Method for determining absolute position of object, interferometer system, projection system and lithographic apparatus

Abstract

The invention provides a method for determining the absolute position of an object using an interferometer system, comprising the steps of: providing a light beam; splitting the light beam into a measurement beam and a reference beam; directing the measurement beam along the measurement path towards a reflective measurement surface on the object; directing the reference beam along a reference path towards a reflective reference surface on the reference object; receiving, at the detector, the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface; a phase signal is measured based on the measurement beam and the reference beam received by the detector, a cyclic error phase component is separated from the phase signal, and an absolute position of the object is determined based on the cyclic error phase component.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22202878.9 filed on October 21, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The invention relates to a method for determining the absolute position of an object using an interferometer system. The invention also relates to an interferometer system and a projection system and / or a lithography apparatus for an optical lithography system comprising such an interferometer system. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually to a target portion of the substrate). For example, a lithographic apparatus can be used to manufacture integrated circuits (ICs). In this case, a pattern forming device (which is alternatively referred to as a mask or reticle) can be used to produce a circuit pattern to be formed on a single layer of an IC. This pattern can be transferred to a target portion (e.g., a portion including a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are patterned continuously. Known lithographic apparatus include: a so-called stepper, in which each target portion is irradiated by exposing the entire pattern to the target portion at one time; and a so-called scanner, in which each target portion is irradiated by scanning the pattern via a radiation beam in a given direction (the "scanning" direction) while simultaneously scanning the substrate parallel to or antiparallel to this direction. The pattern can also be transferred from the pattern forming device to the substrate by imprinting the pattern onto the substrate.

[0005] In an embodiment of a lithographic apparatus, an interferometer system is used to determine the position of a movable object with high precision. Examples of such movable objects are substrate supports and movable optical elements, such as mirrors of a projection optics box. The interferometer system can also be used to accurately determine the path length of a fixed object, such as in a wavelength tracker.

[0006] A disadvantage of most known interferometers is that the interferometers are only able to determine the relative displacement of the movable object relative to a reference object. In order to determine the absolute position of the movable object relative to the reference object, a separate zeroing sensor may be provided. The zeroing sensor is used to determine the absolute starting position of the movable object. Once the absolute starting position is known, the interferometer can determine the relative displacement of the movable object relative to the absolute starting position, so that the absolute position of the movable object can be calculated during the movement of the movable object.

[0007] The zeroing sensor is usually installed at a specific position where the absolute starting position of the movable object can be determined. Therefore, the absolute position of the movable object can be determined only when the movable object is within a relatively small measuring range of the zeroing sensor. The measuring range of the zeroing sensor is usually close to the zeroing sensor, for example within a few centimeters of the zeroing sensor. Each time the measurement of the movable object is started using the interferometer, the movable object must be brought back to the relatively small measuring range of the zeroing sensor of the position measurement system. This may not only be the case when the lithography equipment is started, but also, for example, may be the case when the movable object quickly leaves the field of view of the interferometer, for example when passing behind another movable object.

[0008] WO2019149515A1 discloses a method for determining the absolute position of a movable object relative to a reference object using an interferometer system. The interferometer system includes a measuring axis and a reference axis, the measuring axis includes a reflective measurement surface on the movable object, and the reference axis includes a reflective reference surface on the reference object. In this method, a first light beam and a second light beam originating from a first light source are guided through the measuring axis and the reference axis, respectively. Similarly, another first light beam and another second light beam originating from a second light source are guided through the measuring axis and the reference axis, respectively. The optical frequency of the second light source is adjustable so that the optical frequency of the second light source can be changed during measurement.

[0009] WO2019149515A1 provides an algorithm in which specific measurement values ​​of interferometer signals obtained from measurements in a measuring axis and a reference axis are selected to determine the absolute position of a movable object. The algorithm requires that the length of the reference axis is stable, i.e. does not change during the measurement. In addition, the calculation is relatively complex and may require some specific selection criteria for the measurement data, which may complicate the application of the method.

[0010] In another known method of determining the absolute position of a movable object, the tuning frequency of the tunable light source is required to be high in order to make the measurement less sensitive to movements of the movable object.

[0011] Although both approaches are capable of determining the absolute position of a movable object using an interferometer system, both approaches require additional hardware and / or significant processing time in order to be able to determine the absolute position of the movable object. Summary of the invention

[0012] The present invention is directed to an alternative or improved method of determining the absolute position of an object using an interferometer system. In particular, the present invention is directed to a method of determining the absolute position of a movable object using an interferometer, which method does not require additional hardware and / or a large amount of processing time to determine the absolute position of the movable object. It is also directed to an interferometer system that can implement this method.

[0013] According to an aspect of the present invention, there is provided a method for determining the absolute position of an object using an interferometer system, comprising the following steps:

[0014] Provide a light beam;

[0015] splitting the light beam into a measurement beam and a reference beam;

[0016] directing the measurement beam along a measurement path toward a reflective measurement surface on the object;

[0017] directing the reference beam along a reference path toward a reflective reference surface on a reference object;

[0018] receiving at a detector the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface;

[0019] measuring a phase signal based on the measurement beam and the reference beam received by the detector,

[0020] separating a cyclic error phase component from the phase signal,

[0021] An absolute position of the object is determined based on the cyclic error phase component.

[0022] According to an aspect of the invention, there is provided an interferometer system for determining an absolute position of an object, comprising:

[0023] A light source, the light source is used to provide a light beam;

[0024] a measuring optical path optically connected to the light source to receive a measuring beam and comprising a reflective measuring surface arranged on the object; and

[0025] a reference optical path optically connected to the light source to receive a reference beam and comprising a reflective reference surface disposed on a reference object;

[0026] at least one detector associated with the measurement optical path and the reference optical path to receive the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface;

[0027] a processing unit connected to the detector and configured to:

[0028] receiving a phase signal based on the measurement beam and the reference beam from the at least one detector,

[0029] separating a cyclic error phase component from the phase signal, and

[0030] An absolute position of the object is determined based on the cyclic error phase component.

[0031] According to an aspect of the invention, there is provided a projection system for an optical lithography system and / or a lithography apparatus comprising such an interferometer system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference numerals indicate corresponding parts, and in which:

[0033] - Figure 1 A lithographic apparatus is schematically depicted;

[0034] - Figure 2 An embodiment of an interferometer system according to the present invention is shown;

[0035] - Figure 3 shows the cyclic error phase component separated from the interferometer system; and

[0036] - Figure 4 The relationship between the position of the object and the phase of the cyclic error is shown. DETAILED DESCRIPTION

[0037] Figure 1 A lithographic apparatus according to an embodiment of the invention is schematically depicted. The apparatus comprises an illumination system IL, a support structure MT, a substrate table WT and a projection system PS.

[0038] The illumination system IL is configured to condition the radiation beam B. The support structure MT (e.g. a mask table) is configured to support a patterning device MA (e.g. a mask) and is connected to a first positioner PM configured to accurately position the patterning device according to certain parameters. The substrate table WT (e.g. a wafer stage) is configured to hold a substrate W (e.g. a wafer coated with resist) W and is connected to a second positioner PW configured to accurately position the substrate according to certain parameters. The projection system PS is configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0039] The illumination system IL may include various types of optical components for directing, shaping or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof.

[0040] As used herein, the term "radiation beam" encompasses all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of approximately 365, 355, 248, 193, 157, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0041] The support structure MT supports the patterning device MA, 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 whether 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 table that may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is in a desired position, for example relative to the projection system PS.

[0042] The term “patterning device” as used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section so as to produce a pattern in a target portion C of the substrate W. It should be noted that the pattern imparted to the radiation beam B may not exactly correspond to the desired pattern in the target portion C of the substrate W (for example if the pattern includes phase-shifting features or so called assist features). Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device being formed in the target portion C, such as an integrated circuit.

[0043] 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 to reflect an incident radiation beam B in different directions. The tilted mirrors impart a pattern in the radiation beam B that is reflected by the matrix of mirrors.

[0044] The term "projection system" as used herein should be broadly interpreted as covering any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as is appropriate to the exposure radiation used, or to other factors such as the use of immersion liquid or the use of a vacuum.

[0045] As depicted here, the device is of the transmissive type (eg, employing a transmissive mask). Alternatively, the device may be of the reflective type (eg, employing a programmable mirror array type as described above, or employing a reflective mask).

[0046] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such a "multi-stage" machine, additional tables may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure. The lithographic apparatus may have, in addition to the one or more substrate tables WT, a measurement table which is arranged at a position where the substrate table WT is remote from a position beneath the projection system PS. Instead of supporting the substrate W, the measurement table may be provided with sensors to measure properties of the lithographic apparatus. For example, the projection system may project an image onto a sensor on the measurement table to determine image quality.

[0047] The lithographic apparatus may also be of a type in which at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, for example between the pattern forming device MA and the projection system PS. Immersion techniques are well known in the art for increasing the numerical aperture of the projection system. The term "immersion" as used herein does not mean that structures such as the substrate W are necessarily immersed in the liquid, but only means that the liquid is located between the projection system PS and the substrate W during exposure.

[0048] refer to Figure 1 , the illumination system IL receives a radiation beam B from a radiation source SO. The radiation source SO and the lithographic apparatus may be separate entities, for example when the radiation source SO is an excimer laser. In such a case, the source is not considered to form a part of the lithographic apparatus, and the radiation beam B is transferred from the radiation source SO to the illumination system IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and / or a beam expander. In other cases, the radiation source SO may be an integral part of the lithographic apparatus, for example when the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL together with the beam delivery system BD (where necessary) may be referred to as a radiation system.

[0049] The illumination system IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam B. Typically, at least the outer radial extent and / or the inner radial extent (commonly referred to as "σ-outer" and "σ-inner", respectively) of the intensity distribution in a pupil plane of the illumination system may be adjusted. Additionally, 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.

[0050] The radiation beam B is incident on the patterning device MT held 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 a second positioner PW and a position sensor IF (e.g. an interferometer arrangement, a linear encoder or a capacitive sensor), the substrate table WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and a further position sensor (not in position) may be used, for example after mechanical retrieval from a mask library or during scanning. Figure 1 The support structure MT may be used to accurately position the pattern forming device MA relative to the path of the radiation beam B (as explicitly shown in the figure). In general, the movement of the support structure MT may be achieved with the aid of a long-stroke module and a short-stroke module forming part of the first positioner PM. The long-stroke module may provide coarse positioning of the short-stroke module within a large range of movement. The short-stroke module may provide fine positioning of the support structure MT relative to the long-stroke module within a small range of movement. Similarly, the movement of the substrate table WT may be achieved using a long-stroke module and a short-stroke module forming part of the second positioner PW. The long-stroke module may provide coarse positioning of the short-stroke module within a large range of movement. The short-stroke module may provide fine positioning of the substrate table WT relative to the long-stroke module within a small range of movement. In the case of a stepper (as opposed to a scanner), the support structure MT may be connected only to the short-stroke actuator, or may be fixed. The mask alignment marks M1, M2 and the substrate alignment marks P1, P2 may be used to align the pattern forming device MA with the substrate W. Although the substrate alignment marks P1, P2 as shown occupy dedicated target portions, they may be located in the space between the target portions C (these marks are referred to as scribe alignment marks). Similarly, in case more than one die is provided on the patterning device MA, the mask alignment marks M1 , M2 may be located between the dies.

[0051] The depicted device can be used in at least one of the following modes:

[0052] In a first mode (so called step mode), the support structure MT and the substrate table WT are held substantially stationary while an entire pattern imparted to the radiation beam B is projected at once onto a target portion C (i.e. a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that exposure can be made to a different target portion C. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0053] 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 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 of the target portion (in the non-scanning direction) in a single dynamic exposure, while the length of the scanning movement determines the height of the target portion (in the scanning direction).

[0054] In a third mode, the support structure MT holding the programmable patterning device is held substantially stationary 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 between consecutive radiation pulses during scanning. This mode of operation may be readily applied in maskless lithography using a programmable patterning device, for example a programmable mirror array of the type described above.

[0055] Combinations and / or variations on the above-described modes of use, or entirely different modes of use, may also be employed.

[0056] Figure 2 An interferometer system 100 according to an embodiment of the present invention is depicted. The interferometer system 100 is arranged to measure the position of a movable object 200. The movable object 200 is Figure 1 Part of the lithographic apparatus shown. The interferometer system 100 can, for example, be used to measure the position of a mirror of a projection system PS, a patterning device support MT or a substrate holder WT. The movable object 200 comprises a reflective measurement surface 201. The position of the movable object 200 is determined relative to a reference object 300 having a reference reflective surface 301.

[0057] The interferometer system comprises a light source system 101 to provide a light beam 102. The light source system 101 comprises a light source 103 (eg a stabilized laser source), a first polarization and frequency shift device 104, a second polarization and frequency shift device 105 and a Roger prism 106.

[0058] The interferometer system 100 is a heterodyne interferometer system. Light originating from a 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 and is provided to a first polarization and frequency shifting device 104. The second beam portion has a second polarization and a second wavelength and is provided to a 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 first beam portion is intended to form a measurement beam, and the second beam portion is intended to form a reference beam.

[0059] The first polarization and frequency shifting device 104 and the second polarization and frequency shifting device 105 may each include a separate polarization unit and a frequency shifting unit. The frequency shifting unit includes, for example, a photoacoustic modulator. The first beam portion and the second beam portion are recombined in the Rochon prism 106. Any other optical component besides the Rochon prism 106 may also be used to recombine the first beam portion and the second beam portion.

[0060] In practice, one of the first wavelength of the first beam portion or the second wavelength of the second beam portion may be identical to the wavelength of the light provided by the light source 103, whereas the other of the first wavelength or the second wavelength is shifted by the respective polarization and frequency shifting means 104, 105. It will be clear that also for the non-shifted one of the first wavelength or the second wavelength, means for frequency shifting are not required.

[0061] Hence, the light source system 101 provides a light beam 102 having a first beam portion and a second beam portion.

[0062] The light beam 102 is directed to a polarizing beam splitter 107. The polarizing beam splitter 107 is arranged to split into a first beam portion and a second beam portion to provide a measurement beam based on the first beam portion and a reference beam based on the second beam portion.

[0063] Note that before light beam 102 is received by polarizing beam splitter 107, a portion of light beam 102 is split at non-polarizing beam splitter 108 to direct the portion of the light beam directly to detector 109. The portion of light beam 102 directed directly to detector 109 can be used as a reference signal for the first wavelength and the second wavelength of the first beam portion and the second beam portion.

[0064] 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.

[0065] After reflecting the measuring beam on the reflective measuring surface 201 and reflecting the reference beam on the reflective reference surface 301, the measuring beam and the reference beam are recombined into a reflected beam at the polarization beam splitter 107. The reflected beam is directed to the detector 109. At the detector 109, a phase signal based on the measuring beam and the reference beam is measured.

[0066] The phase signal is introduced into the processing device 110. Based on the phase 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 in the phase signal. Based on these phase shifts in the phase signal, the processing device 110 is able to determine the relative displacement of the movable object 200 relative to the reference object 300. However, it is not possible to directly determine the absolute position of the movable object 200 based on the phase signal. In order to determine the absolute position of the movable object 200 during movement, the starting position of the movable object 200 should be known.

[0067] In prior art embodiments of interferometer systems, a separate zeroing sensor is provided to determine the absolute starting position of the movable object relative to the reference object. Once the absolute starting position is known, the interferometer system can determine the relative displacement of the movable object relative to the absolute starting position to determine the absolute position during movement of the movable object.

[0068] It has been surprisingly found that the cyclic errors of the interferometer system 100 can also be used to determine the absolute position of the movable object 200 without the need for a separate zeroing sensor. The interferometer system has cyclic errors. These cyclic errors are errors with a repeating character. Normally, it is not desirable to have these cyclic errors because they interfere with the phase signal and thus may result in incorrect measurements of changes in the position of the movable object 200.

[0069] One cause of cyclic error is a mismatch between the first polarization and the second polarization of the first beam portion and the second beam portion of the light beam 102 and the polarization direction of the polarization beam splitter 107. Due to this mismatch, there may be incomplete splitting of different polarizations. This means that some light with the second wavelength will be present in the measurement beam and / or some light with the first wavelength will be present in the reference beam. This is also called polarization leakage. The phase of the cyclic error caused by polarization leakage depends on the position of the movable object 200. Therefore, the phase delay can also be used to determine the actual position of the movable object 200.

[0070] By accurately measuring the phase of the cyclic error, the position of the movable object 200 can be determined. In addition, the period of the cyclic error phase signal corresponds to a relatively large displacement of the movable object 200. When this relatively large displacement of the movable object 200 is greater than the maximum stroke of the movable object 200, the position of the movable object 200 that can be determined based on the cyclic error phase signal can be used to determine the absolute position of the movable object 200.

[0071] The cyclic error phase is present in the phase signal measured at the detector 109. The processing device 110 is configured to separate the cyclic error phase component from the phase signal. For example, the cyclic error phase component may be a first order phase component that may be extracted from the phase signal due to the repetition of the cyclic error phase at a known frequency.

[0072] US11287242 B2 and WO2021213750 A1 (the contents of which are incorporated herein by reference in their entirety) disclose interferometer systems that determine cyclic errors by means of processing means in order to correct a phase signal for the occurrence of cyclic errors.

[0073] US 1 1 287 242 B2 describes a method comprising determining a first cyclic error of an optical measurement system when a movable object whose position is to be measured is in a first position, and determining a second cyclic error when the movable object is in a second position.

[0074] WO2021213750 A1 relates to a calibration method that does not require moving objects. Circular errors are determined based on measurements along two interferometer axes, each interferometer axis having two different wavelengths.

[0075] Since the cyclic error phase component is already determined in these interferometer systems in order to correct the phase signal, the cyclic error phase component can also be used to determine the absolute position of the movable object 200 .

[0076] Figure 3 An example of a cyclic error phase component separated from a phase signal measured at the detector 109 is shown. The cyclic error phase component is a first order periodic signal separated from the phase signal based on the periodicity of its known frequency. The known frequency is related to the first wavelength λ of the first beam portion. Figure 2 The interferometer system 100 is a single-pass interferometer system in which the measurement beam propagates once from the interferometer optics to the reflecting measurement surface 201. In such a single-pass interferometer system, the period of the cyclic error is λ / 2.

[0077] In a double-pass interferometer system, in which the measurement beam propagates twice from the interferometer optics to the reflecting measurement surface 201, the period of the cyclic error is, for example, λ / 4. And in a 4-pass interferometer, the period of the cyclic error is λ / 8.

[0078] exist Figure 2 In the embodiment of , for a first wavelength of, for example, 640 nm, the period of the cyclic error is λ / 4=640 / 4=160 nm. Therefore, the first-order cyclic error phase component is a sinusoidal signal with a period of 160 nm. By finding and separating this signal from the phase signal received by the detector 109, the cyclic error phase component can be obtained.

[0079] The separated cyclic error phase component is Figure 3 . The cyclic error caused by polarization leakage is formed by two components. The first component is the interference between the expected reference beam (i.e., the second beam portion in the reference path 305) and the light of the first beam portion leaking into the reference path 305. The second component is the interference between the expected measurement beam (i.e., the first beam portion in the measurement path 205) and the light of the second beam portion leaking into the measurement path 205. The superposition of these two components causes the cyclic error and determines the phase and amplitude of the cyclic error. Since the measurement beam and the reference beam have different wavelengths, the phase between the expected measurement beam and the reference beam leaking into the measurement beam path depends on the length Lx of the measurement axis. This phase difference causes a phase lag L of the cyclic error, and the phase lag L represents the position of the movable object 200. Therefore, the phase lag L can be used to determine the actual position of the movable object 200.

[0080] Figure 4 A graph showing the relationship between the phase lag of the cyclic error signal and the position of the movable object 200 is depicted. As can be seen in the graph, a period of the phase lag corresponds to a displacement of approximately 2.4 meters of the movable object 200. This displacement within one period of the phase lag of the cyclic error signal is substantially greater than the maximum displacement of the movable object 200. Therefore, the phase lag will be maintained within one period, and the absolute position of the movable object 200 corresponding to the phase lag in the period can be determined based on the determined phase lag of the separated cyclic error phase component.

[0081] In the above, the cyclic error phase component of the phase signal is used to determine the absolute position of the movable object 200. In other embodiments, the cyclic error phase component of the phase signal can also be used to determine the absolute position of an immovable object, such as the path length within a wavelength tracker.

[0082] Furthermore, it has been described above that the cyclic error caused by polarization leakage is used to determine the absolute position of the movable object 200. Instead of the cyclic error caused by polarization leakage, any other cyclic error having a direct relationship between the phase of the cyclic error and the position of the movable object 200 may also be applied. Other cyclic errors that do not have this relationship should be minimized.

[0083] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications, such as manufacturing 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 understand that in the context of such alternative applications, any use of the term "wafer" or "die" herein may be considered synonymous with the more general term "substrate" or "target portion", respectively. The substrates referred to herein may be processed in, for example, a track (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a measurement tool, and / or an inspection tool before or after exposure. Where applicable, the disclosure herein may be applied to these and other substrate processing tools. In addition, the substrate may be processed more than once, for example to form 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 is not limited to optical lithography and, where the context permits, may be used in other applications such as imprint lithography. In imprint lithography, the topography of a patterning device defines a pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist provided to the substrate, and the resist is then cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is cured, the patterning device is removed from the resist, leaving the pattern therein.

[0085] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. For example, the present invention may take the form of a computer program containing one or more machine-readable instruction sequences describing the methods described above, or a data storage medium (e.g., a semiconductor memory, a magnetic disk, or an optical disk) in which such a computer program is stored. The above description is intended to be illustrative, not restrictive. Therefore, it will be understood by those skilled in the art that the described invention may be modified without departing from the scope of the claims set forth below. Other aspects of the present invention are set forth in the following numbered clauses.

[0086] 1. A method for determining the absolute position of an object using an interferometer system, comprising the following steps:

[0087] Provide a light beam;

[0088] splitting the light beam into a measurement beam and a reference beam;

[0089] directing the measurement beam along a measurement path toward a reflective measurement surface on the object;

[0090] directing the reference beam along a reference path toward a reflective reference surface on a reference object;

[0091] receiving at a detector the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface;

[0092] measuring a phase signal based on the measurement beam and the reference beam received by the detector,

[0093] separating a cyclic error phase component from the phase signal,

[0094] An absolute position of the object is determined based on the cyclic error phase component.

[0095] 2. A method according to clause 1, wherein the interferometer system is a heterodyne interferometer system.

[0096] 3. A method according to clause 1 or 2, wherein the cyclic error is at least partially caused by polarization leakage.

[0097] 4. A method according to clause 3, wherein splitting the light beam into a measurement beam and a reference beam comprises splitting based on different polarizations within the light beam, and wherein the polarization leakage is caused by the incomplete splitting of the different polarizations.

[0098] 5. A method according to any one of clauses 1-4, wherein the light beam comprises a first beam portion having a first polarization and a second beam portion having a second polarization, wherein the first beam portion has a different wavelength than the second beam portion, wherein the first beam portion is intended to form the measurement beam and the second beam portion is intended to form the reference beam.

[0099] 6. A method according to any of clauses 1-5, wherein the step of separating the cyclic error phase component comprises separating a first-order cyclic error phase component.

[0100] 7. A method according to any of clauses 1-6, wherein the step of separating the cyclic error phase component comprises separating a repetitive phase signal having a known frequency.

[0101] 8. A method according to any of clauses 1 to 7, wherein the step of determining the absolute position of the object comprises determining a phase shift of the first-order cyclic error phase component.

[0102] 9. A method according to any one of clauses 1 to 8, wherein the object is a movable object.

[0103] 10. A method according to clause 9, wherein the movable object is a substrate support, a patterning device support, or a part of a projection system of the lithographic apparatus.

[0104] 11. A method according to any of clauses 1-9, wherein the object is a wavelength tracker.

[0105] 12. An interferometer system for determining the absolute position of an object, comprising:

[0106] A light source, the light source is used to provide a light beam;

[0107] a measuring optical path optically connected to the light source to receive a measuring beam and comprising a reflective measuring surface arranged on the object; and

[0108] a reference optical path optically connected to the light source to receive a reference beam and comprising a reflective reference surface disposed on a reference object;

[0109] at least one detector associated with the measurement optical path and the reference optical path to receive the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface;

[0110] a processing unit connected to the detector and configured to:

[0111] receiving a phase signal based on the measurement beam and the reference beam from the at least one detector,

[0112] separating a cyclic error phase component from the phase signal, and

[0113] An absolute position of the object is determined based on the cyclic error phase component.

[0114] 13. The interferometer system of clause 12, wherein the interferometer system is a heterodyne interferometer system.

[0115] 14. An interferometer system according to clause 12 or 13, wherein the cyclic error is at least partially caused by polarization leakage.

[0116] 15. An interferometer system according to item 14, wherein the interferometer system includes a polarization beam splitter to split the light beam into a measurement beam and a reference beam based on different polarizations within the light beam, and wherein the polarization leakage is caused by incomplete beam splitting of different polarizations.

[0117] 16. An interferometer system according to any one of clauses 12-15, wherein the interferometer system comprises a light source system arranged to provide a first beam portion having a first polarization and a second beam portion having a second polarization, wherein the first beam portion has a different wavelength than the second beam portion, wherein the first beam portion is intended to form the measurement beam and the second beam portion is intended to form the reference beam.

[0118] 17. An interferometer system according to any of clauses 12-16, wherein the cyclic error phase component comprises a first-order cyclic error phase component.

[0119] 18. An interferometer system according to any of clauses 12-17, wherein the processing unit is configured to separate the cyclic error phase component from the phase signal based on separating a repetitive phase signal having a known frequency.

[0120] 19. An interferometer system according to any of clauses 12-18, wherein the processing unit is configured to determine the absolute position of the object based on a phase shift of the first-order cyclic error phase component.

[0121] 20. A projection system for an optical lithography system, the projection system comprising an interferometer system according to any of clauses 12 to 19.

[0122] 21. A lithographic apparatus comprising an interferometer system according to any of clauses 12-19.

[0123] 22. A lithographic apparatus according to clause 21, wherein the object is a movable object.

[0124] 23. A lithographic apparatus according to clause 22, wherein the movable object is a substrate support, a patterning device support, or a part of a projection system of the lithographic apparatus.

Claims

1. A method for determining the absolute position of an object using an interferometer system, comprising the following steps: Provide a light beam; splitting the light beam into a measurement beam and a reference beam; directing the measurement beam along a measurement path toward a reflective measurement surface on the object; directing the reference beam along a reference path toward a reflective reference surface on a reference object; receiving at a detector the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface; measuring a phase signal based on the measurement beam and the reference beam received by the detector, separating a cyclic error phase component from the phase signal, An absolute position of the object is determined based on the cyclic error phase component.

2. The method of claim 1, wherein the interferometer system is a heterodyne interferometer system. The method according to claim 1 , wherein the cyclic error is at least partially caused by polarization leakage. 4 . The method of claim 3 , wherein splitting the light beam into a measurement beam and a reference beam comprises splitting based on different polarizations within the light beam, and wherein the polarization leakage is caused by the incomplete splitting of the different polarizations.

5. The method according to any one of claims 1-4, wherein the light beam comprises a first beam portion having a first polarization and a second beam portion having a second polarization, wherein the first beam portion has a different wavelength than the second beam portion, wherein the first beam portion is intended to form the measurement beam and the second beam portion is intended to form the reference beam.

6. The method according to any one of claims 1 to 5, wherein the step of separating the cyclic error phase component comprises separating a first-order cyclic error phase component.

7. The method according to any one of claims 1 to 6, wherein the step of separating the cyclic error phase component comprises separating a repetitive phase signal having a known frequency.

8. A method according to any one of claims 1 to 7, wherein the step of determining the absolute position of the object comprises determining a phase shift of the first order cyclic error phase component.

9. The method according to any one of claims 1 to 8, wherein the object is a movable object.

10. An interferometer system for determining the absolute position of an object, comprising: A light source, the light source is used to provide a light beam; a measurement optical path optically connected to the light source to receive a measurement beam and comprising a reflective measurement surface arranged on the object; as well as a reference optical path optically connected to the light source to receive a reference beam and comprising a reflective reference surface disposed on a reference object; at least one detector associated with the measurement optical path and the reference optical path to receive the measurement beam after reflection on the reflective measurement surface and the reference beam after reflection on the reflective reference surface; a processing unit connected to the detector and configured to: receiving a phase signal based on the measurement beam and the reference beam from the at least one detector, separating a cyclic error phase component from the phase signal, and An absolute position of the object is determined based on the cyclic error phase component.

11. The interferometer system of claim 10, wherein the cyclic error is caused at least in part by polarization leakage.

12. The interferometer system of claim 11, wherein the interferometer system comprises a polarization beam splitter to split the light beam into a measurement beam and a reference beam based on different polarizations within the light beam, and wherein the polarization leakage is caused by incomplete beam splitting of different polarizations.

13. The interferometer system according to any one of claims 10-12, wherein the processing unit is configured to separate the cyclic error phase component from the phase signal based on separating a repetitive phase signal having a known frequency.

14. A projection system for an optical lithography system, the projection system comprising an interferometer system according to any one of claims 10-13.

15. A lithographic apparatus comprising an interferometer system according to any one of claims 10-13.

Images