provide the measurement beam and the reference beam, and to recombine the measurement beam and the reference beam to provide the reflected light beam after the measurement beam is reflected by the reflective measurement surface and the reference beam is reflected by the reflective reference surface.

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

11. A wavefront analysis system to analyze a wavefront difference of a reflected light beam of an 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 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 representative for 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 to analyze the wavefront difference on the basis of the camera signal.

12. The wavefront analysis system of clause 11, wherein the wavefront analysis system comprises a demodulation signal selection device connected to the reference detector to receive the reference detector signal and connected to the measurement detector to receive the measurement detector signal, wherein the modulation 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 for demodulation of the reflected light beam.

13. A projection system for optical lithography comprising the interferometer system of any of the clauses 1-10.

14. A lithographic apparatus comprising the interferometer system of any of the clauses 1-10.

15. A method to analyze a wavefront difference of a reflected light beam of an heterodyne interferometer system, comprising the steps of: providing a light beam; splitting the light beam in a measurement beam having a first wavelength and a reference beam having a second wavelength, the first wavelength and second wavelength being different; guiding the measurement beam along a measurement path towards a reflective measurement surface on an object of interest; guiding the reference beam along a reference path towards a reflective reference surface on a reference object; recombining the reflected measurement beam and the reflected reference beam to provide a reflected light beam after reflection of the measurement beam on the reflective measurement surface and reflection of the reference beam on the reflective reference surface; receiving the light beam at a reference detector to provide a reference detector signal and/or receiving the reflected light beam at measurement detector to provide a measurement detector signal, receiving the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal at a time-of-flight camera; measuring a camera signal representative for a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal; and analyzing the camera signal to analyze the wavefront difference.

16. The method of clause 13, 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 selecting one of the reference detector signal and the measurement detector signal as the demodulation signal

17. The method of clause 15, comprising the step selectively guiding the reference detector signal or the measurement detector signal as the demodulation signal to the time-of-flight camera for demodulation of the reflected light beam, wherein demodulation of the reflected light beam using the reference detector signal as demodulation signal results in a camera signal representative for a wavefront difference of the reflected light beam including displacement of the reflective measurement surface, and wherein demodulation of the reflected light beam using the measurement detector signal as demodulation signal results in a camera signal representative for a wavefront difference of the reflected light beam without displacement of the reflective measurement surface.

18. The method of clause 15, wherein analyzing the camera signal comprises unwrapping of the camera signal representative for the wavefront difference of the reflected light beam.

19. The method of clause 15, wherein analyzing the camera signal comprises determining wavefront deformation of the wavefronts of the reflected measurement beam and/or the reflected reference beam of the reflected light beam. 20. The method of clause 19, wherein analyzing the camera signal comprises calculating a correction and/or compensation to correct and/or compensate wavefront deformation of the wavefronts of the measurement beam and/or the reference beam of the reflected light beam.

Claims

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 demodulation 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 connected to the measurement detector to receive the measurement detector signal, wherein the modulation 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 for demodulation of the reflected light beam.

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

5. The interferometer system of claim 4, wherein the processing device is arranged to unwrap the camera signal representative for the wavefront difference of the reflected light beam.

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

7. A wavefront analysis system to analyze a wavefront difference of a reflected light beam of an 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 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 representative for 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 to analyze the wavefront difference on the basis of the camera signal.

8. The wavefront analysis system of claim 7, wherein the wavefront analysis system comprises a demodulation signal selection device connected to the reference detector to receive the reference detector signal and connected to the measurement detector to receive the measurement detector signal, wherein the modulation 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 for demodulation of the reflected light beam.

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

10. A lithographic apparatus comprising the interferometer system of any of the claims 1-6.

11. A method to analyze a wavefront difference of a reflected light beam of an heterodyne interferometer system, comprising the steps of: providing a light beam; splitting the light beam in a measurement beam having a first wavelength and a reference beam having a second wavelength, the first wavelength and second wavelength being different; guiding the measurement beam along a measurement path towards a reflective measurement surface on an object of interest; guiding the reference beam along a reference path towards a reflective reference surface on a reference object; recombining the reflected measurement beam and the reflected reference beam to provide a reflected light beam after reflection of the measurement beam on the reflective measurement surface and reflection of the reference beam on the reflective reference surface; receiving the light beam at a reference detector to provide a reference detector signal and/or receiving the reflected light beam at measurement detector to provide a measurement detector signal, receiving the reflected light beam and a demodulation signal based on the reference detector signal or the measurement detector signal at a time-of-flight camera; measuring a camera signal representative for a wavefront difference between the reflected measurement beam and the reflected reference beam of the reflected light beam demodulated with the demodulation signal; and analyzing the camera signal to analyze the wavefront difference.

12. The method of 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 selecting one of the reference detector signal and the measurement detector signal as the demodulation signal

13. The method of claim 11, comprising the step selectively guiding the reference detector signal or the measurement detector signal as the demodulation signal to the time-of-flight camera for demodulation of the reflected light beam, wherein demodulation of the reflected light beam using the reference detector signal as demodulation signal results in a camera signal representative for a wavefront difference of the reflected light beam including displacement of the reflective measurement surface, and wherein demodulation of the reflected light beam using the measurement detector signal as demodulation signal results in a camera signal representative for a wavefront difference of the reflected light beam without displacement of the reflective measurement surface.

14. The method of claim 11, wherein analyzing the camera signal comprises unwrapping of the camera signal representative for the wavefront difference of the reflected light beam.

15. The method of claim 11, wherein analyzing the camera signal comprises determining wavefront deformation of the wavefronts of the reflected measurement beam and/or the reflected reference beam of the reflected light beam.