Interferometer system and lithographic apparatus

Abstract

An interferometer system comprises: - first and second splitters to split first and second beams received from first and second input terminals in first measurement and reference beams and second measurement and reference beams - a second combiner to combine the second measurement and reference beams and to direct the combined beams to a second detector, - a coupling out device to direct the first measurement beam to a retro reflector, a coupling in device to receive the first measurement beam reflected by the retroreflector, - a first combiner to combine the first measurement beam received via a first measurement beam propagation path from the coupling in device and the first reference beam received from the second splitter via a first reference beam propagation path, and to direct the combined measurement beams to a first detector.

Description

INTERFEROMETER SYSTEM AND LITHOGRAPHIC APPARATUSCROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 24159456.3 which was filed on 23February, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to an interferometer system. Furthermore the present invention relates to a lithographic apparatus or a measurement tool comprising such an interferometer system.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] The lithographic apparatus may make use of mirrors to project the patterned radiation beam onto the substrate. The position of the mirrors may be measured by a position measurement system, comprising one or more interferometers. The substrate may be held by a substrate table of the lithographic apparatus. The position of the substrate table may likewise be measured by a position measurement system comprising one or more interferometers.

[0006] In order to measure a position of the proj ection mirror or the substrate table in plural degrees of freedom, the lithographic apparatus may comprise plural interferometers,

[0007] In an EUV lithographic apparatus, use may be made of fiber-fed interferometers (FFI), which measure the displacement of plane-mirror targets on the EUV mirror substrates. The FFIs may make use of a so-called ‘four-pass’ configuration: a laser beam is propagating from the interferometer head to the plane-mirror target and back again four times. This particular four-pass configuration enables a large rotation and translation range of the plane -mirror targets. And thus a large rotation and translation range of the EUV mirrors.

[0008] Furthermore, the four-pass configuration may lead to a complex and costly optical and mechanical design of the FFIs. The number of optical and mechanical parts may be large. These parts have to be produced to a very high degree of accuracy. In addition, during assembly all these parts have to be aligned and fixed relative to each other. Due to the high number of parts and facets, the FFIs may suffer from ghost reflections, which may lead to measurement errors.SUMMARY

[0009] An object of the present invention is to provide an interferometer system that overcomes or mitigates one or more problems associated with the prior art.

[0010] According to an aspect of the invention, there is provided an interferometer system comprising: a first input terminal configured to receive a first beam, a second input terminal configured to receive a second beam, a first splitter configured to split the first beam received from the first input terminal in a first measurement beam and a second measurement beam a second splitter configured to split the second beam received from the second input terminal in a first reference beam and a second reference beam, a second combiner configured to combine the second measurement beam and the second reference beam and to direct the combined second measurement and reference beams to a second detector, a retroreflector, a coupling out device configured to direct the first measurement beam to the retro reflector, a coupling in device configured to receive the first measurement beam reflected by the retroreflector, a first combiner configured to combine the first measurement beam received from the coupling in device and the first reference beam received from the second splitter and to direct the combined first measurement and reference beams to a first detector, wherein the interferometer system is configured to guide the first measurement beam via a first measurement beam propagation path from the first splitter to the coupling out device and from the coupling in device to the first combiner and to guide the first reference beam from the second splitter via a first reference beam propagation path to the first combiner.

[0011] According to another aspect of the invention there is provided a lithographic apparatus or a measurement tool comprising the interferometer system according to the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source; andFigure 2 depicts a highly schematic, perspective view of an interferometer system according to an embodiment of the present invention.Figure 3 depicts a highly schematic side view of the interferometer system according to Figure 2;Figure 4 depicts another highly schematic, perspective view of the interferometer system according to Figure 2 and 3;Figure 5A - 5D depict cross sectional views illustrating beam propagation paths in the interferometer system as described with reference to Figures 2 - 4;Figure 6 depicts a highly schematic, perspective view of an interferometer system according to another embodiment of the present invention; andFigures 7A and 7B depict a highly schematic view of an interferometer system according to further embodiments of the present invention.DETAILED DESCRIPTION

[0013] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0014] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0015] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For thatpurpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0016] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0017] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0018] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0019] Figure 2 depicts a highly schematic, perspective view of an interferometer system according to an embodiment of the invention. The interferometer system comprises first and second input terminals FIT, SIT configured to receive respective first and second beams. The first and second beams may be laser beams that are split from a single laser source beam. The first and second beams may be at a same wavelength or may have different wavelengths. First and second splitters FSP, SSP split the first and second beams received at the respective first and second input terminals into respective second measurement and reference beams and first measurement and reference beams. Thus, the first splitter splits the first beam in a first measurement beam and a second measurement beam. The second splitter splits the second beam in a second reference beam and a first reference beam. A second combiner SC combines the second measurement and reference beams and directs the combined second measurement and reference beams to a second detector (not depicted in Figure 2). The interferometer system further comprises a coupling out device COD which directs the first measurement beam to a retroreflector RRF of the interferometer. The first measurement beam is reflected by the retroreflector and propagates to a coupling in device CID, which receives the first measurement beam as reflected by the retroreflector. The first measurement beam from the coupling in device and the first reference beam from the second splitter are combined by a first combiner FC. The first combiner combines the first measurement and reference beams and directs the combined first measurement and reference beams to a first detector (not depicted in Figure 2). The first measurement beam is guided from the first splitter to the coupling out device and from the coupling in device to the first combiner via a first measurementbeam propagation path of the interferometer system. The first reference beam is guided from the second splitter to the first combiner via a first reference beam propagation path.

[0020] The retroreflector may be attached to an object of which the position is to be measured. Accordingly, a movement of the object translates into a movement of the retroreflector. As the measurement beam propagates to the retroreflector once, i.e. from the coupling out device to the retroreflector and back from the retroreflector to the coupling in device, a single pass interferometer configuration may be provided. An interference pattern resulting from interference of the second measurement and reference beams is provided at the second detector. Similarly, an interference pattern resulting from interference of the first measurement and reference beams is provided at the first detector. A movement of the target, may result in a change of path length from the coupling out device towards the retroreflector and back from the retroreflector to the coupling in device may cause the interference pattern of the first measurement and reference beams at the first detector to change, which may be detected by the first detector. A displacement of the retroreflector, thus a displacement of the object to which the retroreflector is attached, may be derived from the change of the interference pattern at the first detector. The interference patterns at the second detector and at the first detector may comprise a spatial pattern in turn high intensity and low intensity bars. As the object moves, the spatial pattern of in turn high intensity and low intensity bars may move, causing the first detector to in turn detect high and low amplitude one after the other. In an ideal case, one may not have a fringe pattern: the interferogram may consist of a single spot which may vary in intensity as the target moves. A displacement of the object may be derived from a phase difference at the first detector in respect of the second detector.

[0021] The propagation path along which the first measurement beam propagates, may be not- coincident with the propagation path along which the first reference beam propagates. For this reason, an effect of reflections, higher order diffractions etc. may be reduced, translating into a more pronounced interference pattern of high intensity and low intensity, which may enable to more accurately determine a phase difference between the signals detected at the first detector and at the second detector.

[0022] Figure 3 depicts a view of the interferometer system as depicted in and described above with reference to Figure 2, the view from the back side as seen in Figure 2, i.e. seen from the side of the retroreflector. The first and second input terminals are remote from each other thus to enable the beam propagation paths to be separate, i.e. not-coincide. In Figure 3, the first input terminal and the first splitter are arranged at a top left side and the second input terminal and the second splitter are arranged at the top right side. The second combiner is arranged between the first and second input terminals. The first measurement beam propagates from the first splitter diagonally downwardly where it is coupled out by the coupling out device. Similarly, the first reference beam propagates diagonallydownwardly and is subsequently directed to the first combiner. As seen in Figure 3, the first measurement beam propagation path FMP extends from the first splitter to the coupling out device and from the coupling in device to the first combiner. The first reference beam propagation path FRP extends from the second splitter to the first combiner.

[0023] Figure 4 depicts a same interferometer system as has been depicted in and described with reference to Figures 2 and 3. For illustrative purposes, Figure 4 depicts 4 planes, indicated by 5 A, 5B, 5C and 5D respectively. The measurement and reference beams propagating along the planes 5A - 5D, and further features of the interferometer system will be described with reference to Figures 5A- 5D.

[0024] Figure 5A - 5D depict top views of each one of the planes 5A - 5D.The planes may be understood as cross sectional views through an optically transparent block OTB, such as a glass block. The block may be optically transmissive for the wavelength(s) of the measurement and reference beams. The measurement and reference beams may be beams of any suitable wavelength, such as visible light or infrared.

[0025] Figure 5A depicts the plane 5A. The first and second input terminals are arranged at a first plane 1 PL of an exterior surface of the optically transparent block and receive the first and second beams. The first splitter comprises a grating, at the first input terminal, and splits the first beam received from the first input terminal in the second measurement beam SMB and the first measurement beam FMB. The second splitter likewise comprises a grating at the second input terminal, and splits the second beam received from the second input terminal in the second reference beam SRB and the first reference beam FRB. The first and second splitters are likewise arranged at the first plane of the exterior surface of the optically transparent block. The second measurement and reference beams are diffracted by the respective gratings of the splitters and reflect on a second plane 2PL of the exterior surface of the optically transparent block, to coincide at the second combiner at the first plane of the exterior surface of the optically transparent block. The second plane is substantially parallel to the first plane. It will be understood that for each one of Figures 5A - 5D, the first plane is depicted at the left side and the second plane is depicted at the right side.

[0026] Figure 5D depicts the plane 5D in the drawing of Figure 4, depicting the first measurement beam that has passed through the grating of the first splitter, the first measurement beam propagating to a grating GR on the second plane of the exterior surface of the optically transparent block where it is diffracted, propagates back to the first plane of the exterior surface of the optically transparent block and reflects on the first and second planes of the exterior surface of the optically transparent block, to propagate out of the optically transparent block at the first measurement beam coupling out device, comprising a grating at the second plane of the exterior surface of the optically transparent block.

[0027] As seen in Figure 5B, which depicts the plane 5B, the first measurement beam is reflected by the retroreflector, which may be connected to the object of which the position is to be measured, andis diffracted at the coupling in device. Thereto, the coupling in device, which is arranged at the second plane of the exterior surface of the optically transparent block, comprises a respective grating. The first measurement beam, diffracted by the grating of the coupling in device, propagates to the first combiner arranged at the first plane of the exterior surface of the optically transparent block. The first combiner comprises a grating at the first plane of the exterior surface of the optically transparent block and combines the first measurement and reference beams to output the optically transparent block to be detected by the measurement beam detector. .

[0028] As seen in figure 5C, which depicts the plane 5C, the first reference beam that has passed through the grating of the second splitter (as depicted in Figure 5A), is diffracted by a grating GR on the second plane of the exterior surface of the optically transparent block. The diffracted first reference beam reflects on the first and second planes of the exterior surface of the optically transparent block, following which the first reference beam is incident on a grating on the second plane of the exterior surface of the optically transparent block. At the grating, the first reference beam is diffracted and propagates to the first combiner at the first plane of the exterior surface of the optically transparent block, where the first measurement and reference beams are combined and propagate out of the optically transparent block to the first detector.

[0029] The input terminals, the second combiner, the first combiner, the coupling out device and the coupling in device may be transmissive, i.e. transmitting the respective beams in respectively out of the optically transparent block, while the remaining gratings in the optically transparent block may be reflective, hence mirroring the respective beam back into the optically transparent block.

[0030] As seen in Figure 5C, the first reference beam is diffracted on the second plane of the exterior surface the optically transparent block, after which the first reference beam forms a convergent beam, converging to the point where the first reference beam reflects on the second plane of the optically transparent block, following which the first reference beam diverges towards the above described grating on the second plane of the exterior surface of the optically transparent block. The convergence may be achieved in that the grating on the second plane of the exterior surface of the optically transparent block, which diffracts the first reference beam into the plane of Figure 5C, comprises curved grating lines.

[0031] In an embodiment, the first and second beams are non-polarized.

[0032] In an embodiment, the interferometer system comprises an optically transparent block and wherein the first and second splitters, the coupling out device, the coupling in device, the second combiner and the first combiner are arranged at the optically transparent block. The optically transparent block may for example comprise a glass block. The first and second splitters, the coupling out device, the coupling in device, the second combiner and the first combiner may be arranged at a surface of the optically transparent block. Due to the optically transparent block, the first and secondspliters, the coupling out device, the coupling in device, the second combiner and the first combiner may be arranged at well-defined mutual positions, which may facilitate an adjustment and / or calibration of the interferometer system. Furthermore, a part count of the interferometer system may be low, as the first and second spliters, the coupling out device, the coupling in device, the second combiner and the first combiner are arranged at the optically transparent block, i.e. are arranged at a same part of the interferometer system.

[0033] In an embodiment, the interferometer system comprises the optically transparent block and wherein the first and second spliters, the second combiner and the first combiner are arranged on a first plane of an exterior surface of the optically transparent block. As the first and second spliters, the second combiner and the first combiner are arranged at a same plane, mutual positions may be well defined, facilitating an adjustment, calibration as well as athermal stability of the interferometer system.

[0034] For example, the coupling out device and the coupling in device are arranged on a second plane of the exterior surface of the optically transparent block, the second plane being substantially parallel to the first plane. A position of the coupling out device and coupling in device relative to the positions of the first and second spliters, the second combiner and the first combiner may be accurately defined. As a result, an optical path length for the first measurement and reference beams in the optically transparent block may be accurately defined and may accurately match each other.

[0035] As further example, the first measurement and reference beam paths reflect on the first and second planes. Accordingly, the first measurement and reference beam paths may be well defined. Separate reflectors may be omited, hence promoting to reduce a parts count.

[0036] In an embodiment, the grating comprised in the first spliter comprises concave grating lines. As a result of the concave or curved grating lines, the first reference beam may converge to the point where the first reference beam reflects on the second plane of the optically transparent block, following which the first reference beam diverges towards the above described grating on the second plane of the optically transparent block.

[0037] A diffractive interferometer embodiment is described with reference to Figures 2 - 5D and a refractive interferometer embodiment is described with reference to Figure 6. As an example, of a diffractive interferometer system, in an embodiment, the first and second spliters, the coupling out device, the coupling in device, the second combiner and the first combiner each comprise a respective grating.

[0038] Figure 6 depicts an optically transparent block OTB, such as in the present example a rhomb prism, The first and second beams are spatially separated and incident on a first surface of the rhomb prism. The first and second input terminals and the first and second spliters are accordingly arranged at the first surface of the rhomb prism.

[0039] Figure 6 depicts an embodiment of the interferometer system, showing an example of a refractive interferometer system.

[0040] In the refractive interferometer system, as depicted in Figure 6, the first and second splitters FSP, SSP, the coupling out device COD, the coupling in device CID and the first combiner FC are each formed by a respective refractive surface. A refractive surface or refractive plane may be understood as a surface or plane where a refractive optical index exhibits a change. The first and second splitters may be formed by partially reflective surfaces, providing that fractions of the first and second beams incident on the splitters are reflected to form the second measurement and reference beams, while a remainder of the first and second beams propagate through the splitter and are refracted to form the first measurement and reference beams. The reflected second measurement and reference beams propagate to an auxiliary optically transparent block AOTB. the second measurement beam is reflected at a back plane BP of the auxiliary optically transparent block and the second reference beam is reflected at a front plane FP of the auxiliary optically transparent block. At the front plane of the auxiliary optically transparent block, which thus forms the second combiner SC, the second measurement and reference beams are combined to propagate to a second detector of the interferometer system.

[0041] The first measurement and reference beams propagate through the optically transparent block OTB to a second plane 2PL thereof, which is parallel to the first plane 1PL. At the second plane, the first measurement beam is refracted and coupled out towards the retroreflector RRF, while the first reference beam is reflected and propagates through the rhomb prism to a third plane 3PL of the rhomb prism. The second plane, or a transmissive part thereof, thus forms the coupling out device COD. In order for the first reference beam to be reflected at the second plane, a reflective window, such as a reflective coating, may locally be provided where the first reference beam is incident on the second plane. The first measurement beam propagates to the retroreflector, where it is reflected and is incident on the third plane of the rhomb prism forming the coupling in device CID. At the third plane of the rhomb prism, the first measurement beam, propagating from the rhomb prism, is refracted to propagate to a fourth plane 4PL of the rhomb prism, substantially parallel to the third plane of the rhomb prism. At the third plane, the first reference beam is reflected and propagates through the rhomb prism to the fourth plane. A propagation path of the first measurement and reference beams substantially overlaps from the third plane to the fourth plane. At the fourth plane forming the first combiner MBC, the first measurement and reference beams are combined and refracted to propagate out of the rhomb prism to the first detector of the interferometer system.

[0042] In an embodiment, the interferometer system comprises the optically transparent block and wherein the first and second splitters, the coupling out device, the coupling in device and the first combiner are arranged at an exterior surface of the optically transparent block. The exterior surface of the optically transparent block forms a refractive surface, as the refractive index changes at the surface,namely in the block, the refractive index being the refractive index of the optically transparent material of the optically transparent block, while outside of the optically transparent block, the refractive index being the refractive index of the surrounding medium, such as air, vacuum, nitrogen, etc. Due to the fact that the first and second splitters, the coupling out device, the coupling in device and the first combiner are provided at the exterior surface of the optically transparent block, e.g. the rhomb prism in the example described with reference to Figure 6, the positions of these features of the interferometer system are accurately defined in respect of each other. Assembly of the interferometer system may be facilitated. Adjustment and calibration may be facilitated likewise, optical path lengths in the optically transparent block may be well defined.

[0043] In the embodiment described with reference to Figure 6, the optically transparent block comprises a rhomb prism.

[0044] In an embodiment, the first and second splitters are arranged on a first plane of the exterior surface of the rhomb prism, wherein the coupling out device is arranged on a second plane of the exterior surface of the rhomb prism, substantially parallel to the first plane of the exterior surface of the rhomb prism, wherein the coupling in device is arranged on a third plane of the exterior surface of the rhomb prism, and wherein the measurement beam combining device is arranged on a fourth plane of the exterior surface of the rhomb prism, substantially parallel to the third plane of the exterior surface of the rhomb prism. Accordingly - apart from the first measurement beam propagating to the retroreflector and back -the first measurement and reference beams propagate through the rhomb prism, providing that an optical path length of the first measurement and reference beams will substantially correlate, which may make the interferometer system less sensitive to thermal expansion of the rhomb prism due to temperature changes, dimensional tolerances of the rhomb prism, etc.

[0045] In the diffractive interferometer system as well as in the refractive interferometer system, a temperature compensation may be applied to at least partly compensate for a temperature effect in the retroreflector, i.e. for a temperature effect on an optical path length in the retroreflector. Thereto, the interferometer system may further comprise a transmissive device arranged between the coupling out device and the retroreflector and between the retroreflector and the coupling in device . In order to at least partly compensate for a temperature effect on the optical path length in the retroreflector, a change in an optical path length of the measurement beam by the transmissive device may be substantially opposite to a change on the optical path length of the measurement beam by the retroreflector.

[0046] Figure 7A and 7B depict examples of the temperature compensation. A transmissive device TRD is provided in an optical propagation path of the first measurement beam between the optically transparent block OTB and the retroreflector RRF. The transmissive device may be a single pass transmissive device such as depicted in the example of Figure 7A or a multiple pass transmissive device, such as depicted in the example of Figure 7B. The multiple pass transmissive device may enhance acapability of temperature compensation, as the temperature effect on the optical path length of the transmissive device may be enhanced due to the two or more passes.

[0047] As described above, the interferometer system may be a single-pass interferometer system.

[0048] The retroreflector may be a comer cube or a cat’s-eye retroreflector.

[0049] The interferometer system may be comprised in a lithographic apparatus or measurement tool. For example, the interferometer system may be comprised in a position measurement system configured to measure a position of a mirror substrate of a projection mirror of the projection system of the lithographic apparatus. The measurement tool may comprise any measurement tool, such as a measurement tool used in a semiconductor manufacturing process.

[0050] The present development proposes a single pass interferometer. The proposed single-pass interferometer comprises a very limited number of optical and mechanical components. In the embodiment described with reference to Figures 2 to 5D, the interferometer head itself consists of a single parallel window (excluding fiber optics), and the target to be mounted on the EUV mirror substrates consists of two parallel windows. In the embodiment described with reference to Figure 6, the interferometer head may comprise a rhomb prims and a window and the target to be mounted on the EUV mirror substrate may consist of a hollow or solid cube comer retroreflector. Because of the low mass of the optics of the interferometer head, the number of mechanical components can be reduced as well. For example, there may no longer be a need for a tuned mass damper.

[0051] Not only is the number of components reduced (compared to the FFI), these components can also be produced at lower costs. In the embodiment described with reference to Figures 2 to 5D, most optical components are parallel windows, which are relative easy to manufacture to a high degree of accuracy. In the embodiment described with reference to Figure 6, the rhomb prism may comprise two sets of parallel surfaces. The second reason is that the grating structures on the parallel windows can be replicated from a master grating with well-known lithography processes, at low costs.

[0052] The low number of optical components may also facilitate better control of unwanted ghost reflections, which may prevent measurement errors. The proposed single-pass design is not based on polarization encoding of the various beams, as may be the case in the FFI. Instead, the beams may be separated spatially. As a consequence, the design may be less vulnerable to unwanted ghost reflections.

[0053] The beam paths through the parallel window which forms the interferometer head such as in the embodiments described with reference to Figures 2 to 5D, may designed such that the displacement measurement is insensitive to homogeneous temperature changes of this parallel window, as well as to linear temperature gradients. This is accomplished by the point symmetry of the beam paths for the reference and object beams.

[0054] In the refractive embodiment such as described with reference to Figure 6, the beam paths through the rhomb prism which forms the core of the interferometer head, is designed such that thedisplacement measurement is less insensitive to temperature changes of this prism. This is accomplished by commonality of the beam paths for the reference and object beams.

[0055] The diffractive retroreflector target, which has to be mounted on the EUV mirror substrate, may be sensitive to temperature changes (whereas a plane-mirror target is not). This sensitivity may be due to glass expansion as well as a change of refractive index with temperature. The present development enables to use a window in front of the diffractive retroreflector to compensate for the thermal sensitivity of the retroreflector itself.

[0056] The retroreflector target in the embodiment as described with reference to Figure 6, which has to be mounted on the EUV mirror substrate, can be a solid or a hollow cube comer retroreflector, for example. Also other retroreflectors can be used. Hollow cube comers are less sensitive to temperature variations than solid cube comers.

[0057] As stated above, an advantage of the four-pass configuration of the current FFI may be its relatively short signal period as compared to the signal period of the proposed single-pass interferometer. A high-power laser may be used in the interferometer system according to the present development, to enable an increased interpolation factor, without suffering from noise effects.

[0058] Another advantage of the four-pass configuration may be the large rotation and translation ranges. The rotation and translation ranges of the proposed single-pass configuration with a retroreflector as a target on the EUV mirror substrates may be smaller. However, for position measurement of EUV mirrors this may not be limiting.

[0059] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0060] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0061] 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, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0062] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0063] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. Other aspects of the invention are set-out as in the following numbered clauses.1. An interferometer system comprising: a first input terminal configured to receive a first beam, a second input terminal configured to receive a second beam, a first splitter configured to split the first beam received from the first input terminal in a first measurement beam and a second measurement beam a second splitter configured to split the second beam received from the second input terminal in a first reference beam and a second reference beam, a second combiner configured to combine the second measurement beam and the second reference beam and to direct the combined second measurement and reference beams to a second detector, a retroreflector, a coupling out device configured to direct the first measurement beam to the retro reflector, a coupling in device configured to receive the first measurement beam reflected by the retroreflector, a first combiner configured to combine the first measurement beam received from the coupling in device and the first reference beam received from the second splitter and to direct the combined first measurement and reference beams to a first detector,wherein the interferometer system is configured to guide the first measurement beam via a first measurement beam propagation path from the first splitter to the coupling out device and from the coupling in device to the first combiner and to guide the first reference beam from the second splitter via a first reference beam propagation path to the first combiner.2. The interferometer system according to clause 1, wherein the first and second input terminals are spatially separated.3. The interferometer system according to clause 1 or 2, wherein the first measurement and reference beam propagation paths are spatially separated.4. The interferometer system according to any one of the preceding clauses, wherein the first and second beams are non-polarized.5. The interferometer system according to any one of the preceding clauses, wherein the interferometer system comprises an optically transparent block and wherein the first and second splitters, the coupling out device, the coupling in device, the second combiner and the first combiner are arranged at the optically transparent block.6. The interferometer system according to any one of the preceding clauses, wherein the first and second splitters, the coupling out device, the coupling in device, the second combiner and the first combiner each comprise a respective grating.7. The interferometer system according to clause 6 referring to clause 5, wherein the first and second splitters, the second combiner and the first combiner are arranged on a first plane of an exterior surface of the optically transparent block.8. The interferometer system according to clause 7, wherein the coupling out device and the coupling in device are arranged on a second plane of the exterior surface of the optically transparent block, the second plane being substantially parallel to the first plane.9. The interferometer system according to clause 8, wherein the first and second measurement path reflect on the first and second planes.10. The interferometer system according to any one of clauses 6 - 9, wherein the grating comprised in the first splitter comprises concave grating lines.11. The interferometer system according to any one of clauses 1 - 5, wherein the first and second splitters, the coupling out device, the coupling in device and the first combiner are each formed by a respective refractive surface.12. The interferometer system according to clause 11 referring to clause 5, wherein the first and second splitters, the coupling out device, the coupling in device and the first combiner are arranged at an exterior surface of the optically transparent block.13. The interferometer system according to clause 12, wherein the optically transparent block comprises a rhomb prism.The interferometer system according to clause 13, wherein the first and second splitters are arranged on a first plane of the exterior surface of the rhomb prism, wherein the coupling out device is arranged on a second plane of the exterior surface of the rhomb prism, substantially parallel to the first plane of the exterior surface of the rhomb prism, and wherein the coupling in device is arranged on a third plane of the exterior surface of the rhomb prism, and wherein the measurement beam combining device is arranged on a fourth plane of the exterior surface of the rhomb prism, substantially parallel to the third plane of the exterior surface of the rhomb prism. The interferometer system according to any one of the preceding clauses, further comprising a transmissive device arranged between the coupling out device and the retroreflector and between the retroreflector and the coupling in device, wherein a change in an optical path length of the measurement beam by the transmissive device is substantially opposite to a change on the optical path length of the measurement beam by the retroreflector. The interferometer system according to any of the preceding clauses, wherein the interferometer system is a single-pass interferometer system. The interferometer system according to any of the preceding clauses, wherein the retroreflector is a comer cube or a cat’s-eye retroreflector. A lithographic apparatus or measurement tool comprising the interferometer system according to any one of the preceding clauses.

Claims

CLAIMS1. An interferometer system comprising: a first input terminal configured to receive a first beam, a second input terminal configured to receive a second beam, a first splitter configured to split the first beam received from the first input terminal in a first measurement beam and a second measurement beam a second splitter configured to split the second beam received from the second input terminal in a first reference beam and a second reference beam, a second combiner configured to combine the second measurement beam and the second reference beam and to direct the combined second measurement and reference beams to a second detector, a retroreflector, a coupling out device configured to direct the first measurement beam to the retro reflector, a coupling in device configured to receive the first measurement beam reflected by the retroreflector, a first combiner configured to combine the first measurement beam received from the coupling in device and the first reference beam received from the second splitter and to direct the combined first measurement and reference beams to a first detector, wherein the interferometer system is configured to guide the first measurement beam via a first measurement beam propagation path from the first splitter to the coupling out device and from the coupling in device to the first combiner and to guide the first reference beam from the second splitter via a first reference beam propagation path to the first combiner.

2. The interferometer system according to claim 1, wherein the first and second input terminals are spatially separated.

3. The interferometer system according to claim 1 or 2, wherein the first measurement and reference beam propagation paths are spatially separated.

4. The interferometer system according to any one of the preceding claims, wherein the first and second beams are non-polarized.

5. The interferometer system according to any one of the preceding claims, wherein the interferometer system comprises an optically transparent block and wherein the first andsecond spliters, the coupling out device, the coupling in device, the second combiner and the first combiner are arranged at the optically transparent block.

6. The interferometer system according to any one of the preceding claims, wherein the first and second spliters, the coupling out device, the coupling in device, the second combiner and the first combiner each comprise a respective grating.

7. The interferometer system according to claim 6 referring to claim 5, wherein the first and second spliters, the second combiner and the first combiner are arranged on a first plane of an exterior surface of the optically transparent block.

8. The interferometer system according to claim 7, wherein the coupling out device and the coupling in device are arranged on a second plane of the exterior surface of the optically transparent block, the second plane being substantially parallel to the first plane.

9. The interferometer system according to claim 8, wherein the first and second measurement path reflect on the first and second planes.

10. The interferometer system according to any one of claims 6 - 9, wherein the grating comprised in the first spliter comprises concave grating lines.

11. The interferometer system according to any one of claims 1 - 5, wherein the first and second spliters, the coupling out device, the coupling in device and the first combiner are each formed by a respective refractive surface.

12. The interferometer system according to claim 11 referring to claim 5, wherein the first and second spliters, the coupling out device, the coupling in device and the first combiner are arranged at an exterior surface of the optically transparent block.

13. The interferometer system according to claim 12, wherein the first and second spliters are arranged on a first plane of the exterior surface of the rhomb prism, wherein the coupling out device is arranged on a second plane of the exterior surface of the rhomb prism, substantially parallel to the first plane of the exterior surface of the rhomb prism, and wherein the coupling in device is arranged on a third plane of the exterior surface of the rhomb prism, and wherein the measurement beam combining device is arranged on a fourth plane of the exterior surfaceof the rhomb prism, substantially parallel to the third plane of the exterior surface of the rhomb prism.

14. The interferometer system according to any one of the preceding claims, further comprising a transmissive device arranged between the coupling out device and the retroreflector and between the retroreflector and the coupling in device, wherein a change in an optical path length of the measurement beam by the transmissive device is substantially opposite to a change on the optical path length of the measurement beam by the retroreflector.

15. A lithographic apparatus or measurement tool comprising the interferometer system according to any one of the preceding claims.

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