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

Abstract

The present 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 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 the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface with a detector, 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, and determining the absolute position of the object based on the cyclic error phase component.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of European Patent Application No. 22202878.9, filed October 21, 2022, which is incorporated herein by reference in its entirety.

[0002] FIELD OF THE INVENTION The present invention relates to a method for determining the absolute position of an object using an interferometer system, and further to an interferometer system and a projection system for an optical lithography system and / or a lithographic apparatus comprising such an interferometer system. [Background technology]

[0003] A lithographic apparatus is a machine that transfers a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus are used, for example, in the manufacture of integrated circuits (ICs). In such cases, a patterning device, also known as a mask or reticle, can be used to create a circuit pattern to be formed in 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 the substrate (e.g. a silicon wafer). The pattern is typically imaged onto a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned. Known types of lithographic apparatus are called steppers and scanners. In a stepper, each target portion is irradiated by exposing the entire pattern onto it at once. In a scanner, each target portion is irradiated by scanning the radiation beam in a given direction (scan direction) while scanning the substrate parallel or anti-parallel to this direction. Transfer of the pattern from the patterning device to the substrate can also be done by imprinting the pattern onto the substrate.

[0004] In an embodiment of a lithographic apparatus, an interferometer system is used to determine the position of movable objects with high precision. Examples of these movable objects include substrate supports and movable optical elements (e.g., mirrors in a projection optics box). Interferometer systems are also used to accurately determine the path length to fixed objects, for example in wavelength trackers.

[0005] A drawback of most known interferometers is that they can only determine the relative displacement of the movable object relative to a reference object. To determine the absolute position of the movable object relative to the reference object, a separate zeroing sensor can be provided. This zeroing sensor is used to determine the absolute starting position of the movable object. Once this absolute starting position is known, the interferometer can determine the relative displacement of the movable object relative to this absolute starting position, thereby calculating the absolute position of the movable object during its movement.

[0006] The zeroing sensor is typically mounted at a specific location that allows the absolute starting position of the movable object to be determined. Therefore, the absolute position of the movable object can only be determined if the movable object is within a relatively narrow measurement range of the zeroing sensor. The measurement range of the zeroing sensor is typically close to the zeroing sensor, for example, within a few centimeters of the zeroing sensor. Each time a measurement of the movable object is started using the interferometer, the movable target must be brought back within the relatively narrow measurement range of the zeroing sensor of the position measurement system. This may be the case not only during start-up of the lithographic apparatus, but also, for example, when the movable object briefly moves out of the field of view of the interferometer, for example, when it passes behind another movable object.

[0007] WO 2019149515 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 measurement axis including a reflective measurement surface on the movable object and a reference axis including a reflective reference surface on the reference object. In the method, a first beam and a second beam emitted from a first light source are directed through the measurement axis and the reference axis, respectively. Similarly, a further first beam and a second beam emitted from a second light source are directed through the measurement axis and the reference axis, respectively. The optical frequency of the second light source is adjustable, allowing the optical frequency of the second light source to be changed during measurement.

[0008] WO2019149515 provides an algorithm for determining the absolute position of a movable object by selecting specific measurements of an interferometer signal obtained from measurements on a measurement axis and a reference axis. The algorithm requires that the length of the reference axis is stable, i.e., does not change during the measurement. Furthermore, the calculations are relatively complex and may require some specific selection criteria for the measurement data, which can complicate the application of this method.

[0009] Another known method for determining the absolute position of a movable object requires a high variable frequency of the variable light source to make the measurements less sensitive to the movement of the movable object.

[0010] Both methods can use an interferometer system to determine the absolute position of a movable object, but require additional hardware and / or significant processing time to determine the absolute position of the movable object. Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the present invention to provide an alternative or improved method for determining the absolute position of an object using an interferometer system. In particular, it is an object of the present invention to provide a method for determining the absolute position of a movable object using an interferometer that does not require additional hardware and / or substantial processing time to determine the absolute position of the movable object. It is also an object of the present invention to provide an interferometer system capable of implementing such a method. [Means for solving the problem]

[0012] According to one aspect of the present invention, there is provided a method for determining an absolute position of an object using an interferometry system, the method comprising: providing a light beam; splitting a 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 a reference beam along a reference path toward a reflective reference surface on a reference object; receiving at a detector the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface; measuring a phase signal based on the measurement beam and the reference beam received at the detector; separating a cyclic error phase component from the phase signal; determining an absolute position of the object based on the cyclic error phase component; Equipped with.

[0013] According to one aspect of the present invention, there is provided an interferometry system for determining the absolute position of an object, the interferometry system comprising: a light source providing a light beam; a measurement path optically connected to the light source for receiving the measurement beam and including a reflective measurement surface disposed on the object; a reference beam path optically connected to the light source for receiving the reference beam and including a reflective reference surface disposed on the reference object; at least one detector associated with the measurement path and the reference path, the detector receiving the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface; a processing device connected to the detector, receiving a phase signal based on the measurement beam and the reference beam from the at least one detector; Separating the cyclic error phase component from the phase signal; and a processing unit configured to determine an absolute position of the object based on the cyclic error phase component.

[0014] According to one aspect of the invention, there is provided a projection system and / or lithographic apparatus for an optical lithography system, including such interferometric measurements. [Brief explanation of the drawings]

[0015] 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 symbols indicate corresponding parts, and in which: [Figure 1] 1 depicts a schematic diagram of a lithographic apparatus; [Figure 2] 1 illustrates an embodiment of an interferometer system according to the present invention; [Figure 3] FIG. 1 illustrates a cyclic error phase component separated from an interferometer system. [Figure 4] FIG. 10 is a diagram illustrating the relationship between the position of an object and the phase of a cyclic error. DETAILED DESCRIPTION OF THE INVENTION

[0016] Figure 1 schematically depicts a lithographic apparatus according to one embodiment of the invention, the apparatus comprising an illumination system IL, a support structure MT, a substrate table WT, and a projection system PS.

[0017] The illumination system IL is configured to condition a radiation beam B. The support structure MT (e.g. a mask table) is configured to support a patterning device MA (e.g. a mask) and is connected to a first positioner PM configured to accurately position the patterning device according to certain parameters. The substrate table WT (e.g. a wafer table) is configured to hold a substrate W (e.g. a resist-coated wafer) 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 of the substrate W (e.g. comprising one or more dies).

[0018] The illumination system IL may include various optical elements, or combinations thereof, such as refractive optical elements, reflective optical elements, magnetic optical elements, electromagnetic optical elements, electrostatic optical elements or other types of optical elements, to direct, shape or otherwise control the radiation beam B.

[0019] As used herein, the term "radiation beam" encompasses all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., wavelengths around 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., wavelengths in the range of 5 nm to 20 nm), as well as particle beams such as ion beams and electron beams.

[0020] The support structure MT supports, i.e. bears the weight of, the patterning device MA. It holds the patterning device MA depending on the pose of the patterning device MA, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be, for example, a frame or a table, and 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.

[0021] The term "patterning device", as used herein, should be interpreted broadly to refer to any device that can be used to impart a radiation beam B with a pattern in its cross-section to form 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 (e.g. an integrated circuit) being created in the target portion C.

[0022] 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. A programmable mirror array, for example, comprises a matrix arrangement of small mirrors, each individually tiltable so as to reflect an incoming radiation beam B in different directions. The radiation beam B is reflected by the mirror matrix and is patterned by the tilted mirrors.

[0023] The term "projection system" as used herein should be interpreted broadly as encompassing all types of projection systems, including refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic optical systems, or any combination thereof, depending on the exposure radiation used and other factors such as the use of immersion liquids or the use of a vacuum.

[0024] As here depicted, the apparatus may be of a transmissive type (e.g. employing a transmissive mask) or it may be of a reflective type (e.g. employing a programmable mirror array of a type described above, or employing a reflective mask).

[0025] The lithographic apparatus may be of a type with two or more (dual stage) substrate tables WT (and / or two or more mask tables). In such a "multi-stage" apparatus, the 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. In addition to one or more substrate tables WT, the lithographic apparatus may also include a measurement stage arranged to be positioned below the projection system PS when the substrate table WT is away from that position. Instead of supporting the substrate W, the measurement stage may be provided with sensors for measuring properties of the lithographic apparatus. For example, the projection system may project an image onto a sensor on the measurement stage to determine image quality.

[0026] The lithographic apparatus may be of a type in which the substrate W is at least partially covered by a liquid having a relatively high refractive index, such as water, filling a 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 patterning device MA and the projection system PS. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. As used herein, the term "immersion" does not imply that a structure such as the substrate W has to be immersed in liquid, but rather that a liquid is located between the projection system PS and the substrate W during exposure.

[0027] Referring to Figure 1, the illumination system IL receives the radiation beam B from the radiation source SO. The radiation source SO and the lithographic apparatus may be separate entities, for example when the radiation source SO is an excimer laser. In such cases, the radiation source is not considered to form part of the lithographic apparatus and the radiation beam B is passed from the radiation source SO to the illumination system IL with the aid of a beam delivery system BD, which may include, for example, appropriate turning mirrors and / or beam expanders. In other cases, the radiation source SO may be an integral part of the lithographic apparatus, for example when the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0028] 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 "σ-outer" and "σ-inner", respectively) of the intensity distribution in a pupil plane of the illumination system may be adjusted. Furthermore, 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 adjust the radiation beam B so that it has a desired uniformity and intensity distribution in its cross-section.

[0029] 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. 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, a linear encoder, or a capacitive sensor), the substrate table WT can be precisely moved to, for example, position a different target portion C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (not explicitly shown in Figure 1) can be used to precisely position the patterning device MA with respect to the path of the radiation beam B, for example after mechanical retrieval from a mask library or during a scan. In general, movement of the support structure MT can be realized using a long-stroke module and a short-stroke module, which form part of the first positioner PM. The long-stroke module can provide coarse positioning over a large range of movement of the short-stroke module. The short-stroke module can provide fine adjustment 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 over a large range of movement of the short-stroke module. The short-stroke module may provide fine adjustment of the substrate table WT relative to the long-stroke module over a smaller range of movement. In the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short-stroke actuator only, or may be fixed. The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. As shown, the substrate alignment marks P1, P2 occupy dedicated target portions, but they may also 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 M1, M2 may be located between the dies.

[0030] The depicted apparatus could be used in at least one of the following modes:

[0031] In a first mode, known as step mode, the support structure MT and substrate table WT are kept substantially stationary while the entire pattern imparted to the radiation beam B is projected onto one target portion C in one go (i.e. a single static exposure), and the substrate table WT is then moved in the X and / or Y direction to expose a different target portion C. In step mode, the maximum size of the exposure field limits the size of the target portion C that can be imaged in a single static exposure.

[0032] 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 of the target portion (non-scan direction) in a single dynamic exposure, while the length of the scanning movement determines the height of the target portion (scan direction).

[0033] In a third mode, the support structure MT holds the programmable patterning device 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 used and the programmable patterning device is updated as required after each movement of the substrate table WT, or in between successive pulses of radiation during a scan. This mode of operation is readily adaptable to maskless lithography employing a programmable patterning device such as a programmable mirror array of the type described above.

[0034] Combinations and / or variations on the above modes of use or entirely different modes of use may also be employed.

[0035] Figure 2 shows an interferometry system 100 according to an embodiment of the present invention. The interferometry system 100 is configured to measure the position of a movable object 200. The movable object 200 is part of the lithographic apparatus shown in Figure 1. The interferometry system 100 can be used, for example, to measure the position of a mirror of the projection system PS, a patterning device support MT, or a substrate support 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 reflective reference surface 301.

[0036] The interferometer system includes a light source system 101 that provides a light beam 102. The light source system 101 includes a light source 103, such as a stabilized laser light source, a first polarization and frequency shifting device 104, a second polarization and frequency shifting device 105, and a Rochon prism 106.

[0037] The interferometer system 100 is a heterodyne interferometer system. Light emitted by a light source 103 is split into a first light beam portion and a second light beam portion. The first light beam portion is provided to a first polarization and frequency shift device 104 with a first polarization and a first wavelength. The second light beam portion is provided to a second polarization and frequency shift device 105 with a second polarization and a second wavelength. The first and second polarizations are orthogonal to each other. The first and second wavelengths are different. The first light beam portion is intended to form a measurement beam, and the second light beam portion is intended to form a reference beam.

[0038] The first polarization and frequency shifting device 104 and the second polarization and frequency shifting device 105 may comprise separate polarization and frequency shifting units, respectively. The frequency shifting units may comprise, for example, acousto-optic modulators. The first and second light beam portions are recombined by a Rochon prism 106. Optical components other than the Rochon prism 106 may also be used to recombine the first and second light beam portions.

[0039] In practice, one of the first wavelength of the first light beam portion or the second wavelength of the second light beam portion may be the same as the wavelength of the light provided by the light source 103, and the other of the first wavelength or the second wavelength is shifted by the respective polarization and frequency shifting device 104, 105. Obviously, for one of the first wavelength or the second wavelength that is not shifted, no device for frequency shifting is required.

[0040] Thus, the light source system 101 provides a light beam 102 having a first light beam portion and a second light beam portion.

[0041] The light beam 102 is directed to a polarizing beam splitter 107. The polarizing beam splitter 107 is positioned to split the first and second light beam portions to provide a measurement beam based on the first light beam portion and a reference beam based on the second light beam portion.

[0042] It should be noted that before the light beam 102 is received by the polarizing beam splitter 107, a portion of the light beam 102 is split off by the non-polarizing beam splitter 108, and this portion of the light beam is directed directly to the detector 109. This portion of the light beam 102 that is directed directly to the detector 109 can be used as a reference signal for the first and second wavelengths of the first and second light beam portions.

[0043] The measurement beam is directed along measurement path 205 towards a reflective measurement surface 201 on object 200. The reference beam is directed along reference path 305 towards a reflective reference surface 301 on reference object 300.

[0044] After the measurement beam reflects off reflective measurement surface 201 and the reference beam reflects off reflective reference surface 301, the measurement and reference beams are recombined by polarizing beam splitter 107 to form a reflected light beam. The reflected light beam is directed to detector 109, where a phase signal based on the measurement and reference beams is measured.

[0045] 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 optical path length Lx, can be determined with high precision. Movement of the movable object 200 causes phase shifts in the phase signal. Based on these phase shifts in the phase signal, the processing device 110 can determine the relative displacement of the movable object 200 with respect to the reference object 300. However, the absolute position of the movable object 200 cannot be directly determined based on the phase signal. To determine the absolute position of the movable object 200 during movement, the starting position of the movable object 200 needs to be known.

[0046] In conventional interferometry system embodiments, a separate zeroing sensor is provided to determine the absolute starting position of the movable object relative to a reference object. Knowing this absolute starting position, the interferometry system can determine the relative displacement of the movable object relative to this absolute starting position in order to determine the absolute position of the movable object during its movement.

[0047] Surprisingly, it has been discovered 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. Interferometer systems have cyclic errors. These cyclic errors are repetitive in nature. Generally, these cyclic errors are undesirable because they disrupt the phase signal and can lead to erroneous measurements of changes in the position of the movable object 200.

[0048] One source of cyclic errors is a mismatch between the first and second polarizations of the first and second light beam portions of the light beam 102 and the polarization direction of the polarizing beam splitter 107. This mismatch can result in an incomplete splitting of the different polarizations. This means that the measurement beam contains some light at the second wavelength and / or the reference beam contains some light at the first wavelength. This is also called polarization leakage. The phase of the cyclic errors 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.

[0049] By accurately measuring the phase of the cyclic error, the position of the movable object 200 can be determined. Furthermore, the period of the cyclic error phase signal corresponds to a large relative displacement of the movable object 200. If this large relative displacement of the movable object 200 is larger 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 utilized to determine the absolute position of the movable object 200.

[0050] The cyclic error phase is present in the phase signal measured by detector 109. Processing unit 110 is configured to separate the cyclic error phase component from the phase signal. The cyclic error phase component may be, for example, a first order phase component that can be removed from the phase signal due to the recurrence of the cyclic error phase at a known frequency.

[0051] U.S. Pat. No. 1,128,7242 and WO 2021213750, the contents of which are incorporated herein by reference in their entirety, disclose interferometer systems in which cyclic errors are determined by a processing unit to correct for the occurrence of cyclic errors.

[0052] US Patent No. 1,128,7242 describes a method that includes 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.

[0053] WO2021213750 relates to a calibration method that does not require a movable object, where cyclic errors are determined based on measurements along two interferometer axes with two different wavelengths.

[0054] In these interferometer systems, the cyclic error phase component is already determined to correct the phase signal, so that the cyclic error phase component can also be used to determine the absolute position of the movable object 200 .

[0055] FIG. 3 shows an example of a cyclic error phase component separated from the phase signal measured by detector 109. The cyclic error phase component is a first-order periodic signal separated from the phase signal based on its periodicity at a known frequency. This known frequency is related to the first wavelength λ of the first light beam portion. Interferometer system 100 of FIG. 2 is a single-pass interferometer system in which the measurement beam propagates once from the interferometer optics to the reflective measurement surface 201. In such a single-pass interferometer system, the period of the cyclic error is λ / 2.

[0056] In a double-pass interferometer system, where the measurement beam propagates twice from the interferometer optics to the reflectance measurement surface 201, the period of the cyclic error is, for example, λ / 4, and in a four-pass interferometer, the period of the cyclic error is λ / 8.

[0057] In the embodiment of Figure 2, if the first wavelength is, for example, 640 nm, the period of the cyclic error is λ / 4 = 640 / 4 = 160 nm. As a result, the first-order cyclic error phase component is a sinusoidal signal with a period of 160 nm. This signal can be separated from the phase signal received by detector 109 to obtain the cyclic error phase component.

[0058] The separated cyclic error phase components are shown in Figure 3. The cyclic error caused by polarization leakage consists of two components. The first component is the interference between the intended reference beam, i.e., the second light beam portion in reference path 305, and the light of the first light beam portion that leaks into reference path 305. The second component is the interference between the intended measurement beam, i.e., the first light beam portion in measurement path 205, and the light of the second light beam portion that leaks into measurement path 205. The superposition of these two components causes the cyclic error, which determines its phase and amplitude. Because the measurement beam and reference beam have different wavelengths, the phase between the intended measurement beam and the reference beam that leaks into the measurement beam path depends on the length Lx of the measurement axis. This phase difference causes a phase lag L in the cyclic error, which represents the position of movable object 200. As a result, the phase lag L can be used to determine the actual position of movable object 200.

[0059] 4 is a graph illustrating the relationship between the phase delay of the cyclic error signal and the position of movable object 200. As can be seen from this graph, the period of the phase delay corresponds to a displacement of approximately 2.4 meters of movable object 200. This displacement within one period of the phase delay of the cyclic error signal is substantially greater than the maximum displacement of movable object 200. As a result, the phase delay remains within one period, and based on the determined phase delay of the isolated cyclic error phase component, the absolute position of movable object 200 corresponding to the phase delay within this period can be determined.

[0060] 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, this cyclic error phase component of the phase signal can also be used to determine the absolute position of a non-movable object, such as the path length in a wavelength tracker.

[0061] Furthermore, the above describes using the cyclic error caused by polarization leakage to determine the absolute position of the movable object 200. Instead of the cyclic error caused by polarization leakage, other cyclic errors can also be applied, where there is a direct relationship between the phase of the cyclic error and the position of the movable object 200. Other cyclic errors that do not have this relationship should be minimized.

[0062] Although the use of lithography apparatus in the manufacture of ICs is described herein as an example, it should be understood that lithography apparatus may be used in other applications, such as integrated optical systems, guide and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), and thin-film magnetic heads. Those skilled in the art will recognize that in these other applications, the terms "wafer" and "die" herein are considered synonymous with the more general terms "substrate" and "target portion," respectively. The substrate may be processed, either before or after exposure, by a track (a tool that typically applies a layer of resist to the substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein may also apply to these or other substrate processing equipment. Additionally, a substrate may be processed multiple times, for example, to fabricate multi-layer ICs, in which case the term "substrate" herein refers to a substrate that includes multiple processing layers that have already been processed.

[0063] Although particular reference has been made above to the use of embodiments of the invention in connection with optical lithography, it will be appreciated that the invention can also be used in other applications such as imprint lithography and is not limited to optical lithography where circumstances permit. In imprint lithography, a topography in a patterning device defines the pattern to be created on a substrate. The topography of the patterning device is pressed into a layer of resist supplied to the substrate whereafter the resist is cured by electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is then moved away from the resist leaving a pattern in the resist after it has been cured.

[0064] While specific embodiments of the present invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the present invention may take the form of a computer program comprising one or more sequences of machine-readable instructions describing the methods disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic disk, optical disk) having stored thereon such a computer program. The above description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the appended claims. Other aspects of the invention are described in the following numbered paragraphs. 1. A method for determining the absolute position of an object using an interferometer system, comprising: providing 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 the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface with a detector; measuring a phase signal based on the measurement beam and the reference beam received at the detector; separating a cyclic error phase component from the phase signal; determining an absolute position of the object based on the cyclic error phase component; A method for providing the above. 2. The method of claim 1, wherein the interferometer system is a heterodyne interferometer system. 3. The method according to item 1 or 2, wherein the cyclic error is caused at least in part by polarization leakage. 4. The method of claim 3, wherein the step of splitting the light beam into the measurement beam and the reference beam includes splitting based on different polarizations within the light beam, and the polarization leakage is caused by incomplete splitting of the different polarizations. 5. The method of any of items 1 to 4, wherein the light beam comprises a first light beam portion having a first polarization and a second light beam portion having a second polarization, the first light beam portion having a different wavelength from the second light beam portion, the first light beam portion intended to form the measurement beam, and the second light beam portion intended to form the reference beam. 6. The method of any of paragraphs 1 to 5, wherein the step of isolating the cyclic error phase component includes isolating a first-order cyclic error phase component. 7. The method of any of paragraphs 1 to 6, wherein the step of isolating the cyclic error phase component includes isolating a repetitive phase signal of known frequency. 8. The method of any of clauses 1 to 7, wherein the step of determining the absolute position of the object includes determining a phase shift of a first-order cyclic error phase component. 9. The method of any one of paragraphs 1 to 8, wherein the object is a movable object. 10. The method according to paragraph 9, wherein the movable object is a substrate support, a patterning device support, or part of a projection system of a lithographic apparatus. 11. The method of any of paragraphs 1 to 9, wherein the object is a wavelength tracker. 12. An interferometer system for determining the absolute position of an object, comprising: a light source providing a light beam; a measurement optical path optically connected to the light source for receiving a measurement beam and including a reflective measurement surface disposed on the object; a reference beam path optically connected to the light source for receiving a reference beam and including a reflective reference surface disposed on a reference object; at least one detector associated with the measurement path and the reference path, the detector receiving the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface; a processing device connected to the detector, 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; a processor configured to determine an absolute position of the object based on the cyclic error phase component. 13. The interferometer system according to paragraph 12, wherein the interferometer system is a heterodyne interferometer system. 14. An interferometer system according to paragraph 12 or 13, wherein the cyclic error is caused at least in part by polarization leakage. 15. The interferometer system described in paragraph 14, wherein the interferometer system includes a polarizing beam splitter that splits the light beam into a measurement beam and a reference beam based on different polarizations within the light beam, and the polarization leakage is caused by incomplete splitting of the different polarizations. 16. An interferometer system according to any one of clauses 12 to 15, wherein the interferometer system comprises a light source system arranged to provide a first light beam portion having a first polarization and a second light beam portion having a second polarization, the first light beam portion having a different wavelength from the second light beam portion, the first light beam portion being for forming the measurement beam, and the second light beam portion being for forming the reference beam. 17. An interferometer system according to any one of paragraphs 12 to 16, wherein the cyclic error phase component includes a first-order cyclic error phase component. 18. An interferometer system according to any one of clauses 12 to 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 of a known frequency. 19. An interferometer system according to any one of clauses 12 to 18, wherein the processing unit is configured to determine the absolute position of the object based on the phase shift of the first-order cyclic error phase component. 20. A projection system for an optical lithography system, comprising an interferometer system according to any one of paragraphs 12 to 19. 21. A lithographic apparatus comprising an interferometer system according to any one of paragraphs 12 to 19. 22. A lithographic apparatus according to clause 21, wherein the object is a movable object. 23. A lithographic apparatus according to clause 22, wherein the movable object is a substrate support, a patterning device support, or part of a projection system of a lithographic apparatus.

Claims

1. 1. A method for determining an absolute position of an object using an interferometer system, comprising: providing 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 the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface with a detector; measuring a phase signal based on the measurement beam and the reference beam received at the detector; separating a cyclic error phase component from the phase signal; determining an absolute position of the object based on the cyclic error phase component; A method for providing the above.

2. The method of claim 1 , wherein the interferometer system is a heterodyne interferometer system.

3. The method of claim 1 or 2, wherein the cyclic error is caused at least in part by polarization leakage.

4. 4. The method of claim 3, wherein splitting the light beam into the measurement beam and the reference beam comprises splitting based on different polarizations within the light beam, and wherein the polarization leakage results from imperfect splitting of the different polarizations.

5. 5. The method of claim 1, wherein the light beam comprises a first light beam portion having a first polarization and a second light beam portion having a second polarization, the first light beam portion having a different wavelength than the second light beam portion, the first light beam portion intended to form the measurement beam and the second light beam portion intended to form the reference beam.

6. The method of claim 1 , wherein the step of isolating the cyclic error phase component comprises isolating a first order cyclic error phase component.

7. The method of claim 1 , wherein the step of isolating the cyclic error phase component comprises isolating a repetitive phase signal of known frequency.

8. The method of claim 1 , wherein determining the absolute position of the object comprises determining a phase shift of a 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. 1. An interferometer system for determining the absolute position of an object, comprising: a light source providing a light beam; a measurement optical path optically connected to the light source for receiving a measurement beam and including a reflective measurement surface disposed on the object; a reference beam path optically connected to the light source for receiving a reference beam and including a reflective reference surface disposed on a reference object; at least one detector associated with the measurement path and the reference path, the detector receiving the measurement beam after reflection from the reflective measurement surface and the reference beam after reflection from the reflective reference surface; a processing device connected to the detector, 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; a processor configured to determine an absolute position of the object based on the cyclic error phase component.

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

12. 12. The interferometer system of claim 11, wherein the interferometer system comprises a polarizing beam splitter that splits the light beam into a measurement beam and a reference beam based on different polarizations within the light beam, and wherein the polarization leakage results from imperfect splitting of the different polarizations.

13. 13. The interferometer system of claim 10, wherein the processor is configured to separate the cyclic error phase component from the phase signal based on isolating a repetitive phase signal of a known frequency.

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

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

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