Method and system for stabilizing sum frequency generating laser

Abstract

The present disclosure provides a method of operating a sum frequency laser, comprising the steps of: simultaneously emitting a first light beam having a first frequency from a first light source and a second light beam having a second frequency from a second light source; modulating the first frequency in a first direction with a matched tuning frequency and modulating the second frequency in an opposite direction with a matched tuning frequency; and transmitting the first beam and the second beam to a summing device and outputting a stabilized sum frequency beam from the summing device.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from European application No. 23159806.1 filed on March 3, 2023, and the entire contents of said European application are incorporated herein by reference. Technical Field

[0003] The present invention relates to a method and system for stabilizing a sum frequency generation (SFG) laser. The SFG laser can be used, for example, in metrology systems. Conventional metrology systems can be used, for example, to measure displacements of components of lithographic apparatus, including components of projection systems used in optical lithography systems. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (often referred to as a "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] As semiconductor manufacturing processes continue to advance, the number of functional elements (such as transistors) per device has steadily increased for decades, while the size of circuit components has continued to decrease. This trend is often referred to as "Moore's Law." To keep pace with Moore's Law, the semiconductor industry is seeking technologies that can produce smaller and smaller features. To project patterns onto substrates, lithographic equipment can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features patterned on the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithographic equipment using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm, can be used to form smaller features on substrates compared to lithographic equipment using radiation with a wavelength of, for example, 193 nm.

[0006] A sum-frequency generation (SFG) laser is a laser with an output beam whose frequency is equal to the sum of the two mixed input laser beams. Each input laser can have a cavity in which laser light of a specific frequency is generated. The lasing frequency of one or both lasers can be adjusted, for example, by modifying thermal control, current control, piezoelectric actuators, or grating elements. The output of the SFG laser must be stabilized to provide the modulation-free sum-frequency output required for the accuracy of the application. Known methods for stabilizing the output of SFG lasers include locking the total sum-frequency output to a reference, such as a gas absorption peak or a reference laser with a frequency shift.

[0007] Mode hopping, or mode hopping, is a phenomenon in which a laser exhibits abrupt jumps in optical frequency associated with transitions or transitions between different modes of its resonator. Existing methods for laser frequency tuning involve periodic recalibration of thermal and piezoelectric setpoints to find a "safe zone" where sidebands (side modes) are absent, thus avoiding mode hopping. However, even the presence of additional eigenfrequencies or wavelengths in the laser's cavity can cause instabilities in the output of a sum-frequency-generated laser. Even in cases where such instabilities are only temporary, they are desirable to avoid for continuous and high-end applications, such as those in lithography tools.

[0008] Side modes can be detected by observing the presence of free spectral range frequencies in the optical output of the laser. The presence of side modes can be avoided by selecting appropriate thermal and piezoelectric set points.

[0009] During laser calibration using existing methods, the center frequency of the sum-frequency generation laser may vary. Consequently, the laser's operating point may approach the point where side modes appear. Consequently, individual calibration of each laser will impact sum-frequency generation frequency stability and throughput loss. Furthermore, while various measures and control systems are in place to maintain each input laser within a narrow range from the set point, drift in the input laser frequency during operation is still possible. Consequently, the output may be inherently unstable.

[0010] CN101303507A discloses a sum frequency generating laser, whereby it is desired to change the frequency of a first input beam while keeping the wavelength of the sum beam constant. This is achieved by changing the frequency of a second input beam.

[0011] EP 1 650 597 A1 discloses the number of possible combinations of frequencies of a sum frequency generating laser and an input light beam, while keeping the frequency of the sum output light beam constant.

[0012] Systems as described above may all be subject to temporary instabilities in the output of the sum frequency generating laser.An object of the present disclosure is to provide a stable sum frequency generating laser. Summary of the Invention

[0013] The present disclosure provides a method for operating a sum frequency laser, comprising the following steps:

[0014] Simultaneously emitting a first light beam having a first frequency from a first light source and a second light beam having a second frequency from a second light source;

[0015] modulating the first frequency at a matching tuning frequency in a first direction and modulating the second frequency at a matching tuning frequency in an opposite direction; and

[0016] The first light beam and the second light beam are sent to a summing device and a stable sum frequency light beam is output from the summing device.

[0017] In one embodiment, the method comprises the following steps:

[0018] detecting a side mode of one of the first light beam and the second light beam; and

[0019] If the detected side mode exceeds a predetermined threshold, the modulation directions of the first frequency and the second frequency are reversed.

[0020] In an embodiment, the threshold value comprises one or more of the following: the amplitude of the side mode, the power of the side mode, the brightness, the ratio of the amplitude of the side mode to the amplitude of the first frequency or the second frequency, the ratio of the power of the side mode to the power of the first frequency or the second frequency, or the ratio of the brightness to the brightness of the first frequency or the second frequency.

[0021] In an embodiment, the threshold value is a ratio of the amplitude or power of the side mode relative to the amplitude or power of the first frequency or the second frequency.

[0022] In an embodiment, the threshold is approximately 40 dB, which is the ratio between the power of the main mode and the power of the detected side mode.

[0023] In an embodiment, the frequency variation of the first light beam and the frequency variation of the second light beam are 180 degrees out of phase.

[0024] In an embodiment, the step of detecting side modes comprises performing power measurements of free spectral range frequency content of the first light source and the second light source.

[0025] In an embodiment, the step of detecting side modes comprises: diverting a portion of the first light beam to a first sensor and diverting a portion of the second light beam to a second sensor.

[0026] In an embodiment, the first sensor and the second sensor comprise a power detector and an algorithm for providing a signal related to the power of the side mode.

[0027] In an embodiment, modulating the first frequency and modulating the second frequency include: thermally tuning and / or current tuning and / or piezoelectrically tuning the first light source and the second light source, respectively.

[0028] In an embodiment, the matched tuning frequency in the step of modulating the first and second light beams is in the range of mHz to kHz.

[0029] According to another aspect, the present disclosure provides an exposure apparatus comprising one or more sum frequency generating lasers using an operating method according to the present invention.

[0030] According to another aspect, the present disclosure provides a lithographic apparatus comprising one or more sum frequency generating lasers using an operating method according to the present invention.

[0031] According to yet another aspect, the present disclosure provides a projection system for an optical lithography system comprising one or more sum frequency generating lasers using the operating method according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0033] - Figure 1 depicts a schematic overview of a lithographic apparatus;

[0034] - Figure 2 A diagram depicting an embodiment of the system of the present disclosure;

[0035] - Figure 3A and Figure 3B an exemplary graph depicting an example of a mode-hopping region indicating side mode power amplitude observed in the laser power output, wherein the wavelength-controlling piezoelectric voltage (vertical axis) is plotted against the wavelength-controlling temperature of the laser cavity (horizontal axis);

[0036] - Figure 4 depicts an exemplary graph of a laser tuning curve, wherein the vertical axis illustrates the laser wavelength of the laser beam and the horizontal axis illustrates the laser temperature, the exemplary graph indicating an example of laser tuning with a fixed piezoelectric set point and a universal cavity mode;

[0037] - Figure 5 depicts a diagram illustrating modes of a tunable laser;

[0038] - Figure 6 A diagram depicting another embodiment of the system of the present disclosure;

[0039] - Figure 7A and Figure 7B depicts an exemplary graph of laser control according to the disclosed method, the exemplary graph indicating wavelength modulation and resulting side mode power (vertical axis) of a respective laser versus a controllable parameter such as piezoelectric voltage (horizontal axis); and

[0040] - Figure 8 Embodiments of the present disclosure are schematically depicted. DETAILED DESCRIPTION

[0041] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation. Radiation may include (deep) ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0042] As used herein, the terms "reticle," "mask," or "patterning device" should be broadly interpreted 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 the substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0043] A "beamsplitter" (also known as a "beamsplitter") is an optical device that splits a beam of light into two. Beamsplitters are part of many optical systems. In a first variation, a beamsplitter can consist of a cube made from two triangular glass prisms bonded 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 certain wavelength) half of the light incident through one "port" (usually a face of the cube) is reflected and the other half is transmitted due to frustrated total internal reflection (FTIR). Polarizing beamsplitters, 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. A semitransparent mirror consists of an optical substrate, typically a piece of glass or plastic with a thin, partially transparent metal coating. The thin coating can be aluminum or silver deposited using physical vapor deposition. The thickness of the coating is controlled so that a predetermined portion (e.g., half, but this can be any ratio between 0 and 100%) of the light incident at a 45-degree angle and not absorbed by the coating or base material is transmitted and the remainder is reflected. A third variation of a beam splitter is a dichroic mirror prism assembly that uses a dichroic optical coating to split an incident light beam into multiple spectrally distinct output beams.

[0044] A "gas reference cell" is a cell filled with a gas, commonly used in laser absorption spectroscopy. The optical absorption coefficient in the gas, or some other effect resulting from the interaction of the gas with light (such as a frequency-dependent absorption spectrum), allows comparison with a reference beam. Typically, small changes in the light beam caused by the passage of the gas are measured as a function of the laser beam's optical frequency, and the results are presented as a spectrum (e.g., an absorption spectrum). Peaks obtained in this spectrum can be used to identify certain chemical species and measure their concentrations. For such measurements, those skilled in the art typically use a single-frequency laser with a tunable wavelength. Once the spectrum and chemical composition of the cell are known, a gas cell also allows the reverse process of identifying the laser's frequency or wavelength using absorption spectroscopy.

[0045] Gas reference cells are typically provided with appropriate optical windows (with high transmittance over the entire relevant spectral region) for light to enter and leave the cell. Gas reference cells are commercially available with many different gases, including atomic and (usually diatomic) molecular gases. Typical examples are iodine (I2), hydrogen (H2), helium, carbon monoxide (CO), and acetylene (C2H2). This allows for the use of a wide range of standard spectral lines. In some cases, alkali metals such as sodium (Na), potassium (K), rubidium (Rb), or cesium (Cs) are used, as they produce a sufficiently high vapor pressure when electrically heated to some suitable temperature. Such cells may be referred to as vapor cells. Sealed gas cells should be reliably leak-free. Therefore, helium leak testing is often applied.

[0046] An "absorption cell" or "molecular absorption cell" is a device that contains a gas reference cell. The absorption cell can compare the difference in intensity between two parts of the same beam, one part being sent to the gas reference cell and a first sensor that receives the light once it has passed through the reference cell at least once, and the second part being sent unimpeded to a second sensor that serves as a reference.

[0047] The "refractive index" (coefficient of refraction) is a value calculated as the ratio of the speed of light in a vacuum to the speed of light in a second, denser medium. The refractive index variable can be represented by the letters n or n' in descriptive text and mathematical equations.

[0048] An "interferometer" or "laser interferometer" can measure distance or displacement by measuring the phase difference between two light beams: one beam sent to a first reflector or surface at a fixed reference distance, and one beam sent to a second reflector or surface at another distance. When the two reflected signals are recombined in the interferometer, the resulting phase is related to the distance of the second surface from the interferometer. If the distance of the second surface changes, the phase of the combined signal also changes. The utility of these methods lies in the fact that they can measure long distances while maintaining accuracy.

[0049] In a heterodyne interferometer, the measurement and reference beams that interfere at the detector typically originate from the same laser source. The source may have been frequency-shifted to allow heterodyne phase detection. However, the two (split) frequencies can also be generated by two different frequency-locked or phase-locked lasers.

[0050] A "wavelength tracker" is a specific variant of an interferometer that is configured to measure the phase difference between two reflected beams: one beam reflected off a first fixed reflector that provides a first reference axis, and a second beam reflected off a second fixed reflector that provides a second reference axis of a different length than the first. Because the two reflective surfaces are fixed, the measured phase difference will change only if the wavelength of the beams changes. Thus, a wavelength tracker allows monitoring of wavelength deviations from a set point.

[0051] Figure 1A lithographic apparatus LA is schematically depicted and 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 configured 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 wafer stage) WT configured 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., a portion comprising one or more dies) of the substrate W.

[0052] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, 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 illuminator IL may be used to condition the radiation beam B so as to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0053] The term "projection system" PS as used in the present invention should be interpreted broadly to cover various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation 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 may be considered synonymous with the more general term "projection system" PS.

[0054] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g. water) so as to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US 6952253, which is incorporated herein by reference.

[0055] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0056] In addition to the substrate support WT, the lithographic apparatus LA may also include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold multiple sensors. The cleaning devices may be arranged to clean a portion of the lithographic apparatus, such as a portion of the projection system PS or a portion of the system that provides immersion liquid. The measurement platform may be movable beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0057] In operation, the radiation beam B is incident on a patterning device (e.g. a mask) MA held on the mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (in Figure 1 A further position sensor (not explicitly depicted in the figure) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, they can be located in the spaces between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe lane alignment marks.

[0058] To illustrate the present invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes: the x-axis, the y-axis, and the z-axis. Each of the three axes is orthogonal to the other two axes. A rotation about the x-axis is referred to as an Rx rotation. A rotation about the y-axis is referred to as an Ry rotation. A rotation about the z-axis is referred to as an Rz rotation. The x-axis and the y-axis define a horizontal plane, while the z-axis lies in the vertical direction. The Cartesian coordinate system is not limited to the present invention and is used for illustration purposes only. Alternatively, another coordinate system, such as a cylindrical coordinate system, may be used to illustrate the present invention. The Cartesian coordinate system may be oriented differently, for example, so that the z-axis has a component lying in the horizontal plane.

[0059] In a sum frequency generating laser, the frequencies of the first light beam 12 and the second light beam 14 are summed by a summing crystal 16. For example, the first laser beam 12 (having an output at 1064 nm or 282 Thz) and the second laser beam 14 (having an output at, for example, 1562 nm or 192 Thz) can be combined by the summing crystal 16 to obtain a laser beam 20 having a wavelength of 633 nm (474 ​​THz).

[0060] A summing crystal is an optical crystal, typically a single crystal (single-crystal optical material), used as a gain medium in lasers (such as solid-state lasers). Sum frequency generation (SFG) is a second-order nonlinear optical process in which two input photons at angular frequencies ω1 and ω2 are annihilated to simultaneously generate a single photon at frequency ω3. This can be considered a second-order phenomenon in nonlinear optics and typically occurs when light interacts with asymmetric matter (e.g., surfaces and interfaces) and the input light has very high intensity. Sum frequency generation is a "parametric process," meaning that the photons maintain energy conservation while the material of the crystal remains unchanged.

[0061] The summing crystal can be a periodically poled lithium niobate (PPLN) crystal. The crystal can include one or more dopants, typically rare earth ions or transition metal ions. For example, the crystal can be doped with MgO, neodymium, ytterbium, erbium, or chromium.

[0062] Note that the optical crystal 20 can have any optical function, including but not limited to frequency doubling, difference frequency generation, sum frequency generation, optical parametric oscillation, and other nonlinear processes. As an example, the embodiments of the present disclosure will be described with respect to summation, but the concept is applicable to all possible optical crystals and tunable lasers.

[0063] A known method for stabilizing the output of sum-frequency-generated lasers is to lock the laser frequency output to the spectral line of a gas absorption cell. To stabilize the laser to the gas immersion via wavelength modulation spectroscopy, wavelength modulation is typically applied, and the laser gas transmission signal is demodulated with an odd harmonic of the modulation signal (e.g., the first or third harmonic) to generate a control signal that is used to lock the laser frequency to the center of the spectral line.

[0064] In the method of the present disclosure, it is proposed to stabilize the output of the sum frequency generating laser system 10 by ensuring a stable input.

[0065] Overall reference Figure 2 A sum frequency generation (SFG) light source 10 may include a first tunable light source L1 and a second tunable light source L2. The first light source L1 may generate a first light beam 12. The first light beam includes at least radiation having a first frequency f1. The second light source L2 may generate a second light beam 14. The second light beam includes at least radiation having a second frequency f2. The outlets of the first light source L1 and the second light source L2 are connected, for example, via optical fibers, to an inlet of a summing crystal 16. The summing crystal is adapted to receive the first light beam 12 and the second light beam 14. The crystal 16 may provide a third light beam 20 including at least radiation having a third frequency. The third frequency may be the sum of the first frequency and the second frequency.

[0066] The first and second light sources L1 and L2 can be tunable lasers. The first and second light sources can be controlled by respective first and second amplifiers A1 and A2. Lock-in amplifiers A1 and A2 can be controlled by a controller 22. The controller can provide setpoints or drive signals to the respective amplifiers. Amplifiers A1 and A2 provide corresponding actuation signals, such as piezoelectric voltages, to the respective light sources.

[0067] The first and second light sources L1 and L2 can be provided with first and second gas reference cells 24 and 26, respectively. Each reference cell 24 and 26 is provided with a corresponding reference light sensor 28 and 30, a gas cell light sensor 32 and 34, and typically a beam splitter 36 and 38. The respective optical paths receiving the outputs of the respective lasers L1 and L2 can be provided with first and second beam splitters 40 and 42 to direct a small portion of the first and second light beams 12 and 14 to the respective gas reference cells 24 and 26. The small portion can be, for example, in the range of 0.1% to 5% of the received light, such as approximately 1%. The gas cell beam splitters 36 and 38 can direct a portion (typically half) of the received light to the respective reference cells 24 and 26, and another portion to the reference light sensors 28 and 30.

[0068] The corresponding amplifiers A1 and A2 can receive the signals recorded by the respective light sensors 28 to 34. The difference between the gas cell signal and the reference signal can enable locking to the spectral lines of the respective gas cells 24, 26. In response, the first amplifier A1 can therefore add a first offset to the input of the first light source L1. If and when necessary, the second amplifier A2 can add a second offset to the input of the second light source L2 to correct the light source signal.

[0069] When referring to a fraction or portion of a light beam in this document, this can mean a measure of power. In this document, a fraction can be expressed as luminous intensity, which is a measure of the wavelength-weighted power emitted by a light source per unit solid angle in a particular direction. The light source can be based on a luminance function, a standardized model of the sensitivity of the human eye. The international unit of luminous intensity is the candela (cd). Alternatively, the description herein may refer to other suitable measures indicating light power, such as luminous energy [lm.s], luminous flux [lumen], or expressed in the general units of energy [W] or [J / s].

[0070] For example, the method can include stabilizing the first laser output 12 at a first frequency or wavelength, such as 1064 nm. The method can include stabilizing the second laser output 14 at a second frequency or wavelength, such as 1562 nm. The stabilized input beams 12, 14 result in a stabilized sum frequency generated output 20 having a wavelength of, for example, 633 nm. It will be understood that the wavelength outputs provided herein are merely exemplary and that the methods herein can be applied to any combination of frequencies or wavelengths.

[0071] The laser frequency of one or both input lasers L1, L2 may be adjusted, thereby also controlling the frequency of the summed sum frequency generating laser 20. Control of the lasers may involve adjusting controllable parameters, for example using thermal control or current and / or voltage applied to a piezoelectric actuator.

[0072] If the sum-frequency generation output does not require wavelength modulation, it is proposed that the input lasers (i.e., the first laser 10 having a first output wavelength and the second laser L2 having a second output wavelength) be locked to their respective wavelength modulation signals. For example, the first laser L1 having a first laser output of, for example, 1064 nm is locked to the spectral line of the first gas absorption cell 24 via the wavelength modulation signal s1. The second laser L2 having a second laser output of, for example, 1560 nm is locked to the spectral line of the second gas absorption cell 26 via the wavelength modulation signal s2.

[0073] The modulation signals s1 and s2 are chosen to be 180° out of phase with respect to each other. This way, the modulation is cancelled in the sum frequency:

[0074]

[0075] The frequency of the first laser , and the frequency of the second laser .here, is the modulation frequency. The sum frequency generator outputs a frequency of 20 It is given by:

[0076]

[0077] Figure 2 A schematic diagram of an exemplary arrangement for providing such a stable sum frequency generated output is shown, wherein each of the first input laser L1 and the second input laser L2 is stabilized to a respective reference cell 24, 26. The system 10 provides the two input beams with sum frequency lasers having 180 degrees out-of-phase (piezoelectric) modulation so that their wavelengths are stabilized.

[0078] In operation, the sum frequency generating laser system 10 generates a sum frequency laser 20 by summing the frequencies of the first light beam 12 and the second light beam 14 of the two input lasers L1 and L2. The first laser L1 and / or the second laser L2 are stabilized to an absorption peak using the principles of wavelength modulation spectroscopy or frequency modulation spectroscopy. The wavelength modulation signals of lasers L1 and L2 are out of phase and amplitude matched, so that the sum frequency of the sum frequency generating laser 20 is not modulated.

[0079] When the laser is modulated with amplitude dw at modulation frequency ω m Around its center frequency ω L When modulated, the instantaneous frequency is ω=ω L +dω×cos(ω m t). The radiation intensity transmitted through the respective absorption cell can then be expressed as a Fourier series expansion.

[0080] Advantageously, stabilizing the frequency of the input lasers of the sum frequency generation laser 10, rather than locking the sum frequency generation output 20, provides a more time-efficient arrangement because stabilization preparations can be performed before generating the measurement light. Furthermore, by matching the wavelength modulation of the two input lasers (e.g., by locking to a gas cell), the modulation is canceled out in the sum frequency laser wavelength.

[0081] The frequencies of the first and second light beams 12 and 14 can be adjusted using thermal control, current, and / or piezoelectric actuators of the respective lasers L1 and L2. Existing methods involve periodic recalibration of each light source. Recalibration herein can involve, for example, modulating the thermal and piezoelectric set points to find a "safe zone" where sidebands do not occur. The first light source L1 is shown in FIG. Figure 3A , and the second light source L2 sees Figure 3B Herein, the horizontal axis of the graph represents the temperature T of the respective resonant cavity of each laser. The vertical axis represents the voltage V that drives the piezoelectric actuator, which controls the size of the resonant cavity within a certain control range. Figure 3A and Figure 3BIn the figure, dark regions 50 and 56 indicate "safe zones," where the beams generated by their respective light sources primarily consist of light having a setpoint frequency, also known as the main mode. Bright lines 52 and 54, and 58 and 60, respectively, indicate the presence of side modes. Side mode light typically has one wavelength more or less than the main mode. Side modes can arise when standing waves appear in a resonant cavity that have one wavelength more or less than the main mode.

[0082] Side modes can be detected by observing the presence of free spectral range frequencies in the optical output of the respective laser. The presence of side modes can be avoided by selecting appropriate thermal and piezoelectric set points.

[0083] Figure 4 An example of the relationship between the cavity temperature (horizontal axis) and the wavelength of the light beam is shown. Figure 3A Compared to the graph of , varying the temperature while keeping the piezoelectric voltage constant corresponds to crossing e.g. Figure 3A 66. However, at some point, a side mode may also be activated, such as mode 54. Indicated by line 68, this means that there is a risk that the wavelength of the main mode may jump from one wavelength to the next. This can be considered a "mode hop region" 64, i.e., a setting in which the laser output has a certain change in which it jumps or transitions to another setting. If so, then firstly, the wavelength of the main mode will deviate from the set point. In addition, mode hopping will typically introduce temporary jitter or inaccuracy in the output of the sum frequency generating laser output. Here, it is noted that mode hopping can occur in one direction, but can also occur backwards, thus resulting in multiple jitter moments. Multiple jitter moments are to be avoided in high-end applications of sum frequency generating lasers.

[0084] Known methods of calibrating a stable SFM laser to avoid side modes involve periodic recalibration of thermal and piezoelectric set points to find a "safe zone" where no sidebands occur, thereby providing a stable SFM output. Figure 5 The power (vertical axis) of each mode is plotted against the wavelength (horizontal axis). Line 70 may illustrate the arrangement associated with the safety zone. Here, the dominant mode λ L The power is significantly greater than that of L+1 and λ L-1 The power of two adjacent side modes of the wavelength of . Line 72 illustrates the mode hop region. Here, the power of two adjacent modes is of comparable order of magnitude, so that there is a risk that the main mode jumps or transitions from one wavelength to the next, or vice versa.

[0085] The effective total cavity of the input lasers L1 and / or L2 can have a length of about 4 cm and a free spectral range of the order of 2.5 GHz. When the laser temperature changes, the mode selector and the cavity mode are tuned together, and the laser wavelength changes by, for example, about 1 GHz / K. However, due to slightly different expansion coefficients of the fibers used in the respective lasers, the longitudinal mode will shift slightly less than the center wavelength of the spectrally narrow grating. Eventually, the two longitudinal modes experience essentially the same output coupler reflectivity. This is as shown in FIG. Figure 5 This can happen, for example, up to three times across the entire thermal tuning range.

[0086] Side modes can be detected by observing the presence of free spectral range frequencies in the optical output of the laser. The presence of side modes can be avoided by choosing appropriate thermal and piezoelectric set points.

[0087] One issue is that this calibration can only be performed with the laser amplifier turned off, as crossing the sideband will result in mode hopping (IFM lock loss) and potential damage to the laser amplifier. As mentioned in the introduction, the center frequency of the sum-frequency generation laser may change during calibration. Calibration alone can result in lost production.

[0088] The goal is to always stay in a region where the sideband has an amplitude below a set threshold. For example, each laser is controlled to stay in a region where the sideband has an amplitude or power significantly less than that of the main mode. In this context, significantly less, or the threshold, can be equal to or less than 40 dB relative to the main mode.

[0089] Figure 6 Embodiments of the disclosed system 100 are provided that allow for side mode detection during operation of an SGF laser. System 100 is essentially a sum frequency generation laser with two tunable input lasers, L1 and L2. First light source L1 has a first frequency f1, and second light source L2 has a second frequency f2. First and second light beams 12 and 14, i.e., the optical outputs of L1 and L2, respectively, are combined in a frequency summing crystal 16. The crystal generates a light beam 20 having a third frequency f3, where frequency f3 can be f1 + f2. In a practical example, f1 is approximately 192 Thz, and f2 is approximately 281 THz.

[0090] To detect the presence of a side mode of at least one of lasers L1 and L2, the frequencies of first and second beams 12 and 14 of lasers L1 and L2 are swept in opposite directions and 180° out of phase with each other, with matching tuning trajectories. Here, the sum frequency of beam 20 remains constant during the sweep of beams 12 and 14.

[0091] At least a portion of the first light beam 12 and the second light beam 14 can be directed to the light sensors 80, 82, respectively. Directing the first light beam 12 and the second light beam 14 can involve beam splitters 12, 14 or similar optical elements. In practical embodiments, the at least a portion of the first light beam and the second light beam can be about 0.1% to 10% of the light, such as about 0.5% to 2%, such as about 1%.

[0092] The side mode detector may include power sensors 80 and 82. Sensors 80 and 82 may be connected to a processing device 84, such as a computer or dedicated processor. Processor 84 is typically equipped with an algorithm that can calculate the power of the side modes based on the sensor data. Here, the power of the side modes refers to the power at frequencies within the free spectral range. The processor provides a signal 86 to a controller 90 to control the SGF light source 100.

[0093] The first and second light sensors 80, 82 can be adapted to directly detect the power of spectral frequencies in their respective light beams. Sensors 80, 82 can be, for example, high-frequency optical power and / or energy meters. Such sensors are available, for example, from Thorlabs Inc., Newton, NJ (US).

[0094] Figure 7A and Figure 7B The signals 86 of the light sources L1 and L2 are respectively illustrated, that is, the signals 86 L1 and 86 L2 The horizontal axis represents frequency (ω [Hz]) or alternatively wavelength. The vertical axis represents the power of light at a certain frequency. In this paper, ω L1 and ω L2 The main modes or set points of the first and second light sources L1 and L2 are indicated, respectively. At certain times, such as periodically or continuously, the frequencies of the two light sources are modulated in opposite directions around the main mode. Once the power of the detected side mode reaches or exceeds a threshold level 92, the controller 90 reverses the direction of modulation of both light sources L1 and L2.

[0095] Once a side mode of one of the lasers is detected, the direction of frequency or wavelength tuning is reversed. Thus, side modes are avoided while the frequency of sum-frequency-generated beam 20 remains approximately constant. Furthermore, due to the avoidance of side modes, sum-frequency-generated laser 20 is essentially free of jitter or aberrations. In particular, since mode hopping is eliminated, significant jitter due to mode hopping is completely prevented.

[0096] Therefore, side mode detection of L1 and L2 can be based on power measurements of the free spectral range frequency content of each laser.

[0097] Figure 8The system 110 of the present disclosure is shown, wherein the Figure 2 and Figure 6 The system 110 includes a sum frequency generation light source 10 that can generate a stable SGF light beam 20. In this context, the first light source L1 and the second light source L2 can be stabilized, as described with respect to Figure 2 The embodiments shown in FIG.

[0098] In the system 110, the reference sensors 28, 30 can measure the spectral range. In other words, the sensors 28, 30 have a dual function, one function associated with the gas absorption cells 24, 26 as reference sensors, and the other function as spectral power meters. In this article, the measurement outputs of the sensors 28, 30 are provided to the amplifiers A1, A2 and the processing device 84, respectively. The processor 84 then provides a signal 86 to the controller 90. The signal includes a spectral analysis of the light beams 12, 14, respectively. The controller 90 then drives the respective first amplifier A1 and second amplifier A2. For example, the controller 90 can instruct the amplifiers A1 and A2 to modulate the frequencies of the first light source L1 and the second light source L2 in opposite directions, as described above with respect to Figure 6 Descriptive.

[0099] The sum frequency laser 20 can be used as a measurement laser for position or displacement measuring interferometers or wavelength tracker interferometer applications. Such applications are used to measure, for example, Figure 1 The position of the wafer table WT in the lithographic apparatus LA is illustrated in FIG.

[0100] like Figure 8 , the SGF beam 20 may be provided to a beam splitter 98. The beam splitter steers a small portion of the beam 20 to a molecular cell system 94, which includes a gas reference cell 96, a reference light sensor 102, and a gas cell light sensor 104. The molecular cell system 94 may function substantially similarly to the molecular cell systems of the gas reference cells 24, 26 and their respective light sensors.

[0101] The main portion of the beam 20 can be provided to an interferometer system 106. The interferometer system 106 can include one or more interferometers to measure one or more static and / or moving target mirrors. Examples of interferometer systems suitable for the methods and systems of the present disclosure are described in detail in, for example, US2021072088.

[0102] As an example, system 106 can include a stable cavity 108 as a reference axis, with reflectors at its ends. If stable, cavity 108 can allow the interferometer to function as a wavelength tracker. Alternatively, the reflectors at the ends of cavity 108 can be movable. System 106 can also typically include another reference axis that receives the other (typically half) of beam 20. System 106 typically includes an interferometer IFM connected to a phase measurement board PMB. The phase measurement board can output a signal 112 representing the phase difference.

[0103] Signal 112 can be provided to a summing device 118. The summing device can receive other inputs and add them to signal 112. For example, the summing device can be connected to a setpoint trajectory generator 114, which provides a setpoint signal 116. The setpoint generator 114 can be used to map the phase of the ultrastable cavity 108 to the absorption lines of the gas absorption cell 96. The output of the summing device 118 can be provided to the controller 90. The controller, in turn, typically controls the first light source L1 and the second light source L2 via the first amplifier A1 and the second amplifier A2.

[0104] In use, the set point trajectory generator 114 can add an offset to the set points of the first light source L1 and the second light source L2, for example to generate the laser beam 20 with a scanned frequency, for mapping the phase of the ultra-stable interferometer axis 108 relative to a pre-calibrated absorption spectrum of the molecular absorption cell 96.

[0105] After mapping the phase of the interferometer axis to the frequency reference pool, the phase signal 112 of the interferometer 106 can be used for wavelength measurement and / or wavelength control of the light beam 20.

[0106] In use, embodiments of the present disclosure allow for stabilization of the first light source L1 and the second light source L2 .Herein, the power detectors 28 , 30 may be used for side mode detection and for power normalization of the signals passing through the reference cells 24 , 26 .

[0107] In the following, we provide Figure 8 Various use embodiments of the embodiments are used as examples.

[0108] Use of Example 1 involves phase locking the laser 20 to a pre-calibrated ultrastable cavity 108 that has been calibrated for a molecular absorption cell, such as cell 96. Here, the sum frequency generation laser 20 is scanned (e.g., varied over a range of wavelengths between an upper threshold and a lower threshold) by a phase measurement device PMB to determine the relationship between the absorption spectrum of the reference cell 96 and the phase measured in the ultrastable cavity 108.

[0109] Here, lasers L1 and L2 are not scanned, ie are arranged to provide a constant, unchanging output, while allowing the sum frequency of the laser beam 20 to be modulated, for example using generator 114 .

[0110] Once the phase of the ultrastable cavity 108 is mapped to the absorption line of the gas reference cell 96 , the phase of the ultrastable cavity 108 can be used to measure and control the sum frequency or wavelength of the laser beam 20 .

[0111] Sideband detection using seed lasers L1 and L2 including sum frequency generating lasers according to embodiment 2. Here, the method uses opposite frequency sweep directions of the respective seed lasers L1 and L2 so that the sum frequency (ie, the frequency of the sum frequency generating laser beam 20) remains constant.

[0112] The sum frequency of laser L1 and laser L2 is kept constant by “counter-modulation.” This involves modulating the frequencies of laser L1 and laser L2 in opposite directions and at the same sweep rate to detect sidebands and avoid them.

[0113] Herein, the controller 90 can provide a modulation signal to the first amplifier A1 and the second amplifier A2. The modulation frequency, or the rate at which the frequencies of the first light source L1 and the second light source L2 are swept, can typically be selected at a sub-Hz level. The modulation signal provided by the controller 90 can have various waveforms, including but not limited to sinusoidal. Other waveforms may also be suitable. For sum frequency generation lasers in the THz region, the modulation range can typically be in the MHz to GHz range. The modulation range herein refers to the frequency band or range used to modulate the frequencies of the lasers L1 and L2.

[0114] Modulation of the laser L1 may be used to find the frequencies at which sidebands exist and / or to steer the frequency of the laser L1 toward a no-sideband operating set point.

[0115] Modulation of the laser L2 can be used to find the frequency at which sidebands exist and / or to steer the frequency of the laser L2 towards a sideband-free operating point.

[0116] Use embodiment 3 involves stabilizing the frequency of the sum frequency generated lasers by locking the input lasers L1 and L2 to the molecular absorption cells 24, 26 using opposite frequency modulation.

[0117] Here, the sum frequency of lasers L1 and L2 (i.e., the frequency of the sum-frequency generated laser beam 20) is kept constant while modulating the frequencies of lasers L1 and L2 in opposite directions. The modulation frequency is typically selected in the kHz range, and the modulation range can be on the order of several MHz.

[0118] The modulation of laser L1 can be used to determine a locking signal to lock laser L1 to an absorption line of the first molecular absorption cell 24. The modulation of laser L2 can be used to determine a locking signal to lock laser L2 to an absorption line of the second molecular absorption cell 26.

[0119] The third use embodiment enables stabilization of the sum frequency of the beam 20 without using the phase information signal 112 associated with the interferometer 106 and its ultrastable cavity 108 .

[0120] When the frequencies of lasers L1 and L2 are modulated in opposite directions with a certain sweep frequency, then preferably, the sweep frequency should not be present in the sum frequency of beam 20. Therefore, the modulation will not be detectable by phase measurement using ultrastable cavity 108. If frequency modulation is observed, then the control signals for lasers L1 and L2 are not completely out of phase, and small adjustments to timing and or gain can be made until the frequency modulation is no longer detectable by observing phase signal 112 measured using ultrastable cavity 108.

[0121] The disclosed system can be switched between the use embodiments described above. Furthermore, the system can be designed for a specific use embodiment. If so, components can be discarded if they are not required for the respective use embodiment. For example, reference cells 24 and 26 may not be required for use embodiment 1 and / or use embodiment 2.

[0122] In the above description, reference may be made to the wavelength, or frequency, of a light beam. Generally speaking, in a vacuum, wavelength and frequency are related by the following relationship:

[0123]

[0124] Where λ is the wavelength, f is the frequency, and c is the speed of light. However, note that if a light wave moves from a first medium with a first density to a second medium with a higher density, the wavelength becomes shorter. However, the frequency of light does not change when the wave moves from one medium to another. The speed of the wave, v, is related to both the frequency f and the wavelength λ:

[0125]

[0126] Combining the above velocity expression 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:

[0127]

[0128] where n1 is the refractive index of the first medium.

[0129] When the tunable laser providing the input to the SGF laser is set to a selected set point, the wavelength of the laser light may be determined as it passes through the medium. An alternative is to tune the frequency of the laser light in the laser itself. Either tuning method may be applicable to the systems and methods of the present disclosure. For example, for Figure 2 For the embodiment depicted in , wavelength tuning may be preferred. Figure 6 In the embodiment depicted, frequency tuning may be preferred.

[0130] Although specific reference may be made herein to the use of lithographic apparatus in IC fabrication, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the fabrication of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0131] While specific reference is made herein to embodiments of the present invention in the context of lithographic equipment, embodiments of the present invention may be used with other equipment. Embodiments of the present invention may form part of mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These equipment are generally referred to as lithographic tools. Such lithographic tools may utilize vacuum conditions or ambient (non-vacuum) conditions.

[0132] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0133] Where context permits, embodiments of the present invention may 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 that can be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, machine-readable magnetic storage media may include read-only memory (ROM); random-access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and the like. Furthermore, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only and that these actions are actually performed by a computing device, processor, controller, or other device executing the firmware, software, routines, instructions, etc., and in doing so, may cause actuators or other devices to interact with the physical world.

[0134] Although specific embodiments of the present invention have been described above, it will be understood that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be understood by those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

Claims

1. A method for operating a sum frequency laser, comprising the following steps: Simultaneously emitting a first light beam having a first frequency from a first light source and a second light beam having a second frequency from a second light source; modulating the first frequency at a matching tuning frequency in a first direction and modulating the second frequency at a matching tuning frequency in an opposite direction; The first light beam and the second light beam are sent to a summing device and a stable sum frequency light beam is output from the summing device.

2. The method according to claim 1, comprising the steps of: detecting a side mode of one of the first light beam and the second light beam; If the detected side mode exceeds a predetermined threshold, the modulation directions of the first frequency and the second frequency are reversed.

3. The method according to claim 2, wherein: The threshold value includes one or more of the following: the amplitude of the side mode, the power of the side mode, the brightness, the ratio of the amplitude of the side mode to the amplitude of the first frequency or the second frequency, the ratio of the power of the side mode to the power of the first frequency or the second frequency, or the ratio of the brightness to the brightness of the first frequency or the second frequency.

4. The method according to claim 2 or 3, wherein: The threshold value is a ratio of the amplitude or power of the side mode to the amplitude or power of the first frequency or the second frequency.

5. The method according to claim 2, 3 or 4, wherein: The threshold is approximately 40 dB and is the ratio between the power of the main mode and the power of the detected side modes.

6. The method according to claim 1, wherein: The frequency variation of the first light beam and the frequency variation of the second light beam are 180 degrees out of phase.

7. The method according to any one of claims 2 to 6, wherein: The step of detecting side modes includes performing power measurements on free spectral range frequency contents of the first light source and the second light source.

8. The method according to claim 2, wherein: The step of detecting the side mode includes redirecting a portion of the first light beam to a first sensor and redirecting a portion of the second light beam to a second sensor.

9. The method according to claim 7, wherein: The first sensor and the second sensor include a power detector and an algorithm for providing a signal related to the power of the side mode.

10. The method according to one of the preceding claims, wherein Modulating the first frequency and modulating the second frequency include thermally tuning and / or current tuning and / or piezoelectrically tuning the first light source and the second light source, respectively.

11. The method according to one of the preceding claims, wherein The matched tuning frequencies in the step of modulating the first and second light beams are in the range of mHz to kHz.

12. An exposure apparatus comprising one or more sum frequency generating lasers using the operating method according to any one of claims 1 to 10.

13. A lithographic apparatus comprising one or more sum frequency generating lasers using the operating method according to any one of claims 1 to 10.

14. A projection system for an optical lithography system comprising one or more sum frequency generating lasers using an operating method according to any one of claims 1 to 10.

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