Method and system for stabilizing a sum-frequency generating laser

Abstract

This disclosure provides a method for operating a sum-frequency laser. The method includes 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 and the second frequency in the opposite direction at a matched tuning frequency; and transmitting the first and second light beams to an adder device, from which a stabilized sum-frequency light beam is output.

Description

[Technical Field]

[0001] Cross-references to related applications

[0001] This application claims priority to European Application No. 23159806.1, filed on 3 March 2023, which is incorporated in its entirety for reference.

[0002]

[0002] The present invention relates to a method and system for stabilizing a sum-frequency generating (SFG) laser. SFG lasers can be used, for example, in metrology systems. State-of-the-art metrology systems can be used to measure the displacement of components of a lithography apparatus, for example, including components of a projection system for an optical lithography system. [Background technology]

[0003]

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

[0004]

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

[0005]

[0005] A sum-frequency generating (SFG) laser is a laser having an output beam whose frequency is equal to the sum of two mixed input laser beams. Each input laser may have a cavity from which laser light of a specific frequency can be generated. The laser frequencies of one or both lasers can be adjusted, for example, by changing thermal control, by current control, by a piezo actuator, or by a lattice element. The output of the SFG laser should be stabilized to provide an unmodulated sum-frequency output required for the accuracy of the application. Known techniques for stabilizing the output of an SFG laser include locking the sum-frequency output to a reference, for example, a gas absorption peak, or a reference laser having a frequency offset.

[0006]

[0006] Mode hopping is a phenomenon in which a laser exhibits a sudden jump in optical frequency related to the transition between different modes of its resonator. Existing methods of laser frequency tuning include regular recalibration of thermal and piezo setpoints to find a "safe zone" in which no sidebands (side modes) occur, in order to avoid mode hopping. However, even if additional natural frequencies or additional wavelengths occur within the laser cavity, instability can be introduced into the output of an SFG laser. Even if only such instability is temporary, they need to be avoided for continuous and high-end applications such as lithography machines.

[0007]

[0007] Side modes can be detected by observing the presence of free spectral region frequencies in the laser's optical output. The presence of side modes can be avoided by selecting appropriate thermal and piezoelectric setpoints.

[0008]

[0008] During laser calibration using existing methods, the center frequency of the SFG laser may change. As a result, the operating point of the laser can approach conditions in which side modes appear. Separate calibration for each laser affects SFG frequency stability and throughput loss as a result. Furthermore, despite various measurement and control systems to keep each input laser within a narrow range from the setpoint, drift in the input laser frequency is still possible during operation. Therefore, the output may be inherently unstable.

[0009]

[0009] CN101303507A discloses an SFG laser in which it is desirable to change the frequency of a first input optical beam while keeping the wavelength of the Wako beam constant. This is achieved by changing the frequency of a second input optical beam.

[0010]

[0010] EP1650597A1 discloses an SFG laser and a number of possible combinations of input light beam frequencies while keeping the frequency of the sum output light beam constant.

[0011]

[0011] All of the above systems may be subject to transient instability in the SFG laser output. The object of this disclosure is to provide a stabilized sum-frequency generating laser. [Overview of the Initiative]

[0012]

[0012] This disclosure provides a method for operating a sum-frequency laser, the method of which The steps include 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, The steps include: modulating the first frequency in the first direction and modulating the second frequency in the opposite direction at the matched tuning frequency; The steps include transmitting a first light beam and a second light beam to an adder device, and outputting a stabilized sum-frequency light beam from the adder device. Methods that include...

[0013]

[0013] In one embodiment, the method is A step of detecting a side mode in one of the first and second light beams, If the detected side mode exceeds a predetermined threshold, the modulation direction of the first frequency and the second frequency is reversed. Includes.

[0014]

[0014] In one embodiment, the threshold includes one or more of the side mode amplitude, side mode power, brightness, or the ratio thereof to the same aspect of a first frequency or a second frequency.

[0015]

[0015] In one embodiment, the threshold is the ratio of the amplitude or power of the side mode to the power or amplitude of the first frequency or the second frequency.

[0016]

[0016] In one embodiment, the threshold is approximately 40 dB and is the ratio between the power of the main mode and the power of the detected side mode.

[0017]

[0017] In one embodiment, the change in the frequency of the first optical beam and the change in the frequency of the second optical beam are 180 degrees out of phase.

[0018]

[0018] In one embodiment, the step of detecting the side mode includes power measurement of the free spectral range frequency content of the first light source and the second light source.

[0019]

[0019] In one embodiment, the step of detecting the side mode includes redirecting a portion of the first optical beam to the first sensor and redirecting a portion of the second optical beam to the second sensor.

[0020]

[0020] In one embodiment, the first sensor and the second sensor include a power detector and an algorithm for providing a signal correlated with the power of the side mode.

[0021]

[0021] In one embodiment, modulating the first frequency and modulating the second frequency includes thermal and / or current and / or piezo tuning of the first light source and the second light source, respectively.

[0022]

[0022] In one embodiment, the matched tuning frequencies of the step of modulating the first optical beam and the second optical beam are in the range of mHz to kHz.

[0023]

[0023] According to another aspect, the present disclosure provides an exposure apparatus including one or more sum-frequency generation lasers using the operation method described in the present invention.

[0024]

[0024] According to another aspect, the present disclosure provides a lithography apparatus including one or more sum-frequency generation lasers using the operation method described in the present invention.

[0025]

[0025] In a further different 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 described in the present invention. [Brief explanation of the drawing]

[0026]

[0026] Some embodiments of the present invention will be described below, merely as examples, with reference to the attached schematic diagrams.

[0027] [Figure 1] - A schematic diagram of a lithography apparatus is shown. [Figure 2] - A diagram of one embodiment of the system of this disclosure is shown. [Figure 3A] - An exemplary figure of the side-mode power amplitude observed at a laser power output with wavelength-controlled piezoelectric voltage (vertical axis) versus wavelength-controlled temperature of the laser cavity (horizontal axis) is shown, illustrating an example of the mode-hopping region. [Figure 3B] - An exemplary figure of the side-mode power amplitude observed at a laser power output with wavelength-controlled piezoelectric voltage (vertical axis) versus wavelength-controlled temperature of the laser cavity (horizontal axis) is shown, illustrating an example of the mode-hopping region. [Figure 4] - An exemplary diagram of a laser tuning curve is shown, with the vertical axis illustrating the laser wavelength of the laser beam and the horizontal axis illustrating the laser temperature, illustrating an example of laser tuning with a fixed piezoelectric setpoint and cavity mode broadening. [Figure 5] - A diagram illustrating the modes of a tunable laser is shown. [Figure 6] - A diagram of another embodiment of the system of this disclosure is shown. [Figure 7A] - An illustrative diagram of laser control by the method of this disclosure is shown, illustrating the wavelength modulation of the laser and controllable parameters such as the piezoelectric voltage (horizontal axis) with respect to the resulting side-mode power (vertical axis). [Figure 7B]- An illustrative diagram of laser control by the method of this disclosure is shown, illustrating the wavelength modulation of the laser and controllable parameters such as the piezoelectric voltage (horizontal axis) with respect to the resulting side-mode power (vertical axis). [Figure 8] - An embodiment of the present disclosure is illustrated. [Modes for carrying out the invention]

[0028]

[0027] In this document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation. Radiation may include (deep) ultraviolet (e.g., having wavelengths of about 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet, e.g., having wavelengths in the range of 5 to 100 nm).

[0029]

[0028] The terms “reticle,” “mask,” or “patterning device” as used in this document should be broadly interpreted to refer to general patterning devices that can be used to give an incident radiation beam a patterned cross section corresponding to a pattern generated within a target portion of a substrate. The term “light bulb” can also be used in this context. In addition to classic masks (transmissive or reflective, binary, phase-shifted, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0030]

[0029] A beam splitter (also spelled beamsplitter) is an optical device that splits a beam of light into two and is part of many optical systems. In the first version, a beam splitter may be a cube made of two triangular glass prisms bonded at their respective bottom surfaces using, for example, a polyester, epoxy, or urethane-based adhesive. The thickness of the adhesive resin layer is adjusted so that half of the light incident from one "port" (i.e., face of the cube) is reflected (for a particular wavelength) 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 of orthogonal polarization. A second option is to use a translucent mirror, which is often an optical substrate that is a sheet of glass or plastic with a thin coating of partially transparent metal. The thin coating may be aluminum or silver deposited using physical vapor deposition. The thickness of the coating is controlled so that a predetermined portion of the light incident at a 45-degree angle (e.g., half, but this can be any ratio from 0 to 100%) that is not absorbed by the coating or substrate material is transmitted, and the remainder is reflected. A third version of the beam splitter is a dichroic mirror prism assembly that uses a dichroic optical coating to split an incident light beam into multiple spectrally different output beams.

[0031]

[0030] A "gas reference cell" is a cell filled with some gas, typically used in laser absorption spectroscopy. Comparison with a reference beam is possible due to some effect resulting from the interaction between the gas and light, such as the absorption coefficient of light in the gas, or a frequency-dependent absorption spectrum. 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 result is presented in the form of a spectrum, such as an absorption spectrum. 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 within the cell are known, the gas cell also allows for the reverse process of identifying the frequency or wavelength of the laser light using the absorption spectrum.

[0032]

[0031] Gas reference cells are typically provided with a suitable optical window (having high transmittance across the entire relevant spectral region) for light to enter and exit the cell. Reference gas cells are commercially available in a variety of gases, including both atomic gases and (often diatomic) molecular gases. Typical examples include iodine (I2), hydrogen (H2), helium, carbon monoxide (CO), and acetylene (C2H2). Thus, a wide range of standard spectral lines can be used. In some cases, alkali metals such as sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) are used, and when electrically heated to at least a suitable temperature, they generate a sufficiently high vapor pressure. Such cells are sometimes called vapor cells. Sealed gas cells must be reliably leak-free. Therefore, helium leak tests are frequently applied.

[0033]

[0032] An "absorption cell" or "molecular absorption cell" is a device that includes a gas reference cell. The absorption cell can compare the intensity difference of two parts of the same beam. One part is sent to the gas reference cell and then to a first sensor to receive light that has passed through the reference cell at least once, and the second part is sent unobstructed to a second sensor used as a reference.

[0034]

[0033] The 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 medium with a higher density. The refractive index variable is sometimes denoted by the letter n or n' in explanatory texts and mathematical formulas.

[0035]

[0034] An interferometer or laser interferometer can measure distance or displacement by measuring the phase difference between two light beams: a light beam sent to a first reflector or first surface at a fixed reference distance and a light beam sent to a second reflector or surface at a different distance. When the two reflected signals are recombined within the interferometer, the resulting phase is related to the distance from the interferometer to the second surface. As the distance to the second surface changes, the phase of the combined signals also changes. The usefulness of these methods is that measurements can be made over long distances while maintaining accuracy.

[0036]

[0035] In a heterodyne interferometer, the measurement beam and the reference beam interfering at the detector typically originate from the same laser source. A frequency offset may be given to the source to enable heterodyne phase detection. However, the two (divided) frequencies may also be generated by two different frequency-locked or phase-locked lasers.

[0037]

[0036] A "wavelength tracker" is a specific version of an interferometer set up to measure the phase difference between two reflected light beams. One beam is reflected by a first fixed reflector that provides a first reference axis, and a second beam is reflected by a second fixed reflector that provides a second reference axis having a different length from the first axis. Since the two reflective surfaces are fixed, the phase difference measured changes only if the wavelength of the light beam changes. Thus, a wavelength tracker makes it possible to monitor the wavelength deviation from a setpoint.

[0038]

[0037] Figure 1 schematically shows a lithography apparatus LA. The lithography apparatus LA includes an illumination system (also called an illuminator) IL configured to adjust a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., a mask table) MT connected to a first positioner PM constructed to support a patterning device (e.g., a mask) MA and configured to precisely position the patterning device MA according to specific parameters, a substrate support (e.g., a substrate table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and configured to precisely position the substrate support according to specific parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project the pattern applied to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (e.g., including one or more dies).

[0039]

[0038] During operation, the illumination system IL receives the radiated 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 guide, shape, and / or control the radiation. The illuminator IL may be used to adjust the radiated beam B so that it has a desired spatial and angular intensity distribution in its cross-section, in the face of the patterning device MA.

[0040]

[0039] As used herein, the term “projection system” PS should be broadly interpreted to encompass all types of projection systems, including refractive, reflective, reflective-refracting, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, that are appropriate for the exposure radiation used and / or for other factors such as the use of immersion liquid or vacuum. As used herein, all terms “projection lens” should be considered synonymous with the more general term “projection system” PS.

[0041]

[0040] The lithography apparatus LA may be of a type in which at least a portion of the substrate is covered with a liquid such as water having a relatively high refractive index to fill the space between the projection system PS and the substrate W (also known as immersion lithography). Further information relating to immersion technology is provided in US Patent No. 6952253, which is incorporated herein by reference.

[0042]

[0041] The lithography apparatus LA may be of a type having two or more substrate support WTs (also called a “dual-stage” machine). In such a “multi-stage” machine, the substrate support WTs can be used in parallel, and / or, while the preparation steps for subsequent exposure of the substrate W are being performed on the substrate W located on one substrate support WT, the other substrate W may be used to expose a pattern on the other substrate W.

[0043]

[0042] In addition to the substrate support WT, the lithography apparatus LA may include a measurement stage. The measurement stage is positioned to hold sensors and / or a cleaning device. The sensors may be positioned to measure the characteristics of the projection system PS or the characteristics of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be positioned to clean a part of the lithography apparatus, for example, a part of the projection system PS or a part of the system that provides the immersion fluid. The measurement stage may move under the projection system PS when the substrate support WT is away from the projection system PS.

[0044]

[0043] During operation, the radiating beam B is incident on a patterning device (e.g., mask MA) held on a mask support MT, and a pattern is formed by the pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiating beam B passes through a projection system PS, which focuses the beam onto a target portion C on the substrate W. A second positioner PW and a position measuring system IF can be used to precisely move the substrate support WT to position, for example, various target portions C in a focused and aligned position within the path of the radiating beam B. Similarly, a first positioner PM and optionally another position sensor (not explicitly shown in Figure 1a) can be used to precisely position the patterning device MA relative to the path of the radiating beam B. The patterning device MA and the substrate W may be aligned using mask alignment marks M1 and M2 and substrate alignment marks P1 and P2. In the example, the substrate alignment marks P1 and P2 occupy dedicated target areas, but the substrate alignment marks can also be placed in the space between target areas. When the substrate alignment marks P1 and P2 are positioned between target areas C, they are known as scribe line alignment marks.

[0045]

[0044] To illustrate the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, namely the x-axis, y-axis, and z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is called an Rx rotation. A rotation around the y-axis is called a Ry rotation. A rotation around the z-axis is called an Rz rotation. The x-axis and y-axis define the horizontal plane, while the z-axis defines the vertical direction. The Cartesian coordinate system is not intended to limit the invention and is used only for illustrative purposes. Alternatively, other coordinate systems, such as a cylindrical coordinate system, may be used to illustrate the invention. For example, the orientation of the Cartesian coordinate system may be different, such that the z-axis has a component along the horizontal plane.

[0046]

[0045] In an SFG laser, the frequencies of the first light beam 12 and the second light beam 14 are added together by an additive crystal 16. For example, the first laser beam 12 (having an output of 1064 nm or 282 THz) and the second laser beam 14 (having an output of 1562 nm or 192 THz) can be combined by the additive crystal 16 to obtain a laser beam 20 having a wavelength of 633 nm (474 ​​THz).

[0047]

[0046] Addition crystals are optical crystals, typically single crystals (single-crystal optical materials), and are used as gain media for lasers such as solid-state lasers. Sum-frequency generation (SFG) is a second-order nonlinear optical process based on the annihilation of two input photons at angular frequencies ω1 and ω2 while simultaneously generating one photon at frequency ω3. This can be considered a second-order phenomenon in nonlinear optics and typically occurs under conditions where light interacts with a material that is asymmetric (e.g., surface and interface) and the input light has very high intensity. Sum-frequency generation is a "parametric process," meaning that the photons satisfy energy conservation and the material of the crystal does not change.

[0048]

[0047] The additive crystal may be a periodically polarized reversal lithium niobate (PPLN) crystal. The crystal may contain one or more dopants, typically rare earth ions or transition metal ions. For example, the crystal may be doped with MgO, neodymium, ytterbium, erbium, or chromium.

[0049]

[0048] It should be noted that the optical crystal 20 may have any optical functions, including but not limited to frequency doubling, difference frequency generation, sum frequency generation, optical parametric oscillation, and other nonlinear processes. As an example, embodiments of this disclosure are described with respect to addition, but the concept is applicable to all possible optical crystals and tunable lasers.

[0050]

[0049] A known method for stabilizing the output of an SFG laser is to lock the laser frequency output to the spectral lines of the gas absorption cell. Typically, wavelength modulation is applied to stabilize the laser in the gas depression by wavelength modulation spectroscopy, and the laser gas transmission signal is demodulated with odd harmonics (e.g., 1st or 3rd harmonic) of the modulated signal to generate a control signal to lock the laser frequency to the center of the spectral lines.

[0051]

[0050] The present disclosure proposes stabilizing the output of the SFG laser system 10 by ensuring a stabilized input.

[0052]

[0051] Generally, referring to Figure 2, the sum frequency generator (SFG) light source 10 may include a first tunable light source L1 and a second tunable light source L2. The first light source L1 can generate a first light beam 12. The first light beam includes at least radiation having a first frequency f1. The second light source L2 can 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 to the inlet of an additive crystal 16, for example, via an optical fiber. The additive crystal is adapted to receive the first light beam 12 and the second light beam 14. The crystal 16 can provide a third light beam 20 including radiation having at least a third frequency. The third frequency may be the sum of the first frequency and the second frequency.

[0053]

[0052] The first light source L1 and the second light source L2 may be tunable lasers. The first and second light sources may be controlled by first and second amplifiers Al and A2, respectively. The lock-in amplifiers Al and A2 may be controlled by a controller 22. Here, the controller may provide setpoint or drive signals to each amplifier. Amplifiers Al and A2 provide operating signals such as piezoelectric voltages corresponding to each light source.

[0054]

[0053] The first light source L1 and the second light source L2 may be provided with first and second gas reference cells 24 and 26, respectively. Each gas 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 output of the respective lasers L1 and L2 may be provided with first and second beam splitters 40 and 42 to direct small portions of the first and second light beams 12 and 14 to their respective gas reference cells 24 and 26. This small portion may be, for example, in the range of 0.1 to 5% of the received light, for example, about 1%. The gas cell beam splitters 36 and 38 can direct a portion, typically half, of the received light to their respective gas reference cells 24 and 26, and the other portion to the reference light sensors 28 and 30.

[0055]

[0054] Each amplifier Al and A2 can receive signals recorded by their respective photosensors 28-34. The difference between the gas cell signal and the reference signal can be locked onto the spectral lines of their respective gas reference cells 24 and 26. In response, the first amplifier Al can consequently add a first offset to the input to the first light source L1. The second amplifier A2 can add a second offset to the input to the second light source L2 to correct the light source signal if necessary.

[0056]

[0055] Wherever a small portion or part of a light beam is referred to herein, it may mean a measure of power. Wherever a small portion is referred herein, it may be expressed as luminous intensity, which is a measure of wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. The latter may be based on a luminous intensity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is candela (cd). Alternatively, the description herein may relate to light power, for example, light energy [lm.s], luminous flux [lumens], or other appropriate measures expressed in common energy units [W] or [J / s].

[0057]

[0056] For example, the method may include stabilizing a first laser output 12 at a first frequency or a first wavelength, for example, 1064 nm. The method may also include stabilizing a second laser output 14 at a second frequency or a second wavelength, for example, 1562 nm. The stabilized input beams 12, 14 result in a stabilized SFG output 20 having a wavelength of, for example, 633 nm. The wavelength outputs provided herein are merely examples, and it will be understood that the methods herein may be applied to any combination of frequency or wavelength.

[0058]

[0057] The laser frequencies of one or both input lasers LI, L2 can be adjusted, thereby controlling the combined frequency of the SFG laser 20. Laser control may involve adjusting controllable parameters, for example, by thermal control or by using current and / or voltage applied to a piezo actuator.

[0059]

[0058] When wavelength modulation is not desired for the SFG output, it is proposed that each of 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 1064 nm is locked to the spectral line of the first gas absorption cell 24 having a wavelength modulation signal s1. For example, the second laser L2 having a second laser output of 1560 nm is locked to the spectral line of the second gas absorption cell 26 having a wavelength modulation signal s2.

[0060]

[0059] The modulated signals s1 and s2 are selected so that they are 180° out of phase with respect to each other. In this way, the modulation is canceled at the sum frequency.

number

Equation

[0061]

[0060] FIG. 2 shows a schematic diagram of an exemplary configuration for providing such a stabilized SFG output, and each of the first and second input lasers L1, L2 is stabilized to respective gas reference cells 24, 26. The system 10 provides a sum-frequency laser having a 180-degree phase shift (piezo) modulation to the two input optical beams for its wavelength stabilization.

[0062]

[0061] During operation, the SFG laser system 10 generates a sum-frequency laser 20 by summing the frequencies of the first optical beam 12 and the second optical beam 14 of the two input lasers L1 and L2. The first laser L1 and / or the second laser L2 is stabilized to an absorption peak using the principle of wavelength modulation spectroscopy or frequency modulation spectroscopy. The wavelength modulation signals for the lasers L1 and L2 are phase-shifted and amplitude-matched so that the sum frequency of the SFG laser 20 is not modulated.

[0063]

[0062] When a laser is modulated at a modulation frequency ω m with a modulation amplitude dω around its central frequency ω L , the instantaneous frequency is ω = ω L + dω * cos(ω m t). And the intensity of the radiation transmitted through each absorption cell can be expressed as a Fourier series expansion.

[0064]

[0063] Advantageously, instead of locking the SFG output 20, a more time-efficient arrangement is presented, as stabilization preparation can be performed before the measurement light is generated by stabilizing the frequency of the input laser of the SFG laser 10. Furthermore, by matching the wavelength modulations of both input lasers (e.g., by locking to the gas cell), the modulations cancel each other out at the sum-frequency laser wavelength.

[0065]

[0064] The frequencies of the first light beam 12 and the second light beam 14 may be adjusted using thermal control, current, and / or piezoelectric actuators of the respective lasers L1 and L2. Existing methods include normal recalibration of each light source. Recalibration as described herein includes, for example, modulating the thermal setpoint and piezoelectric setpoint to find a “safe zone” in which no sidebands are generated. Refer to Figure 3A for the first light source L1 and Figure 3B for the second light source L2. Here, the horizontal axis of the figures represents the temperature T of the respective resonant cavity of each laser. The vertical axis represents the voltage V driving the piezoelectric actuator that controls the size of the resonant cavity within a certain control range. In Figures 3A and 3B, the dark regions 50, 56 indicate the “safe zone,” and the light beams generated by the respective light sources contain light that mainly has a setpoint frequency, also called the main mode. Emission lines 52, 54 and 58, 60 indicate the generation of side modes, respectively. Side modes, typically light, have one or fewer wavelengths than the main mode. Side modes can occur when standing waves are generated within a resonant cavity.

[0066]

[0065] Side modes can be detected by observing the presence of free spectral region frequencies at the optical output of each laser. The presence of side modes can be avoided by selecting appropriate thermal and piezoelectric setpoints.

[0067]

[0066] Figure 4 shows an example of the relationship between cavity temperature (horizontal axis) and the wavelength of the light beam. Compared to the diagram in Figure 3A, changing the temperature while the piezoelectric voltage remains constant corresponds, for example, to a horizontal scan across the diagram in Figure 3A. In this specification, as the temperature rises, the wavelength may initially increase gradually along, for example, line 66, but at some point, a specific side mode, for example mode 54, may also be activated. As shown by line 68, this means 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-hopping region” 64, i.e., a setting where the laser output has a particular change that jumps or hops to another setting. If so, first, the wavelength of the main mode deviates from the setpoint. Furthermore, mode hopping typically results in transient jitter or inaccuracy in the output of the SFG laser output. In this specification, it should be noted that mode hopping can occur in one direction, but can also occur in the reverse direction, and therefore cause multiple moments of jitter. The latter is to be avoided in high-end applications of SFG lasers.

[0068]

[0067] Known methods for calibrating a stabilized SFG laser to avoid side modes include regular recalibration of thermal and piezoelectric setpoints to find a "safe zone" where sidebands do not occur, thereby providing a stable SFG output. Figure 5 illustrates the power (vertical axis) versus wavelength (horizontal axis) for each mode. Line 70 can illustrate the settings for the safe zone. Here, the main mode λ L The power of is wavelength λ L+1 and λ L-1 The power of the main mode far exceeds that of two adjacent side modes, each having a power of . Line 72 illustrates a mode-hopping region. In this specification, the powers of two adjacent modes are of comparable magnitude, and there is a risk that the main mode will jump or hop from one wavelength to the next, or vice versa.

[0069]

[0068] The effective total cavity of the input lasers LI and / or L2 may have a length of about 4 cm and a free spectral region of about 2.5 GHz. When the laser temperature is changed, the mode selector and the cavity modes are tuned together, and the laser wavelength changes, for example, within about 1 GHz / K, but due to the slightly different expansion coefficients of the fibers used in each laser, the longitudinal modes move somewhat less than the center wavelength of the spectrally narrow grating. Eventually, a situation arises in which the two longitudinal modes experience substantially the same output coupler reflectance. This is shown as the situation in Figure 5. This can occur, for example, up to three times over the total thermal tuning range.

[0070]

[0069] Side modes can be detected by observing the presence of free spectral region frequencies in the laser's optical output. The presence of side modes can be avoided by selecting appropriate thermal and piezoelectric setpoints.

[0071]

[0070] The problem is that this calibration can only be performed with the laser amplifier turned off, as passing through the sidebands would result in mode hopping (IFM loss of lock) and potential damage to the laser amplifier. As mentioned in the introduction, the center frequency of the SFG laser may change during calibration. A separate calibration may result in throughput loss.

[0072]

[0071] The goal is to keep the sidebands in a zone where the amplitude is below a set threshold. For example, each laser is controlled so that the sidebands remain in a zone where the amplitude or power is significantly smaller than that of the main mode. In this specification, the significantly smaller or threshold may be 40 dB or less relative to the main mode.

[0073]

[0072] Figure 6 provides an embodiment of the system 100 of the present disclosure, enabling side-mode detection during operation of an SGF laser. The system 100 is essentially a sum-frequency generating laser having two tunable input lasers, L1 and L2. The first light source L1 has a first frequency f1, and the second light source L2 has a second frequency f2. The optical outputs of the first light beam 12 and the second light beam 14, i.e., LI and L2 respectively, are combined in a sum-frequency crystal 16. The crystal generates a light beam 20 having a third frequency f3, where frequency f3 may be f1 + f2. In a practical example, f1 is approximately 192 THz and f2 is approximately 281 THz.

[0074]

[0073] In order to detect the appearance of at least one side mode of lasers LI and L2, the frequencies of the first and second optical beams 12 and 14 of lasers LI and L2 are scanned in opposite directions and out of phase by 180° relative to each other using matched tuned trajectories. In this specification, the sum frequency of optical beam 20 remains constant during the scanning of beams 12 and 14.

[0075]

[0074] At least portions of the first and second light beams 12 and 14 may be diverted to light sensors 80 and 82, respectively. Diverting the first and second light beams 12 and 14 may involve beam splitters 12 and 14 or equivalent optical elements. At least portions of the first and second light beams may, in actual embodiments, represent about 0.1 to 10% of the light, for example, about 0.5 to 2%, or for example, about 1%.

[0076]

[0075] The side-mode detector may include power sensors 80, 82. The sensors 80, 82 may be connected to a processing device 84 such as a computer or a dedicated processor. The processor 84 is typically provided with an algorithm that can calculate the side-mode power from the sensor data. As used herein, side-mode power means power at free-spectrum frequencies. The processor provides a signal 86 to a controller 90 for controlling the SGF light source 100.

[0077]

[0076] The first optical sensor 80 and the second optical sensor 82 may be suitable for directly detecting the power of spectral frequencies in their respective light beams. The sensors 80 and 82 may be, for example, high-frequency optical power and / or power meters. Such sensors can be obtained, for example, from Thorlabs Inc., Newton, NJ (US).

[0078]

[0077] Figures 7A and 7B show the signals 86 for light sources LI and L2, respectively, i.e., signal 86 LI and 86 L2 This is an example. The horizontal axis represents frequency (ω [Hz]) or alternatively, wavelength. The vertical axis shows the power of light at a certain frequency. Here, ω LI and ω L2 These indicate the main modes or setpoints for the first light source L1 and the second light source L2, respectively. At specific timings (e.g., periodically or continuously), the frequencies of both light sources are modulated in opposite directions around the main mode. When the detected power of the side mode reaches or exceeds the threshold level 92, the controller 90 reverses the modulation direction for both light sources L1 and L2.

[0079]

[0078] When a side mode is detected for one of the lasers, the direction of frequency or wavelength tuning is reversed. This avoids the side mode while keeping the frequency of the sum-frequency generated light beam 20 nearly constant. Furthermore, when the side mode is avoided, the SFG laser 20 is substantially free of jitter or aberrations. In particular, significant jitter caused by mode hopping is completely prevented as the mode hopping is eliminated.

[0080]

[0079] Therefore, side-mode detection of LI and L2 can be based on power measurements of the free spectral region frequency content of each laser.

[0081]

[0080] Figure 8 shows a system 110 of the present disclosure, which may combine features of the embodiments of Figures 2 and 6. The system 110 includes an SFG light source 10 capable of generating a stable SGF light beam 20. The first and second light sources LI and L2 can be stabilized as described herein with respect to the embodiment shown in Figure 2.

[0082]

[0081] In system 110, reference sensors 28 and 30 can perform measurements in the spectral region. In other words, sensors 28 and 30 have a dual function: one is related to the gas absorption cells 24 and 26 as reference sensors, and the other is functioning as a spectral power meter. In this specification, the measurement outputs of sensors 28 and 30 are provided to both amplifiers Al and A2 and processing device 84, respectively. Next, processor 84 provides a signal 86 to controller 90, which includes spectral analysis of light beams 12 and 14, respectively. Next, controller 90 drives the first amplifier A1 and the second amplifier A2, respectively. For example, controller 90 can instruct amplifiers Al 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.

[0083]

[0082] The sum-frequency laser 20 may be used as a metrology laser for position or displacement measuring interferometer or wavelength tracker interferometer applications. Such applications are useful, for example, for measuring the position of a wafer table WT in a lithography apparatus LA, as illustrated in Figure 1.

[0084]

[0083] As shown in Figure 8, the SGF light beam 20 may be supplied to a beam splitter 98. The beam splitter diverts a small portion of the light beam 20 to a molecular absorption cell system 94, which includes a gas reference cell 96, a reference photosensor 102, and a gas cell photosensor 104. The molecular absorption cell system 94 may function substantially similarly to a molecular absorption cell system of gas reference cells 24, 26 and their respective photosensors.

[0085]

[0084] The main portion of the light beam 20 may be supplied to the interferometer system 106. The interferometer system 106 may comprise one or more interferometers for measuring one or more static and / or moving target mirrors. An example of an interferometer system suitable for the method and system of this disclosure is described in detail, for example, US Patent No. 2021072088.

[0086]

[0085] As an example, system 106 may include a stable cavity 108 having a reflector at its end as a reference axis. If stable, the cavity 108 may allow the interferometer to function as a wavelength tracker. Alternatively, the reflector at the end of the cavity 108 may be a movable device. System 106 may typically include another reference axis that receives another (typically half) of the light beam 20. System 106 typically includes an interferometer IFM connected to a phase measurement board PMB. The PMB may output a signal 112 representing the phase difference.

[0087]

[0086] Signal 112 may be supplied to an adder device 118. The adder device can receive other inputs and add them to signal 112. For example, the adder device may be connected to a setpoint orbit generator 114 and provide a setpoint signal 116. The setpoint generator 114 can be used to map the phase of the ultrastable cavity 108 to the absorption line of the gas absorption cell 96. The output of the adder device 118 may be supplied to a controller 90. The controller then typically controls the first light source LI and the second light source L2 via a first amplifier A1 and a second amplifier A2.

[0088]

[0087] During use, the setpoint orbit generator 114 can scan the sum-frequency generated laser beam 20 by adding an offset to the setpoints of the first light source L1 and the second light source L2, for example, to map the phase of the ultrastable interferometer axis 108 to the pre-calibrated absorption spectrum of the molecular absorption cell 96.

[0089]

[0088] After mapping the phase of the interferometer axis to a frequency reference cell, the phase signal 112 of the interferometer 106 may be used for wavelength measurement and / or wavelength control of the light beam 20.

[0090]

[0089] During use, embodiments of the present disclosure enable stabilization of the first light source LI and the second light source L2. In this specification, power detectors 28, 30 may be used for side-mode detection and for power normalization of signals passing through gas reference cells 24, 26.

[0091]

[0090] Below, as an example, various use cases of the embodiment shown in Figure 8 are provided.

[0092]

[0091] Use case 1 involves phase-locking a laser 20 to a pre-calibrated superstable cavity 108 calibrated to a molecular absorption cell such as cell 96. Here, the sum-frequency generating laser 20 is scanned 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 superstable cavity 108 (e.g., varying over a wavelength range between an upper threshold and a lower threshold).

[0093]

[0092] In this specification, lasers LI and L2 are not scanned, i.e., they are set to provide a constant, non-changing output, while a generator 114, for example, is used to enable modulation of the sum frequency of the SFG laser beam 20.

[0094]

[0093] When 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.

[0095]

[0094] Example 2 of use involves sideband detection of seed lasers LI and L2 of an SFG laser. Here, the method uses the inverse frequency scanning directions of the respective seed lasers LI and L2, thereby keeping the sum frequency, i.e., the frequency of the SFG laser beam 20, constant.

[0096]

[0095] The sum frequency of lasers L1 and L2 is kept constant by "reverse modulation". This involves modulating the frequencies of lasers L1 and L2 in opposite directions and at the same scanning speed to detect and avoid sidebands.

[0097]

[0096] In this specification, the controller 90 may provide a modulated signal to the first amplifier Al and the second amplifier A2. The modulation frequency, or the scanning speed of the frequencies of the first light source L1 and the second light source L2, can typically be selected at the sub-Hz level. The modulated signal provided by the controller 90 may have a variety of waveforms, including but not limited to sine waves. Other waveforms may also be preferred. For SFG lasers in the THz region, the modulation range may typically be in the range of MHz to GHz. In this specification, the modulation range means the frequency band or range used to modulate the frequencies of lasers L1 and L2.

[0098]

[0097] The modulation of laser L1 can be used to determine the frequency at which sidebands exist and / or to guide the frequency of laser L1 to an operating setpoint without sidebands.

[0099]

[0098] The modulation of laser L2 can be used to determine the frequency at which sidebands exist and / or to guide the frequency of laser L2 to an operating setpoint without sidebands.

[0100]

[0099] Example 3 includes frequency stabilization of the SFG laser by using inverse frequency modulation to lock the input lasers L1 and L2 to molecular absorption cells 24 and 26.

[0101]

[0100] In this specification, the sum frequency of lasers L1 and L2, i.e., the frequency of the SFG laser beam 20, is kept constant by modulating the frequencies of lasers L1 and L2 in opposite directions. Here, the modulation frequency can typically be selected at the kHz level. The modulation range may be on the order of a few MHz.

[0102]

[0101] Modulation of laser L1 may be used to determine a lock signal for locking laser L1 to the absorption line of the first molecular absorption cell 24. Modulation of laser L2 may be used to determine a lock signal for locking laser L2 to the absorption line of the second molecular absorption cell 26.

[0103]

[0102] The third use case makes it possible to stabilize the sum frequency of the beam 20 without using the interferometer 106 and the phase information signal 112 associated with its ultrastable cavity 108.

[0104]

[0103] When the frequencies of lasers L1 and L2 are modulated in the opposite direction to a certain scanning frequency, preferably this scanning frequency is missing from the sum frequency of the light beam 20. Therefore, phase measurements using the ultrastable cavity 108 cannot detect the modulation. If frequency modulation is observed, the control signals of lasers L1 and L2 are not completely out of phase, and small adjustments to the timing and / or gain can be made until the frequency modulation is no longer detectable by observing the phase signal 112 measured using the ultrastable cavity 108.

[0105]

[0104] The system of the present disclosure can switch between the above use cases. The system may also be designed in particular for one of the use cases. If so, parts of the setup may be discarded if these parts are not required for each use. For example, gas reference cells 24, 26 may not be required for use case 1 and / or use case 2.

[0106]

[0105] In the above description, either the wavelength or frequency of the light beam may be referred to. Generally speaking, in a vacuum, wavelength and frequency are related as follows:

number

number

[0107]

[0105] When the above equation for velocity is combined with the definition of refractive index, the relationship between the wavelength λ0=c / f in a vacuum and the wavelength λ1=v1 / f in a first medium other than a vacuum is as follows.

number

[0108]

[0106] When setting a tunable laser that provides input to an SGF laser to a selected set point, one option is to determine the wavelength of the laser light while it is moving through the medium. Alternatively, the frequency of the laser light in the laser itself can be tuned. Either tuning method may be suitable for the systems and methods of this disclosure. For example, in the embodiment shown in Figure 2, wavelength tuning may be preferred. In the embodiment shown in Figure 6, frequency tuning may be preferred.

[0109]

[0107] While this specification specifically refers to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance patterns and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and the like.

[0110]

[0108] While embodiments of the present invention are specifically mentioned in relation to lithography apparatus, embodiments of the present invention can also be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses are sometimes commonly referred to as lithography tools. Such lithography tools can be used under vacuum conditions or ambient (non-vacuum) conditions.

[0111]

[0109] As described above, specific references have been made to the use of embodiments of the present invention in relation to optical lithography. However, the present invention may, of course, be used in other applications, such as imprint lithography, and is not limited to optical lithography, where circumstances permit.

[0112]

[0110] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof, where circumstances permit. Embodiments of the present invention may also be implemented as instructions that can be stored in a machine-readable medium and read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read by a machine (e.g., a computer device). For example, a machine-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic storage medium, optical storage medium, flash memory device, or propagating signals in the form of electricity, light, sound, or other (e.g., carrier waves, infrared signals, digital signals, etc.). Furthermore, in this specification, firmware, software, routines, and instructions may be described as performing some kind of operation. However, such descriptions are merely for convenience, and it should be understood that such operation is actually performed by a computer device, processor, controller, or other device that executes the firmware, software, routines, instructions, etc., and that in doing so, actuators or other devices may interact with the physical world.

[0113]

[0111] Although specific embodiments of the present invention have been described above, it is clear that the present invention can be implemented in forms other than those described above. The above description is intended to be illustrative and not restrictive. Accordingly, as will be clear to those skilled in the art, modifications to the invention described herein may be made without departing from the scope of the appended claims.

Claims

1. A method for operating a sum-frequency laser, wherein the method is The steps include 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, The steps include modulating the first frequency in the first direction and modulating the second frequency in the opposite direction at a matched tuning frequency, The steps include transmitting the first light beam and the second light beam to an adding device and outputting a stabilized sum-frequency light beam from the adding device. Methods that include...

2. A step of detecting a side mode in one of the first light beam and the second light beam, If the detected side mode exceeds a predetermined threshold, the modulation direction of the first frequency and the second frequency is reversed. The method according to claim 1, including the method described in claim 1.

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

4. The method according to claim 2 or 3, wherein the threshold is the ratio of the amplitude or power of the side mode to the power or amplitude 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 mode.

6. The method according to any one of claims 1 to 5, wherein the change in the frequency of the first light beam and the change in the frequency of the second light beam are out of phase by 180 degrees.

7. The method according to any one of claims 2 to 6, wherein the step of detecting the side mode includes power measurement of the free spectral domain frequency content 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 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.

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 correlated with the power of the side mode.

10. The method according to any one of claims 1 to 9, wherein modulating the first frequency and modulating the second frequency includes thermal and / or current and / or piezoelectric tuning of the first light source and the second light source, respectively.

11. The method according to any one of claims 1 to 10, wherein the matched tuning frequency in the step of modulating the first light beam and the second light beam is in the range of MHz to kHz.

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

13. A lithography apparatus comprising one or more sum-frequency generating lasers using the operating method described in any one of claims 1 to 10.

14. A projection system for a photolithography system, comprising one or more sum-frequency generating lasers, using the operating method described in any one of claims 1 to 10.

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