Interferometer system without walk-off, method for using an interferometer system

Abstract

The present disclosure provides an interferometer system, comprising: an input terminal configured to receive an input beam from a source of radiation, a polarizing beam splitter which is configured to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.

Description

INTERFEROMETER SYSTEM WITHOUT WALK-OFF, METHOD FOR USING AN INTERFEROMETER SYSTEMCROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 24155770.1 which was filed on 5 February, 2024 and EP application 24157145.4 which was filed on 12 February, 2024 which are incorporated herein in their entirety by reference.FIELD

[0002] The present disclosure relates to an interferometer system, and a method of using the interferometer system. The disclosure also relates to a lithographic apparatus including the 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 (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’ s law’ . To keep up with Moore’ s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. 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 are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 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] Interferometers are used for various measurements in the lithographic apparatus and in other equipment related to the semiconductor fabrication process, including metrology. For instance, wavelength trackers may be used to control stability of laser beam inputs, while the position of moveable equipment such as wafer stages may be controlled up to nm level using one or more phase tracking interferometers.

[0006] The interferometer design used in known lithographic systems is typically based on a two-pass principle. Herein, the target is a flat mirror. Four-pass designs are also known.

[0007] CN-218443724-U discloses a miniaturized laser interferometer characterized by comprising a miniaturized frequency-stabilized light source module, an interference light path module, a corner reflector and a data processing module, wherein the interference light path module comprises: a miniaturized beam splitter, a miniaturized corner reflector, a glass slide and a miniaturized photodetector; the miniature frequency stabilization light source module generates a frequency stabilization laser with narrow line width, and the frequency stabilization laser is divided into signal light and reference light with equal power by a miniature beam splitter; the signal light returns to the miniature beam splitter through a corner reflector mounted on the target; the reference light returns to the miniaturized beam splitter through the miniaturized corner reflector; the signal light reflected by the corner reflector enters the interference light path module, interferes with the reference light to generate two paths of interference phase orthogonal signals, is incident on the miniaturized photoelectric detector, and inputs the interference signals obtained by detection into the data processing module.

[0008] US-20030197870-A1 discloses an interferometer which returns parallel beams that are subject to walk-off caused by reflector misalignment for an additional pass through the interferometer optics and thereby eliminates beam walk-off. A return reflector can be a plane mirror that directs returning beams to retrace paths through the interferometer optics to combine and exit along the axis of the input beam. Separation optics can separate the combined beam from the input beam. Alternatively, a return reflector such as an isosceles prism or a trapezoidal prism reflects and offsets returning beams so that the combined beam is offset from the input beam. The return reflector more generally responds to a shift in incident beam position with a matching shift of the reflected beam in contrast to a retroreflector, which shifts a reflected beam in a direction opposite to the shift in the incident beam.

[0009] US-20060017933-A1 discloses an interferometer system for measuring a displacement along a first direction including a measurement roof optic (e.g., a porro prism) mounted to a stage translatable along the first direction, a polarizing beam splitter having (a) a first face opposite the measurement roof optic and (b) a second face opposite the first face, a first wave plate located between the measurement roof optic and the first face of the polarizing beam splitter, and a redirecting optic located opposite the first face of the polarizing beam splitter. A measurement path through the system includes only segments located substantially in a plane defined by the first direction and a second direction orthogonal to the first direction.

[0010] Two-pass interferometers typically suffer from measurement or reference-beam walk-off when the flat target is rotated around an axis perpendicular to the direction of the incident light beam. Walk-off in combination with low laser power causes deterioration of signal quality.

[0011] US-6806960-B2 and US-6897962-B2 disclose a number of four-pass interferometer designs. A multi-axis interferometer uses a combined beam for a first pass through the interferometer optics. Measurement and reference components of the combined beam that exit the interferometeroptics are subject to walk-off that measurement or reference reflector misalignment can cause. A return reflector and non-polarizing beam splitter system split the combined beam into separated input beams for the various axes of the interferometer and return the separated beams for respective second passes through the interferometer optics. Walk-off for the separated beams in the interferometer optics cancels the walk-off for the combined beam to eliminate beam walk-off in separated output beams.

[0012] The four-pass design obviates walk-off, but is significantly more complex and expensive. In addition, the four-pass design includes more optical elements, and any flaws in any one of these optical elements may cause imperfections in the measurement signals. Consequently, the four-pass interferometers as disclosed in US6806960B2 often suffer from severe disturbances, such as ghost rays and cyclic errors.

[0013] Although the systems and methods disclosed in the publications referenced above may generally provide decent results for their respective purpose as stated, interferometer designs including corner cubes in practice still experience walk-off and the associated problems. Walk-off is more prominent when measuring over large distances. Walk-off causes deteriorating signal quality, a problem which is even more pronounced in combination with relatively low laser power.

[0014] The present disclosure aims to provide an improved interferometer system obviating walk-off.SUMMARY

[0015] The disclosure provides an interferometer system, comprising: an input terminal configured to receive an input beam from a source of radiation, a polarizing beam splitter which is configured to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.

[0016] In an embodiment, the interferometer system is a two-pass interferometer.

[0017] In an embodiment, the reference path is directed towards a second optical reflector and back to the beam splitter, then towards the translating reflector and back to the beam splitter.

[0018] In an embodiment, the first optical reflector is selected from a retro reflector, a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cat eye retroreflector.

[0019] In an embodiment, the second optical reflector is a flat mirror.

[0020] In an embodiment, the translating reflector is selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.

[0021] The interferometer system may comprise: a first quarter wave plate arranged between the beam splitter and the first optical reflector; and a second quarter wave plate arranged between the beam splitter and the second optical reflector.

[0022] In an embodiment, the beam splitter comprises a first polarizing beam splitter and a second polarizing beam splitter, wherein the first polarizing beam splitter is configured to reflect the first portion of the input beam to follow the measurement path and to reflect the second portion of the input beam to follow the reference path, wherein the measurement path is directed towards the first optical reflector and back to the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter, and wherein the reference path is directed towards the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter.

[0023] In an embodiment, the input beam comprises radiation of at least two distinct frequencies.

[0024] The interferometer system may comprise frequency adjustment means for adjusting at least one of the at least two distinct frequencies.

[0025] In an embodiment, the beam spitter is adapted to polarize the first portion to a first polarization, and the second portion to a second polarization other than the first polarization.

[0026] In an embodiment, the movable measurement target is a substrate support, a patterning device support or an optical element of a projection system of a lithographic apparatus.

[0027] According to another aspect, the disclosure provides a lithographic apparatus or measurement tool, comprising at least one interferometer system as described above.

[0028] According to another aspect, the disclosure provides a method comprising the steps of: providing an input beam of radiation to a polarizing beam splitter, using the beam splitter to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.

[0029] In an embodiment, the first optical reflector is selected from a retro reflector, a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cateye retroreflector; and / or the translating reflector is selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.

[0030] In an embodiment, the reference path is directed towards a second optical reflector and back to the beam splitter, then towards the translating reflector and back to the beam splitter.

[0031] In an embodiment, the beam splitter comprises a first polarizing beam splitter and a second polarizing beam splitter, the method comprising the steps of: using the first polarizing beam splitter to reflect the first portion of the input beam to follow the measurement path and to reflect the second portion of the input beam to follow the reference path, wherein the measurement path is directed towards the first optical reflector and back to the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter, and wherein the reference path is directed towards the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter.

[0032] The system and method as claimed herein enable a two-pass interferometer substantially obviating walk-off. The reversal prism instead of a corner cube obviates walk-off. The system of the disclosure includes only a relatively limited number of optical elements, thereby limiting the associated optical errors.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] 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 schematic overview of a lithographic apparatus, and Figures la to lb depict a schematic overview of a projection exposure apparatus designed for operation in EUV (Fig. la) and a DUV lithographic apparatus (Fig. lb);Figure 2 depicts a schematical side-view of a conventional system;Figure 3 depicts a schematical side-view of another conventional system;Figure 4 depicts a schematical side-view of an embodiment of the system of the disclosure;Figure 5 depicts a schematical perspective view of an embodiment of the system of the disclosure;Figure 6 depicts a schematical perspective view of an embodiment of the system of the disclosure;Figure 7 depicts a schematical side-view of another embodiment of the system of the disclosure;Figure 8 depicts a schematical perspective view of the embodiment of Figure 7; and Figures 9 to 11 depict schematical side views of respective variants of a beam shifter.DETAILED DESCRIPTION

[0034] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0035] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0036] A “beam splitter” (also spelled beamsplitter) is an optical device that splits a beam of light in two.

[0037] A "polarizing beam splitter" typically refers to a beam splitter provided with one or more optical elements to polarize one or more of the incoming or outgoing beams. The optical elements may include a waveplate or retarder. A waveplate is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates are a half-wave plate, which shifts the polarization direction of linearly polarized light, and a quarter-wave plate, which converts linearly polarized light into circularly polarized light and vice versa.

[0038] A "retroreflector" (sometimes called a retroflector or cataphote) is a device or optical element that reflects radiation, such as light, back to its source with minimum scattering. In the ideal situation, the incident beam and the outgoing beam are substantially parallel. This works at a wide range of angle of incidence, unlike a planar mirror, which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence. Corner reflectors or corner cubes, and cat's eye reflectors are the most used kinds.

[0039] A "beam shifter" or "translating reflector" is a device or optical element that reflects radiation, such as light, such that a mutual distance between the incident beam and the outgoing beam remains substantially the same. The mutual distance remains the same, even when the position of the incident beam shifts or translates with respect to the beam shifter. Contrary to a retroreflector, the incoming beam and outgoing beam are not necessarily parallel to each other. For instance, when the incident beam is directed under an angle with respect to the beam shifter, the outgoing beam will be directed under the same angle yet in the opposite direction.

[0040] "Walk-off1herein refers to misalignment of two or more radiation beams, such as laser light beams. Misalignment herein typically refers to a deviation from a design mode of operation.

[0041] “Refractive Index” (Index of Refraction) is a value calculated from the ratio of the speed of light in a vacuum to that in a second medium of greater density. The refractive index variable may be symbolized by the letter n or n' in descriptive text and mathematical equations.

[0042] An “interferometer” or a “laser interferometer” can measure distance or displacement by measuring the phase difference between two light beams, one sent to a first reflector or first surface at a fixed reference distance, and one sent to a second reflector or surface at another distance. When the two reflected signals are recombined in the interferometer, the resulting phase is related to the distance of the second surface from the interferometer. If the distance of the second surface changes, so does the phase of the combined signal. The utility of these methods are that the measurement can be made over long distances while maintaining accuracy.

[0043] A "homodyne interferometer" measures phase by comparing the intensities of two sinusoidal signals (sine and cosine). A single frequency laser source is used in homodyne systems, having for instance frequency Fl. The laser beam from the stationary reference path is returned with frequency Fl, but the beam from the (moving) measurement path is returned with a Doppler shifted frequency. These beams are interfered together in the detector to give a beat frequency of zero when the optics are stationary, whilst the beat frequency rises as the optics move in either direction.

[0044] A "heterodyne interferometer" uses a dual frequency laser source. The laser source may provide a single laser beam, which can be split in two wherein one of the two beams is given a frequency offset to allow for heterodyne phase detection. The two frequencies may however also be generated by two different, frequency-locked or phase-locked lasers.

[0045] In a heterodyne interferometer, the output beam from the dual frequency laser source may comprise two polarizations, one with a first frequency Fl, the other with a different, second frequency F2. The beat frequency or frequency difference between them is F2 - Fl. A polarizing beam-splitter reflects the light with first frequency Fl into the reference path. Light with the second frequency F2 passes through the splitter into the measurement path where it strikes a moving reflector causing the frequency of the reflected beam to be Doppler shifted by 5F. This reflected beam is then combined with the Fl frequency light at the interferometer, and returned to the laser detector unit with a new beat frequency of F2 - Fl +5F. The beat frequency of a heterodyne laser system varies with the velocity of the moving reflector. When the optics are stationary, the beat frequency is F2 - Fl. As the optics move apart, the beat frequency rises by 5F, if they move together it falls.

[0046] A “wavelength tracker” is a specific version of an interferometer set up to measure a phase difference between two reflected light beams, one beam reflecting on a first fixed reflector providing a first reference axis and a second beam reflecting on a second fixed reflector providing a second reference axis having a different length than the first axis. As the two reflective surfaces are fixed, the measured phase difference will change only if the wavelength of the light beam changes. Thus, a wavelength tracker allows to monitor deviations of the wavelength from a setpoint.

[0047] " Polarization" of light refers to an intensity difference between respective polarization states of a laser beam. P-polarized light (from the German word 'parallel') has its electric field polarized parallel to the plane of incidence. S-polarized light (from the German word 'senkrechf) is perpendicular to the plane of incidence.

[0048] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0049] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0050] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0051] As depicted in Figure 1, the lithographic apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0052] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0053] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0054] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0055] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system PMS, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0056] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0057] Figure la shows a schematic illustration of an exemplary projection exposure apparatus 600 designed for operation in the EUV, in which the present invention is implementable. However, the invention can also be used in other nanopositioning systems. In accordance with Figure la, an illumination device in a projection exposure apparatus 600 designed for EUV comprises a field facet mirror 603 and a pupil facet mirror 604. The light from a light source unit comprising a plasma light source 601 and a collector mirror 602 is directed at the field facet mirror 603. A first telescope mirror 605 and a second telescope mirror 606 are arranged downstream of the pupil facet mirror 604 in the light path. Arranged downstream in the light path is a deflection mirror 607, which directs theradiation incident on it at an object field in the object plane of a projection lens comprising six mirrors 651- 656. At the location of the object field, a reflective structure -bearing mask 621 is arranged on a mask stage 620 and with the aid of the projection lens is imaged into an image plane, in which a substrate 661 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 660.

[0058] The invention can likewise be used in a DUV apparatus, as illustrated in Figure lb. A DUV apparatus is set up in principle like the above-described EUV apparatus from Figure la, wherein mirrors and lens elements can be used as optical elements in a DUV apparatus and the light source of a DUV apparatus emits used radiation in a wavelength range of 100 nm to 400 nm. The DUV lithography apparatus 700 illustrated in Figure lb has a DUV light source 701. By way of example, an ArF excimer laser that emits radiation 702 in the DUV range at 193 nm, for example, can be provided as the DUV light source 701. A beam shaping and illumination system 703 guides the DUV radiation 702 onto a photomask 704. The photomask 704 is embodied as a transmissive optical element and can be arranged outside the systems 703. The photomask 704 has a structure which is imaged onto a wafer 706 or the like in a reduced fashion by means of the projection system 705. The projection system 705 has a plurality of lens elements 707 and / or mirrors 708 for imaging the photomask 704 onto the wafer 706. In this case, individual lens elements 707 and / or mirrors 708 of the projection system 705 can be arranged symmetrically with respect to the optical axis 709 of the projection system 705. It should be noted that the number of lens elements 707 and mirrors 708 of the DUV lithography apparatus 700 is not restricted to the number illustrated. A greater or lesser number of lens elements 707 and / or mirrors 708 can also be provided. In particular, the beam shaping and illumination system 703 of the DUV lithography apparatus 700 comprises a plurality of lens elements 707 and / or mirrors 708.Furthermore, the mirrors are generally curved on their front side for beam shaping purposes. An air gap 710 between the last lens element 707 and the wafer 706 can be replaced by a liquid medium having a refractive index of > 1. The liquid medium can be high-purity water, for example. Such a setup is also referred to as immersion lithography and has an increased photolithographic resolution. The actuators according to the invention can be used for the adjustment of the lens elements 707 and or mirrors 708 and / or for the deformation thereof in the DUV lithography apparatus 700, in particular in the projection system 705 thereof.

[0059] The projection optical system, e.g. the EUV or DUV projection optical apparatus, may comprise at least one position measurement system, e.g. the interferometer according to the invention. The at least one position measurement system is configured to measure the position of at least one optical element of the projection optical apparatus. As an alternative or an addition, the illumination apparatus may comprise at least one position measurement system, e.g. the interferometer according to the invention. The at least one position measurement system is configured to measure the position of at least one optical element of the illumination apparatus.

[0060] Generally referring to Figures 2 and 3, a conventional two-pass design of an interferometer 1 may include an input terminal 2 for receiving an input beam 4. The input beam may typicallyinclude a reference beam 6 and a measurement beam 8. The respective beams 6 and 8 may each have a feature distinguishing one with respect to the other, such as, but not limited to, frequency, wavelength, phase, or polarization. For more detail, see the description herein below.

[0061] The interferometer 1 includes a beam splitter 10 for receiving the input beam 4. The beam splitter 10 may be provided with a first wave plate 12. The beam splitter 10 may be provided with a second wave plate 14. The first wave plate and the second wave plate may be selected from, for instance, a quarter wave plate, and a half wave plate. The beam splitter 10 may have a reflection surface 16. The reflection surface 16 herein may typically be reflective for radiation having certain characteristics, while being transmissive for radiation having other characteristics. For instance, the reflection surface 16 may be reflective for radiation having certain polarization directions, while being transmissive for radiation having other polarization directions.

[0062] A first reflector 20 may be connected to a target of choice, such as a reference cavity or a movable object. The interferometer typically includes a second reflector. The interferometer may include a third reflector 24.

[0063] As exemplified in Figure 2, typically the first reflector may be a flat mirror. As exemplified in Figure 3, the first reflector may also be a corner cube, a species of a retroreflector.

[0064] The beam splitter is a polarizing beam splitter. A polarization beam splitter produces two polarized output beams. One beam is reflected at an angle, and the other beam is let through. The reflected beam is typically called the S-polarization beam, while the beam that is let through is the P- polarization. The beam splitter may comprise two right-angle triangular prisms stacked against each other with a beam-splitting coating dividing them in between. This will make for a reflective surface that will stand 45% of the original unpolarized light beam when set properly. There are different types of reflective coatings used in polarized beam splitters. The type of beam splitter coating dictates how the polarizing beam splitter works. The coating will dictate how much of the original unpolarized light is reflected and how much is let through. Polarized beam splitter cubes may feature a dielectric coating that can be intended for different uses. How much of the original light beam is let through or reflected in the s and p polarization may be controlled through this coating.

[0065] Due to the combination of the polarizing beam splitter (PBS) 10 and the first and second wave plates 12, 14, a beam re-entering the PBS takes a different turn at the internal splitting surface 16 of the PBS. In use, the beam splitter 10 directs the reference beam 6 towards the second reflector 22. Herein, the reference beam 6 is directed through the second quarter wave plate 14. As a result, the polarization of the reference beam changes. The reference beam 6 is reflected on the second reflector 22 and returned to the beam splitter 10. Due to the changed polarization, the reflection surface 16 will be transmissive for the reflected reference beam and therefore continue towards the third reflector 24. The third reflector may be a retroreflector. On the other hand, the beam splitter 10 directs the measurement beam 8 towards the first reflector 20. Herein, the measurement beam 8 passes through the first quarter wave plate 12. As a result, the polarization of the measurement beam changes. Themeasurement beam 8 is reflected on the first reflector 20 and returned to the beam splitter 10. Due to the changed polarization, the reflection surface 16 will be reflective for the reflected measurement beam. As a result, the measurement beam is also directed towards the third reflector 24.

[0066] In the designs exemplified in Figures 2 and 3, both the reference beam 6 and the measurement beam 8 are directed towards the third reflector, are reflected towards the beam splitter 10, and are subsequently once again directed towards their respective targets. Thus, the measurement beam 8 is directed to the first reflector 20 twice, and the reference beam 6 is directed to the second reflector 22 twice. Then, both beams exit the interferometer 1 indicated by exit beam 26. The exit beam is typically directed towards equipment for further processing, typically including detectors, phase measurement etc. As both the measurement and the reference beam may pass the interferometer twice, the interferometer 1 may be referred to as a two-pass interferometer.

[0067] In the setup of Figure 2, the first reflector 20 is a flat mirror. In the ideal situation, the measurement beam 8 and the reference beam 6 exiting the beam splitter 10 towards a detector (not shown) coincide. In the exit beam 26, the reference beam 6 and the measurement beam 8A travel along the same axis and in the same direction. A detector subsequently can process the two beams and provide measurement data.

[0068] A problem arises when the mirror of first reflector 20 rotates around an axis perpendicular to the direction of travel of the measurement beam 8. Such rotation is exemplified by arrow 28. Due to the rotation, exiting measurement beam 8B and the reference beam 6 travel in slightly different, non-parallel directions. This results in a mutual distance 30 between the reference beam 6 and the measurement beam 8B. Said distance 30 may be referred to as walk-off. Walk-off herein is a deviation from the design value.

[0069] As exemplified in Figure 3, the first reflector 20 may also be a retroreflector. If the retroreflector is in its design location, as exemplified by retroreflector 20A, the measurement beam 8 and the reference beam 6 coincide when exiting the beam splitter 10 towards a detector (not shown). As indicated by exiting measurement beam 8A, herein the reference beam 6 and the measurement beam 8A travel along the same axis and in the same direction.

[0070] A problem arises when the retroreflector of the first reflector 20 is translated in a direction perpendicular to the direction of travel of the measurement beam 8, as exemplified by retroreflector 20B. Due to the translation, exiting measurement beam 8B is also translated with respect to the design, as indicated by beam 8A. The latter results in a mutual distance 30 between the reference beam 6 and the exiting measurement beam 8B. Thus, a retroreflector as first reflector may also cause walk-off.

[0071] Generally referring to Figure 4, an interferometer 40 according to the present disclosure has an input terminal 2 adapted to receive an input beam 4 provided by a light source 32. The light source 32 typically comprises one or more laser light sources. The interferometer 40 is adapted to provide an output beam or exit beam 26. The output beam 26 can be directed to a detector 34. The interferometer40 includes a beam shifter 42 as third reflector 24. The interferometer 40 may include a first reflector 20. In an embodiment, the first reflector 20 is a retroreflector. The interferometer 40 may include a second reflector 22. In an embodiment, the second reflector 22 is a flat mirror 44. The interferometer includes a polarizing beam splitter 46. The polarizing beam splitter 46 is adapted to receive the input beam 4 and direct its components towards the respective reflectors. The respective beams may pass the first quarter wave plate 12 and second quarter wave plate 14.

[0072] If the first reflector 20 is in its predetermined design location, as indicated by reflector 20A, the measurement beam 8 follows a path as indicated by first reflected measurement beam 8A. If the first reflector however is translated, as exemplified by the second reflector 20B, the measurement beam follows a path as indicated by second measurement beam 8B. In the interferometer 40 of the present disclosure, both the first measurement beam 8A and the second measurement beam 8B result in the same exit beam 26. Despite the translation of the first reflector 20, in the output beam 26 the measurement beam 8 and the reference beam 6 are aligned. In other words, the interferometer 40 as exemplified in Figure 4 substantially obviates walk-off.

[0073] The perspective view of Figure 5 depicts a use example, wherein the reference beam 6 and measurement beam 8 may be reflected on one or more of the respective first, second and third reflectors at a different level, i.e. out of plane of the input beam 4. Herein, the interferometer 40 of the present disclosure obviates walk-off in the output beam 26 both for in-plane and for out-of-plane reflections.

[0074] Generally referring to Figures 6 and 7, in another embodiment, an interferometer 50 of the present disclosure is provided with a beam splitter 10 comprising two polarizing beam splitters 52, 54. Each polarizing beam splitter 52, 54 may be provided with polarizing means (not shown, but comparable to the first and second waveplates 12, 14 as shown in Figure 4) and with a reflection surface 16. The interferometer 50 includes a first reflector 20. The first reflector may be a retroreflector. The interferometer 50 includes a third reflector 24, functioning as a beam shifter 42.

[0075] The interferometer 50 basically functions as two single-pass interferometers in conjunction. Herein, the beam splitter 10 directs the reference beam 6 towards the third reflector 24. The third reflector shifts the reference beam and reflects the reference beam back towards the beam splitter, which subsequently directs the reference beam towards the design location of the output beam 26. The beam splitter 10 allows the measurement beam 8 to pass and continue towards the first reflector 20. In a preferred embodiment, the first reflector is a retroreflector. The first reflector 20 reflects to measurement beam towards the beam splitter 10, which allows the reflected measurement beam to pass and continue towards the third reflector 24. The third reflector shifts the measurement beam and reflects the measurement beam back towards the beam splitter 10. The measurement beam 8 passes the beam splitter 10 and continues towards the first reflector. At the first reflector 20, the measurement beam is reflected for a second time. The second reflection of the measurement beam is reflected to the beam splitter 10, passes the beam splitter 10, and continues towards the output of theinterferometer 50. Herein, the reflected measurement beam 8 is combined with the reference beam 6 in the output beam 26.

[0076] If the first reflector 20 is in its predetermined design location, as indicated by reflector 20A, the measurement beam 8 follows a path as indicated by first reflected measurement beam 8A. If the first reflector however is translated, as exemplified by the second reflector 20B, the measurement beam follows a path as indicated by second measurement beam 8B. In the interferometer 50, both the first measurement beam 8A and the second measurement beam 8B result in the same exit beam 26. Despite the translation of the first reflector 20, in the output beam 26 the measurement beam 8 and the reference beam 6 are aligned. The interferometer 50 as exemplified in Figures 6 and 7 substantially obviates walk-off.

[0077] The perspective view of Figure 8 indicates that the reference beam 6 and measurement beam 8 may be reflected at different levels, i.e. out of plane with the input beam 4. Herein, the measurement beam 8 may be reflected at different levels at the first reflector 20 and at the third reflector. The reference beam 6 may be reflected at different levels at the third reflector. The interferometer 50 of the present disclosure obviates walk-off in the output beam 26 both for in-plane and for out-of-plane reflections.

[0078] The beam shifter 42 of the third reflector may have various layouts, as exemplified in Figures 8 to 11. The beam shifter 42 of Figures 8 and 9 includes three mirrors 60, 62, 64. The three mirrors 60 to 64 can have a multitude of configurations and locations, as exemplified in Figure 8 and 9 respectively. Figure 10 exemplifies a beam shifter 42 comprising a prismatic optical element 70, combined with a reflective surface 72, basically a flat mirror. The beam shifter 42 of Figure 11 includes a truncated prism or cone 80.

[0079] The present disclosure basically provides an interferometer which obviates walk-off yet is relatively simple and includes only a limited amount of optical elements. In use, the interferometer of the present disclosure limits passage of the measurement beam through the respective optical elements to two times only, in effect providing a two-pass interferometer. Due to the limited amount of optical elements and the limited number of passes of the measurement beam, optical aberrations are limited to a minimum. Consequently, noise and aberrations in the output signal are limited as well, supporting the accuracy of measurements.

[0080] In a practical embodiment, the interferometer may include a second reflector which is a simple flat mirror. A flat mirror is relatively simple to fabricate and can be made with relatively high optical accuracy.

[0081] In use, the polarizing beam splitter 10 is configured to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path. The measurement path is directed towards the first optical reflector. The first reflector may be mounted on a measurement target. The first reflector reflects the first portion back to the beam splitter, then towards the third reflector which is a translating reflector or beam shifter. The thirdreflector reflects the first portion back to the beam splitter. The reference path is also directed towards the translating reflector and back to the beam splitter. The reference path may also include a second optical reflector, for reflecting the second portion or reference beam back to the beam splitter.

[0082] In a preferred embodiment, the first optical reflector is retro reflector. The retro reflector may be selected from a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cat eye retroreflector.

[0083] The third reflector is a translating reflector. The translating reflector or beam shifter may be selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.

[0084] In a practical embodiment, the beam spitter is adapted to polarize the first portion to a first polarization, and the second portion to a second polarization other than the first polarization.

[0085] In a practical embodiment, the first reflector 20 may be connected to a target of choice. The target may be selected from a movable measurement target, a substrate support, a patterning device support, or an optical element of a projection system of a lithographic apparatus.

[0086] To indicate utility and exemplary setup of an interferometer system, and for technical details, options, and methods to operate and use the interferometer system, reference is made to US2021072088.

[0087] In a practical embodiment, the reference beam section 6 may be provided by a first light source to provide a first laser beam having a first frequency. The measurement beam section 8 may be provided by a second light source for providing a second laser beam having a second frequency. Alternatively, the measurement beam section and the reference beam section may be split from one single laser beam, wherein one or both are provided with an offset, such as a frequency offset, a phase offset, and / or a different polarization.

[0088] The first light source and / or the second light source may be tuneable lasers, able to provide a laser beam having a tuneable or adjustable frequency. Herein, adjusting the frequency of one or more of the laser sources may be referred to as frequency adjustment means for adjusting the first frequency and / or the second frequency. In practice, tuneable lasers are available on the market. Examples of tuneable lasers include laser diodes, lasers having an adjustable resonator cavity length, or the use of adjustable optical elements to adjust the laser output.

[0089] Alternatively, one or both of the first light source and the second light source may provide a laser beam with a fixed frequency. The first light source and / or the second light source may comprise, for instance, a stabilized HeNe laser.

[0090] Although specific reference may be made in this text to the use of a 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.

[0091] 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.

[0092] 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.

[0093] 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. For instance, features of various embodiments as described above may be combined. Other aspects of the invention are set-out as in the following numbered clauses.1. An interferometer system, comprising: an input terminal configured to receive an input beam from a source of radiation, a polarizing beam splitter which is configured to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.2. The interferometer of clause 1, wherein the interferometer system is a two-pass interferometer.3. The interferometer system of clause 1 or 2, wherein the first optical reflector is selected from a retro reflector, a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cat eye retroreflector.4. The interferometer of one of the previous clauses, wherein the reference path is directed towards a second optical reflector and back to the beam splitter, then towards the translating reflector and back to the beam splitter.5. The interferometer system of clause 4, wherein the second optical reflector is a flat mirror.6. The interferometer system of one of the previous clauses 1-3, wherein the beam splitter comprises a first polarizing beam splitter and a second polarizing beam splitter,wherein the first polarizing beam splitter is configured to reflect the first portion of the input beam to follow the measurement path and to reflect the second portion of the input beam to follow the reference path, wherein the measurement path is directed towards the first optical reflector and back to the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter, and wherein the reference path is directed towards the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter.7. The interferometer system of one of the previous clauses, wherein the translating reflector is selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.8. The interferometer system of one of the previous clauses referring to clause 4, comprising: a first quarter wave plate arranged between the beam splitter and the first optical reflector; and a second quarter wave plate arranged between the beam splitter and the second optical reflector.9. The interferometer system of one of the previous clauses, wherein the input beam comprises radiation of at least two distinct frequencies.10. The interferometer system of clause 9, comprising frequency adjustment means for adjusting at least one of the at least two distinct frequencies.11. The interferometer system of one of the previous clauses, wherein the beam spitter is adapted to polarize the first portion to a first polarization, and the second portion to a second polarization other than the first polarization.12. The interferometer system of one of the previous clauses, wherein the movable measurement target is a substrate support, a patterning device support or an optical element of a projection system of a lithographic apparatus.13. Lithographic apparatus or measurement tool or inspection tool, comprising at least one interferometer system according to one of the previous clauses.14. Method comprising the steps of: providing an input beam of radiation to a polarizing beam splitter, using the beam splitter to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.15. The method of clause 14, wherein the first optical reflector is selected from a retro reflector, a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cat eye retroreflector; and / or wherein the translating reflector is selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.16. The method of clause 14 or 15, wherein the reference path is directed towards a second optical reflector and back to the beam splitter, then towards the translating reflector and back to the beam splitter.17. The method of clause 14 or 15, wherein the beam splitter comprises a first polarizing beam splitter and a second polarizing beam splitter, the method comprising the steps of: using the first polarizing beam splitter to reflect the first portion of the input beam to follow the measurement path and to reflect the second portion of the input beam to follow the reference path, wherein the measurement path is directed towards the first optical reflector and back to the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter, and wherein the reference path is directed towards the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter.18. Projection optical system comprising at least one interferometer system according to one of the previous clauses 1-12.19. Illumination apparatus comprising at least one interferometer system according to one of the previous clauses 1-12.

Claims

CLAIMS1. An interferometer system, comprising: an input terminal configured to receive an input beam from a source of radiation, a polarizing beam splitter which is configured to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.

2. The interferometer system of claim 1, wherein the first optical reflector is selected from a retro reflector, a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cat eye retroreflector.

3. The interferometer of one of the previous claims, wherein the reference path is directed towards a second optical reflector and back to the beam splitter, then towards the translating reflector and back to the beam splitter.

4. The interferometer system of claim 1 or 2, wherein the beam splitter comprises a first polarizing beam splitter and a second polarizing beam splitter, wherein the first polarizing beam splitter is configured to reflect the first portion of the input beam to follow the measurement path and to reflect the second portion of the input beam to follow the reference path, wherein the measurement path is directed towards the first optical reflector and back to the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter, and wherein the reference path is directed towards the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter.

5. The interferometer system of one of the previous claims, wherein the translating reflector is selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.

6. The interferometer system of claim 5 referring to claim 3, comprising: a first quarter wave plate arranged between the beam splitter and the first optical reflector; and a second quarter wave plate arranged between the beam splitter and the second optical reflector.

7. The interferometer system of one of the previous claims, wherein the input beam comprises radiation of at least two distinct frequencies.

8. The interferometer system of one of the previous claims, wherein the beam spitter is adapted to polarize the first portion to a first polarization, and the second portion to a second polarization other than the first polarization.

9. The interferometer system of one of the previous claims, wherein the movable measurement target is a substrate support, a patterning device support or an optical element of a projection system of a lithographic apparatus.

10. Lithographic apparatus or measurement tool or inspection tool, comprising at least one interferometer system according to one of the previous claims.

11. Method comprising the steps of: providing an input beam of radiation to a polarizing beam splitter, using the beam splitter to reflect a first portion of the input beam to follow a measurement path and to reflect a second portion of the input beam to follow a reference path, wherein the measurement path is directed towards a first optical reflector mounted on a movable measurement target and back to the beam splitter, then towards a translating reflector and back to the beam splitter, and wherein the reference path is directed towards the translating reflector and back to the beam splitter.

12. The method of claim 11, wherein the first optical reflector is selected from a retro reflector, a solid corner cube retroreflector, a hollow corner cube retroreflector, a diffractive retroreflector, and a cat eye retroreflector; and / or wherein the translating reflector is selected from a reversal prism, a periscope prism, and a reflective assembly comprising an odd number of at least three mirrors.

13. The method of claim 11 or 12, wherein the reference path is directed towards a second optical reflector and back to the beam splitter, then towards the translating reflector and back to the beam splitter.

14. The method of claim 11 or 12, wherein the beam splitter comprises a first polarizing beam splitter and a second polarizing beam splitter, the method comprising the steps of: using the first polarizing beam splitter to reflect the first portion of the input beam to follow the measurement path and to reflect the second portion of the input beam to follow the reference path, wherein the measurement path is directed towards the first optical reflector and back to the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter, and wherein the reference path is directed towards the second polarizing beam splitter, then towards the translating reflector and back to the second polarizing beam splitter.

15. Projection optical system comprising at least one interferometer system according to one of the previous claims 1-9.

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