WO2025214738A1

WO2025214738A1 - Method to determine ghost reflections in an interferometer system, interferometer system, projection system and exposure apparatus

Info

Publication number: WO2025214738A1
Application number: PCT/EP2025/057675
Authority: WO
Inventors: Maarten Jozef JANSEN; Patrick KWEE
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-04-12
Filing date: 2025-03-20
Publication date: 2025-10-16
Anticipated expiration: 2026-10-12

Abstract

A method to determine ghost reflections in an interferometer system, comprising: providing a first light beam with a fixed light frequency from a first light source; providing a second light beam with a changeable light frequency from a second light source; guiding the first and second light beams at least partially along a first measurement axis to a reflective measurement surface arranged on a first movable object to obtain first and second interferometer signals, while changing the frequency of the second light frequency, detecting at a light detector the first and second interferometer signals, using the first interferometer signal to maintain the movable object in a fixed position, measuring the second light frequency and/or a frequency change of the second light frequency, determining one or more ghost reflections using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency.

Description

METHOD TO DETERMINE GHOST REFLECTIONS IN AN INTERFEROMETER SYSTEM. INTERFEROMETER SYSTEM. PROIECTION SYSTEM AND EXPOSURE APPARATUS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 24169870.3 which was filed on 12

April, 2024 and which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a method to determine ghost reflections in an interferometer system. The invention further relates to an interferometer system and to a projection system for optical exposure systems and/or an exposure apparatus comprising such interferometer system.

BACKGROUND ART

[0003] An exposure apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. An exposure apparatus can be a lithographic apparatus. An exposure 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 exposure 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 an exposure 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 movable optical elements, for example mirrors of the projection optics box.

[0005] In interferometer systems ghost reflections may occur. These ghost reflections are reflections that are not intended to occur, but result from the non-ideal set-up of optical elements of the interferometer system. Although ghost reflections may to some extent be avoided by reducing the number of optical interfaces, applying anti reflective coatings and wedged surfaces, ghost reflections may in practice still occur. Some of these ghost reflections may reach the first light detector and, as a consequence, lead to position measurement errors. These position measurement errors may for example occur when the path length of part of the measurement beam changes due to a ghost reflection. The position measurement errors introduced by ghost reflections may be substantial.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to provide a method that can be used to determine measurements errors in an interferometer system caused by one or more ghost reflections. It is a further object to provide a method and interferometer system in which measurement errors due to ghost reflections can be at least partially corrected.

[0007] According to an aspect of the invention, there is provided a method to determine ghost reflections in an interferometer system, said method comprising: providing a first light beam with a first light frequency from a first light source, wherein the first light frequency is a fixed light frequency; providing a second light beam with a second light frequency from a second light source, wherein the second light frequency is a changeable light frequency; guiding the first light beam at least partially along a first measurement axis to a reflective measurement surface arranged on a first movable object to obtain a first interferometer signal, guiding the second light beam at least partially along the first measurement axis to the reflective measurement surface arranged on the first movable object to obtain a second interferometer signal, while changing the frequency of the second light frequency, detecting at at least one light detector the first interferometer signal and the second interferometer signal, using the first interferometer signal to maintain the movable object in a fixed position, measuring the second light frequency and/or a frequency change of the second light frequency, determining one or more ghost reflections using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency.

[0008] According to an aspect of the invention, there is provided a method to correct for ghost reflections in an interferometer system, comprising: determining one or more ghost reflections according to the invention, and correcting for the one or more determined ghost reflections.

[0009] According to an aspect of the invention, there is provided an interferometer system, comprising: a first light source configured to provide a first light beam with a first light frequency, wherein the first light frequency is a fixed light frequency; a second light source configured to provide a second light beam with a second light frequency, wherein the second light frequency is a changeable light frequency; interferometer optics configured to guide the first light beam at least partially along a first measurement axis to a reflective measurement surface arranged on a first movable object to obtain a first interferometer signal, interferometer optics configured to guide the second light beam at least partially along the first measurement axis to the reflective measurement surface arranged on the first movable object to obtain a second interferometer signal, at least one light detector configured to detect the first interferometer signal and the second interferometer signal, a light frequency sensor system configured to measure the second light frequency and/or a frequency change of the second light frequency, and a processing unit configured to determine one or more ghost reflections using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency, while the movable object is held in a fixed position using the first interferometer signal. [0010] According to an aspect of the invention, there is provided a projection system for an optical exposure system comprising the interferometer system according to the invention.

[0011] According to an aspect of the invention, there is provided an exposure apparatus, comprising the interferometer system according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] 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 an exposure apparatus, such as a lithographic apparatus;

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

Figure 3 shows an example of a ghost reflection between two optical elements;

Figure 4 shows the effect of ghost reflections on position measurement accuracy in dependence of the light frequency used for position measurement;

Figure 5 shows a detrended graph of the effect shown in Figure 4;

Figure 6 shows a graph showing a wavelength dependent position measurement difference between a first interferometer signal and a second interferometer signal; and

Figure 7 shows a graph with amplitudes and optical path length difference of ghost reflections.

DETAILED DESCRIPTION

[0013] Figure 1 schematically depicts an exposure apparatus according to one embodiment of the invention. The exposure apparatus can be a lithographic apparatus. The apparatus comprises an illumination system IL, a support structure MT, a substrate table WT and a projection system PS.

[0014] 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

[0015] 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.

[0016] 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.

[0017] 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 exposure 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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).

[0022] The exposure 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 exposure 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 exposure apparatus. For example, the projection system may project an image on a sensor on the measurement stage to determine an image quality.

[0023] The exposure 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 exposure 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.

[0024] Referring to figure 1, the illumination system IL receives a radiation beam B from a radiation source SO. The radiation source SO and the exposure 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 exposure 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 exposure 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.

[0025] 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 o-o liter and c -inner, 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.

[0026] 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.

[0027] The depicted apparatus could be used in at least one of the following modes:

[0028] 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.

[0029] 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. [0030] 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.

[0031] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0032] Figure 2 shows an embodiment of an interferometer system 100. The interferometer system 100 is arranged to determine a position of a movable object 200, for example a part of a projection system PS, a substrate support WT or a patterning device support MT of a lithographic apparatus. The movable object 200 comprises a reflective measurement surface 201.

[0033] The interferometer system 100 of Figure 2 is a heterodyne interferometer system. In alternative embodiments, the interferometer system may be a homodyne interferometer system or another type of interferometer system.

[0034] The interferometer system 100 comprises a light source device 101. The light source device 101 comprises a first light source 102, a first polarization and frequency shift device 104, a second polarization and frequency shift device 105, a Rochon prism 106, a second light source 107, a third polarization and frequency shift device 108, and a fourth polarization and frequency shift device 109.

[0035] The first light source 102 is configured to provide a first light beam with a first fixed light frequency and is for example a stabilized HeNe laser source configured to provide a light beam with a stable light frequency.

[0036] In the light source device 101, a first light beam originating from the first light source 102 is split into a first first light beam part and a second first light beam part. The first first light beam part is provided in the first polarization and frequency shift device 104 with a first polarization and shifted with a first frequency shift. The second first 2d with a second frequency shift. The first polarization and second polarization are orthogonal to each other. The first frequency shift and the second frequency shift are different, for example opposite to each other, such that there is a first frequency difference between the first first light beam part and the second first light beam part. The first first light beam part is intended to form a measurement beam and the second first 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 first light beam part and the second first light beam part are recombined in a Rochon prism 106. Any other optical component than a Rochon prism 106 may also be used to recombine the first first light beam part and the second first 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 first light source 102, while the other of the first wavelength or second wavelength is shifted by the respective first and second 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] The second light source 107 is configured to provide a second light beam with a tunable light frequency, for example from a tunable laser source. The second light beam originating from the second light source 107 is split into a first second light beam part and a second second light beam part. The first second light beam part is provided in a third polarization and frequency shift device 108 with a first polarization and shifted with a third frequency shift. The second second light beam part is provided in the fourth polarization and frequency shift device 109 with a second polarization and shifted with a fourth frequency shift. The first polarization and second polarization are orthogonal to each other. The third frequency shift and the fourth frequency shift are different, for example opposite to each other, such that there is a second frequency difference between the first second light beam part and the second second light beam part. The first second light beam part is intended to form a measurement beam and the second second light beam part is intended to form a reference beam. The first second light beam part and the second second light beam part are recombined in the Rochon prism 106.

[0040] The first frequency difference between the first first light beam part and the second first light beam part and the second frequency difference between the first second light beam part and the second second light beam part are different to enable simultaneous measurement with the light originating from the first light source 102 and light originating from the second light source 107.

[0041] The light source device 101 thus provides a combined light beam comprising the first first light beam part and the second first light beam part and/or the first second light beam part and the second second light beam part. In alternative embodiments, the light source device 101 may provide a combined measurement beam comprising the first first light beam part and the first second light beam part and a combined reference beam comprising the second first light beam part and the second second light beam part.

[0042] The first frequency difference between the first first light beam part and the second first light beam part and the second frequency difference between the first second light beam part and the second second light beam part are different to enable simultaneous measurement with the first light beam originating from the first light source 102 and the second light beam originating from the second light source 107, when they are propagated along the same optical path.

[0043] The combined light beam is guided to interferometer optics 210 of a first measurement axis 202. The interferometer optics 210 are arranged to split the light originating from the first light source 102 into a measurement beam and a reference beam, based on the polarization of the light. The measurement beam is guided along a measurement path, having the measurement path length Lx, to a reflective measurement surface 201 on the first movable object 200. The reflective measurement surface 201 in Figure 2 is depicted as a comer-cube reflector to show the light beam parts propagating in opposite directions. In practice, the reflective measurement surface 201 may be a flat reflective surface arranged perpendicular to the incident measurement beam. In such case the reflected measurement beam will normally travel along the same path as the incident measurement beam.

[0044] After the measurement beam is reflected by the reflective measurement surface 201, the measurement beam is recombined with the reference beam in the interferometer optics 210 to form a first interferometer signal. The first interferometer signal is directed to a first light detector 103a which is part of a light sensor device 103.

[0045] Correspondingly, the interferometer optics 210 are arranged to split the light originating from the second light source 107 into a measurement beam and a reference beam, based on the polarization of the light. The measurement beam is also guided along the measurement path having the measurement path length Lx, to the reflective measurement surface 201 on the first movable object 200. After the measurement beam is reflected by the reflective measurement surface 201, the measurement beam is recombined with the reference beam in the interferometer optics 210 to form a second interferometer signal. The second interferometer signal is also directed to the first light detector 103a.

[0046] Thus, the light beams of the first light source 102 and the second light source 107 follow generally the same paths along the first measurement axis 202.

[0047] In the embodiment of Figure 2, the first interferometer signal and the second interferometer signal are both guided to the first light detector 103a. In an alternative embodiment, the first interferometer signal and the second interferometer signal may be guided to two separate detectors after optical separation of the first interferometer signal and the second interferometer signal.

[0048] The measurements at the first light detector 103a, e.g. a light diode, are fed via the light sensor device 103 into a processing unit 120. The processing unit 120 may be configured to process the first interferometer signal and the second interferometer signal.

[0049] An embodiment of an interferometer system having a first light source 102 that provides a light beam with a fixed light frequency and a second light source 107 that provides a light beam with a tunable light frequency is disclosed in further detail in WO2019149515A1, the contents of which are herein incorporated by reference in its entirety. In this configuration, the first interferometer signal based on light originating from the first light source 102 is used to accurately track the change in a position of the movable object 200, while the second interferometer signal based on light originating from the second light source 107 may be used to determine an absolute position of the movable object 200.

[0050] In the embodiment of Figure 2, part of the combined light beam, i.e. light originating from the first light source 102 and/or light originating from the second light source 107 is split off at a semitransparent mirror 110 and guided to the interferometer optics 310 of a second measurement axis 302. The second measurement axis 302 has a fixed measurement path length Lr and can be used to accurately determine the frequency of the light originating from the second light source 107.

[0051] Corresponding to the first measurement axis 202, a light beam reaching the interferometer optics 310 of the second measurement axis 302 is split by the interferometer optics 310 into a measurement beam and a reference beam based on the polarization of the respective light beam. The measurement beam is guided along the second measurement path 302 to a fixed reflective surface 301 arranged at a fixed location. After the measurement beam is reflected by the fixed reflective surface 301, the measurement beam is recombined with the reference beam in the interferometer optics 310 to form a further first interferometer signal with respect to light originating from the first light source 102 and a further second interferometer signal with respect to light originating from the second light source 107. The further first interferometer signal and the further second interferometer signal are guided to a second light detector 103b connected to the light sensor device 103.

[0052] Since the second measurement axis 302 has a fixed measurement path length Lr, the processing unit 120 can be configured to determine a light frequency of the light of the further first interferometer signal and the further second interferometer signal and therewith of the light originating from the first light source 102 and the second light source 107.

[0053] The second measurement axis 302 can be used as a light frequency sensor system to measure the second light frequency and/or a frequency change of the second light frequency.

[0054] WO2019149515 Al, the contents of which are herein incorporated in its entirety by reference, discloses further details of a wavelength tracking system that can be used to measure the second light frequency and/or a frequency change of the second light frequency.

[0055] In interferometer systems, such as the interferometer system shown in Figure 2, ghost reflections may occur. These ghost reflections are reflections that are not intended to occur, but result from the non-ideal set-up of the optical elements of the interferometer system 100. Although ghost reflections may to some extent be avoided by reducing the number of optical interfaces, applying anti reflective coatings and wedged surfaces, ghost reflections may in practice still occur. Some of these ghost reflections may reach the first light detector 103a and, as a consequence lead to measurement errors. These measurement errors may for example occur when the path length of part of the measurement beam changes due to a ghost reflection.

[0056] Figure 3 shows an example of a ghost reflection between two optical components in the measurement path of the measurement beam. In particular, part of the measurement beam is unintentionally reflected back and forth between the interferometer optics 210 and the reflective measurement surface 201 as indicated in dashed lines. Ghost reflections may also occur at other locations in the interferometer system 100. Generally, a ghost reflection is any reflection that causes a light beam, for example measurement beam or reference beam, to follow an undesired path However, not all ghost reflections lead to measurement errors as not all ghost reflections reach the light detector. [0057] The position measurement errors due to ghost reflections may be periodic with displacement of the movable object 200, frequency of light used for position measurement and tilt of the reflective measurement surface 201 (when using a flat reflective measurement surface).

[0058] It has been found that the position measurement with light of the second light source with changeable, e g. tunable light frequency can be used to identify ghost reflections. Due to the periodic nature of the measurement error due to ghost reflections, the measurement errors will average out when measuring a position using light over a plurality of light frequencies, for example when sweeping the second light frequency during the measurement. In position measurements based on light originating from the first light source 102, the wavelength dependent measurement errors cannot be determined.

[0059] It is proposed to determine one or more ghost reflections within the interferometer system 100 using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency and eventually the first interferometer signal, while the movable object 201 is held in a fixed position using the first interferometer signal.

[0060] In a first embodiment, the presence of ghost reflections can be identified by obtaining an unwrapped phase of the second interferometer signal, and determining one or more deviations from a linear trend of the signal that correlates with the frequency of the tunable laser in the unwrapped phase of the second interferometer signal.

[0061] Figure 4 shows an unwrapped phase IFM of the second interferometer signal in dependence of the associated change in frequency Af of the light originating from the second light source 107. Figure 4 also shows a linear trend line indicating a linear trend of this unwrapped phase IFM in dependence of the change in frequency Af.

[0062] Figure 5 shows a detrended second interferometer signal, i.e. the signal of shown in Figure 4 compensated for the trend line. It is clear that there are wavelength dependent deviations of the unwrapped phase IFM with respect to the trend line. The deviations can be attributed to one or more ghost reflections in the interferometer system 100.

[0063] In a second embodiment, the presence of ghost reflections can be identified on the basis of inconsistencies between the first interferometer signal and the second interferometer signal, while the movable object 201 is held in a fixed position using the first interferometer signal.

[0064] Figure 6 shows, for example, inconsistencies in the measurement path length Lx determined with the first interferometer signal and the measurement path length Lx determined with the second interferometer signal, while the movable object 201 is held in a fixed position using the first interferometer signal. As the phase determined with the second interferometer signal is measured while changing the second light frequency over a relatively wide frequency range, measurement errors due to ghost reflections will average out in the second interferometer signal. At the same time, the phase determined on the basis of the first interferometer signal may comprise measurement errors due to ghost reflections, since a single light frequency is used for these measurements. The inconsistencies between the first interferometer signal and the second interferometer signal shown in Figure 6 can therefore be attributed to one or more ghost reflections in the interferometer system 100.

[0065] The frequency content of the detrended interferometer signal, shown in Figure 5, or the frequency content of the inconsistencies between the first interferometer signal and the second interferometer signal, shown in Figure 6, can further be used to determine the optical path length difference (OPD) of the one or more ghost reflections using the equation OPD = c/Af, wherein Af is the frequency change that is required to measure one cycle of the wavelength dependent error for a ghost reflection that has an optical pathlength difference of OPD inside the interferometer system 100.

[0066] Figure 7 shows a graph, wherein the frequency content is shown in dependence of the optical pathlength OPD of the ghost reflections identified as described above. This graph shows that for each ghost reflection the optical path length, the frequency and amplitude of the ghost reflections are determined.

[0067] To identify the locations of the optical elements that cause the ghost reflections, the determined optical pathlengths OPD of ghost reflections may be compared with a model of the optical system that includes the optical path length with respect to all optical elements used in the optical system. Knowledge with respect to the optical elements that cause ghost reflections can be used to improve the optical system, for example by improving the manufacturing process of a respective optical element such that a ghost reflection caused by that optical element may be reduced in amplitude or avoided. The magnitude of the respective ghost reflection may for example be reduced by improving antireflection coating or by introducing additional wedging of optical surfaces.

[0068] Next to the frequency and amplitude of the ghost reflections, the phase of each ghost reflection can also be determined by Fourier analysis of the respective signals.

[0069] Further, it is possible to determine a correlation between a phase and a target set point of an identified ghost reflection to determine the number of interactions of the identified ghost reflection with the reflective measurement surface. The number of interactions of the identified ghost reflection with the reflective measurement surface 201, gives additional information for the identification of the locations of the optical elements that cause the ghost reflections.

[0070] As an alternative or in addition to identification and improvement of optical elements that cause ghost reflections, the identification of ghost reflections may also be used to correct for the one or more determined ghost reflections. Such correction may be applied during actual position measurement of the movable object 200.

[0071] The position measurement errors caused by ghost reflections are position dependent and wavelength dependent. By identifying the position dependency of the wavelength dependent measurement error, e.g. dependency on tilt and/or translation of the movable object 200, a correction model may be constructed that can be used for error correction of position measurements of the interferometer system.

[0072] The measurements based on light originating from the first light source 102 are based on a fixed light frequency as the first light source 102 provides light with a stabilized frequency. The position measurement errors based on this light of the first light beam will only be position dependent at a fixed wavelength. These position measurements may be used to keep the movable object 200, in particular the reflective measurement surface 201 at a static location while measuring the position with light originating from the second light source 107. The position measurements using light originating from the second light source 107 can be used to measure the position of the movable object 200, in particular the reflective measurement surface 201, for a range of light frequencies, such that the impact of ghost reflections are averaged out or can be fitted to a correction model.

[0073] By this approach the position and tilt dependent errors that are caused by ghost reflections are compensated for by the position measurements made with light originating from the second light source 107. The difference between the position measurements with light originating from the first light source 102 with and without this compensation can be used to determine a correction map that maps the measurement errors of position measurements made with light originating from the first light source 102 as a function of tilt and translation of the movable object 200. The correction map may be stored in a memory and used in a feedforward compensation to compensate for the effect of ghost reflections.

[0074] Hereinabove, it has been described that the impact of ghost reflections in an interferometer system may be significant depending on the quality of the interferometer optics. In interferometer systems the position and wavelength dependent errors caused by ghost reflections may be at least 1 nm. [0075] It is proposed to identify the presence of ghost reflections using a light source with a tunable light frequency. By determining the presence the ghost reflections, for example by determining the amplitude and optical path distance of the ghost reflections the optical elements that cause the ghost reflections may be identified. This identification of the ghost reflections can be used to replace and/or improve the interferometer optics.

[0076] In addition or as an alternative, it is proposed to provide a correction model that can be used to reduce or avoid the effect of ghost reflections on the position measurement accuracy of the inte rferomete r system 100.

[0077] In the embodiment of Figure 2, the second light source is a tunable laser source. In an alternative embodiment the second light source may be a broadband light source, e.g. a broadband frequency comb laser. When such broadband frequency comb laser is used, the impact of ghost reflections as function of light frequency may be analysed directly by a broadband high resolution spectrometer. An interferometer system comprising a broadband high resolution spectrometer may be able to measure the wavelength dependent inconsistencies as a result of ghost reflections in a single acquisition.

[0078] In an embodiment, the broadband light source, e.g. a broadband frequency comb laser, may be used as a single combined light source combining the first light source and the second light source.

[0079] In an embodiment, correcting for the one or more determined ghost reflections comprises selecting two light frequencies to measure a position of the movable object such that a phase of a ghost reflection related position measurement error for one selected light frequency is opposite to a phase of the ghost reflection related position measurement error for the other selected light frequency to compensate for the ghost reflection related position measurement error.

[0080] Further, in the embodiment of the interferometer system 100 of Figure 2, the first light source 102 and the second light source 107 are shown as part of a light source device 101. This should not be regarded as limiting the device to a single unit. The first light source 102 and the second light source 107 may be provided as separate light source units with or without polarization and frequency shift devices and/or Rochon prisms or other optical elements or devices.

[0081] The interferometer system 100 may comprise additional measurement axes to measure the position of further movable objects within for example a lithographic apparatus.

[0082] Embodiments of interferometer systems are described in which some optical paths are defined in free space and some are defined by optical fibers. In alternative embodiments, optical paths in free space may also be provided by optical fibers, and optical paths defined by optical fibers may be provided in free space.

[0083] 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.

[0084] 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. [0085] 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.

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. A method to determine ghost reflections in an interferometer system, said method comprising: providing a first light beam with a first light frequency from a first light source, wherein the first light frequency is a fixed light frequency; providing a second light beam with a second light frequency from a second light source, wherein the second light frequency is a changeable light frequency; guiding the first light beam at least partially along a first measurement axis to a reflective measurement surface arranged on a first movable object to obtain a first interferometer signal, guiding the second light beam at least partially along the first measurement axis to the reflective measurement surface arranged on the first movable object to obtain a second interferometer signal, while changing the frequency of the second light frequency, detecting at at least one light detector the first interferometer signal and the second interferometer signal, using the first interferometer signal to maintain the movable object in a fixed position, measuring the second light frequency and/or a frequency change of the second light frequency, determining one or more ghost reflections using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency.

2. The method of clause 1, wherein determining ghost reflections comprises: obtaining an unwrapped phase of the second interferometer signal, and determining one or more deviations from a linear trend of a signal that correlates with the frequency of the tunable laser in the unwrapped phase of the second interferometer signal, and attributing the one or more deviations to one or more ghost reflections in the interferometer system.

3. The method of clause 1 or 2, wherein determining ghost reflections comprises: determining one or more inconsistencies between the first interferometer signal and the second interferometer signal, and attributing the one or more inconsistencies to one or more ghost reflections in the interferometer system.

4. The method of any of the clauses 1-3, wherein the method comprises: determining the one or more deviations and/or the one or more inconsistencies as a function of the second light frequency and/or the frequency change of the second light frequency, and determining the optical path length, amplitude and/or phase of a ghost reflection on the basis of the one or more deviations and/or the one or more inconsistencies as a function of the second light frequency and/or the frequency change of the second light frequency.

5. The method of clause 4, wherein the method comprises: determining a correlation between a phase and a target set point of a determined ghost reflection to determine the number of interactions of the determined ghost reflection with the reflective measurement surface.

6. The method of any of the preceding clauses, wherein measuring the second light frequency and/or a frequency change of the second light frequency comprises: splitting off a part of the first light beam and a part of the second light beam, and guiding the split-off part of the first light beam and/or split-off part the second light beam along a second measurement axis with a fixed optical path length.

7. The method of clause 6, wherein the second measurement axis comprises a wavelength tracker.

8. The method of any of the preceding clauses, wherein guiding the first light beam and/or the second light beam at least partially along the first measurement axis comprises: splitting the first light beam and/or the second light beam into a measurement beam and a reference beam; guiding the measurement beam along a measurement path towards the reflective measurement surface; guiding the reference beam along a reference path towards a reflective reference surface on a reference object; recombining the measurement beam and reference beam after reflection on the reflective measurement surface and the reflective reference surface to obtain the first interferometer signal and/or the second interferometer signal, respectively, and guiding the first interferometer signal and/or the second interferometer signal to the at least one light detector.

9. The method of clause 8, the method comprising: splitting the first light beam into a first first light beam part and a second first light beam part, shifting the first light frequency of at least one of the first first light beam part and the second first light beam part with a first frequency shift, splitting the second light beam into a first second light beam part and a second second light beam part, shifting the second light frequency of at least one of the first second light beam part and the second second light beam part with a second frequency shift, using the first first light beam part and the first second light beam part as the respective measurement beams, and using the second first light beam part and the second second light beam part as the respective reference beams.

10. The method of any of the preceding clauses, wherein the second light source is a tunable laser light source or a broadband frequency comb laser light source.

11. A method to correct for ghost reflections in an interferometer system, comprising: determining one or more ghost reflections according to any of the preceding clauses, and correcting for the one or more determined ghost reflections.

12. The method of clause 11, wherein correcting for the one or more determined ghost reflections comprises: fitting ghost reflection related position errors determined for a plurality of different second light frequencies in a ghost reflections correction model, and using the ghost reflections correction model to correct a measured position.

13. The method of clause 12, wherein the ghost reflection correction model comprises an average ghost reflection related position error of the ghost reflection related position errors determined for the plurality of different second light frequencies.

14. The method of clause 10, wherein correcting the one or more determined ghost reflections comprises selecting two light frequencies to measure a position of the movable object such that a phase of a ghost reflection related position measurement error for one selected light frequency is opposite to a phase of the ghost reflection related position measurement error for the other selected light frequency to compensate for the ghost reflection related position measurement error.

15. An interferometer system, comprising: a first light source configured to provide a first light beam with a first light frequency, wherein the first light frequency is a fixed light frequency; a second light source configured to provide a second light beam with a second light frequency, wherein the second light frequency is a changeable light frequency; interferometer optics configured to guide the first light beam at least partially along a first measurement axis to a reflective measurement surface arranged on a first movable object to obtain a first interferometer signal, the interferometer optics further configured to guide the second light beam at least partially along the first measurement axis to the reflective measurement surface arranged on the first movable object to obtain a second interferometer signal, at least one light detector configured to detect the first interferometer signal and the second interferometer signal, a light frequency sensor system configured to measure the second light frequency and/or a frequency change of the second light frequency, and a processing unit configured to determine one or more ghost reflections using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency, while the movable object is held in a fixed position using the first interferometer signal.

16. The interferometer system of clause 15, wherein the processing unit is configured to: obtain an unwrapped phase of the second interferometer signal, and determine one or more deviations from a linear trend in the unwrapped phase of the second interferometer signal, and attribute the one or more deviations to one or more ghost reflections in the interferometer system.

17. The interferometer system of clause 15 or 16, wherein the processing unit is configured to: determine one or more inconsistencies between the first interferometer signal and the second interferometer signal for a plurality of different second light frequencies, and attribute the one or more inconsistencies to one or more ghost reflections in the interferometer system.

18. The interferometer system of any of the clauses 15-17, wherein the processing unit is configured to: determine the one or more deviations and/or the one or more inconsistencies as a function of the second light frequency and/or the frequency change of the second light frequency, and determine the optical path length, power and/or phase of a ghost reflection on the basis of the one or more deviations and/or the one or more inconsistencies as a function of the second light frequency and/or the frequency change of the second light frequency.

19. The interferometer system of clause 18, wherein the processing unit is configured to: determine a correlation between a phase and a target set point of a determined ghost reflection to determine the number of interactions of the determined ghost reflection with the reflective measurement surface.

20. The interferometer system of any of the clauses 15-19, wherein the frequency sensor system comprises: a light splitter configured to split off a part of the first light beam and a part of the second light beam, and a second measurement axis with a fixed optical path length along which the split -off part of the first light beam and/or split-off part the second light beam is guided.

21. The interferometer system of clause 20, wherein the second measurement axis comprises a wavelength tracker.

22. The interferometer system of any of the clauses 15-21, wherein the interferometer optics comprises a light splitter configured to split the first light beam and/or the second light beam into a measurement beam and a reference beam; the interferometer optics configured to guide the measurement beam along a measurement path towards the reflective measurement surface; guide the reference beam along a reference path towards a reflective reference surface on a reference object; recombine the measurement beam and reference beam after reflection on the reflective measurement surface and the reflective reference surface to obtain the first interferometer signal and/or the second interferometer signal, respectively, and guide the first interferometer signal and/or the second interferometer signal to the at least one light detector.

23. The interferometer system of clause 22, a first light beam splitter configured to split the first light beam into a first first light beam part and a second first light beam part, at least one first frequency shift device to shift the first light frequency of at least one of the first first light beam part and the second first light beam part with a first frequency shift, a second light beam splitter configured to split the second light beam into a first second light beam part and a second second light beam part, and at least one second frequency shift device to shift the second light frequency of at least one of the first second light beam part and the second second light beam part with a second frequency shift, and wherein the first first light beam part and the first second light beam part are used as the respective measurement beams and wherein the second first light beam part and the second second light beam part are used as the respective reference beams.

24. The interferometer system of any of the clauses 15-22, wherein the second light source is a tunable laser light source or a broadband frequency comb laser light source.

25. The interferometer system of any of the clauses 15-24, wherein the processing unit is configured to correct for one or more determined ghost reflections.

26. The interferometer system of clause 25, wherein the processing unit is configured to: fit ghost reflection related position errors determined for a plurality of different second light frequencies in a ghost reflections correction model, and use the ghost reflections correction model to correct a measured position.

27. The interferometer system of clause 25, wherein the ghost reflection correction model comprises an average ghost reflection related position error of the ghost reflection related position errors determined for the plurality of different second light frequencies.

28. The interferometer system of clause 25, wherein the processing unit is configured to select two light frequencies to measure a position of the movable object such that a phase of a ghost reflection related position measurement error for one selected light frequency is opposite to a phase of the ghost reflection related position measurement error for the other selected light frequency to compensate for the ghost reflection related position measurement error.

29. A projection system for an optical exposure system comprising the interferometer system of any of the clauses 15-28. 30. An exposure apparatus or lithographic apparatus, comprising the interferometer system of any of the clauses 15-28.

31. The exposure apparatus of clause 30, wherein the first movable object is part of a projection system of the exposure apparatus.

Claims

2. The method of claim 1, wherein determining ghost reflections comprises: obtaining an unwrapped phase of the second interferometer signal, and determining one or more deviations from a linear trend of a signal that correlates with the frequency of the tunable laser in the unwrapped phase of the second interferometer signal, and attributing the one or more deviations to one or more ghost reflections in the interferometer system.

3. The method of claim 1 or 2, wherein determining ghost reflections comprises: determining one or more inconsistencies between the first interferometer signal and the second interferometer signal, and attributing the one or more inconsistencies to one or more ghost reflections in the interferometer system.

4. A method to correct for ghost reflections in an interferometer system, comprising: determining one or more ghost reflections as claimed in any of the preceding claims, and correcting for the one or more determined ghost reflections.

5. The method of claim 4, wherein correcting for the one or more determined ghost reflections comprises: fitting ghost reflection related position errors determined for a plurality of different second light frequencies in a ghost reflections correction model, and using the ghost reflections correction model to correct a measured position.

6. An interferometer system, comprising: a first light source configured to provide a first light beam with a first light frequency, wherein the first light frequency is a fixed light frequency; a second light source configured to provide a second light beam with a second light frequency, wherein the second light frequency is a changeable light frequency; interferometer optics configured to guide the first light beam at least partially along a first measurement axis to a reflective measurement surface arranged on a first movable object to obtain a first interferometer signal, the interferometer optics further configured to guide the second light beam at least partially along the first measurement axis to the reflective measurement surface arranged on the first movable object to obtain a second interferometer signal, at least one light detector configured to detect the first interferometer signal and the second interferometer signal, a light frequency sensor system configured to measure the second light frequency and/or a frequency change of the second light frequency, and a processing unit configured to determine one or more ghost reflections using the second interferometer signal and the second light frequency and/or the frequency change of the second light frequency, while the movable object is held in a fixed position using the first interferometer signal.

7. The interferometer system of claim 6, wherein the processing unit is configured to: obtain an unwrapped phase of the second interferometer signal, and determine one or more deviations from a linear trend in the unwrapped phase of the second interferometer signal, and attribute the one or more deviations to one or more ghost reflections in the interferometer system.

8. The interferometer system of claim 6 or 7, wherein the processing unit is configured to: determine one or more inconsistencies between the first interferometer signal and the second interferometer signal for a plurality of different second light frequencies, and attribute the one or more inconsistencies to one or more ghost reflections in the interferometer system.

9. The interferometer system of any of the claims 6-8, wherein the processing unit is configured to: determine the one or more deviations and/or the one or more inconsistencies as a function of the second light frequency and/or the frequency change of the second light frequency, and determine the optical path length, power and/or phase of a ghost reflection on the basis of the one or more deviations and/or the one or more inconsistencies as a function of the second light frequency and/or the frequency change of the second light frequency.

10. The interferometer system of claim 9, wherein the processing unit is configured to: determine a correlation between a phase and a target set point of a determined ghost reflection to determine the number of interactions of the determined ghost reflection with the reflective measurement surface.

11. The interferometer system of any of the claims 6-10, wherein the frequency sensor system comprises: a light splitter configured to split off a part of the first light beam and a part of the second light beam, and a second measurement axis with a fixed optical path length along which the split-off part of the first light beam and/or split-off part the second light beam is guided.

12. The interferometer system of claim 11, wherein the second measurement axis comprises a wavelength tracker.

13. The interferometer system of any of the claims 6-12, wherein the processing unit is configured to correct for one or more determined ghost reflections.

14. A projection system for an optical exposure system comprising the interferometer system of any of the claims 6-13.

15. An exposure apparatus or lithographic apparatus, comprising the interferometer system of any of the claims 6-13.