An optical filter, a heterodyne interferometer system comprising the filter, and a method for filtering an input beam for a heterodyne interferometer

Abstract

A heterodyne interferometer system comprises: a first light source providing a first laser beam having a first frequency f1; a second light source providing a second laser beam having a second frequency f2 which differs from the first frequency; a combiner for polarizing the first and second laser beams and for providing an input beam wherein the polarized first and second laser beams are combined; an optical resonator cavity comprising an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, the cavity being adapted for receiving the input beam and providing an output beam comprising a first spatial mode having the first frequency f1 and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization; and a heterodyne interferometer for receiving the output beam.

Description

AN OPTICAL FILTER, A HETERODYNE INTERFEROMETER SYSTEM COMPRISING THE FILTER, AND A METHOD FOR FILTERING AN INPUT BEAM FOR A HETERODYNE INTERFEROMETERCROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of EP application 23196954.4 which was filed on 12 September, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a system and method for mode cleaning heterodyne interferometer input. The invention also relates to a lithographic apparatus including the system, and to projection systems for optical lithography systems including the 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.Interferometers are also used in projection systems for optical lithography systems, and in other exposure apparatus. 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 performance of plane mirror interferometers typically depend on the laser beam quality of the input. A range of causes can negatively impact the laser beam quality of the input, leading to residual errors in the interferometer output due to beam quality issues of the input.

[0007] Conventional technology has some problems. In case of heterodyne interferometers, using two laser beams having different frequencies, problems may include one or more of, for instance:Collinearity and cobore (a position difference) errors of the two combined laser beams coming out of the remote optical combiner lead to reduced ac / dc ratio. Eventually, this leads to a reduced signal to noise ratio and higher noise in general.Polarization clocking errors lead to a larger cyclic error with larger residuals after SW correction.Wavefront errors lead to interferometer errors.Intensity noise of the laser at the heterodyne frequency, i.e. the frequency difference between the two laser beams, leads to phase errors. If the typical power of the interferometers is relatively low, shot noise dominates. For higher power lasers however, intensity noise becomes an important factor.High frequency phase noise can lead to interferometer errors.Drift of components in remote optical combiners lead to suboptimal alignment of the interferometers.Replacement of the remote optical combiner causes extra downtime because all interferometers need to be realigned.Wavelength fluctuations lead to interferometer errors due to the highly asymmetric arm lengths of the interferometer.

[0008] The article "High-extinction-ratio resonant cavity polarizer for quantum-optics measurements", Saraf et al., APPLIED OPTICS Vol. 46, No. 18, 20 June 2007, pages 3850 to 3855, describes, see Figure 4, a quantum-noise measuring system of a Nd:YAG free-space saturated amplifier. A high-power beam saturates the gain of a 100 W class slab amplifier and is separated from the precisely overlapped probe beam after the amplifier. Three Fabry-Perot cavities in the setup function as spatial and spectral filters and resonant polarizers. This allows generation of a single- spatial-mode and single-polarization shot-noise-limited probe beam, precision overlap of the high power and probe beam, and finally, precision separation of the beams after amplification.

[0009] The PhD Thesis "Signal Extraction and Optical Design for an Advanced Gravitational Wave Interferometer", James E. Mason, California Institute of Technology, 2001, describes a signal extraction scheme in section 3.2.4. Herein, a single mode cleaner is proposed capable of passing two sideband frequencies of a certain carrier frequency.

[0010] CN105571516A discloses a heterodyne interferometer wherein two beams of light of different frequencies are combined through a beam splitter prism and the combined light beam is filtered through a spatial filter.

[0011] US2003147083A1 discloses a scanning interferometric near-field confocal microscopy system that can use heterodyne interference. The system comprises an input beam that can have two frequencies, a spatial filter in the shape of a pin hole and an electronic processor at the detector to analyse signals from multiple polarizations.

[0012] Although the systems and methods disclosed in the publications referenced above may generally provide satisfactory results for their respective purpose as stated, these systems fail to solve at least some of the challenges relating to heterodyne laser input as stated above.

[0013] The present disclosure aims to provide an improved heterodyne interferometer system having improved input beam quality.SUMMARY

[0014] The disclosure provides an optical filter for a heterodyne interferometer, the optical filter comprising: an optical resonator cavity adapted to receive an input beam comprising radiation having at least a first frequency fl and a second frequency f2 which differs from the first frequency, an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, wherein the optical resonator cavity is configured to provide an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization.

[0015] In an embodiment, the optical resonator cavity comprises length adjustment means able to adjust a length of the cavity.

[0016] In an embodiment, the length adjustment means comprise one or more of: a piezo actuator and a heater.

[0017] In an embodiment, the filter comprises frequency adjustment means for adjusting the first frequency and / or the second frequency.

[0018] In an embodiment, the first polarization is an s-polarization and the second polarization is a p-polarization.

[0019] In an embodiment, the odd number of mirrors comprises:- a first mirror having a partial transmission and reflectivity, the first mirror being adapted for receiving the input beam;- a second mirror having substantially the same partial transmission and reflectivity as the first mirror, the second mirror being adapted for providing the output beam; and- at least one third mirror having substantially full reflectivity.

[0020] The first mirror and the second mirror may have the same composition.

[0021] According to another aspect, the disclosure provides a heterodyne interferometer system, comprising: a first light source for providing a first laser beam having a first frequency fl; a second light source for providing a second laser beam having a second frequency f2 which differs from the first frequency; a combiner for polarizing the first laser beam and the second laser beam and for providing an input beam wherein the polarized first laser beam and the polarized second laser beam are combined; an optical resonator cavity comprising an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, the optical resonator cavity being adapted for receiving the input beam and for providing an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization; and a heterodyne interferometer for receiving the output beam.

[0022] According to another aspect, the disclosure provides a lithographic apparatus, comprising an optical filter according to the invention.

[0023] According to another aspect, the disclosure provides a lithographic apparatus, comprising a heterodyne interferometer system according to the invention.

[0024] According to another aspect, the disclosure provides a projection system for an optical lithography system, comprising an optical filter according to the invention.

[0025] According to another aspect, the disclosure provides a projection system for an optical lithography system, comprising a heterodyne interferometer system according to the invention.

[0026] According to another aspect, the disclosure provides a method for filtering an input beam for a heterodyne interferometer, the method comprising the steps of: providing an optical resonator cavity adapted to receive an input beam comprising radiation having at least a first frequency fl and a second frequency f2 which differs from the first frequency, wherein an odd number of at least partially reflective mirrors is arranged in the cavity for reflecting and circulating the input beam in the cavity, using the optical resonator cavity to provide an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization.

[0027] In an embodiment, the method comprises the step of providing the output beam to the heterodyne interferometer.

[0028] In an embodiment, the method comprises the step of adjusting a length of the optical resonator cavity to have at least one mode resonant of at least the first frequency and the second frequency.

[0029] In an embodiment, the step of adjusting the length comprises at least one of activating a piezo actuator to move at least one of the mirrors, and heating the cavity.

[0030] In an embodiment, the method comprises the step of adjusting at least one of the first frequency and the second frequency to have at least one mode resonant in the cavity.BRIEF DESCRIPTION OF THE DRAWINGS

[0031] 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;Figure 2 depicts a schematic representation of an exemplary heterodyne interferometer system;Figure 3 depicts a detail of the system of Figure 2, including an embodiment of a filter according to the present disclosure; andFigure 4 depicts a detail of the system of Figure 2, including an embodiment of a filter according to the present disclosure.DETAILED DESCRIPTION

[0032] 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).

[0033] 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.

[0034] A “beam splitter” (also spelled beamsplitter) is an optical device that splits a beam of light in two.

[0035] ‘ ‘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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] A "spatial filter" is an optical device which uses the principles of Fourier optics to alter the structure of a beam of light or other electromagnetic radiation, typically coherent laser light. Spatial filtering is commonly used to clean the output of lasers, removing aberrations in the beam due to imperfect optics, or due to variations in the laser gain medium itself. Spatial filtering can be applied to transmit a pure transverse mode from a multimode laser while blocking other modes emitted from the optical resonator. The term "filtering" indicates that the desirable structural features of the original source pass through the filter, while the undesirable features are blocked.

[0042] "Spatial modes" or laser modes are wavelike properties of the beam of light that evolve while the beam passes back and forth through an amplifier of the laser source, bouncing between themirrors. The beam grows as long as the gain in the amplifier exceeds the losses within the cavity. The development of modes involves an attempt by competing portions of the light beam having slightly different frequencies, to fit an exact number of their waves into the optical cavity with the constraint that the oscillating electric field of the light beam is zero at each of the mirrors. Most laser sources have a number of modes operating simultaneously, in the form of both longitudinal and transverse modes, which give rise to a complex frequency and spatial structure within the beam in what might otherwise appear as a relatively simple, pencil-like beam of light. For instance, in a cross section of a laser light beam, different spatial modes typically have different intensity distributions. A spatial mode having a mere round intensity distribution may for instance be referred to as the "00 spatial mode".

[0043] "Cobore" as used herein refers to a position difference between two laser beams. When combining two laser beams, it is generally intended to output a combined beam wherein the two original beams propagate in combination along the exact same path or axis. When paths of respective laser beams are offset with respect to each other, the distance between the respective paths is referred to as a cobore error.

[0044] "Colinearity" relates to a direction of propagation of two or more combined laser beams. Colinear laser beams propagate in parallel, at least over a certain distance. Non-colinear laser beams propagate at an angle with respect to each other, and as a result will diverge.

[0045] " 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 senkrecht) is perpendicular to the plane of incidence.

[0046] In the description below, reference may be made to either the wavelength of a light beam or its frequency. Generally speaking, in vacuum, wavelength and frequency are related according to:

[0047] wherein is wavelength, f is frequency, and c is the speed of light. However, the wavelength becomes shorter if the wave of light moves from a first medium having a first density to a second, denser medium. The frequency of the light however does not change when the wave moves from one medium to another. The speed of the wave v is related to both the frequency f and the wavelength : v = A

[0048] Combining the above expression for velocity with the definition of index of refraction, the relationship between the wavelength A0 = c / f in vacuum and the wavelength Al = vl / f in a first medium other than vacuum, wherein nl is the refractive index of the first medium, is: cA0 f fcAl yl fvlf

[0049] When setting tuneable lasers to a selected setpoint, it is an option to determine the wavelength of the laser light while travelling through the first medium. An alternative is to tune the frequency of the laser light in the laser itself.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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).

[0054] 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.

[0055] 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 WTmay 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.

[0056] 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.

[0057] 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 IF, 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.

[0058] 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.

[0059] Figure 2 shows an exemplary overview of an interferometer system 1. The system 1 will be described herein below to indicate utility and exemplary setup of the filter of the present disclosure. For additional technical details, options and alternative embodiments, and methods to operate and use the interferometer system 1, reference is made to US20220048119.

[0060] The interferometer system 1 comprises a first light source 2 to provide a first laser beam 4 having a first frequency. The interferometer system 1 may comprise a second light source 6 for providing a second laser beam 8 having a second frequency. Alternatively, the second laser beam may be split from the first laser beam and provided with an offset, such as a frequency offset or a phase offset.

[0061] The first light source 2 and / or the second light source 6 may be tuneable lasers, able to provide a laser beam having a tuneable or adjustable frequency.

[0062] 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.

[0063] Alternatively, one or both of the first light source 2 and the second light source 6 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.

[0064] The system 1 may comprise a recombiner 10 for combining the first laser beam 4 and the second laser beam 8. The combiner 10 is also known as a remote optical combiner (ROC). The first laser beam and the second laser beam may propagate along the same path(s) after being combined in the recombiner 10. The recombiner 10 may be, for instance, a Rochon prism. The combiner 10 may be designed or adapted to polarize the first laser beam 4 and / or the second laser beam 8. Herein, in a preferred embodiment, the combiner 10 may be adapted to provide an output wherein the first laser beam has a first polarization and wherein the second laser beam has another, second polarization. The first and second polarizations may typically be selected from s-polarization and p-polarization. So, the combiner 10 may combine as well as polarize the first laser beam 4 and the second laser beam 8.

[0065] One or both of the first and second beams of radiation may be guided towards one or both of a first interferometer 20 and / or a second interferometer 30. The first interferometer 20 may be set up to measure a location or movement of a moveable target 22 having a reflective surface 24. The first interferometer may be used to measure the length Lx, or a change thereof, of a first axis 26. The second interferometer 30 may be a wavelength tracker, having a fixed target 32 provided with a second reflective surface 34. Herein, a second axis 36 may have a fixed length Lref which can be used as a reference. Other elements of the system 1 may include, but are not limited to, detector means 40 including one or more sensors 42, 44, 46, a controller 50, and one or more beam splitters.

[0066] The present disclosure provides an optical filter 60. The filter 60 may be adapted for spatial filtering of the input to the interferometer system 1. The optical filter 60 may be included, for instance, at one of the locations as indicated in Figure 2. Both options are described in more detail below with reference to Figures 3 and 4 respectively.

[0067] Generally referring to Figure 3, the optical filter 60 may be included in the input section of a heterodyne interferometer system. The optical filter 60 comprises an optical resonator cavity 62 adapted to receive an input beam 64. The input beam comprises radiation having at least a first frequency fl and a second frequency f2. Herein, typically the second frequency differs from the first frequency fl . In addition, the radiation at the first frequency may have a first polarization. The radiation at the second frequency may have a second polarization which is different than the first polarization.

[0068] In a practical embodiment, the first frequency and the second frequency may be in the order of hundreds of terahertz. The frequency difference 5f = (f2 - fl) may be in the range of megahertz, for instance 5f may be about 1 to 50 MHz.

[0069] As described above with reference to Figure 2, the input beam 64 may comprise a combination of the first laser beam 4 and the second laser beam 8.

[0070] The resonator cavity 62 comprises an odd number of at least partially reflective mirrors 66, 68, 70. The mirrors are arranged in the cavity 62 for reflecting and circulating the input beam 64 in the cavity. In a practical embodiment, the cavity comprises three mirrors. The skilled artisan will recognise that any odd-number of mirrors exceeding three may be used. The cavity may comprise, for instance, five mirror, or seven mirrors. Due to the odd number of mirrors, the cavity is able to resonate and provide an output for polarized light.

[0071] The optical resonator cavity 64 is configured to provide an output beam 72. The output beam may comprise a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization. The second polarization may differ from the first polarization. The first polarization may be an s-polarization. The second polarization may be a p-polarization.

[0072] The mirrors 66 and 68 may have partial transmission. Herein, a part of the input beam 64 passes the first mirror 66 and is reflected inside the cavity 62 via the other mirrors. The second mirror 68 is also partially transmissive and allows a part of the light resonating inside the cavity to pass and continue as output beam 72. In a preferred embodiment, the two mirrors 66, 68 have substantially the same partial transmission and reflectivity. For instance, the two mirrors 66 and 68 may be fabricated during the same production run, cut from the same base material, and / or include the same coatings. The coatings are, preferably, applied during the same coating procedure and at the same time. Thus, the first mirror 66 and the second mirror 68 may the same composition. The third mirror 70 and any further mirrors (not shown) may have substantially full reflectivity, or at least maximum reflectivity as enabled with available technology. The mirrors may have a reflectivity of exceeding 90%. The first and second mirror 66, 68 may have a transmissivity in the range of 0.5 to 10%, for instance in the order of 1%.

[0073] The optical resonator cavity 62 may be provided with length adjustment means 74, which are able to adjust a length Lcavof the resonator cavity 62. The length of the cavity herein impacts theround trip length L. The round trip length, or length L, determines the number of wavelengths fitting inside the cavity and thus which wavelengths or frequencies resonate. This will be explained in more detail below.

[0074] The length adjustment means 74 may comprise, for instance, an actuator 76 (Fig. 3), such as a piezo actuator. The actuator can be connected to one or more of the mirrors, for instance to the third mirror 70. Activation of the actuator can move the respective mirror, and thereby adjust the length L. The length adjustment means 74 may include a heater 78 (Fig. 4) for heating the cavity 62 and thereby adjusting the length Lcavof the cavity 62.

[0075] Herein below operation and functionality of the optical filter 60 will be described in more detail.

[0076] The optical filter 60 constitutes a Fabry-Perot resonator. Unlike conventional resonators however, the filter 60 of the present disclosure is adapted to receive, and provide an output, comprising at least two different frequencies, each having a different polarization. The optical resonator or cavity 62 comprises an odd number, for instance at least three, highly reflecting mirrors. As the polarization of reflected light shifts at each reflection, the odd number of mirrors results in polarized light at the output. At least two of the mirrors have some small transmissivity, as explained above. The transmission through the resonator cavity 62 exhibits relatively sharp resonances and is very small between those. The distance between resonant peaks is referred to as the Free Spectral Range (FSR). The FSR can be expressed as a length (between wavelength) or a frequency range (between resonant peaks in the frequency domain). The resonance frequencies can be tuned, for instance, by changing the cavity length Lcavusing the length adjustment means 74. In effect, the adjustment means 74 may adjust the mirror distance of at least the third mirror 70 with respect to the other mirrors 66, 68.

[0077] For heterodyne operation, a substantial circulating optical power in the resonator cavity 62 is possible if the input wave 64 has at least two optical frequencies each close to a resonance frequency of the resonator cavity 62. Alternatively worded, the input beam 64 has at least two optical wavelengths close to one of the resonance wavelengths of the resonator cavity 62. Although the input frequencies fl and f2 differ (by 5f), the length of the cavity 62 significantly exceeds the corresponding first wavelength i and second wavelength z.2as included in the input beam 64. As a result, it is possible to adjust the length of the cavity Lcavto match both a multitude of the first wavelength i and second wavelength X2, resulting in a standing wave and resonance for both the first wavelength i and second wavelength z.2simultaneously.

[0078] In resonance, the contribution of the input wave leaking through the input mirror 66 adds constructively to the circulating wave. And, there is destructive interference for the reflected field. The input field reflected at the input mirror 66 is cancelled by the field leaking out of the resonator. Therefore, there is effectively no reflection in resonance. In anti-resonance, the circulating field isquite weak, and most radiation is reflected at the input mirror. The weak field leaking out of the resonator towards the input source adds constructively to the reflected field.

[0079] The invention is to transmit the laser beam 64 coming out of the remote optical combiner (ROC) 10 through an impedance matched, planar optical cavity with an odd number of reflections per roundtrip. The optical cavity 62 acts as a mode cleaner. The mode cleaning effect is described below.

[0080] The difference of the filter 60 and the related method described herein, compared to a conventional mode cleaner, is that usually only a single frequency laser beam is (and can be) passed through the cavity, giving the mode cleaning effect.

[0081] By choosing a cavity with an odd number of reflections, the s-polarized and p-polarized fundamental mode frequencies are separated, due to the pi phase shift difference between s- polarization and p-polarization at non-normal incidence reflection.

[0082] The free spectral range of the optical cavity 62 is FSR = c / L, wherein c is the speed of light, and L is the roundtrip length of optical cavity. Radiation having an s-polarization or a p-polarization is usually separated by half a FSR.

[0083] In a practical embodiment, for instance for a heterodyne frequency difference 5f of about 15 MHz the roundtrip length L may be, for instance, about 10 m. Herein, the FSR would be about 30MHz. In practice, this is feasible, the interferometers 20, 30 may have a total length of the order of 8m.

[0084] To set the resonance of the cavity 62, it is possible, for instance, to lock the cavity 62 to the laser frequencies and / or to adjust the laser frequencies to the round trip length L. To adjust the length L, the length adjustment means 74 can be used. Adjusting the laser frequency can be done, for instance, using an appropriate tuneable laser source and corresponding locking scheme.

[0085] Radiation is resonant in the cavity 62 is an integer number of wavelengths fits in the round trip length L. Alternatively worded, a laser beam is resonant in the optical cavity 62 if the round-trip phase is a multiple of 2pi. Round trip phase herein is the phase difference with respect to the phase after passage of the first mirror 66 of the radiation after traversing the entire length L.

[0086] It is an aim to have both the first frequency fl and the second frequency f2 as included in the input beam 64 resonant at the same time. In a practical embodiment, frequencies fl and f2 differ in frequency and in polarization. In practice, the input beams may have been polarized in the remote optical combiner 10.

[0087] Furthermore, in a preferred embodiment, the cavity has different resonances for the two polarization states. The optical filter 60 provides a polarization filter effect, see above, due to the odd count of mirrors. Due to an odd number of mirrors, one of two polarization states will be substantially cancelled, providing a polarized output in the output beam 72. In a practical embodiment, the resonance frequencies are different for the two beams, one in s-polarization and the other in p- polarization. Thus, the filter 60 provides a polarization filtering effect. The filter 60 comprises an odd number of mirrors of at least three. As a result, the filter provides substantially a pi phase shiftbetween the s-polarization and p-polarization and the resonance frequencies between the s- polarization and p-polarization are shifted by about half the FSR (FSR / 2).

[0088] In a practical embodiment, effects that cause a different phase delay for fl and f2 are limited or preferably obviated. A common phase delay for fl and f2 is less relevant.

[0089] The cavity length causes a different phase delay of fl and f2 because they have a different frequency and therefore wavelength. The phase due to the round trip length through the cavity is 2pi*L*fl / c for the first frequency fl and 2pi*L*f2 / c for the second frequency f2. The difference is therefore L*2pi / c*(fl-f2).

[0090] The mirror coatings may cause different phase delays for the different polarization states (assuming non-normal incidence). This comes from the Fresnel equations. S-polarized light has a sign flip (extra pi phase) compared to p-polarized light. This phase delay of pi may be relatively strongly influenced by the coating design. To optimize the filter 60, the differential phase delay can be specified as requirement for the mirror coatings. For simplicity, we assume a pi phase shift. The phase shift may be different however. Whatever the value of the phase shift at a respective mirror with a particular composition and coating, said value(s) are preferably taken into account for calculating the round trip length L. For example, if the coating design is much better at pi + 0.1, then this would be also fine. Let’s call the differential phase delay dpi, dp2, dp3 for the three mirrors of the cavity. For instance, if the coatings have a different phase shift, this can compensated by adjusting the cavity length.

[0091] The filter 60 can be set up as follows. First, the round trip length L and / or the first frequency is adjusted until the first frequency fl is resonant in the cavity 62. Herein, the round-trip phase delay for the first frequency is a multiple of 2pi. This can for instance be achieved by changing the laser frequency of the first light source 2. If so, the second frequency f2 will also be resonant if the difference phase delay is a multiple of 2pi. I.e., L*2pi / c*(fl-f2) + dpi + dp2 + dp3 = n*2pi. The parameters L, dpi, dp2, dp3 can be designed and selected, such that this equation is true. As an example, in a practical embodiment: With dpl=dp2=dp3=pi, (fl - f2) = 15MHz, c = 3e8 m / s, n = 2, the round trip length is L=c / (2*15MHz) = 10 m.

[0092] A practical approach to design the filter 60 may include the steps of: Select dpi, dp2, dp3. Manufacture the mirrors. Measure the actual phase delay caused by the mirrors. Calculate L using the steps provided above. And manufacture the filter 60 and its cavity 62 to provide the round trip length L.

[0093] Tolerances may be compensated by making L adjustable and / or to allow an adjustable f 1 -f2 frequency difference.

[0094] To have an impedance matched cavity, the first mirror 66 and the second mirror 68 preferably have the same transmission and reflectivity. In a practical embodiment, the first mirror and the second mirror are produced at the same time and during the same process. This may include producing both mirrors from the same coating run to reduce manufacturing tolerances. The thirdmirror preferably has relatively high reflection, to minimize losses. Thus, an impedance matched cavity can be provided with limited power loss.

[0095] Reflectivity of the mirrors will be different for s- polarized and p-polarized radiation. This effect may be ignored. The higher reflectivity (usually s-polarization) can be limited for the input and output mirrors in order to limit sensitivity for cavity losses. In a practical embodiment, the cavity finesse can be selected at around 100. The latter would allow a few percent losses.

[0096] The angle of incidence need to be considered in the coating design.

[0097] In a practical embodiment, the filter 60 comprises planar cavity 62. 1.e. the beam path inside the cavity 62 is planar. For a three mirror cavity this is inherent. For more than three mirrors, such as five or seven, the beam path may be non-planar. The out-of-plane reflection will rotate the polarization state. If there is a reason to choose an out-of-plane configuration, this needs to be taken into account for the round-trip phase.

[0098] The optical filter of the present disclosure provides the advantages as described below.

[0099] Only one spatial mode can be resonant in the optical cavity 62. Therefore, frequencies fl and f2 would have the exact same spatial profile after transmission through the optical filter 60. This means perfect collinearity and perfect cobore.

[0100] The polarization clocking of the transmitted beam 72 is defined by the plane of the optical cavity 62. Since the cavity 62 is a relatively large in comparison to the cross section of the input beam 64, good accuracies should be easy to achieve. The filter 60 will provide, for instance, significantly better results than the relatively small combiner crystals as typically included in the remote optical combiners, such as combiner 10. Consequently, the polarization clocking error is reduced.

[0101] It may be possible to use two mode cleaning cavities arranged in parallel. However, if two separate cavities instead of one are used, at least the above two advantages would not work. These advantages really rely on the use of a single cavity for mode cleaning of at least two frequencies simultaneously.

[0102] Since only one spatial mode is resonant in the cavity 62, any wavefront distortions due to optical fibres and outcoupling optics are strongly suppressed. The transmitted mode will have a much better wavefront quality than the input beam 64 and the wavefront quality will be substantially equal for both frequencies fl and f2. In a practical embodiment, the system of the disclosure is adapted such that the output beam 72 comprises the 00 spatial mode for at least one or both of the frequencies fl and 12. The latter can be achieved by, for instance, one or more of adjusting the respective frequencies, adjusting the cavity length, adjusting coatings, adjusting the polarization of respective input beams 4, 8.

[0103] The optical cavity acts as low pass filter for intensity fluctuations. Therefore intensity fluctuations of the laser beams 4, 8 will be suppressed. The optical cavity 62 also acts as low pass filter for phase fluctuations.

[0104] Since the spatial mode of the cavity 62 is only given by the relative position of the cavity mirrors, any beam pointing or position drift in front of the cavity will be strongly suppressed in the transmitted beam.

[0105] The cavity can act as reference for the spatial mode of the laser beam. If the remote optical combiner 10 needs to be replaced, for instance because an optical fibre was damaged, differences between respective optical combiners will be filtered out by the optical cavity. The cavity 62 acts as isolator for laser beam parameters. Therefore, the optical filter obviates re-alignment of the interferometers 20, 30 downstream of the cavity, because the output beam 72 has not changed.

[0106] The optical filter 60 can have a very similar function as a wavelength tracker. The length of the cavity, or the round trip length L, can act as length reference.

[0107] In theory, all the issues listed in the introduction can be solved in many different ways. However, the optical filter 60 described herein is especially attractive because it combines so many improvements within a single component.

[0108] 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.

[0109] 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.

[0110] 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.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 optical filter for a heterodyne interferometer, the optical filter comprising: an optical resonator cavity adapted to receive an input beam comprising radiation having at least a first frequency fl and a second frequency f2 which differs from the first frequency,an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, wherein the optical resonator cavity is configured to provide an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization.2. The filter of clause 1, the optical resonator cavity comprising length adjustment means able to adjust a length of the cavity.3. The filter of clause 2, the length adjustment means comprising one or more of: a piezo actuator and a heater.4. The filter of clause 1, comprising frequency adjustment means for adjusting the first frequency and / or the second frequency.5. The filter of clause 1, wherein the first polarization is an s-polarization and the second polarization is a p-polarization.6. The filter of clause 1, the odd number of mirrors comprising:- a first mirror having a partial transmission and reflectivity, the first mirror being adapted for receiving the input beam;- a second mirror having substantially the same partial transmission and reflectivity as the first mirror, the second mirror being adapted for providing the output beam; and- at least one third mirror having substantially full reflectivity.7. The filter of clause 6, the first mirror and the second mirror having the same composition.8. A heterodyne interferometer system, comprising: a first light source for providing a first laser beam having a first frequency fl; a second light source for providing a second laser beam having a second frequency f2 which differs from the first frequency; a combiner for polarizing the first laser beam and the second laser beam and for providing an input beam wherein the polarized first laser beam and the polarized second laser beam are combined; an optical resonator cavity comprising an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, the optical resonator cavity being adapted for receiving the input beam and for providing an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization; and a heterodyne interferometer for receiving the output beam.9. A lithographic apparatus, comprising an optical filter according to any of clauses 1-7.10. A lithographic apparatus, comprising a heterodyne interferometer system according to clause 8.11. A projection system for an optical lithography system, comprising an optical filter according to any of clauses 1-7.12. A projection system for an optical lithography system, comprising a heterodyne interferometer system according to clause 8.13. A method for filtering an input beam for a heterodyne interferometer, the method comprising the steps of: providing an optical resonator cavity adapted to receive an input beam comprising radiation having at least a first frequency fl and a second frequency f2 which differs from the first frequency, wherein an odd number of at least partially reflective mirrors is arranged in the cavity for reflecting and circulating the input beam in the cavity, using the optical resonator cavity to provide an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization.14. The method of clause 13, comprising the step of providing the output beam to the heterodyne interferometer.15. The method of clause 13, comprising the step of adjusting a length of the optical resonator cavity to have at least one mode resonant of at least the first frequency and the second frequency.16. The method of clause 15, the step of adjusting the length comprising at least one of activating a piezo actuator to move at least one of the mirrors, and heating the cavity.17. The method of one of clauses 13 to 16, comprising the step of adjusting at least one of the first frequency and the second frequency to have at least one mode resonant in the cavity.

Claims

CLAIMS1. An optical filter for a heterodyne interferometer, the optical filter comprising: an optical resonator cavity adapted to receive an input beam comprising radiation having at least a first frequency fl and a second frequency f2 which differs from the first frequency, an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, wherein the optical resonator cavity is configured to provide an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization.

2. The filter of claim 1, the optical resonator cavity comprising length adjustment means able to adjust a length of the cavity.

3. The filter of claim 2, the length adjustment means comprising one or more of: a piezo actuator and a heater.

4. The filter of claim 1, comprising frequency adjustment means for adjusting the first frequency and / or the second frequency.

5. The filter of claim 1, wherein the first polarization is an s-polarization and the second polarization is a p-polarization.

6. The filter of claim 1, the odd number of mirrors comprising:- a first mirror having a partial transmission and reflectivity, the first mirror being adapted for receiving the input beam;- a second mirror having substantially the same partial transmission and reflectivity as the first mirror, the second mirror being adapted for providing the output beam; and- at least one third mirror having substantially full reflectivity.

7. The filter of claim 6, the first mirror and the second mirror having the same composition.

8. A heterodyne interferometer system, comprising: a first light source for providing a first laser beam having a first frequency fl; a second light source for providing a second laser beam having a second frequency f2 which differs from the first frequency;a combiner for polarizing the first laser beam and the second laser beam and for providing an input beam wherein the polarized first laser beam and the polarized second laser beam are combined; an optical resonator cavity comprising an odd number of at least partially reflective mirrors arranged in the cavity for reflecting and circulating the input beam in the cavity, the optical resonator cavity being adapted for receiving the input beam and for providing an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization; and a heterodyne interferometer for receiving the output beam.

9. A projection system for an optical lithography system, comprising an optical filter according to any of claims 1-7.

10. A projection system for an optical lithography system, comprising a heterodyne interferometer system according to claim 8.

11. A method for filtering an input beam for a heterodyne interferometer, the method comprising the steps of: providing an optical resonator cavity adapted to receive an input beam comprising radiation having at least a first frequency fl and a second frequency f2 which differs from the first frequency, wherein an odd number of at least partially reflective mirrors is arranged in the cavity for reflecting and circulating the input beam in the cavity, using the optical resonator cavity to provide an output beam comprising a first spatial mode having the first frequency fl and a first polarization and a second spatial mode having the second frequency f2 and a second polarization which differs from the first polarization.

12. The method of claim 11, comprising the step of providing the output beam to the heterodyne interferometer.

13. The method of claim 11, comprising the step of adjusting a length of the optical resonator cavity to have at least one mode resonant of at least the first frequency and the second frequency.

14. The method of claim 13, the step of adjusting the length comprising at least one of activating a piezo actuator to move at least one of the mirrors, and heating the cavity.

15. The method of one of claims 11 to 14, comprising the step of adjusting at least one of the first frequency and the second frequency to have at least one mode resonant in the cavity.

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