Method for stabilizing wavelength of a tunable laser device, tunable laser device, and position measurement system including a tunable laser device

Abstract

The wavelength of the tunable laser device is stabilized by providing a laser beam from a tunable laser source to an interferometer having a stable reference axis and to a gas absorption cell; scanning the laser beam between a first wavelength and a second wavelength to determine an absorption cell transmission spectrum as a function of wavelength difference; determining a phase change as a function of wavelength difference using the interferometer; determining a transmission spectrum as a function of phase change using the gas absorption cell transmission spectrum and the phase change as a function of wavelength difference; correlating the transmission spectrum determined as a function of phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide an absolute laser wavelength as a function of phase change; defining a phase setpoint corresponding to the wavelength setpoint; and tuning the tunable laser device to the wavelength setpoint using the phase setpoint.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application No. 22190984.9 filed on August 18, 2022 and European Patent Application No. 22194604.9 filed on September 8, 2022, both of which are incorporated by reference in their entireties.

[0002]

[0002] The present technology relates to controlling the wavelength of a laser output of a tunable laser device. The tunable laser device can be used, for example, to monitor equipment displacement in a lithography apparatus. For example, the disclosed tunable laser device and method for controlling its wavelength can be applied to a projection system for an optical lithography system. [Background technology]

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

[0004]

[0004] Due to continuous advances in semiconductor manufacturing processes, for decades the number of functional elements, such as transistors, per device has steadily increased while the dimensions of circuit elements have continuously decreased, following a trend commonly referred to as "Moore's Law." To keep up with Moore's Law, the semiconductor industry is pursuing technologies that enable the creation of ever smaller features. To project a pattern onto a substrate, a lithography system may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features patterned on the substrate. Common wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithography systems using extreme ultraviolet (EUV) radiation with wavelengths in the 4 nm to 20 nm range, e.g., 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than lithography systems using radiation with a wavelength of, e.g., 193 nm.

[0005]

[0005] Tunable lasers have been widely used over the past few decades to measure the wavelength response of optical systems. Early applications were primarily in spectroscopy. Since the advent of wavelength division multiplexing in optical communications, tunable lasers have increasingly been applied in the field of communications component measurement.

[0006]

[0006] In semiconductor manufacturing processes, tunable lasers are often used to precisely measure the position of various elements of a lithography apparatus. The elements may be movable, such as a wafer stage. Here, the laser measurement system can provide information about the exact position of each element relative to other elements in the optical system, such as a fiducial, a mask, or a radiation source. Elements such as the radiation source, mask, lenses, and / or mirrors may also remain stationary. Here, the laser measurement system can be used to monitor undesired deviations from a predetermined position, for example due to thermal drift.

[0007] The term "tunable laser" refers to a laser whose wavelength is tunable. A tunable laser measurement system typically includes a laser and a measurement device and / or a method for determining the wavelength of the laser. A tunable laser control system includes a mechanism for calculating an error signal corresponding to the difference between the current wavelength and the desired setpoint. The error signal is used to drive one or more laser actuators.

[0008]

[0008] A key element of a tunable laser measurement or control system is the wavelength measurement itself. Various types of instruments and methods can be used for this, including Fizeau interferometers, grating spectrum analyzers, Fourier spectrometers, and etalon / wavelength reference combinations, among others. Additional methods include mapping the wavelength response to the angle or position of the actuator.

[0009]

[0009] US20200182702A1 discloses a tunable laser system including a tunable laser scanned over a range of frequencies and an interferometer having multiple interferometer outputs. At least two of the multiple interferometer outputs have a phase difference. A wavelength reference has spectral characteristics within the frequency range, and the spectral characteristics do not change in an expected operating environment of the tunable laser. A processing circuit uses the spectral characteristics and the multiple interferometer outputs to generate an absolute measurement of the wavelength of the tunable laser and controls the tunable laser based on a comparison of the absolute measurement of the wavelength of the tunable laser to a setpoint wavelength.

[0010]

[0010] As described in US20200182702A1, it may be desirable for the measured wavelength signal to exhibit one or more of the following characteristics: 1. Directionality: The wavelength measurement contains information about the direction of laser tuning. 2. Continuous: The measurement can be made continuously in time and throughout the tuning range of the laser. 3. High Accuracy: For applications such as spectroscopy and photodetection, accuracy down to the sub-picometer (pm) level is desirable, and in some cases is crucial. 4. High Precision: Accuracy is of little importance unless it is at most equal to the accuracy of the control. Many applications benefit from precision several orders of magnitude better than the accuracy. 5. Absolute Information: Without absolute information, only relative wavelength measurements are obtained. There will be an unknown wavelength offset between the measured wavelength and the actual wavelength. 6. Zero or Negligible Drift: Many applications are sensitive to short-term or long-term drift. 7. Low Latency: This requirement is particularly applicable when using wavelength measurements in control systems. Latency is the time it takes from the moment light leaves the laser to the moment the appropriate correction signal is applied to the laser actuator. The time it takes to calculate the error signal affects the total latency, which is inversely proportional to the feedback loop bandwidth. Reducing latency allows for higher frequency control.

[0011]

[0011] Although the system of US20200182702A1 meets all of these characteristics, the system uses a feedback loop that introduces delays and results in an iterative process of adjusting the actual laser wavelength to a setpoint based on error correction.

[0012]

[0012] US20020043616A1 discloses a system and method for calibrating a tunable laser, such as a widely tunable laser (WTL). This is done by sending the output of the laser through a gas cell and an etalon, varying the tuning parameters of the laser, and comparing the spectra of the two signals. This can be used to determine the absolute transmission wavelength of the WTL as a function of the tuning parameter (such as temperature). The solution described can be used, for example, to calibrate an etalon for different temperatures.

[0013]

[0013] A significant drawback of the system of US20020043616A1 relates to tuning: the system compares and locks to peaks in the spectrum, and as a result, calibration of the laser is essentially limited to the specific frequency or wavelength of the peak.

[0014]

[0014] Furthermore, known methods usually require wavelength modulation or frequency sideband generation using an electro-optic modulator (EOM). When locking to a gas absorption cell, it is not possible to lock to frequencies between spectral lines. Sideband locking provides some flexibility between spectral lines, but it requires a wide frequency tuning range of the EOM, making it impossible to freely select any position between absorption lines.

[0015]

[0015] Existing methods stabilize the laser frequency in the source, rather than the laser wavelength of the interferometer itself. Such methods require additional, complex wavelength stabilization, for example, by software-based compensation, which can introduce errors, especially periodic errors. They also require knowledge of the optical path difference (OPD) ratio between the interferometer axes. Wavelength trackers traditionally measure the laser frequency and refractive index noise to provide data for use in software-based compensation.

[0016]

[0016] The present disclosure seeks to overcome at least one or more of the shortcomings of the prior art and to provide an improved method and system for enabling laser wavelength control. Summary of the Invention

[0017]

[0017] The present disclosure provides a method for stabilizing the wavelength of a tunable laser device, comprising: simultaneously providing a laser beam from a tunable laser source to a first interferometer having a mechanically stable reference axis and to a gas absorption cell; scanning a wavelength range from a first wavelength to a second wavelength with a laser beam; determining a transmission spectrum of the gas absorption cell as a function of wavelength difference relative to the first wavelength; determining a phase change as a function of a wavelength difference for a first wavelength using a first interferometer; determining a transmission spectrum as a function of phase change using the transmission spectrum of the gas absorption cell and the phase change as a function of wavelength difference; correlating the determined transmission spectrum as a function of phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide the absolute laser wavelength as a function of phase change; defining a phase setpoint corresponding to the wavelength setpoint; and tuning the tunable laser device to a wavelength setpoint using the phase setpoint.

[0018] In one embodiment, the method includes measuring the displacement of a device in a lithographic apparatus using a laser beam set at a setpoint wavelength, the device may include a wafer table, a wafer, a mask, a mask table, or any other equipment in an optical stage of the lithographic apparatus.

[0019]

[0019] The step of measuring the displacement using the laser beam may include providing the laser beam to a second interferometer.

[0020]

[0020] The step of correlating the determined transmission spectrum as a function of phase change to a pre-calibrated transmission spectrum of the wavelength-tunable laser device may include fitting the determined transmission spectrum to the pre-calibrated transmission spectrum using a fitting algorithm.

[0021]

[0021] The step of tuning the tunable laser device to the setpoint wavelength using the phase setpoint may include continuously using the first interferometer to determine a phase change relative to the phase setpoint.

[0022] According to another aspect, the present disclosure provides a tunable laser device, comprising: a tunable laser source for providing a laser beam; a first interferometer having a mechanically stable reference axis adapted to receive the laser beam; a gas absorption cell adapted to receive the laser beam simultaneously with the interferometer; a processor, The processor scanning a wavelength range from a first wavelength to a second wavelength with a laser beam; determining the transmission spectrum of the gas absorption cell as a function of time; determining the phase change as a function of time using a first interferometer; determining a transmission spectrum as a function of phase change using the transmission spectrum of the gas absorption cell and the phase change as a function of time; correlating the determined transmission spectrum as a function of phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide the absolute laser wavelength as a function of phase change; defining a phase setpoint corresponding to the wavelength setpoint; and A tunable laser device is provided that is adapted to tune the tunable laser device to a wavelength setpoint using a phase setpoint.

[0023]

[0023] Here, determining the transmission spectrum of the gas absorption cell as a function of time may include correlating the time with the wavelength difference relative to the first wavelength.

[0024]

[0024] Determining the phase change as a function of time using the first interferometer may include correlating the time with a wavelength difference for the first wavelength.

[0025]

[0025] The laser device may be adapted to measure displacement of a device in a lithographic apparatus using a laser beam set to a wavelength setpoint.

[0026] The device may include a second interferometer adapted to receive the laser beam and measure the displacement.

[0027]

[0027] A device in a lithographic apparatus may include a wafer table, a wafer, a mask, a mask table, or any other equipment in the optical stage of the lithographic apparatus.

[0028]

[0028] The processor may be adapted to correlate the determined transmission spectrum to the pre-calibrated transmission spectrum by fitting the determined transmission spectrum to the pre-calibrated transmission spectrum using a fitting algorithm.

[0029]

[0029] The tunable laser device may be adapted to have an operating mode, and the first interferometer is adapted to continuously determine a phase change relative to a phase setpoint.

[0030]

[0030] According to yet another aspect, the present disclosure provides a position measurement system including a tunable laser device adapted to perform the method described in clause 1.

[0031] According to another aspect, the present disclosure provides a lithographic apparatus including at least one position measurement system as described above.

[0032]

[0032] The disclosed method and device uses a phase-measuring interferometer. Using phase to tune the wavelength of a laser eliminates the need for calibration limitations on transmission lines, such as with an etalon. Phase measurement allows for continuous measurement and adjustment (instead of discrete steps). This allows for more accurate wavelength control. Also, because phase measurements can be made continuously (in time), any deviation from a set wavelength can be immediately corrected without the need for sweeping or other repetitive steps. The disclosed method eliminates the need for active control of the length of the optical cavity to perform calibration.

[0033]

[0033] The cavity of the wavelength tracker part of the interferometer is preferably as stable and fixed as possible, in this way making it unnecessary to detect the transmission line of the optical cavity.

[0034]

[0034] The disclosed method and apparatus can achieve laser wavelength stabilization by calibrating the phase of an interferometric wavelength tracker to the spectral absorption spectrum of a molecular absorption cell. To effectively stabilize the laser wavelength instead of the laser frequency, it is proposed to lock to the phase of the wavelength tracker (which has a mechanically stable reference axis). A control loop that keeps the phase of the wavelength tracker signal constant prevents the occurrence of periodic errors. No wavelength modulation is required, and locking is not limited to a specific wavelength value. Therefore, wavelength compensation is inherent and does not need to be applied by software, nor does it require OPD ratio information. [Brief explanation of the drawings]

[0035]

[0035] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0036] [Figure 1] 1 shows a schematic diagram of an exemplary lithographic apparatus; [Figure 2] 1 shows a schematic diagram of an embodiment of a tunable laser device. [Figure 3A] 1 illustrates steps in one embodiment of the method of the present disclosure. [Figure 3B] 1 illustrates an exemplary embodiment of a method of the present disclosure that combines various inputs to arrive at a diagram relating wavelength tracker phase to absolute wavelength. [Figure 3C] 1 shows an exemplary diagram illustrating steps in the method of the present disclosure. [Figure 4] 1 shows an exemplary diagram relating wavelength tracker phase to absolute wavelength. [Figure 5] 1 shows a schematic diagram of a tunable laser device in a laser device operation mode according to the method of the present disclosure. [Figure 6] 1 shows a schematic diagram of another embodiment of a tunable laser device. DETAILED DESCRIPTION OF THE INVENTION

[0037]

[0036] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet (e.g., having a wavelength of about 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet, e.g., having a wavelength in the range of 5 to 100 nm).

[0038]

[0037] The terms "reticle," "mask," or "patterning device," as used herein, should be interpreted broadly to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be produced in a target portion of a substrate. The term "light valve" may also be used in this context. In addition to the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0039]

[0038] "Refractive index" (refractive index) is a value calculated from the ratio of the speed of light in a vacuum to the speed of light in a second, denser medium. The refractive index variable is sometimes denoted by the letter n or n' in descriptions and mathematical formulas.

[0040]

[0039] An "interferometer" or "laser interferometer" can measure distance by measuring the phase difference between two light beams: one sent to a first reflector or surface at a fixed reference distance, and the other 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 from the interferometer to the second surface. As the distance of the second surface changes, the phase of the combined signal also changes. The utility of these methods is that measurements can be made over long distances while maintaining accuracy.

[0041] In a heterodyne interferometer, the measurement and reference beams that interfere at the detector typically originate from the same laser source. To enable heterodyne phase detection, the sources may be frequency offset. However, the two (split) frequencies may also be generated by two different frequency-locked or phase-locked lasers.

[0042]

[0041] A "wavelength tracker" is a specific version of an interferometer configured to measure the phase difference between two reflected light beams. One beam reflects off a first fixed reflector that provides a first reference axis, and the second beam reflects off a second fixed reflector that provides a second reference axis that has a different length from the first axis. Because the two reflecting surfaces are fixed, the measured phase difference changes only if the wavelength of the light beams changes. Thus, a wavelength tracker allows for monitoring the deviation of the wavelength from a setpoint.

[0043]

[0042] For a general description of interferometers and further details, see, for example, "A tutorial on laser interferometry for precision measurements" (Conference Paper, Proceedings of the American Control Conference, June 2013, DOI: 10.1109 / ACC.2013.6580402, Daniel Abramovitch et al.).

[0044] A "beam splitter" (also spelled beamsplitter) is an optical device that splits a light beam into two and is part of many optical systems. In the first version, the beam splitter can be a cube made of two triangular glass prisms glued together at their bases using, for example, a polyester-, epoxy-, or urethane-based adhesive. The thickness of the adhesive resin layer is adjusted so that (for a specific wavelength) half of the light incident from one "port" (i.e., face of the cube) is reflected and the other half is transmitted by FTIR (Frustrated Total Internal Reflection). Polarizing beam splitters, such as Wollaston prisms, use birefringent materials to split light into two beams with orthogonal polarization states. A second option is to use a semitransparent mirror. This consists of an optical substrate, often a sheet of glass or plastic, coated with a thin coating of a partially transparent metal. The thin coating can be aluminum or silver deposited using physical vapor deposition. The thickness of the coating is controlled so that a portion (usually half) of the light incident at a 45 degree angle and not absorbed by the coating or substrate material is transmitted, and the rest is reflected.A third version of the beamsplitter is the dichroic mirror prism assembly, which uses a dichroic optical coating to split an incoming light beam into multiple spectrally distinct output beams.

[0045]

[0044] A "gas reference cell" is a cell filled with some gas, typically used in laser absorption spectroscopy. Some effect resulting from the interaction of the gas with the light, such as the absorption coefficient of the light in the gas or a frequency-dependent phase change, allows comparison with a reference beam. Typically, small changes in the light beam caused by passing through the gas are measured as a function of the optical frequency of the laser beam, and the results are presented in the form of a spectrum, e.g., an absorption spectrum. The peaks obtained in such a spectrum can be used to identify specific chemical species and measure their concentration. A tunable single-frequency laser is often used for such measurements. Once the spectrum and chemical composition in the cell are known, the gas cell also allows the reverse process of using the absorption spectrum to identify the frequency or wavelength of the laser light.

[0046]

[0045] Gas reference cells are usually equipped with an appropriate optical window (with high transmittance over the entire relevant spectral range) to allow light to enter and exit the cell. Reference gas cells are commercially available for a variety of gases, including both atomic and (often diatomic) molecular gases. Typical examples include iodine (I2), hydrogen (H2), helium, carbon monoxide (CO), and acetylene (C2H2). A wide range of standard spectral lines can therefore be used. In some cases, alkali metals such as sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) are used, which, when electrically heated to at least a suitable temperature, generate a sufficiently high vapor pressure. Such cells are sometimes called vapor cells. Sealed gas cells must be reliably leak-tight. For this reason, a helium leak test is often applied.

[0047] An "absorption cell" or "molecular absorption cell" is a device that includes a gas reference cell. An absorption cell can compare the difference in intensity of two portions of the same beam. One portion is sent to the gas reference cell and to a first sensor to receive light that has passed through the reference cell at least once, and the second portion is sent unobstructed to a second sensor that is used as a reference.

[0048] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also referred to as an 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 according to certain parameters, a substrate support (e.g. a substrate 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 according to 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 the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0049]

[0048] In operation, the illumination system IL receives a radiation beam from the radiation source SO, for example via the 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, to direct, shape and / or control the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0050]

[0049] The term "projection system" PS as used herein should be interpreted broadly to encompass any type of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optics, or any combination thereof, appropriate for the exposure radiation being used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein should be considered as synonymous with the more general term "projection system" PS.

[0051]

[0050] The lithographic apparatus LA may be of a type (also known as immersion lithography) in which at least a part of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill a space between the projection system PS and the substrate W. Further information about immersion techniques is provided in US6952253, which is incorporated herein by reference.

[0052] The lithographic apparatus LA may be of a type having 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 while preparation steps for a subsequent exposure of the substrate W are being performed on a substrate W located on one substrate support WT, another substrate W on another substrate support WT may be used to expose a pattern onto another substrate W.

[0053] 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 a plurality of sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for providing immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0054] In operation, the radiation beam B is incident on a patterning device (e.g. mask MA), which is held on the mask support MT, and is patterned according to a pattern (design layout) present on the patterning device MA. After passing through 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. Using the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, to position different target portions C at focused and aligned positions in the path of the radiation beam B. Similarly, the patterning device MA can be accurately positioned relative to the path of the radiation beam B using the first positioner PM and possibly further position sensors (not explicitly shown in Figure 1a). The patterning device MA and substrate W may be aligned using mask alignment marks M1 and M2 and substrate alignment marks P1 and P2. Although substrate alignment marks P1 and P2 as illustrated occupy dedicated target portions, the substrate alignment marks may be located in spaces between the target portions. When located between target portions P1 and P2, they are known as scribe-lane alignment marks.

[0055]

[0054] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes: x, y, and z. Each of the three 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- and y-axes define a horizontal plane, while the z-axis defines a vertical direction. The Cartesian coordinate system is not a limitation of the invention and is used for illustration purposes only. Alternatively, other coordinate systems, 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, so that the z-axis has a component along the horizontal plane.

[0056]

[0055] In a vacuum, wavelength and frequency are related by:

number

number

[0057] Combining the above velocity equation with the definition of refractive index, the relationship between the wavelength in vacuum, λ0=c / f, and the wavelength in the first medium other than vacuum, λ1=v1 / f, is

number

[0058]

[0057] Although laser light may pass through a low-pressure environment, the "vacuum" inside a machine such as a lithography tool is typically denser than a theoretical vacuum. As a result, for improved accuracy in setting a tunable laser to a selected setpoint, it is generally preferred to determine the wavelength of the laser light as it passes through the medium, rather than tuning the frequency of the laser light within the laser itself.

[0059] In one embodiment, the disclosed method applies wavelength stabilization instead of laser frequency stabilization, where it is assumed that the refractive index of the medium is the same in the mechanically ultra-stable wavelength tracker and the interferometer axis in the measurement system.

[0060] 2, a tunable laser device 1 according to the present disclosure comprises a tunable laser source 2 for providing a laser beam 4. The laser source is tunable, meaning that the laser beam 4 can be adjusted or tuned between at least a first wavelength and a second wavelength, where the second wavelength may be greater than the first wavelength.

[0061] The laser 2 may be connected to a controller 3 to control or tune the wavelength or frequency of the laser beam 4. The controller may in turn be connected to a setpoint trajectory generator 5. The setpoint generator 5 may be configured to provide a sweep to the controller 3, effectively sweeping the laser wavelength from a first wavelength to a second wavelength and / or vice versa. The output of the setpoint trajectory generator may be provided to the controller via a summer or summing device 7. The summing device allows a feedback signal or feedback loop signal to be added or subtracted from the setpoint set by the generator 5. Feedback signals are described below.

[0062] Wavelength modulation can be achieved by open-loop control of the laser (usually by changing the temperature of the laser cavity) or by closed-loop control. In closed-loop control, the wavelength of the laser can be adjusted, for example, by controlling the temperature of the laser cavity to a desired setpoint trajectory (not shown in FIG. 2) or by controlling the phase of a wavelength tracker to a desired setpoint trajectory (as shown in FIG. 2). The setpoint generator can also be directly connected to the tuning element of the laser (usually a TEC controller) in either an open-loop or closed-loop manner. A direct connection may include a thermistor that measures the laser cavity temperature. The main objective is to tune the wavelength of the laser, and several options are considered within the scope of the disclosure. FIG. 2 shows a wavelength control loop based on the measured WLT phase. In practical embodiments, tuning can be performed using a closed-loop or open-loop setpoint of the temperature of the laser cavity.

[0063]

[0062] Actual embodiments of tunable laser sources are provided, for example, in Research Disclosure database number 689021, "High Power Metrology Lasers in Interferometry," published in the September 2021 journal and digitally released on August 5, 2021.

[0064] The laser beam is directed to a first beam splitter 6. The first beam splitter directs one part of the beam 4 to an interferometer device 8 and another part to an absorption cell 10.

[0065]

[0064] The absorption cell 10 includes a second beam splitter 12. The second beam splitter 12 directs a portion of the light incident on the absorption cell 10 to a gas reference cell 14 having a first sensor 16. Another portion is directed to a second sensor 18.

[0066] The interferometer device 8 may comprise a first interferometer 20. The first interferometer 20 may be a wavelength tracker. The interferometer device 8 may also comprise a second interferometer 22. The second interferometer may be a distance measurement device. A third beam splitter 24 may be provided to direct a portion of the light entering the interferometer device 8 to the first interferometer 20 and a second portion of the light to the second interferometer 22.

[0067]

[0066] The first interferometer 20 is a wavelength tracker. The first interferometer comprises a first reflector 30 at a first distance and a second reflector at a second distance greater than the first distance. Both the first reflector 30 and the second reflector 32 have fixed positions. Thus, each of the first distance and the second distance is constant, providing two fixed reference axes, respectively. It is desirable that both the first reference axis and the second reference axis are as stable as possible. Stable in this context means that the lengths of both axes remain substantially constant, at least within a predetermined set of operating conditions.

[0068] In practical embodiments, the second distance may exceed the first distance by about 0.5-10 m, for example, about 1 m. In practical embodiments, the difference in optical path length may be about 6-12 meters, for example, about 8-10 meters. This can be achieved in a compact housing by employing a folded-beam interferometer with multiple paths within the measurement cavity.

[0069] The second interferometer 22 may be for distance or position measurement of the device 40. Here, the second interferometer includes a third reflector 42 arranged at a fixed position to provide a third reference axis. The device 40 may function as or be equipped with a fourth reflector.

[0070]

[0069] Equipment 40 may be any part of interest, including but not limited to a wafer table, a lens or mirror, a mask table or mask, or other equipment in the optical section of lithographic apparatus LA.

[0071]

[0070] Actual embodiments of the first interferometer and the second interferometer suitable for the method and system of the present disclosure are provided in, for example, US2021072088.

[0072] The first and second output signals 34, 44 of the first and second interferometers, respectively, may be provided to a phase measurement device 50, also referred to as a phase measurement board (PMB). The PMB provides an output 52. The output 52 may include a first phase difference 54 of the two reflections in the first interferometer 20.

[0073] The output 52 of the PMB may also include a second phase difference of the two reflections in the second interferometer 22, thereby enabling a position measurement of the device 40. For a detailed description, see, for example, the documents cited herein above.

[0074] The first phase difference 54 is defined as the wavelength tracker phase or Φ WLT The wavelength tracker phase 54 may be provided to a data logger 56. The data logger logs, or records, the measured wavelength tracker phase.

[0075]

[0074] The wavelength tracker phase 54 may also be provided to the additional device 7 to act as a feedback signal for correcting the setpoint of the laser source 2, the function of which will be explained below.

[0076]

[0075] The data logger 56 may also be provided with an absorption cell output 58, ie the output of the absorption cell 10. The output of the absorption cell typically comprises an intensity spectrum I as a function of the frequency or wavelength of the laser beam 4.

[0077] The wavelength tracker phase 54 and the absorption cell output 58 may be provided to a processor 60. The processor 60 may be any type of suitable data processor, such as a computer or computer module. Another input 62 to the processor 60 may include a reference transmission spectrum of the absorption cell 10. The input 62 may include a pre-calibrated transmission spectrum I of the absorption cell 10 as a function of the wavelength λ of the laser 2. The pre-calibrated spectrum is typically provided by the producer or manufacturer of the respective absorption cell. Because the pre-calibrated spectrum is typically created in a controlled laboratory environment with state-of-the-art accuracy, the wavelength X here may also be referred to as the actual wavelength, absolute wavelength, or reference wavelength.

[0078]

[0077] The output 66 of the processor 50 can be used in step 68 of the method of the present disclosure.

[0079] The method of the present disclosure will be described with reference to Figures 3A, 3B, 3C, and 4. Several general steps will be designated herein as (1), (2), and (3). These general indicators are also included in the system schematic of Figure 2 to indicate where in the system these steps take effect.

[0080] 3A, the method of the present disclosure includes at least steps labeled (1), (2), and (3). In a first step 80, the method measures the phase difference 54 of the wavelength tracker and the transmission spectrum 58 of the gas reference cell 14 while tuning the laser. Here, tuning the laser means, for example, controlling the laser source 2 to sweep the wavelength of the laser beam 4 from a first wavelength λ1 to a second wavelength λ2. In an actual embodiment, the sweeping may include repeatedly increasing and decreasing the wavelength from the first wavelength λ1 to the second wavelength λ2, and vice versa.

[0081]

[0080] In a second step 82, the absorption cell intensity spectrum 58, the wavelength tracker phase 54 and the predetermined intensity spectrum 64 are related to determine a function 84 of the wavelength tracker phase as a function of absolute wavelength.

[0082]

[0081] The first step 80 and the second step 82 may be referred to as a tuning mode or a calibration mode.

[0083] In the third step 86, also called the operating mode, the setpoint wavelength λ sp The wavelength of the laser source 2 is selected by the wavelength tracker phase 54 (Φ WLT ), where the wavelength tracker phase 54 (Φ WLT ) is the setpoint wavelength λ determined using function 84 (see FIG. 4). sp The wavelength tracker phase setpoint (Φ sp ), the wavelength tracker phase begins to deviate from the wavelength tracker phase setpoint (Φ sp ) can be adjusted until the wavelength returns to

[0084] 3B collectively shows an exemplary diagram illustrating the measurement output 58 of the absorption cell 10 and the wavelength tracker output 54. The first output 58 is a representation of the intensity I as a function of time (expressed in or related to the wavelength difference Δλ relative to a starting point, such as the first wavelength). The second output 54 is a representation of the phase difference ΔΦ of the two light beams in the wavelength tracker 20 as a function of time. WLT (expressed in or related to the wavelength difference Δλ relative to the start of the sweep). Combining the two outputs 54 and 58 gives the phase difference ΔΦ of the wavelength tracker 20 WLT A function 88 is obtained that shows the absorption cell intensity I as a function of

[0085]

[0084] If we do not assume a linear wavelength trajectory, the traced measurements of gas permeability 58 and WLT phase 54 are all traced as a function of time. Combining 58 and 54, we can plot 58 as a function of 54, which can be correlated with 64 to find the linear dependence of WLT phase and absolute wavelength.

[0086]

[0085] A function 88 can be fitted to a pre-calibrated diagram 64 of the absorption cell intensity I as a function of absolute wavelength. This fitting results in a phase difference ΔΦ of the wavelength tracker 20, as illustrated in FIG. WLT and the absolute wavelength λ, a function 84 is obtained.

[0087]

[0086] Suitable fitting methods include, but are not limited to, the method described in "A novel laser intensity function and its fitting method," Optics & Laser Technology, Vol. 47, April 2013, pp. 183-188, by Yuan Wen-quan et al.

[0088] 3C shows a schematic of an exemplary method for fitting a function 88 to a pre-calibrated diagram 64 of absorption cell intensity I as a function of absolute wavelength λ. Here, the peaks, valleys, and generally recognizable portions of each curve can be fitted to one another using a selected fitting method. The fitting results in a wavelength tracker phase difference ΔΦ WLT and absolute wavelength λ 84[ΔΦ WLT (λ)] is obtained.

[0089] 5 shows the components of the laser device 1 active in an operating mode, where the wavelength tracker interferometer 20 is operating continuously and the wavelength tracker phase setpoint (Φ sp ) wavelength tracker phase difference ΔΦ WLTThe continuous operation of the wavelength tracker 20 is both an advantage and a prerequisite. The advantage is that the wavelength tracker phase setpoint (Φ sp ) can immediately lead to an appropriate correction of the associated wavelength λ. Thus, the laser source can be controlled to adjust the setpoint wavelength λ SP The wavelength tracker phase difference ΔΦ WLT Since the wavelength tracker essentially moves in a 360-degree cycle, continuous operation is a prerequisite and measurements of deviation from the setpoint must be continuous to be meaningful. However, as long as the wavelength tracker is active, a new sweep of the trajectory generator 5 is not required, and the system can and does continue to operate uninterrupted for relatively long periods of time, where long periods can refer to periods spanning multiple days, weeks, or even months.

[0090]

[0089] It should be noted that the system of Figure 5 can, and in fact often will, also include a second interferometer 22 for measuring the movement and position of selected equipment 40. In the operational mode, the second interferometer 22 is also typically active, its setup and functionality being similar to the embodiment shown in and described with reference to Figure 2.

[0091] 6 shows an embodiment in which the tunable laser source 2 includes at least two lasers 90, 92. One or both of the first laser 90 and the second laser 92 can be tunable, i.e., have a variable frequency or wavelength. The first laser beam 94 and the second laser beam 96 output from the lasers 90, 92, respectively, are combined. For example, both beams can be directed to a summing device 98. The device 98 can be a quartz crystal. Suitable crystals include periodically poled lithium niobate (PPLN).

[0092]

[0091] For details and practical examples of summation crystals 98, see, for example, "Double-pass high-efficiency sum-frequency generation of a broadband orange laser in a single MgO:PPLN crystal," Optical Materials Express, Vol. 9, No. 2, pp. 837-844 (2019) [https: / / doi.org / 10.1364 / OME.9-000837], Dismas K. Choge et al.

[0093]

[0092] The first laser 90 may be a source that outputs light at approximately 282 THz. The second laser 92 may be a source that outputs light at approximately 192 THz. The combined output coupled to the light beam 4 may have a frequency on the order of 474 THz (wavelength on the order of 633 nm).

[0094]

[0093] Below, information is provided regarding actual embodiments of the disclosed methods and systems, as well as additional advantages and methods of use.

[0095]

[0094] The laser source includes, for example, a tunable laser such as a DFB laser, an ELC laser, a DBR laser, an FP laser (Fabry-Perot laser), an SFG (sum frequency generation) laser, an SHG (second harmonic generation) laser, or a gas laser.

[0096]

[0095] Depending on the type of laser, wavelength tuning can typically be achieved by adjusting the temperature, current, and / or control of a controllable filter (stretching a fiber Bragg, tilting a free-space grating (ECL), or a combination thereof).

[0097]

[0096] If the laser is a sum frequency laser (see Figure 6 for an example), wavelength control can be obtained by tuning one of the input lasers or by tuning both input lasers simultaneously.

[0098] The method begins by calibrating the phase of a mechanically ultrastable interferometer axis (wavelength tracker 20) to the spectral absorption spectrum of a molecular absorption gas cell 10. In a practical embodiment, the absorption cell's gas reference cell 14 contains iodine vapor. Once calibrated, the wavelength tracker's phase setpoint can be freely chosen to match the phase corresponding to the desired laser frequency or wavelength.

[0099] By locking to the phase of the wavelength tracker, the method and system effectively stabilizes the laser wavelength rather than the laser frequency. A control loop that keeps the phase of the wavelength tracker signal constant prevents periodic errors from occurring.

[0100]

[0099] A laser beam is used as input to a highly stable phase interferometer axis (axis of interferometer 20), preferably with a long OPD. The same laser beam is used to measure the transmittance of molecular absorption cell 10. The laser can receive a control input signal to tune the laser frequency.

[0101]

[0100] Step 1: Introduce a frequency sweep of the laser source (open or closed loop). Measure the phase of the interferometer axis and the transmittance of the absorption cell. During this measurement, it is assumed that the refractive index of the medium inside the wavelength tracker remains constant.

[0102] Step 2: Identify the absorption dip position and fit (part of) the measured transmission curve to a pre-calibrated curve 64, where the absolute laser frequency is known as a function of the phase of the stable reference 20.

[0103] Step 3: Define a new phase setpoint relative to the stable reference axis and tune the laser to the desired laser frequency or wavelength. From this point on, the laser frequency control loop actually stabilizes the laser wavelength within the ultra-stable wavelength tracker cavity (inside interferometer 20).

[0104]

[0103] The phase interferometer 20 may be, for example, a homodyne interferometer or a heterodyne interferometer.

[0105]

[0104] The molecular absorption cell 10 can be used in a single-pass, multi-pass configuration, or positioned within an optical cavity to increase the circulating power. A signal power reference detector is typically used to compensate for laser power variations when determining the reference cell transmission curve.

[0106]

[0105] By determining the laser frequency or wavelength as a function of the phase of an ultra-stable reference axis and then locking the laser to the phase of this ultra-stable interferometer axis, no wavelength modulation is required to keep the laser locked.

[0107]

[0106] Locking the laser wavelength to the phase of the ultrastable cavity allows the laser wavelength to be locked to any wavelength value, not just to frequencies that coincide with the spectral lines of the molecular absorption cell.

[0108]

[0107] Here, ultrastability is, for example, 5 × 10 -17 This may involve systems that use optical cavities with thermal noise limitations. Such systems may use a 30 cm long ULE cavity, a fused silica mirror substrate, and a crystalline mirror coating. As another example, an ultrastable laser system may include an optical cavity with reduced thermal noise. Such systems may have a thermal noise of 5×10 -16 The latter is so low that the respective cavity can be called superstable.

[0109]

[0108] Below we compare this with the conventional system mentioned in the introduction.

[0110]

[0109] US20020043616A1 uses an etalon without phase measurement, so the linear calibration line is not and cannot be determined.

[0111]

[0110] US20200182702A1 describes a tunable laser system in which a laser is tuned over a range of frequencies and output to an interferometer with multiple outputs. A wavelength reference, such as a gas cell, can be used in combination with at least two interferometer outputs to generate an absolute measurement of the tunable laser wavelength. This document describes using a "continuous phase signal" from the interferometer as a measurement of the laser wavelength, but does not disclose the use of a single stable reference. As a result, there is no prior "locking" of the wavelength. Instead, the system performs continuous, repeated calibrations. In contrast, the systems and methods disclosed herein use a laser without the need to calibrate it once and perform subsequent wavelength tunings.

[0112] This method and system avoids periodic errors: Locking the laser frequency or wavelength to the phase of the wavelength tracker eliminates the effect of periodic errors due to refractive index changes.

[0113]

[0112] By locking the laser frequency or wavelength to the phase of the wavelength tracker, the laser wavelength becomes insensitive to changes in the refractive index of the surrounding medium.

[0114]

[0113] Wavelength trackers are traditionally intended to measure laser frequency and refractive index noise. This noise is assumed to be subtracted from the interferometer axis, which shares the same source and medium, using an appropriate gain factor. The gain factor is equal to the OPD ratio between the interferometer axis and the wavelength tracker axis. By stabilizing the laser wavelength in the wavelength tracker rather than the laser frequency in the source, wavelength compensation becomes inherent and does not need to be applied by software. This means that no information about the OPD ratio is needed to make the compensation available.

[0115]

[0114] The disclosed method allows tracking of the actual wavelength of the laser output. In other words, it is not the set wavelength that is adjusted, but the actual wavelength of the laser output (which may deviate slightly from the set wavelength). Here, the phase difference in the interferometer can be continuously measured, providing a continuous signal. The latter allows for immediate and accurate correction and adjustment of deviations from the setpoint wavelength. Therefore, this method and a system including this method allow for more precise adjustment and control of the laser wavelength.

[0116] In a practical embodiment, the laser source 2 may include one or more helium-neon (HeNe) lasers. The lasers may have an operating wavelength of about 633 nm and an output power on the order of 1 mW. Such lasers are suitable for displacement interferometry applications, such as lithography machines (including DUV and EUV), mask inspection, and mask writing equipment. Note that these values ​​are merely examples. Other wavelengths for interferometer systems are also contemplated within the scope of this disclosure.

[0117] For long-distance displacement interferometry, stabilized laser sources with coherence lengths greater than the measured path length difference are typically used. Zeeman-stabilized and two-mode frequency-stabilized HeNe lasers have shown instabilities on the order of about 100 kHz and long-term stability of several MHz or more. HeNe lasers with internal (or external) iodine reference cells stabilized to the Doppler-free iodine molecular hyperfine line can exhibit instabilities down to sub-pm per meter levels over several hours.

[0118] HeNe laser frequencies are inherently sensitive to vibrations from the environment, such as from an electronics rack, which can excite the eigenmodes of the laser tube. For fiber-coupled interferometers, pointing drift of the HeNe laser can cause coupling loss over time, necessitating periodic checks and coupling optimization. To compensate for wavelength stability noise that may arise from the laser source or refractive index noise, HeNe lasers can be operated using a dedicated wavelength tracker in accordance with the present disclosure.

[0119]

[0118] Other suitable wavelength-tunable laser sources include various types of frequency-doubled lasers, which are capable of producing output powers of the order of a few watts with high beam quality. For example, a frequency-doubled Nd:YAG laser can be used to produce a beam with a wavelength of 660 nm, ND:YLF can be used to produce a beam with a wavelength of 656.5 nm, and Nd:YVO4 or Nd:GdVO4 can be used to produce a beam with a wavelength of 671 nm.

[0120]

[0119] All of the above lasers require frequency or wavelength stabilization to be useful for long-range metrology applications. Here, the present disclosure proposes to replace known and commercially available laser stabilization methods with the methods described herein above.

[0121]

[0120] Fast laser frequency or wavelength control can typically be achieved by one or more of piezo control (cavity length / grating tilt), current control, and thermal control (slower), depending on the type of laser.

[0122]

[0121] Although specific reference is made in this specification to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0123] Although specific reference is made herein to embodiments of the invention in relation to lithography apparatus, embodiments of the invention can also 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 objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus are sometimes referred to generically as lithography tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0124]

[0123] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention may also be used in other applications, for example imprint lithography, and furthermore is not limited to optical lithography where circumstances permit.

[0125]

[0124] Embodiments of the present invention may, where circumstances permit, be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium and readable and executable by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic storage media, optical storage media, flash memory devices, or electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Also, firmware, software, routines, or instructions may be described herein as performing certain actions. However, it should be understood that such description is for convenience only, and that such actions may actually be due to a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and in so doing, may cause actuators or other devices to interact with the physical world.

[0126]

[0125] While specific embodiments of the present invention have been described above, it will be apparent that the present invention can be practiced in other ways than those described above. The above description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the appended claims. Features of each embodiment can, for example, be combined. Other aspects of the present invention are set forth in the following numbered clauses. 1. A method for stabilizing the wavelength of a tunable laser device, comprising: simultaneously providing a laser beam from a tunable laser source to a first interferometer having a mechanically stable reference axis and to a gas absorption cell; scanning a wavelength range from a first wavelength to a second wavelength with a laser beam; determining a transmission spectrum of the gas absorption cell as a function of wavelength difference relative to the first wavelength; determining a phase change as a function of wavelength difference for a first wavelength using a first interferometer; determining a transmission spectrum as a function of phase change using the transmission spectrum of the gas absorption cell and the phase change as a function of wavelength difference; correlating the determined transmission spectrum as a function of phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide the absolute laser wavelength as a function of phase change; defining a phase setpoint corresponding to the wavelength setpoint; and tuning the tunable laser device to a wavelength setpoint using the phase setpoint. 2. The method of clause 1, comprising measuring the displacement of a device in a lithographic apparatus using a laser beam set at a setpoint wavelength. 3. The method of clause 2, wherein the step of measuring the displacement using the laser beam includes providing the laser beam to a second interferometer. 4. The method of clause 2 or 3, wherein the device comprises a wafer table, a wafer, a mask, a mask table, or any other equipment in an optical stage of a lithographic apparatus. 5. The method of clause 1, wherein the step of correlating the determined transmission spectrum as a function of phase change to a pre-calibrated transmission spectrum of the tunable laser device includes fitting the determined transmission spectrum to the pre-calibrated transmission spectrum using a fitting algorithm. 6. The method of any of clauses 1-5, wherein the step of tuning the tunable laser device to the setpoint wavelength using the phase setpoint includes continuously using the first interferometer to determine the phase change relative to the phase setpoint. 7. A wavelength tunable laser device, comprising: a tunable laser source for providing a laser beam; a first interferometer having a mechanically stable reference axis adapted to receive the laser beam; a gas absorption cell adapted to receive the laser beam simultaneously with the interferometer; a processor, The processor scanning a wavelength range from a first wavelength to a second wavelength with a laser beam; determining the transmission spectrum of the gas absorption cell as a function of time; determining the phase change as a function of time using a first interferometer; determining a transmission spectrum as a function of phase change using the transmission spectrum of the gas absorption cell and the phase change as a function of time; correlating the determined transmission spectrum as a function of phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide the absolute laser wavelength as a function of phase change; defining a phase setpoint corresponding to the wavelength setpoint; and A tunable laser device adapted to tune the tunable laser device to a wavelength setpoint using the phase setpoint. 8. The tunable laser device of clause 7, wherein determining the transmission spectrum of the gas absorption cell as a function of time includes correlating the time with the wavelength difference relative to the first wavelength. 9. A tunable laser device as described in clause 7 or 8, wherein determining the phase change as a function of time using the first interferometer includes correlating the time with a wavelength difference for the first wavelength. 10. A tunable laser device according to clause 7, wherein the laser device is adapted to measure displacement of a device in a lithographic apparatus using a laser beam set at a wavelength setpoint. 11. A tunable laser device as described in clause 10, including a second interferometer adapted to receive the laser beam and measure the displacement. 12. A tunable laser device according to clause 10 or 11, wherein the device in a lithographic apparatus comprises a wafer table, a wafer, a mask, a mask table, or any other equipment in an optical stage of a lithographic apparatus. 13. A tunable laser device according to any of clauses 7 to 12, wherein the processor is adapted to correlate the determined transmission spectrum to the pre-calibrated transmission spectrum by fitting the determined transmission spectrum to the pre-calibrated transmission spectrum using a fitting algorithm. 14. A tunable laser device according to any of clauses 7 to 13, wherein the tunable laser device is adapted to have an operating mode and the first interferometer is adapted to continuously determine a phase change relative to a phase setpoint. 15. A position measurement system comprising a tunable laser device as described in clause 7 and / or adapted to perform the method described in clause 1. 16. A lithographic apparatus comprising at least one position measurement system according to clause 15.

Claims

1. 1. A method for stabilizing the wavelength of a tunable laser device, comprising: simultaneously providing a laser beam from a tunable laser source to a first interferometer having a mechanically stable reference axis and to a gas absorption cell; scanning the laser beam through a wavelength range from a first wavelength to a second wavelength; determining a transmission spectrum of the gas absorption cell as a function of wavelength difference relative to the first wavelength; determining a phase change using the first interferometer as a function of the wavelength difference for the first wavelength; using the transmission spectrum of the gas absorption cell and the phase change as a function of the wavelength difference to determine a transmission spectrum as a function of the phase change; correlating the determined transmission spectrum as a function of the phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide an absolute laser wavelength as a function of the phase change; defining a phase setpoint corresponding to the wavelength setpoint; and tuning the tunable laser device to the wavelength setpoint using the phase setpoint.

2. The method of claim 1 , comprising measuring displacement of a device in a lithographic apparatus using the laser beam set at a setpoint wavelength.

3. The method of claim 2 , wherein measuring displacement using the laser beam comprises providing the laser beam to a second interferometer.

4. The method of claim 2 or 3, wherein the device comprises a wafer table, a wafer, a mask, a mask table, or any other equipment in an optical stage of the lithographic apparatus.

5. 2. The method of claim 1 , wherein correlating the determined transmission spectrum as a function of the phase change to the pre-calibrated transmission spectrum of the tunable laser device comprises fitting the determined transmission spectrum to the pre-calibrated transmission spectrum using a fitting algorithm.

6. 6. The method of claim 1, wherein the step of tuning the tunable laser device to the setpoint wavelength using the phase setpoint comprises continuously using the first interferometer to determine a phase change relative to the phase setpoint.

7. 1. A wavelength tunable laser device, comprising: a tunable laser source for providing a laser beam; a first interferometer having a mechanically stable reference axis adapted to receive the laser beam; a gas absorption cell adapted to receive the laser beam simultaneously with the interferometer; a processor, The processor: scanning the laser beam over a wavelength range from a first wavelength to a second wavelength; determining a transmission spectrum of the gas absorption cell as a function of time; determining a phase change as a function of time using the first interferometer; determining a transmission spectrum as a function of the phase change using the transmission spectrum of the gas absorption cell and the phase change as a function of time; correlating the determined transmission spectrum as a function of the phase change with a pre-calibrated transmission spectrum of the tunable laser device to provide an absolute laser wavelength as a function of the phase change; defining a phase setpoint corresponding to the wavelength setpoint; and A tunable laser device adapted to use said phase setpoint to tune said tunable laser device to said wavelength setpoint.

8. 8. The tunable laser device of claim 7, wherein determining the transmission spectrum of the gas absorption cell as a function of time comprises correlating time with a wavelength difference for the first wavelength.

9. 9. The tunable laser device of claim 7 or 8, wherein determining the phase change as a function of time using the first interferometer comprises correlating time with the wavelength difference for the first wavelength.

10. 8. The tunable laser device of claim 7, wherein the laser device is adapted to measure displacement of a device in a lithographic apparatus using the laser beam set to the wavelength setpoint.

11. The tunable laser device of claim 10 including a second interferometer adapted to receive the laser beam and measure the displacement.

12. 12. The tunable laser device of claim 10 or 11, wherein the device in the lithographic apparatus comprises a wafer table, a wafer, a mask, a mask table, or any other equipment in an optical stage of the lithographic apparatus.

13. 13. The tunable laser device of claim 7, wherein the processor is adapted to correlate the determined transmission spectrum to a pre-calibrated transmission spectrum by fitting the determined transmission spectrum to the pre-calibrated transmission spectrum using a fitting algorithm.

14. 14. The tunable laser device of claim 7, wherein the tunable laser device is adapted to have an operating mode and the first interferometer is adapted to continuously determine a phase change relative to the phase set point.

15. A position measurement system comprising a tunable laser device according to claim 7 and / or adapted to perform the method according to claim 1.

16. A lithographic apparatus comprising at least one position measurement system according to claim 15.

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