CN120752581A

CN120752581A - Interferometer system, wavefront analysis system, projection system, lithography apparatus and method for analyzing the wavefront of a light beam of a heterodyne interferometer system

Info

Publication number: CN120752581A
Application number: CN202480013213.XA
Authority: CN
Inventors: M·J·詹森
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2023-02-23
Filing date: 2024-01-22
Publication date: 2025-10-03
Also published as: WO2024175282A1

Abstract

An interferometer system includes a light source that provides a light beam; an optical system for separating the light beam into a measurement beam and a reference beam, each having a different wavelength, and the optical system is arranged to direct the measurement beam to a reflective measurement surface, direct the reference beam to a reflective reference surface, and recombine the reflected measurement beam and the reflected reference beam 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 representing a wavefront difference between the reflected measurement beam and the reflected reference beam, demodulated using the demodulation signal. The interferometer system analyzes the wavefront difference based on the camera signal.

Description

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

Cross Reference to Related Applications

The present application claims priority from European application 23158336.0 filed on 2.23 of 2023, the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates to an interferometer system and a wavefront analysis system for analyzing a wavefront difference of a reflected light beam of a heterodyne interferometer system (interferometer system). The invention also relates to a projection system for an optical lithography system comprising such an interferometer system and/or to a lithographic apparatus comprising such an interferometer system, and to a method for analysing the wavefront difference of a reflected light beam of a heterodyne interferometer system.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. For example, lithographic apparatus can be used in the manufacture of Integrated Circuits (ICs). In this case, a patterning device (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. The pattern may be transferred to a target portion (e.g., including a portion of 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) provided on the substrate. Typically, a single substrate will include a network of adjacent target portions that are continuously 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 the radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. The pattern may also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

In an embodiment of the lithographic apparatus, the interferometer system is for determining the position of the movable object with high accuracy. Examples of such movable objects are substrate supports and optical elements, such as mirrors of projection optics. Interferometer systems can also be used to accurately determine path lengths to a fixed object, such as in a wavelength tracker.

The 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 directed to the optical system. The optical system is arranged to split the light beam into a measurement beam and a reference beam, to direct the measurement beam along a measurement path to the reflective measurement surface, and to direct the reference beam along a reference path to the 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 beam. The measurement detector is arranged to receive the reflected light beam to provide a measurement detector signal. The measurement detector signal is representative of 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.

The interferometer system may have a measurement error that depends on the tilt of the reflective measurement surface. These tilt-related errors are generally 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. Other causes may also affect the wavefront quality of the reflected beam, such as alignment of optical elements of the optical system, manufacturing tolerances, and fiber noise.

A SHACK HARTMAN sensor may be provided to measure the gradient of the wavefront using a segmented microlens array and by integrating the gradient into the wavefront map. However, SHACK HARTMANN sensors can only measure wavefront gradients, and cannot measure the phase shift of the beam.

Disclosure of Invention

It is an object of the present invention to provide an improved system for analyzing (e.g., characterizing) the wavefront of a reflected measurement beam and a reflected reference beam of interferometer optics. In particular, it is an object of the present invention to provide an interferometer system that can be used for analyzing the wavefront difference of a reflected measurement beam and a reflected reference beam of a heterodyne interferometer system to determine wavefront deformation for diagnosing and/or calibrating the interferometer system.

It is a further object to provide an improved method for analyzing the wavefront difference of a reflected light beam of a heterodyne interferometer system, or at least to provide an alternative.

According to one aspect of the present 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 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, to direct the reference beam along a reference path to a reflective reference surface, and to recombine the reflected measurement beam and 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 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 analyze the wavefront difference based on the camera signal.

According to one aspect of the present invention there is provided a wavefront analysis system for analyzing a wavefront difference of a reflected beam of a heterodyne interferometer system, the interferometer system providing a reflected beam and a reference detector signal and/or a measurement detector signal, wherein the reflected 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 for analyzing the wavefront difference based on the camera signal.

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

According to one aspect of the invention, there is provided a method for analysing the wavefront difference of a reflected beam of a heterodyne interferometer system, the method comprising the steps of:

Providing a light beam;

Splitting the light beam into a measuring beam having a first wavelength and a reference beam having a second wavelength, the first wavelength and the second wavelength being different;

directing a measurement beam along a measurement path towards a reflective measurement surface on the object of interest;

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

after the measurement beam is reflected on the reflective measurement surface and the reference beam is reflected on the reflective reference surface, recombining the reflected measurement beam and the reflected reference beam to provide a reflected beam;

the light beam is received at the reference detector to provide a reference detector signal and/or the reflected light beam is received at the measurement detector to provide a measurement detector signal,

Receiving the reflected light beam at the time-of-flight camera, and a demodulation signal based on the reference detector signal or the measurement detector signal;

Measuring a camera signal representing a wavefront difference between a reflected measurement beam and a reflected reference beam of a reflected light beam demodulated with the demodulation signal, and

The camera signal is analyzed to analyze the wavefront difference.

Drawings

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:

FIG. 1 schematically depicts a lithographic apparatus;

figure 2 shows an embodiment of an interferometer system according to the present 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 the interferometer system according to the present invention.

Detailed Description

FIG. 1 schematically depicts a lithographic apparatus according to an embodiment of the invention. The apparatus includes an illumination system IL, a support structure MT, a substrate table WT, and a projection system PS.

The illumination system IL is configured to condition a radiation beam B. The support structure MT (e.g. a mask table) is configured to support a patterning device MA (e.g. a mask), and is 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 configured to hold a substrate W (e.g., a resist-coated wafer) W and is 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.

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.

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

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 may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be, for example, a frame or a table, 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.

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. In general, the pattern applied to the radiation beam will correspond to a particular functional layer in a device (such as an integrated circuit) created in the target portion C.

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. One 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 create a pattern in the radiation beam B which is reflected by the mirror matrix.

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.

As shown, the apparatus is transmissive (e.g., employing a transmissive mask). Alternatively, the apparatus may be reflective (e.g. employing a programmable mirror array of the type described above, or employing a reflective mask).

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 also have a measurement table, which is arranged at a position below the projection system PS when the substrate table WT is remote from that position. 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 the image quality.

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. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between patterning device MA and 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.

Referring to fig. 1, the illumination system IL receives a radiation beam B from a radiation source SO. For example, when the source SO is an excimer laser, the source SO and the lithographic apparatus may be separate entities. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam B is passed from the 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 source SO may be an integral part of the lithographic apparatus, for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illumination system IL may include an adjuster AD for adjusting the angular intensity distribution of the radiation beam B. In general, at least the outer and/or inner radial extent (commonly referred to as outer σ and inner σ, respectively) of the intensity distribution in a pupil plane of the illumination system can be adjusted. IN addition, 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 as to have a desired uniformity and intensity distribution in cross-section.

The radiation beam B is incident on the patterning device MT fixed on 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 aid of the second positioner PW and position sensor IF (e.g. an interferometer, 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. Likewise, the first positioner PM and another position sensor (which is not explicitly depicted in fig. 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 in a smaller 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. The patterning device MA and the substrate W may be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as shown occupy dedicated target portions, they may be located in spaces between target portions C (these are referred to as scribe-lane alignment marks). Similarly, in the case where more than one die is provided on the patterning device MA, the mask alignment marks M1, M2 may be located between the dies.

The depicted apparatus may be used in at least one of the following modes:

In a first mode (i.e., a 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 moved in the X and/or Y direction so that a different target portion C may 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.

In a second mode (i.e., a so-called scanning 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 speed 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, while the length of the scanning motion determines the height (in the scanning direction) of the target portion.

In the third mode, the support structure MT is kept essentially stationary to hold 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, 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 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.

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

FIG. 2 depicts an interferometer system 100 according to an embodiment of the present invention. The interferometer system 100 is arranged to measure a change in position of the movable object 200. The movable object 200 is part of the lithographic apparatus shown in fig. 1. The interferometer system 100 can be used, for example, to measure the 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 the starting position of the movable object 200 is known, the actual absolute position of the movable object 200 may be determined based on the starting position and the measured position change.

Interferometer system 100 includes a light source device 101 for providing a light beam 102. The light source device 101 comprises a light source 103 (e.g. a stabilized laser source), a first polarization and frequency shifting device 104, a second polarization and frequency shifting device 105 and a Rochon prism 106.

Interferometer system 100 is a heterodyne interferometer system. Light originating from the 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 the first polarization and frequency shifting device 104. The second beam portion has a second polarization and a second wavelength in the second polarization and frequency shifting device 105. The first polarization and the second polarization are orthogonal to each other. The first wavelength and the second wavelength are different. The difference between the first frequency of the first beam portion and the second frequency of the second beam portion may be in the range of 0.5MHz-50MHz, for example in the range of 5MHz-20 MHz. The first beam portion is intended to form a measuring beam and the second beam portion is intended to form a reference beam.

The first and second polarization and frequency shifting devices 104 and 105 may each comprise separate polarization and frequency shifting units. The frequency shift unit includes, for example, a photoacoustic modulator. The first and second beam portions are recombined in the Rochon prism 106. Any other suitable optical component other than the Rochon prism 106 may be used to recombine the first and second beam portions.

In practice, one of the first wavelength of the first beam portion or the second wavelength of the second beam portion may be the same as 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. It is clear that no means for frequency shifting is needed for one of the first wavelength or the second wavelength that is not shifted.

As an alternative to this configuration, a free space Zeeman (Zeeman) beam-splitting laser may be applied. Such a free-space Zeeman beam-splitting laser may provide a beam having a first beam portion and a second beam portion, the first beam portion having orthogonal polarizations and different wavelengths than the second beam portion.

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

The light beam 102 is directed to an optical system comprising a polarizing beam splitter 107. The polarizing beam splitter 107 is arranged to split the first and second beam portions to provide a measuring beam based on the first beam portion and a reference beam based on the second beam portion.

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.

After reflection of the measurement beam on the reflective measurement surface 201 and reflection of 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 the reflected beam 108. The reflected light beam 108 is directed to a measurement detector 109, such as an avalanche photodiode. At the measurement detector 109, a measurement detector signal based on the reflected light beam 108 is measured.

The measurement detector signal may be directed to the processing device 110. Based on the measurement detector signal, 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 of the phase signal. Based on these phase shifts of the phase signal, the processing device 110 is able to determine the relative displacement of the movable object 200 with respect 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.

A part of the light beam 102 of the light source device 101 is guided by a half mirror 111 to a reference detector 112, e.g. an avalanche photodiode. This portion of the light beam 102 does not interact with either 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. The reference detector signal may be directed to the processing device 110 for further processing. The reference detector signal may 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, as it represents the light beam 102 directed to 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 of the light beam 102.

The interferometer system 100 of FIG. 2 can have a measurement error that depends on the tilt of the reflective measurement surface 201. These tilt-related errors are typically related to the wavefront quality of the reflected beam 108, such as the wavefront of the measurement beam relative to the reference beam. In addition, other causes may also affect the quality of the wavefront difference of the reflected beam 108, such as alignment of optical elements of the optical system, manufacturing tolerances, and fiber noise.

It is desirable to acquire knowledge about the 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 direct 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 dividing the reflected light beam 108 into a first portion directed to the measurement detector 109 and a second portion 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.

Note that the measurement detector 109 and the time-of-flight camera 115 may include at least one polarizer to create interference between the orthogonally polarized reflected measurement beam and the reflected reference beam. Accordingly, the reference detector 112 may include at least one polarizer to create interference between the orthogonally polarized measurement and reference beams.

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

By demodulating the reflected beam 109 using the measurement detector signal, the wavefront difference of the reflected 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 may be used as a demodulation signal to demodulate the reflected light beam 108 received by the time-of-flight camera 115.

The time-of-flight camera 115 provides a camera signal representing a wavefront difference between the wavefront of the reflected measurement beam and the wavefront of 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. The processing device for processing the camera signals may also be another processing device than the processing device 110 for processing the measurement detector signals and the reference detector signals received directly from the measurement detector and the reference detector.

The camera signal includes information of the wavefront difference of the reflected light beam 108. By analyzing the camera signal, this information can be determined. For example, the processing device may be arranged to spread (upwrap) a camera signal representing a wavefront difference of the reflected light beam. By unwrapping the camera signal, a spatial representation of the wavefront can be determined. Such spatial representation facilitates analysis of wavefront quality.

The processing device 110 may be arranged to determine a wavefront deformation of the reflected measurement beam of the reflected light beam and/or the wavefront of the reflected reference 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 may be the result of, for example, air turbulence and/or position/tilt related deformations, which are the result of alignment and/or manufacturing tolerances of the optical elements of the optical system, such as imperfections in the reflection measurement surface 201, which may, for example, create ghost reflections.

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 the new interferometer setup to determine the product quality relative to the wavefront difference of the reflected beam 108.

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

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

The wavefront of the measurement or reference beam may be pre-collimated to know the wavefront of the beam. The time-of-flight camera 115 may then be used to determine the absolute wavefront of the other of the measurement or reference beams. For example, when the wavefront of the reference beam is pre-calibrated, the time-of-flight camera 115 may be used to determine the absolute wavefront of the measurement beam. One of the measurement or reference beams may be pre-calibrated using a perfect or known reference wavefront, or using another type of absolute wavefront measuring device, such as a Shack-Hartman sensor, before it is mixed at the wavefront difference sensor 115. The sensor for absolute measurement should then be used for measuring or referencing the beam.

Furthermore, only one of the first and second beam portions may be directed to the optical system of interferometer 100, while the other of the first and second beam portions is directed directly to time-of-flight camera 115. For example, a first beam portion intended for forming a measurement beam may be directed to the polarizing beam splitter 107 to follow the measurement path 205, while a second beam portion intended for forming a reference beam is not directed to the polarizing beam splitter 107, but directly to the time-of-flight camera 115. The reflected measurement beam and the second beam portion may be mixed prior to detection at the time-of-flight camera 115. The second beam portion may then be a fully collimated beam originating from a high quality or pre-collimated collimator, or the second beam portion may emerge from the end of the fiber tip without the need for optics to emerge as a perfect spherical wavefront of a known wavefront reference. It should be noted that the first and second beam portions may be reversed, e.g., the second beam portion is directed to the optics of interferometer 100 and the first beam portion is directed directly to time-of-flight camera 115 to analyze the wavefront quality of the wavefront of the reference beam.

Fig. 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 may be used as demodulation signals. In this embodiment, the wavefront analysis is provided as a separate wavefront analysis system 400 comprising a time-of-flight camera 115, a demodulation signal selection device 116 and a separate processing device 117.

The wavefront analysis system 400 is provided with the reflected light beam 108 via the half mirror 113 and 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 polarizing beam splitter, or a combination of a wave plate and a polarizing beam splitter, may be provided, which are aligned such that the reflected measuring beam and the reflected reference beam interfere. The wavefront analysis system 400 may then be implemented without additional signal loss to the detector 109. Measurement detector 109 and time-of-flight camera 115 will then detect heterodyne signals 180 out of phase.

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

Demodulation of the reflected beam using the reference detector signal as demodulation signal yields a camera signal representing the wavefront difference between the measured beam of the reflected beam and the reference beam, including the displacement of the reflective measurement surface 201. Demodulation of the reflected beam using the measurement detector signal as demodulation signal yields a camera signal representing the wavefront of the reflected beam without displacement of the reflective measurement surface 201.

The demodulation signal selection device 116 may 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, as indicated by the dashed arrow in fig. 3. In alternative embodiments, the demodulation signal selection device 116 may be controlled by another device (e.g., the processing device 110).

Fig. 4 shows a second alternative embodiment of an interferometer system 100 comprising a wavefront analysis system. In this embodiment, both the reference detector signal and the measurement signal may be used as demodulation signals. Corresponding to the embodiment of fig. 3, 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 the interferometer system in the present embodiment, but may also be provided as separate systems as shown in fig. 3.

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, such as the processing device 110 shown by the dashed arrow in fig. 4.

In the embodiment of fig. 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 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 reflected measurement surface 201 (i.e. the movable object 200).

In order to determine the displacement of the reflective measurement surface 201, a reference detector signal is used as demodulation signal. If it is desired to continuously measure the displacement of the reflective measurement surface 201 and without the need to analyze the wavefront 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 directed as a demodulation signal to the time-of-flight camera 115.

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

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. Those skilled in the art will appreciate that any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively, in the context of such alternative applications. 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. In addition, the substrate may be processed multiple times, for example, in order to fabricate a multi-layer IC, so the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

While the use of embodiments of the invention in the context of optical lithography may be specifically mentioned above, 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. After the resist is cured, the patterning device is moved out of the resist to leave a pattern therein.

While specific embodiments of the invention have been described above, it should 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.

The above description is intended to be illustrative, and not restrictive. Accordingly, it will be apparent to those 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 as described in the numbered clauses below.

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, to direct the reference beam along a reference path to a reflective reference surface, and to recombine 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 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,

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 to the measurement detector to receive the measurement detector signal, wherein the modulation 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 clause 1, wherein the interferometer system comprises a processing device for analyzing the wavefront difference of the reflected light beam.

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

6. The interferometer system of clause 4, wherein the processing device is 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.

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 for a wavefront deformation of the reflected measurement beam of the reflected light beam and/or the wavefront of the reflected reference beam.

8. The interferometer system of clause 7, wherein the light beam comprises a first light beam portion having a first polarization and a second light beam portion having a second polarization, wherein the first light beam portion has a different wavelength than the second light beam portion, wherein the first light beam portion is intended to form the measurement beam and the second light beam portion 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 and second beam portions 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 clauses 1-9, wherein the movable object is a substrate support of a lithographic apparatus, a patterning device support, or an optical element of a projection system.

11. A wavefront analysis system for analyzing a wavefront difference of a reflected beam of a heterodyne interferometer system, the interferometer system providing a reflected beam and a reference detector signal and/or a measurement detector signal, wherein the reflected 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, an

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

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

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

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

Providing a light beam;

dividing the light beam into a measurement beam having a first wavelength and a reference beam having a second wavelength, the first and second wavelengths being different;

Directing the measurement beam along a measurement path towards a reflective measurement surface on the object of interest;

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

Recombining the reflected measurement beam and the reflected reference beam after the measurement beam is reflected on the reflective measurement surface and the reference beam is reflected on the reflective reference surface to provide a reflected light beam;

the light beam is received at a reference detector to provide a reference detector signal and/or the reflected light beam is received at a measurement detector to provide a measurement detector signal,

Receiving the reflected light beam at a time-of-flight camera and a demodulation signal based on the reference detector signal or the measurement detector signal;

Measuring 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

The camera signal is analyzed 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, an

One of the reference detector signal and the measurement detector signal is selected as the demodulation signal.

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

Selectively directing either 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 demodulating 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 comprising 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 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 the camera signal representing the wavefront difference of the reflected light beam.

19. The method of clause 15, wherein analyzing the camera signal comprises determining a wavefront deformation of the wavefront 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 for a wavefront deformation of the wavefront 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,

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;

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; and

The camera signal is analyzed to analyze the wavefront difference.

12. The method according to claim 11, comprising the steps of:

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.

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

15 . The method of claim 11 , 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.