Optical filter, heterodyne interferometer system comprising the filter and method for filtering an input beam of a heterodyne interferometer

Abstract

A heterodyne interferometer system comprising: 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 different 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 reflecting mirrors arranged in the cavity for reflecting the input beam and circulating it in the cavity, the cavity being adapted to receive the input beam and provide 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 different from the first polarization; and a heterodyne interferometer for receiving the output beam.

Description

Cross-references to related applications

[0001] This application claims priority to European Patent Application No. 23196954.4, filed on September 12, 2023, which is incorporated herein by reference in its entirety. Technical Field

[0002] This invention relates to a system and method for pattern-cleaning heterodyne interferometer inputs. The invention also relates to a photolithography apparatus including the system, and a projection system for an optical photolithography system including the system. Background Technology

[0003] A lithography apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithography apparatus can be used in the manufacture of integrated circuits (ICs). For instance, a lithography apparatus may project a pattern (also commonly referred to as a “design layout” or “design”) from a patterning device (such as a mask) onto a layer of radiation-sensitive material (such as a photoresist) provided on a substrate (such as a wafer).

[0004] For decades, as semiconductor manufacturing processes have continued to advance, the size of circuit elements has steadily decreased, while the number of functional components (such as transistors) in each device has steadily increased, following a trend commonly known as "Moore's Law." To keep pace with Moore's Law, the semiconductor industry is working to develop technologies that can produce continuously shrinking features. To project patterns onto a substrate, photolithography apparatuses may use electromagnetic radiation. The wavelength of this radiation determines the minimum feature size that can be patterned on the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Photolithography apparatuses using extreme ultraviolet (EUV) radiation (with wavelengths in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm) may be used to form even smaller features on the substrate than those using radiation with a wavelength of, for example, 193 nm.

[0005] Interferometers are used for various measurements in lithography apparatuses and other equipment related to semiconductor manufacturing processes, including metrology. Interferometers are also used in projection systems of optical lithography systems and other exposure apparatuses. For example, wavelength trackers may be used to control the stability of laser beam input, while one or more phase-tracking interferometers may be used to control the position of movable devices such as wafer stages with precision up to the nanometer scale.

[0006] The performance of a plane mirror interferometer typically depends on the quality of the input laser beam. A number of factors can adversely affect the quality of the input laser beam, leading to residual errors in the interferometer output due to beam quality issues.

[0007] Known techniques have several problems. In the case of heterodyne interferometers, using two laser beams with different frequencies, the problems may include one or more of the following: Collinearity and coaxiality (positional deviation) errors of the two combined radiation beams from the remote optical combiner result in a reduced AC / DC ratio. Ultimately, this typically leads to a decreased signal-to-noise ratio and higher noise. - Polarization clock errors lead to larger periodic errors and larger residual errors after SW correction. - Wavefront error leads to interferometer error. Intensity noise at the heterodyne frequency (i.e., the frequency difference between two laser beams) causes phase errors. If the typical power of the interferometer is relatively low, shot noise is dominant. However, for higher power lasers, intensity noise becomes a significant factor. High-frequency phase noise can cause interferometer errors. - Drifting of components in the remote optical combiner leads to suboptimal alignment of the interferometer. The replacement of the remote optical combiner resulted in additional downtime because all interferometers needed to be realigned. -Due to the asymmetry in the arm length and height of the interferometer, wavelength fluctuations can cause interferometer errors.

[0008] Saraf et al. APPLIED OPTICS (Applied Optical Components) (Vol. 46, No. 18, June 20, 2007, pp. 3850-3855) High-extinction-ratio resonant cavity polarizer for quantum-optics measurements (High Extinction Ratio Resonant Cavity Polarizer for Quantum Optical Component Measurement) describes a quantum noise measurement system for an Nd:YAG free-space saturated amplifier (see...) Figure 4 The high-power beam saturates the gain of a 100W-class planar amplifier and separates it from the precisely coincident probe beam after amplification. Three Fabry-Perot cavities in the setup serve as spatial and spectral filters, as well as resonant polarizers. This allows for the generation of a single spatial mode and a single polarization, shot-noise-limited probe beam, precise coincidence of the high-power beam with the probe beam, and ultimately, precise separation of the beam after amplification.

[0009] James E. Mason's doctoral dissertation Signal Extraction and Optical Design for an Advanced Gravitational Wave Interferometer The signal extraction scheme in Section 3.2.4 of (Signal Extraction and Optical Design of Advanced Gravitational Wave Interferometers) (Caltech, 2001) is described. In this paper, a single-mode cleaner capable of allowing two sideband frequencies of a given carrier frequency to pass through is proposed.

[0010] CN105571516A discloses a heterodyne interferometer in which two beams of different frequencies are combined and passed through a beam splitter prism, and the combined beams are filtered by a spatial filter.

[0011] US2003147083A1 discloses a scanning interferometric near-field confocal microscopy system that can use heterodyne interferometry. The system includes: an input beam that may have two frequencies, a pinhole-shaped spatial filter, and an electronic processor at the detector to analyze signals from multiple polarizations.

[0012] While the systems and methods disclosed in the literature referenced above may generally provide satisfactory results for their respective purposes, these systems cannot address at least some of the challenges associated with heterodyne laser inputs described above.

[0013] This disclosure aims to provide an improved heterodyne interferometer system with improved input beam quality. Summary of the Invention

[0014] This 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 f1 and a second frequency f2 different from the first frequency; An odd number of mirrors, each capable of at least partial reflection, are arranged in the cavity to reflect the input beam and circulate it within the cavity; The optical resonator cavity is configured to provide an output beam, which includes a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization different from the first polarization.

[0015] In one embodiment, the optical resonator cavity includes a length adjustment device capable of adjusting the length of the cavity.

[0016] In an embodiment, the length adjustment device includes one or more of the following: a piezoelectric actuator and a heater.

[0017] In one embodiment, the filter includes a frequency adjustment device for adjusting a first frequency and / or a second frequency.

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

[0019] In an embodiment, an odd number of reflectors includes: A first reflector having partial transmittance and reflectance is adapted to receive an input beam. A second reflector, having substantially the same partial transmittance and reflectance as the first reflector, is adapted to provide an output beam; and At least one third reflecting mirror has substantially total reflectivity.

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

[0021] According to another aspect, this disclosure provides a heterodyne interferometer system, comprising: A first light source is used to provide a first laser beam having a first frequency f1; The second light source is used to provide a second laser beam with a second frequency f2 that is different from the first frequency; A combiner for polarizing a first laser beam and a 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 includes an odd number of at least partially reflective mirrors arranged within the cavity for reflecting an input beam within the cavity and circulating it within the cavity. The optical resonator cavity is adapted to receive the input beam and to provide an output beam, the output beam including a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization different from the first polarization. A heterodyne interferometer is used to receive the output beam.

[0022] According to another aspect, this disclosure provides a photolithography apparatus including an optical filter according to the invention.

[0023] According to another aspect, this disclosure provides a photolithography apparatus including a heterodyne interferometer system according to the invention.

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

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

[0026] According to another aspect, this disclosure provides a method for filtering an input beam for a heterodyne interferometer, the method comprising the following steps: An optical resonator cavity is provided, which is adapted to receive an input beam comprising radiation having at least a first frequency f1 and a second frequency f2 different from the first frequency, wherein an odd number of at least partially reflective mirrors are arranged in the cavity for reflecting the input beam in the cavity and circulating it in the cavity; An optical resonator cavity is used to provide an output beam, which includes a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization different from the first polarization.

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

[0028] In one embodiment, the method includes the step of adjusting the length of the optical resonator cavity to cause at least one mode of at least a first frequency and a second frequency to resonate.

[0029] In an embodiment, the length adjustment step includes at least one of the following: activating a piezoelectric actuator to move at least one of the reflectors; and heating the cavity.

[0030] In one embodiment, the method includes the step of adjusting at least one of a first frequency and a second frequency to cause at least one mode to resonate in the cavity. Attached Figure Description

[0031] Embodiments of the present invention will be described below by way of example only, with reference to the accompanying schematic diagrams, wherein:

[0032] Figure 1 This is a schematic overview diagram showing the photolithography apparatus;

[0033] Figure 2 This is a schematic diagram illustrating an exemplary heterodyne interferometer system;

[0034] Figure 3 Show Figure 2 Details of the system, including embodiments of the filters according to this disclosure; and

[0035] Figure 4 Show Figure 2 Details of the system, including embodiments of the filters according to this disclosure. Detailed Implementation

[0036] In the current document, the terms “radiation” and “beam” are used to cover all types of electromagnetic radiation, including deep ultraviolet radiation (e.g., with wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., with wavelengths in the range of about 5 nm to 100 nm).

[0037] The terms “mask,” “mask,” or “patterning apparatus” as used herein may be interpreted broadly to refer to a general patterning apparatus that may be used to impart a patterned cross-section to an incident radiation beam, corresponding to a pattern to be created in a target portion of a substrate. The term “optical valve” may also be used in this context. Examples of other such patterning apparatuses, besides classic masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

[0038] A beam splitter is an optical device that splits a beam of light into two.

[0039] The "index of refraction" is a numerical value calculated from the ratio of the speed of light in a vacuum to the speed of light in a second medium with a greater density. In descriptive texts and mathematical equations, the variable of refraction may be symbolized by the letter n or n'.

[0040] An interferometer, or laser interferometer, measures distance or displacement by measuring the phase difference between two beams. One beam is sent to a first reflector or surface at a fixed reference distance, and the other beam is sent to a second reflector or surface at a different 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 to the second surface changes, the phase of the combined signal also changes. The practicality of these methods lies in their ability to perform measurements over long distances while maintaining accuracy.

[0041] A "zero-difference interferometer" measures phase by comparing the intensities of two sinusoidal signals (sine and cosine). A single-frequency laser source with, for example, frequency F1 is used in the zero-difference system. The laser beam from the stationary reference path returns at frequency F1, while the beam from the (moving) measurement path returns at a Doppler-shifted frequency. When the optics are stationary, these beams interfere with each other in the detector to obtain a zero beat frequency, while the beat frequency increases as the optics move in either direction.

[0042] A heterodyne interferometer uses a dual-frequency laser source. The laser source may provide a single laser beam, which can be split into two beams, one of which is frequency-shifted to allow for heterodyne phase detection. However, the two frequencies may also be generated by two different, frequency-locked or phase-locked lasers.

[0043] In a heterodyne interferometer, the output beam from a dual-frequency laser source may contain two polarizations, one with a first frequency F1 and the other with a different second frequency F2. The beat frequency or frequency difference between the two is F2. F1. The polarization beam splitter reflects light with a first frequency F1 into the reference path. Light with a second frequency F2 passes through the beam splitter into the measurement path and strikes a moving reflector in that path, causing a Doppler frequency shift of δF in the reflected beam. This reflected beam is then combined with light at the interferometer with a frequency of F1, resulting in a Doppler shift of F2. F1 The new beat frequency δF is returned to the laser detector unit. The beat frequency of the heterodyne laser system varies with the speed of the moving reflector. When the optical element is stationary, the beat frequency is F2. F1. When the optical elements move apart, the beat frequency increases by δF; if the optical elements move together, the beat frequency decreases.

[0044] A wavelength tracker is a specific version of an interferometer designed to measure the phase difference between two reflected beams. One beam is reflected on a first fixed reflector that provides a first reference axis, and the other beam is reflected on a second fixed reflector that provides a second reference axis of a different length from the first reference axis. Because the two reflecting surfaces are fixed, the measured phase difference will only change when the wavelength of the beam changes. Therefore, a wavelength tracker allows monitoring of the deviation of the wavelength from a set point.

[0045] A spatial filter is an optical device that uses the principles of Fourier optics to alter the structure of a beam or other electromagnetic radiation (typically coherent laser). Spatial filtering is often used to clean the output of a laser, removing imperfections in the optics or aberrations caused by variations in the laser gain medium itself. Spatial filters can be applied to transmit pure transverse modes from a multimode laser while blocking other modes emitted from the optical resonator. The term "filter" implies that desired structural features of the original source pass through the filter while unwanted features are blocked.

[0046] A "spatial mode," or laser mode, is a wave-like property of a beam as it travels back and forth through an amplifier of the laser source and bounces between mirrors. Beam growth occurs as long as the gain in the amplifier exceeds the losses within the cavity. The formation of a mode involves attempting to make portions of the beam with slightly different frequencies compete to fit the exact number of their waves into the optical cavity, constrained so that the oscillating electric field of the beam is zero at each mirror. Most laser sources have multiple modes operating simultaneously, in the form of both longitudinal and transverse modes, which cause complex frequencies and spatial structures within the beam, whereas in other cases the beam may present itself as a relatively simple pencil-shaped beam. For example, different spatial modes typically have different intensity distributions in the cross-section of a laser beam. A spatial mode with only a circular intensity distribution might, for example, be called a "00 spatial mode."

[0047] As used in this article, "cobore" refers to the positional difference between two laser beams. When combining two laser beams, it is generally desirable to output a combined beam in which the two original beams propagate along exactly the same path or axis. The distance between the corresponding paths of the laser beams when they are offset relative to each other is called the cobore error.

[0048] "Collometry" refers to the propagation directions of two or more combined laser beams. Collinear laser beams propagate in parallel at least over a certain distance. Non-collinear laser beams propagate at an angle to each other, resulting in divergence.

[0049] The "polarization" of light refers to the intensity difference between the different polarization states of a laser beam. P-polarized light (derived from the German word "parallel") has an electric field that is parallel to the plane of incidence. S-polarized light (derived from the German word "senkrecht") is perpendicular to the plane of incidence.

[0050] In the following description, the wavelength or frequency of the light beam may be referenced. Generally, in a vacuum, wavelength and frequency are related according to the following formula:

[0051] in Let λ be the wavelength, f be the frequency, and c be the speed of light. However, if a light wave travels from a first medium with a first density to a second medium with a greater density, the wavelength becomes shorter. However, when a wave propagates from one medium to another, the frequency of the light does not change. The wave speed v is related to both the frequency f and the wavelength λ:

[0052] Combining the above velocity expression with the definition of refractive index, the relationship between the wavelength λ0 = c / f in vacuum and the wavelength λ1 = v1 / f in the first medium other than vacuum is as follows, where n1 is the refractive index of the first medium:

[0053] When a tunable laser is set to a selected setpoint, the wavelength of the laser as it propagates in the first medium can be determined. Another alternative is to adjust the frequency of the laser light within the laser itself.

[0054] Figure 1 A lithography apparatus LA is schematically illustrated. The lithography apparatus LA includes: an irradiation system (also called an irradiator) IL configured to modulate a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT configured to support a patterning apparatus (e.g., a mask) MA and connected to a first locator PM configured to precisely position the patterning apparatus MA according to specific parameters; a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a wafer coated with resist) W and connected to a second locator PW configured to precisely position the substrate support according to specific parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the radiation beam B by the patterning apparatus MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0055] In operation, the irradiation system IL receives a radiation beam from the radiation source SO, for example via the beam delivery system BD. The irradiation system IL may include multiple types of optical components for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The irradiator IL may be used to adjust the radiation beam B to have a desired spatial and angular intensity distribution in the cross-section of the plane in which the patterning device MA is located.

[0056] The term "projection system" (PS) as used herein should be interpreted broadly to encompass multiple types of projection systems, including refractive, reflective, catadioptric, variable, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, adapted to the type of exposure radiation used and / or other factors such as immersion or vacuum environments. Any term used herein, "projection lens," may be considered equivalent to the more general term "projection system" (PS).

[0057] like Figure 1 As shown, the lithography apparatus is of the transmission type (e.g., using a transmission mask). Alternatively, the lithography apparatus may be of the reflection type (e.g., using a programmable mirror array of one of the types mentioned above, or using a reflection mask).

[0058] A lithography apparatus LA can be of the type in which a portion of the substrate is covered by a liquid (e.g., water) with a relatively high refractive index to fill the space between the projection system PS and the substrate W—this is also known as immersion lithography. More information on immersion technology can be found in US6952253, which is incorporated herein by reference.

[0059] The lithography apparatus LA may also be of the type with two or more substrate supports WT (also known as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or subsequent exposure preparation steps may be performed on a substrate W located on one of the substrate supports WT while another substrate W on the other substrate support WT is used to expose a pattern on that other substrate W.

[0060] In addition to the substrate support WT, the lithography apparatus LA may include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning equipment. The sensors may be arranged to measure characteristics of the projection system PS or the radiation beam B. The measurement stage may hold multiple sensors. The cleaning equipment may be arranged to clean part of the lithography apparatus, such as part of the projection system PS or part of a system providing immersion solutions. The measurement stage may move below the projection system PS as the substrate support WT moves away from the projection system PS.

[0061] In operation, a radiation beam B is incident on a patterning device MA (e.g., a mask) held on a mask support MT and patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved, for example, to position different target portions C along the path of the radiation beam B at the focused and aligned positions. Similarly, a first positioner PM and possibly another position sensor (not shown in the image)... Figure 1 (As explicitly shown in the diagram) This may be used to precisely position the patterning apparatus MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the patterning apparatus MA and the substrate W. Although the substrate alignment marks P1, P2 in the diagram occupy dedicated target portions, they may also be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribing alignment marks.

[0062] To illustrate this invention, a Cartesian coordinate system is used. A Cartesian coordinate system has three axes: the x-axis, the y-axis, and the z-axis. Any one of these axes is orthogonal to the other two. A rotation about the x-axis is called an Rx rotation. A rotation about the y-axis is called an Ry rotation. A rotation about the z-axis is called an Rz rotation. The x-axis and y-axis define a horizontal plane, while the z-axis is vertical. The Cartesian coordinate system is not intended to limit the invention but is merely illustrative. Alternatively, another coordinate system (e.g., a cylindrical coordinate system) may be used to illustrate the invention. The orientation of the Cartesian coordinate system may differ, for example, by giving the z-axis a component along the horizontal plane.

[0063] Figure 2 An exemplary overview of interferometer system 1 is shown. System 1 will be described below to indicate the utility and exemplary setup of the filters of this disclosure. For additional technical details, options and alternative embodiments, and methods of operating and using interferometer system 1, see US20220048119.

[0064] Interferometer system 1 includes a first light source 2 for providing a first laser beam 4 having a first frequency. Interferometer system 1 may also include 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 a shift (such as a frequency shift or phase shift).

[0065] The first light source 2 and / or the second light source 6 may be tunable lasers, which are capable of providing laser beams with tunable or adjustable frequencies.

[0066] In this paper, adjusting the frequency of one or more laser sources may be referred to as a frequency adjustment device for adjusting a first frequency and / or a second frequency. In practice, tunable lasers are commercially available. Examples of tunable lasers include laser diodes, lasers with adjustable resonator cavity lengths, or those using adjustable optical elements to adjust the laser output.

[0067] 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 include, for example, a stabilized HeNe laser.

[0068] System 1 may include a combiner 10 for combining a first laser beam 4 with a second laser beam 8. Combiner 10 is also referred to as a remote optical combiner (ROC). The first and second laser beams may be combined in combiner 10 and propagate along the same (or multiple) paths. Combiner 10 may be, for example, a Rochon prism. Combiner 10 may be designed or adapted to polarize the first laser beam 4 and / or the second laser beam 8. In this document, in a preferred embodiment, combiner 10 may be adapted to provide an output where the first laser beam has a first polarization and the second laser beam has another second polarization. The first and second polarizations are typically selected from s-polarization and p-polarization. Therefore, combiner 10 may combine the first laser beam 4 with the second laser beam 8 and polarize the first laser beam 4 and the second laser beam 8.

[0069] One or both of the first and second radiation beams may be directed to one or both of the first interferometer 20 and / or the second interferometer 30. The first interferometer 20 may be configured to measure the position or movement of a movable target 22 having a reflective surface 24. The first interferometer may be used to measure the length L of the first axis 26. x Or its modifications. The second interferometer 30 may be a wavelength tracker having a fixed target 32 ​​with a second reflective surface 34 disposed thereon. In this document, the second axis 36 may have a fixed length L that can be used as a reference. ref Other components of System 1 may include, but are not limited to: a detector device 40 including one or more sensors 42, 44, 46; a controller 50; and one or more beam splitters.

[0070] This 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 in, for example... Figure 2 One of the locations shown. Refer to each location below. Figure 3 and Figure 4 Please describe the two options in more detail.

[0071] General Reference Figure 3An optical filter 60 may be included in the input section of a heterodyne interferometer system. The optical filter 60 includes an optical resonator cavity 62 adapted to receive an input beam 64. The input beam includes radiation having at least a first frequency f1 and a second frequency f2. Typically, the second frequency differs from the first frequency f1. Additionally, the radiation at the first frequency may have a first polarization. The radiation at the second frequency may have a second polarization different from the first polarization.

[0072] In practical implementations, the first and second frequencies may be on the order of hundreds of terahertz. The frequency difference δf = (f2 - f1) may be in the megahertz range, for example, δf may be approximately 1 to 50 MHz.

[0073] As referenced above Figure 2 The input beam 64 may include a combination of a first laser beam 4 and a second laser beam 8.

[0074] The resonator cavity 62 includes an odd number of mirrors 66, 68, and 70 that are at least partially reflective. These mirrors are arranged in the cavity 62 to reflect the input beam 64 and circulate it within the cavity. In a practical embodiment, the cavity includes three mirrors. Those skilled in the art will understand that any odd number of mirrors, more than three, may be used. The cavity may include, for example, five or seven mirrors. Due to the odd number of mirrors, the cavity is able to resonate and provide an output for polarized light.

[0075] Optical resonator cavity 64 is configured to provide output beam 72. The output beam may include a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization. The second polarization may differ from the first polarization. The first polarization may be s-polarized. The second polarization may be p-polarized.

[0076] Reflectors 66 and 68 may be partially transmissive. Herein, a portion of the input beam 64 passes through the first reflector 66 and is reflected within cavity 62 via other reflectors. The second reflector 68 is also partially transmissive and allows a portion of the light resonating within the cavity to pass through and continue as the output beam 72. In a preferred embodiment, the two reflectors 66 and 68 have substantially the same partial transmittance and reflectivity. For example, the two reflectors 66 and 68 may be manufactured in the same production batch, cut from the same substrate material, and / or include the same coating. Preferably, the coating is applied simultaneously in the same coating process. Therefore, the first reflector 66 and the second reflector 68 may have the same composition. The third reflector 70 and any other reflector (not shown) may have substantially total reflectivity, or at least the maximum reflectivity achievable with the prior art. The reflectors may have a reflectivity exceeding 90%. The first reflector 66 and the second reflector 68 may have a transmittance in the range of 0.5% to 10% (e.g., about 1%).

[0077] The optical resonator cavity 62 may be provided with a length adjustment device 74, which can adjust the length L of the resonator cavity 62. cav The length of the resonant cavity in this paper affects the round-trip length L. The round-trip length, or length L, determines the number of wavelengths that fit inside the cavity and thus determines which wavelengths or frequencies resonate. This will be explained in more detail below.

[0078] The length adjustment device 74 may include, for example, an actuator 76 ( Figure 3 Such as a piezoelectric actuator. The actuator can be connected to one or more of the reflectors, for example, to a third reflector 70. Activation of the actuator can move the corresponding reflector, thereby adjusting the length L. The length adjustment device 74 may include a heater 78. Figure 4 (This is used to heat the cavity 62, thereby adjusting the length L of the cavity 62) cav .

[0079] The operation and functionality of the optical filter 60 will be described in more detail below.

[0080] Optical filter 60 constitutes a Fabry-Pérot resonator. However, unlike conventional resonators, filter 60 of this disclosure is adapted to receive and provide an output comprising at least two different frequencies, each with a different polarization. The optical resonator or cavity 62 includes an odd number (e.g., at least three) of highly reflective mirrors. Because the polarization of the reflected light is shifted at each reflection, the odd number of mirrors results in polarized light at the output. As explained above, at least two of the mirrors have low transmittance. Transmission through resonant cavity 62 exhibits relatively sharp resonances and is minimal between these resonances. The distance between the resonant peaks is called the free spectral range (FSR). FSR can be expressed as a length (between wavelengths) or a frequency range (between resonant peaks in the frequency domain). The resonant frequency can be varied, for example, by using a length adjustment device 74 to change the resonant cavity length L. cav To adjust. In fact, the adjustment device 74 may adjust the mirror distance of at least the third mirror 70 relative to the other mirrors 66, 68.

[0081] For heterodyne operation, a large circulating optical power in resonator cavity 62 is possible if the input wave 64 has at least two optical frequencies, each close to the resonant frequency of resonator cavity 62. In other words, the input beam 64 has at least two optical wavelengths close to one of the resonant wavelengths of resonator cavity 62. Although the input frequencies f1 and f2 differ by (δf), the length of resonator cavity 62 significantly exceeds the corresponding first wavelength λ1 and second wavelength λ2 included in the input beam 64. As a result, the length L of the cavity can be adjusted. cav To match an integer multiple of the first wavelength λ1 and the second wavelength λ2, thereby simultaneously generating standing waves and resonances for both the first wavelength λ1 and the second wavelength λ2.

[0082] In resonance, the contribution of the input wave leaking through input mirror 66 constructively increases to the cyclic wave. Furthermore, destructive interference exists in the reflected field. The input field reflected at input mirror 66 is canceled out by the field leaking from the resonator. Therefore, there is practically no reflection in resonance. In anti-resonance, the cyclic field is quite weak, and most of the radiation is reflected at the input mirror. The weak field leaking from the resonator towards the input source constructively increases to the reflected field.

[0083] This invention enables a laser beam 64 originating from a remote optical combiner (ROC) 10 to be transmitted through an impedance-matched planar optical cavity with an odd number of reflections per round trip. The optical cavity 62 acts as a mode cleaner. The mode cleaning effect is described below.

[0084] Compared to conventional mode cleaners, the filter 60 differs from the related methods described herein in that it typically uses only a single-frequency laser beam (and can) pass through the cavity to achieve the mode cleaning effect.

[0085] By selecting a cavity with an odd number of reflections, the fundamental mode frequencies of the s-polarization and p-polarization are separated, attributed to the pi phase shift difference between the s-polarization and p-polarization at the non-normal incident reflection.

[0086] The free spectral range of optical cavity 62 is FSR = c / L, where c is the speed of light and L is the round-trip length of the optical cavity. Radiation with s-polarization or p-polarization typically has half of the FSR separated.

[0087] In a practical embodiment, for example, for a heterodyne frequency difference δf of approximately 15 MHz, the round-trip length L might be, for example, approximately 10 meters. In this paper, the FSR will be approximately 30 MHz. In practice, this is feasible, and interferometers 20 and 30 might have a total length on the order of approximately 8 meters.

[0088] To establish resonance in cavity 62, cavity 62 may be locked to a laser frequency and / or the laser frequency may be adjusted according to the round-trip length L. To adjust the length L, a length adjustment device 74 can be used. Adjusting the laser frequency can be done, for example, using a suitable tunable laser source and a corresponding locking scheme.

[0089] The radiation resonates in cavity 62 with integer wavelengths matching the round-trip length L. In other words, the laser beam resonates in optical cavity 62 if the round-trip phase is an integer multiple of 2π. The round-trip phase in this paper is the phase difference relative to the phase of the radiation after it has traversed the entire length L and passed through the first reflector 66.

[0090] The goal is to make both the first frequency f1 and the second frequency f2 included in the input beam 64 resonate simultaneously. In a practical embodiment, frequencies f1 and f2 differ in both frequency and polarization. In practice, the input beam may already be polarized in the remote optical combiner 10.

[0091] Furthermore, in a preferred embodiment, the cavity has different resonances for the two polarization states. Referring above, since the number of mirrors is odd, the optical filter 60 provides a polarization filtering effect. Due to the odd number of mirrors, one of the two polarization states will be substantially canceled out, thus providing a polarized output in the output beam 72. In a practical embodiment, the two beams have different resonant frequencies: one is s-polarized and the other is p-polarized. Therefore, the filter 60 provides a polarization filtering effect. The filter 60 includes an odd number of mirrors, at least three. As a result, the filter provides approximately a pi phase shift between s-polarization and p-polarization, and the resonant frequency between s-polarization and p-polarization is shifted by approximately half the FSR (FSR / 2).

[0092] In practical embodiments, the effects causing different phase delays of f1 and f2 are limited, or preferably eliminated. The correlation between the common phase delays of f1 and f2 is low.

[0093] The cavity length causes different phase delays for f1 and f2 because they have different frequencies and therefore different wavelengths. The phase difference due to the round-trip length through the cavity is 2π for the first frequency f1. L f1 / c, for the second frequency f2 is 2π L f2 / c. Therefore, the difference is L. 2pi / c (f1-f2).

[0094] The mirror coating may introduce different phase delays for different polarization states (assuming non-normal incidence). This stems from the Fresnel equations. s-polarized light has a sign flip compared to p-polarized light (an additional pi phase). This pi phase delay can be significantly affected by the coating design. To optimize filter 60, the differential phase delay can be specified as a requirement for the mirror coating. For simplicity, we assume a pi phase shift. However, this phase shift can vary. Regardless of the phase shift value at the corresponding mirror with a specific composition and coating, it is preferable to consider this value(s) when calculating the round-trip length L. For example, this is also acceptable if the coating design is better at pi+0.1. We refer to the differential phase delays of the three mirrors used in the cavity as dp1, dp2, and dp3. For example, if the coatings have different phase shifts, this can be compensated for by adjusting the cavity length.

[0095] The filter 60 can be configured as follows. First, adjust the round-trip length L and / or the first frequency until the first frequency f1 resonates in the cavity 62. In this paper, the round-trip phase delay used for the first frequency is a multiple of 2π. This can be achieved, for example, by changing the laser frequency of the first light source 2. If so, if the difference phase delay is a multiple of 2π, then the second frequency f2 will also resonate. That is, L 2pi / c (f1-f2)+dp1+dp2+dp3=n 2pi. Parameters L, dp1, dp2, and dp3 can be designed and selected to make this equation hold. As an example, in a practical implementation: under the conditions of dp1=dp2=dp3=pi, (f1-f2)=15MHz, c=3e8m / s, and n=2, the round-trip length is L=c / (2ππ / 2π). 15MHz) = 10m.

[0096] A practical method for designing filter 60 may include the following steps: selecting dp1, dp2, and dp3; fabricating a mirror; measuring the actual phase delay caused by the mirror; calculating L using the steps provided above; and fabricating filter 60 and its cavity 62 to provide the round-trip length L.

[0097] Tolerances can be compensated by making L adjustable and / or allowing an adjustable f1-f2 frequency difference.

[0098] To achieve an impedance-matched cavity, the first reflector 66 and the second reflector 68 preferably have the same transmittance and reflectivity. In practical embodiments, the first and second reflectors are manufactured simultaneously and during the same process. This may involve producing two reflectors from the same coating batch to reduce manufacturing tolerances. The third reflector preferably has a relatively high reflectivity to minimize losses. Therefore, the impedance-matched cavity can have limited power losses.

[0099] The reflectivity of the mirrors will differ for s-polarized and p-polarized radiation. This effect may be negligible. To limit sensitivity to cavity loss, the higher reflectivity (typically s-polarized) of the input and output mirrors can be limited. In practical embodiments, the cavity fineness can be chosen to be around 100. The latter will allow for a few percent of loss.

[0100] The angle of incidence needs to be considered in the coating design.

[0101] In a practical embodiment, filter 60 includes a planar cavity 62. That is, the beam path within cavity 62 is planar. This is inherent for a three-mirror cavity. For more than three (such as five or seven) mirrors, the beam path may be non-planar. Out-of-plane reflections will cause the polarization state to rotate. If there is a reason to choose an out-of-plane configuration, this needs to be taken into account in the round-trip phase.

[0102] The optical filter disclosed herein offers the advantages described below.

[0103] Only one spatial mode can resonate in optical cavity 62. Therefore, frequencies f1 and f2 will have exactly the same spatial profile after transmission through optical filter 60. This implies perfect collinearity and perfect coaxiality.

[0104] The polarization clock of the transmitted beam 72 is defined by the plane of the optical cavity 62. Since the cross-section of cavity 62 is relatively large compared to the input beam 64, good accuracy should be easily achieved. The filter 60 will provide, for example, significantly better results compared to the relatively small combiner crystal typically included in remote optical combiners (such as combiner 10). Therefore, the polarization clock error is reduced.

[0105] It is possible to use two mode cleaning cavities arranged in parallel. However, if two separate cavities are used instead of a single cavity, at least the two advantages mentioned above will no longer apply. These advantages actually rely on using a single cavity to perform mode cleaning on at least two frequencies simultaneously.

[0106] Since only one spatial mode resonates in cavity 62, any wavefront distortion caused by the optical fiber and externally coupled optical elements is strongly suppressed. The transmission mode will have a wavefront quality far superior to that of the input beam 64, and the wavefront quality will be approximately equal for frequencies f1 and f2. In a practical embodiment, the system of this disclosure is configured such that the output beam 72 includes a 00 spatial mode for at least one or both of frequencies f1 and f2. The latter can be achieved by, for example, one or more of the following: adjusting the corresponding frequency, adjusting the cavity length, adjusting the coating, and adjusting the polarization of the corresponding input beams 4 and 8.

[0107] The optical cavity acts as a low-pass filter for intensity fluctuations. Therefore, intensity fluctuations in laser beams 4 and 8 will be suppressed. Optical cavity 62 also acts as a low-pass filter for phase fluctuations.

[0108] Since the spatial pattern of cavity 62 is given only by the relative positions of the cavity mirrors, any beam pointing or positional deviation in front of the cavity will be strongly suppressed in the transmitted beam.

[0109] The cavity can serve as a reference for the spatial pattern of the laser beam. If the remote optical combiner 10 needs to be replaced (e.g., because the fiber is damaged), the differences between the corresponding optical combiners will be filtered out by the optical cavity. Cavity 62 acts as an isolator for the laser beam parameters. Therefore, this optical filter avoids the need for realignment of the interferometers 20 and 30 downstream of the cavity, since the output beam 72 remains unchanged.

[0110] The optical filter 60 can function similarly to a wavelength tracker. The cavity length or round-trip length L can serve as a length reference.

[0111] In theory, all the problems listed can be solved in different ways. However, the optical filter 60 described herein is particularly attractive because it incorporates numerous improvements in a single component.

[0112] While this article may specifically cite applications of photolithography equipment in IC manufacturing, it should be understood that the photolithography equipment described herein may also have other applications. Other possible applications include: the manufacturing of integrated optical systems, the guiding and detection of patterns in magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0113] While specific embodiments of the invention may be referenced herein in the section concerning lithography apparatus, these embodiments may also be used in other devices. Embodiments of the invention may form part of mask inspection equipment, metrology tools, or any apparatus for measuring or processing objects such as wafers (or other substrates), masks (or other patterning apparatus). These apparatuses may be commonly referred to as lithography tools. Such lithography tools may be used in a vacuum environment or in ambient (non-vacuum) environments.

[0114] Although the foregoing may have specifically referenced the use of embodiments of the invention in the context of optical lithography, it should be understood that the invention is not limited to optical lithography in this context and may be used in other applications such as imprint lithography.

[0115] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in ways other than those described. The above description is intended to be illustrative and not limiting. Therefore, those skilled in the art will understand that modifications may be made to the invention without departing from the scope of the claims set forth below. For example, features of the various embodiments described above may be combined. Several aspects of the invention are described in the following numbered clauses: 1. An optical filter for a heterodyne interferometer, the optical filter comprising: An optical resonator cavity is adapted to receive an input beam comprising radiation having at least a first frequency f1 and a second frequency f2 different from the first frequency. An odd number of at least partially reflective mirrors are arranged in the cavity to reflect the input beam and circulate it within the cavity. The optical resonator cavity is configured to provide an output beam, the output beam including a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization different from the first polarization. 2. The filter according to item 1, wherein the optical resonator cavity includes a length adjustment device capable of adjusting the length of the cavity. 3. The filter according to clause 2, wherein the length adjustment device comprises one or more of the following: a piezoelectric brake and a heater. 4. The filter according to item 1, including a frequency adjustment device for adjusting the first frequency and / or the second frequency. 5. The filter according to item 1, wherein the first polarization is s-polarization and the second polarization is p-polarization. 6. The filter according to item 1, wherein the odd number of reflectors comprises: A first reflector having partial transmittance and reflectivity is adapted to receive the input beam. A second reflector, having substantially the same partial transmittance and reflectance as the first reflector, is adapted to provide the output beam; and At least one third reflecting mirror has substantially total reflectivity. 7. The filter according to item 6, wherein the first reflector and the second reflector have the same composition. 8. A heterodyne interferometer system, comprising: A first light source is used to provide a first laser beam having a first frequency f1; The second light source is used to provide a second laser beam having a second frequency f2 that is different 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 includes an odd number of at least partially reflective mirrors arranged in the cavity to reflect and circulate an input beam within the cavity. The optical resonator cavity is adapted to receive the input beam and to provide an output beam, the output beam including a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization different from the first polarization. A heterodyne interferometer is used to receive the output beam. 9. A photolithography apparatus comprising an optical filter according to any one of items 1 to 7. 10. A photolithography apparatus comprising a heterodyne interferometer system as described in claim 8. 11. A projection system for an optical lithography system, comprising an optical filter according to any one of items 1 to 7. 12. A projection system for an optical lithography system, comprising the heterodyne interferometer system as described in clause 8. 13. A method for filtering the input beam of a heterodyne interferometer, the method comprising the following steps: An optical resonator cavity is provided, the optical resonator cavity being adapted to receive an input beam comprising radiation having at least a first frequency f1 and a second frequency f2 different from the first frequency, wherein an odd number of at least partially reflecting mirrors are arranged in the cavity for reflecting the input beam and circulating it in the cavity; The optical resonator cavity is used to provide an output beam, the output beam including a first spatial mode having a first frequency f1 and a first polarization, and a second spatial mode having a second frequency f2 and a second polarization different from the first polarization. 14. The method according to clause 13, comprising the step of providing the output beam to the heterodyne interferometer. 15. The method according to clause 13, comprising the step of adjusting the length of the optical resonator cavity to cause at least one mode of at least the first frequency and the second frequency to resonate. 16. The method according to clause 15, wherein the step of adjusting the length includes at least one of: activating a piezoelectric actuator to move at least one of the reflectors; and heating the cavity. 17. The method according to any one of clauses 13 to 16, comprising the step of adjusting at least one of the first frequency and the second frequency to cause at least one mode to resonate in the cavity.

Claims

1. An optical filter for a heterodyne interferometer, the optical filter comprising: An optical resonator cavity is adapted to receive an input beam, said input beam comprising radiation having at least a first frequency f1 and a second frequency f2, the second frequency being different from the first frequency. An odd number of at least partially reflective mirrors are arranged in the cavity to reflect the input beam and circulate it within the cavity. The optical resonator cavity is configured to provide an output beam, the output beam including a first spatial mode and a second spatial mode, the first spatial mode having a first frequency f1 and a first polarization, and the second spatial mode having a second frequency f2 and a second polarization different from the first polarization.

2. The filter according to claim 1, wherein the optical resonator cavity includes a length adjustment device capable of adjusting the length of the cavity.

3. The filter according to claim 2, wherein the length adjustment device comprises one or more of the following: a piezoelectric actuator and a heater.

4. The filter according to claim 1, comprising a frequency adjustment device for adjusting the first frequency and / or the second frequency.

5. The filter according to claim 1, wherein the first polarization is s-polarization and the second polarization is p-polarization.

6. The filter according to claim 1, wherein the odd number of the reflectors comprises: A first reflector having partial transmittance and reflectivity is adapted to receive the input beam. The second reflector has a portion of the same transmittance and reflectance as the first reflector, and the second reflector is adapted to provide the output beam. as well as At least one third reflecting mirror has substantially total reflectivity.

7. The filter according to claim 6, wherein the first reflector and the second reflector have the same composition.

8. A heterodyne interferometer system, comprising: A first light source is used to provide a first laser beam having a first frequency f1; A second light source is used to provide a second laser beam having a second frequency f2, the second frequency f2 being different 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 includes an odd number of at least partially reflective mirrors arranged in the cavity to reflect the input beam and circulate the input beam within the cavity. The optical resonator cavity is adapted to receive the input beam and to provide an output beam, the output beam including a first spatial mode and a second spatial mode, the first spatial mode having a first frequency f1 and a first polarization, and the second spatial mode having a second frequency f2 and a second polarization different from the first polarization. as well as A heterodyne interferometer is used to receive the output beam.

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

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

11. A method for filtering the input beam of a heterodyne interferometer, the method comprising the following steps: An optical resonator cavity is provided, the optical resonator cavity being adapted to receive an input beam, the input beam comprising radiation having at least a first frequency f1 and a second frequency f2, the second frequency being different from the first frequency, wherein an odd number of at least partially reflecting mirrors are arranged in the cavity for reflecting the input beam and circulating the input beam in the cavity; The optical resonator cavity is used to provide an output beam, the output beam including a first spatial mode and a second spatial mode, the first spatial mode having a first frequency f1 and a first polarization, and the second spatial mode having a second frequency f2 and a second polarization different from the first polarization.

12. The method according to claim 11, comprising the following steps: The output beam is provided to the heterodyne interferometer.

13. The method according to claim 11, comprising the following steps: The length of the optical resonator cavity is adjusted so that at least one mode of at least the first frequency and the second frequency resonates.

14. The method of claim 13, wherein the step of adjusting the length comprises at least one of: activating a piezoelectric actuator to move at least one of the reflectors; and heating the cavity.

15. The method according to any one of claims 11 to 14, comprising the following steps: Adjust at least one of the first frequency and the second frequency so that at least one mode resonates in the cavity.

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