System and method for growing pseudo-phase-matched strontium tetraborate and lithium triborate crystals for frequency conversion

Abstract

A method for growing a periodically polarized nonlinear crystal may include placing a seed crystal in a melt to form a seed crystal melt mixture, the seed crystal comprising at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and the melt comprising at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. The method may further include heating the seed crystal melt mixture to a predetermined temperature until a periodically polarized nonlinear crystal is formed.

Description

[Technical Field]

[0001] Cross-reference of related applications This application invokes, by reference to, the entirety of U.S. Provisional Patent Application No. 63 / 521,880, filed on June 20, 2023, under Section 119(e) of the U.S. Patent Act.

[0002] This disclosure generally relates to systems and methods for growing nonlinear optical crystals, and more particularly to systems and methods for growing periodically polarized nonlinear optical crystals. [Background technology]

[0003] As semiconductor devices shrink in size, the size of the smallest particles or pattern defects that can cause device failure also decreases. Therefore, there is a growing need to detect smaller particles and defects on patterned and unpatterned semiconductor wafers and reticles. The intensity of light scattered by particles smaller than the wavelength of light generally increases or decreases as a large power of the particle's size (for example, the total scattering intensity of light from a small, isolated spherical particle is proportional to the sixth power of its diameter and inversely proportional to the fourth power of its wavelength). Because the intensity of scattered light increases, shorter wavelengths generally result in improved sensitivity to detecting smaller particles and defects than longer wavelengths.

[0004] Because the scattered light intensity from small particles and defects is generally very low, high illumination intensity is required to generate a signal that can be detected in a very short time. For example, an average light source output level of 1W or more may be required. At such high average output levels, a high pulse repetition frequency is desirable because the higher the repetition frequency, the lower the energy per pulse, and therefore the lower the risk of damage to the system optics or the object under inspection. The illumination demands in inspection and measurement are often met by continuous wave (CW) light sources. CW light sources have a constant output level, which avoids the problem of damage from peak output and allows for continuous acquisition of images or data. However, in many cases, a mode-locked laser (also called a pseudo-CW laser) with a repetition frequency of about 50 MHz or higher may be useful because a high repetition frequency means that the energy per pulse can be low enough to avoid damage in many measurement and inspection applications. Compared to a CW laser with the same average output level, a mode-locked laser has a higher peak output, which can enable more efficient and simpler wavelength conversion.

[0005] Pulsed lasers for generating vacuum ultraviolet (VUV) light are known in the art. Conventional lasers that generate light at 133 nm are well known (see, for example, G.W. Faris and M.J. Dyer, "Two-photon excitation of neon at 133 nm", Opt. Lett. 18, 382 (1993), and A. Tuennermann, C. Momma, K. Mossavi, C. Windolph, and B. Wellegehausen, "Generation of tunable short pulse VUV radiation by four-wave mixing in xenon with femtosecond KrF-excimer laser pulses", IEEE J. Quantum Electron. 29, 1233 (1993)). Unfortunately, such lasers are not well suited for inspection applications because of their low laser pulse repetition frequencies and the high cost of ownership due to the use of toxic and corrosive gases in their laser oscillation media.

[0006] Solid deep ultraviolet (DUV) lasers are desirable because they allow for higher repetition frequencies, can operate in continuous wave (CW) mode, and do not require toxic liquids or gases. There are several crystals that can be used for DUV frequency conversion. For example, beta barium borate (BBO) and cesium lithium borate (CLBO) crystals are common crystals for ultraviolet (UV) frequency conversion. Both materials have some ability to phase-match UV light, but have various drawbacks for high-power VUV frequency conversion. The damage threshold of BBO is relatively low when exposed to high-intensity DUV radiation. Furthermore, BBO is not transmissive below about 190 nm. CLBO may have a higher damage threshold than BBO, but is hygroscopic, requiring great care during handling, processing, and operation. In addition, CLBO has increased absorption at wavelengths shorter than about 185 nm.

[0007] For DUV frequency conversion, other less common crystals have also been investigated. For example, potassium beryllium fluoroborate (KBBF) (KBe2BO3F2), and other compounds of the beryllium fluoroborate (ABe2BO3F2) system where A = Na, K, Rb, Cs, Tl, NH4, have absorption edges in the range of 147 - 155 nm, and have a moderate nonlinear coefficient and a sufficient birefringence to enable phase matching at 161 - 202 nm. In KBBF, second harmonic generation of 200 nm light has been demonstrated at an output of 1.2 W, but the transmittance begins to decrease at wavelengths shorter than 200 nm, reducing the possibility of high-power generation at shorter wavelengths. Furthermore, the reported maximum growth size of KBBF crystals is 3.7 mm (Wang, X.Y.; Yan, X.; Luo, S.Y.; Chen, C.T. "Flux Growth of Large KBBF Crystals by Localized Spontaneous Nucleation" J. Cryst. Growth, 318, 610 - 612 (2011)), limiting the applicability of this material.

[0008] Other DUV-transparent crystals exist, namely, among others, strontium beryllium borate (SBBO) (Sr2Be2B2O7), strontium aluminum pentafluoride (SrAlF5), and boron phosphate (BPO4), but these crystals have issues such as unstable crystal structures, difficult growth, toxic precursors, or require further development of growth methods, as well as research on damage thresholds and nonlinear processes.

[0009] Other nonlinear crystals that are transparent in DUV do not have a birefringence large enough to enable birefringence phase matching in DUV. However, pseudo-phase matching is possible in many of these crystals. For example, magnesium barium fluoride (BaMgF2) and magnesium strontium tetrafluoride (SrMgF4) have high transmittance down to about 125 nm and are ferroelectrics, so periodic polarization is possible for pseudo-phase matching. However, the nonlinear coefficients of these materials are too small to overcome losses due to surface scattering or absorption within the material. Furthermore, periodic polarization using the ferroelectric properties of crystals does not always result in perfectly straight boundaries between polarization domains. This is acceptable for pseudo-phase matching in infrared (IR) or visible light, but it has a negative impact on VUV / DUV pseudo-phase matching because the mismatch in refractive indices between the short wavelengths involved is large, as seen in Selmeyer's refractive index model in the transparent region of dielectric nonlinear frequency conversion crystals, resulting in a shorter polarization period. Currently, periodically polarized crystals for VUV / DUV frequency conversion of any size are not commercially available.

[0010] To obtain a sufficiently large conversion efficiency (requiring approximately tens to thousands of polarization periods), it is necessary to fabricate VUV / DUV-transmitting frequency conversion crystals grown to a sufficient size (effective aperture diameter of approximately 5 to 10 mm). However, unless the material is ferroelectric to the required degree, there is currently no method for growing high-quality periodically polarized VUV / DUV nonlinear bulk crystals. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] International Publication No. 2020 / 131652 [Patent Document 2] International Publication No. 2023 / 107298 [Overview of the project] [Problems that the invention aims to solve]

[0012] Therefore, there is a need for a commercially scalable method for growing periodically polarized nonlinear crystals that generates DUV radiation at wavelengths around 120-200 nm, avoiding many or all of the drawbacks of conventional crystals, and suitable for use in systems configured for sample inspection, systems configured for exposing patterns onto photoresists on substrates, or systems configured for drilling, cutting, and ablation of materials including biological tissues. [Means for solving the problem]

[0013] Methods for growing a periodically polarized nonlinear crystal are disclosed according to one or more embodiments of the present disclosure. In embodiments, the method comprises placing a seed crystal in a melt to form a seed crystal melt mixture, wherein the seed crystal comprises at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and the melt comprises at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. In embodiments, the method comprises heating and cooling the seed crystal melt mixture to a predetermined temperature until a periodically polarized nonlinear crystal is formed.

[0014] One or more embodiments of the present disclosure are disclosed, which are periodically polarized nonlinear crystals. In the embodiments, the periodically polarized nonlinear crystal comprises a plurality of crystal plates arranged in a stacked configuration, the plurality of crystal plates comprising at least a first crystal plate and a second crystal plate, the first crystal plate adjacent to the second crystal plate, the plurality of crystal plates comprising at least one of one or more strontium tetraborate (SBO) plates or one or more lithium triborate (LBO) plates, the plurality of crystal plates are configured to form a periodic structure, the periodic structure achieving pseudo-phase matching (QPM) of light.

[0015] Optical systems according to one or more embodiments of the present disclosure are disclosed. In an embodiment, the optical system comprises an illumination source configured to produce illumination having a wavelength of 120 nm to 200 nm. In an embodiment, the optical system comprises an optical subsystem configured to direct illumination from the illumination source onto a sample. In an embodiment, the illumination source includes a first fundamental laser configured to produce a fundamental laser beam having a corresponding fundamental frequency and a fundamental wavelength of 720 nm to 800 nm. In an embodiment, the illumination source includes two or more frequency doubling stages, the two or more frequency doubling stages including at least an intermediate frequency doubling stage and a final frequency doubling stage, the intermediate frequency doubling stage configured to receive a first fundamental frequency and generate second harmonic light having a second harmonic frequency, the final frequency doubling stage configured to generate laser output light from the second harmonic light, the final frequency doubling stage includes a nonlinear crystal configured to double the frequency of the second harmonic light, the nonlinear crystal includes a plurality of crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates, and the plurality of crystal plates are coordinately configured to form a periodic structure that achieves pseudo-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

[0016] Laser assemblies according to one or more embodiments of the present disclosure are disclosed. In embodiments, the laser assembly includes a first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency and a fundamental wavelength of 720 nm to 800 nm. In an embodiment, the laser assembly includes two or more frequency doubling stages, the two or more frequency doubling stages including at least an intermediate frequency doubling stage and a final frequency doubling stage, the intermediate frequency doubling stage configured to receive a first fundamental frequency and generate second harmonic light having a second harmonic frequency, the final frequency doubling stage configured to generate laser output light from the second harmonic light, the final frequency doubling stage includes a nonlinear crystal configured to double the frequency of the second harmonic light, the nonlinear crystal includes a plurality of crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates, and the plurality of crystal plates are coordinately configured to form a periodic structure that achieves pseudo-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

[0017] Methods for growing a periodically polarized nonlinear crystal are disclosed according to one or more embodiments of the present disclosure. In embodiments, the method comprises contacting a periodically polarized seed crystal with a molten mixture from a Pt nozzle connected to a platinum (Pt) crucible containing the molten mixture, wherein the periodically polarized seed crystal comprises at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and the molten mixture comprises at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. In embodiments, the method comprises withdrawing the periodically polarized seed crystal from the Pt nozzle at a predetermined rate while maintaining contact with the molten mixture until a periodically polarized nonlinear crystal is formed.

[0018] It will be understood that both the above general description and the following detailed description are illustrative and descriptive only and do not necessarily limit the invention as described in the claims. The accompanying drawings incorporated herein and forming part thereof illustrate embodiments of the invention and, together with the general description, help illustrate the principles of the invention.

[0019] Many of the advantages of this disclosure can be better understood by those skilled in the art by referring to the accompanying drawings. [Brief explanation of the drawing]

[0020] [Figure 1] This is a schematic block diagram of an optical system according to one or more embodiments of the present disclosure. [Figure 2] This is a schematic block diagram showing a laser assembly according to one or more embodiments of the present disclosure. [Figure 3] This is a schematic diagram showing a furnace for growing periodically polarized strontium tetraborate (SBO) according to one or more embodiments of the present disclosure. [Figure 4A] This is a schematic diagram showing a periodically polarized SBO seed crystal according to one or more embodiments of the present disclosure. [Figure 4B] This is a schematic diagram showing a periodically polarized SBO seed crystal according to one or more embodiments of the present disclosure. [Figure 5] This is a schematic diagram showing a periodically polarized SBO crystal grown from a periodically polarized SBO species according to one or more embodiments of the present disclosure. [Figure 6] This flowchart shows a method for growing a periodically polarized nonlinear crystal according to one or more embodiments of the present disclosure. [Modes for carrying out the invention]

[0021] Hereinafter, the subject matter of the disclosed invention will be given in detail with reference to the accompanying drawings. This disclosure is shown and described in particular with respect to several embodiments and their particular features. The embodiments described herein are to be construed as illustrative and not limiting. Those skilled in the art will readily understand that various modifications and changes in form and detail can be made without departing from the spirit and scope of this disclosure.

[0022] Embodiments of this disclosure relate to improvements in the nonlinear crystal growth of periodically polarized strontium tetraborate (SBO) (SrB4O7) and lithium triborate (LBO) (LiB3O5) for semiconductor optical systems.

[0023] SBO is a material that is attracting increasing attention for its potential to generate DUV frequencies. The space group of SBO is the Pnm21 space group, the point group is mm2, and it has non-zero d 33 This demonstrates the existence of nonlinear coefficients that can be used for pseudo-phase matching. For SBO, the natural crystal coordinates are a=4.4255Å, b=10.709Å, and c=4.2341Å (Oseledchik, YS et al., "New nonlinear optical crystals: strontium and lead tetraborates," Optical Materials 4, 669-674 (1995), this entire document is cited and incorporated into this application). The crystallographic coordinates in the Cartesian coordinate system are X, Y, Z, where X, Y, Z correspond to a, b, and c. The optical coordinates are x, y, z, where x, -y, and z correspond to b, a, and c, respectively. Refractive index n z The corresponding z optical coordinates are along the 21 symmetry axis of the SBO. In the case of pseudo-phase matching (QPM) of the SBO, high d 33 The axis that is alternately flipped to access the nonlinear coefficients is the c-axis. 33The nonlinear coefficient has been measured at 1.5 pm / V during frequency doubling from 800 nm to 400 nm (Petrov, V. et al., "Application of the nonlinear crystal SrB4O7 for ultrafast diagnostics converting to wavelengths as short as 125 nm," Optics Letters, 29, 4 (2004), this entire document is cited and incorporated into this application). Furthermore, SBO exhibits DUV transmittance to wavelengths up to 125 nm, and frequency conversion to this wavelength has been demonstrated. SBO has a UV light-induced damage threshold of 16.4 J / cm² at 266 nm. 2 This is calcium fluoride (CaF2) (11.4 J / cm³). 2 ) and silica (4.8 J / cm³) 2 ) is significantly higher than (Tanaka et al., "High surface laser-induced damage threshold of SBO single crystals under 266-nm (DUV) laser irradiation," Optics Express, 28, 20 29239 (2020), this entire document is cited and incorporated into this application). Although SBO is a biaxial crystal, it is nearly isotropic (Oseledchik, YS et al., "New nonlinear optical crystals: strontium and lead tetraborates," Optical Materials 4, 669-674 (1995)), making birefringence phase matching for frequency conversion in DUV impossible. 33 Due to nonlinear coefficients, SBO is a candidate for pseudo-phase matching where the fundamental wave and second harmonic are polarized parallel to each other and parallel to the c-axis. Phase mismatch caused by the different refractive indices of the fundamental and harmonics is compensated by alternately reversing the direction of the material's c-crystal axis by 180°, thereby reducing the phase difference between harmonics due to the difference in sign of the nonlinear coefficients.

[0024] It should be noted that SBO exhibits unique optical and mechanical properties. The transmission range of SBO is 130 - 3200 nm in wavelength. See Y. S. Oseledchik, A. L. Prosvirnin, A. I. Pisarevskiy, V. V. Starshenko, V. V. Osadchuk, S. P. Belokrys, N. V. Svitanko, A. S. Korol, S. A. Krikunov, and A. F. Selevich, "New nonlinear optical crystals: strontium and lead tetraborates", Opt. Mater. 4, 669 (1995), which is hereby incorporated by reference in its entirety. This broad transmission range covers the VUV, DUV, visible, and near-infrared (IR) wavelength ranges. The VUV and DUV ranges are of particular interest in semiconductor inspection and metrology. Also, note that the transmittance is high. For example, the transmittance exceeds 80% from about 250 nm to about 2500 nm. This high transmittance makes SBO a suitable candidate for frequency generation, especially in the UV wavelength range. When SBO is grown under optimal conditions, a better transmission curve can be obtained, and the transmittance can reach over 80% at wavelengths above 200 nm and over 50% at wavelengths of 130 - 200 nm. The dielectric and optical properties of strontium tetraborate glass are described by M. V. Shankar and K. B. R. Barma in "Dielectric and Optical Properties of strontium tetraborate Glass", Journal of Materials Science Letters 15 (1996) 858 - 860, which is hereby incorporated by reference in its entirety.

[0025] Here, it is further contemplated that LBO is a well-studied and commercially available nonlinear optical material. LBO belongs to the Pna21 space group and the mm2 point group, and d 31 、d 32 、d 33 、d 24 and d 15This demonstrates that a nonlinear coefficient may exist and can be used for pseudo-phase matching. For LBOs, following the conventions of Roberts, D. "Simplified characterization of uniaxial and biaxial nonlinear optical crystals: a plea for standardization of nomenclature and conventions," IEEE Journal of Quantum Electronics, 28, 10 (1992), Table I "IEEE / ANSI" column (this entire document is cited and incorporated into this application), the natural crystallographic coordinates are a=7.3788 Å, b=8.4473 Å, and c=5.1395 Å. The crystallographic coordinates in the Cartesian coordinate system are X, Y, and Z, where X, Y, and Z correspond to a, b, and c. The optical coordinates are x, y, and z, where x, y, and z correspond to b, c, and a, respectively. The refractive index n y The corresponding y optical coordinates are along the 21 symmetry axes of the LBO. In the case of QPM of LiB3O5, high d 31 and d 15 The axis that is alternately flipped to access the nonlinear coefficients is the c-axis. 31 d 32 and d 33 The nonlinear coefficients at 1064 nm have been measured to be 0.85 pm / V, -0.67 pm / V, and 0.04 pm / V, respectively ("Simplified Characterization of Uniaxial and Biaxial Nonlinear Optical Crystals: A Plea for Standardization of Nomenclature and Conventions," Roberts, DA, IEEE J. of Quantum Electron., 28, 10 (1992), this entire document is cited and incorporated into this application). Assuming Kleinman symmetry and neglecting absorption, d 24 The nonlinear coefficient is d 32 Equal to the nonlinear coefficient, d 15 The nonlinear coefficient is d 31It is equal to the nonlinear coefficient. The transmittance range of LBO is 160-2300 nm, and it has sufficient birefringence to perform phase-matched frequency conversion for wavelengths up to approximately 266 nm. Birefringent phase matching is not possible for wavelengths below 200 nm, but pseudo-phase matching can be used. d 33 The nonlinear coefficients are too small to obtain practical conversion efficiency, but d 31 and d 15 The nonlinear coefficients are large enough for practical pseudo-phase matching. 31 In Type I phase matching, which utilizes nonlinear coefficients, the fundamental wave is polarized parallel to the crystallographic axis a, generating a second harmonic polarized parallel to the crystallographic axis c. 15 In Type II phase matching utilizing nonlinear coefficients, one or part of the fundamental beam is polarized parallel to the c crystallographic axis, and one or part of the fundamental beam is polarized parallel to the a crystallographic axis, generating a second harmonic polarized parallel to the a crystallographic axis. The phase mismatch caused by the different refractive indices of the fundamental and harmonics is compensated by alternately reversing the direction of the c crystal axis of the material by 180° increments, thereby reducing the phase difference between harmonics due to the difference in sign of the nonlinear coefficients. The damage threshold of the LBO is approximately 18 J / cm² at 355 nm. 2 And it's more expensive than BBO.

[0026] Ferroelectric materials such as lithium niobate (PPLN) and magnesium barium fluoride (MgBaF2), which undergo periodic polarization reversal, can have their crystal axes reversed by applying an electrostatic field, thereby making it possible to easily design pseudo-phase-matched materials. As mentioned above, MgBaF2 is not suitable for pseudo-phase matching because it has small nonlinear coefficients, and PPLN is not transparent under DUV. Since SBO and LBO are not known to exhibit ferroelectric effects along the c-axis, a different method must be used to pseudo-phase match SBO and LBO. For example, one method is to use a large single-crystal SBO as the starting material. This matter is generally described in U.S. Patent No. 11,543,732, entitled "Frequency Conversion Using Stacking Strontium Tetraborate Plates," published on January 3, 2023; U.S. Patent No. 11,237,455, entitled "Frequency Conversion Using Stacked Strontium Tetraborate Plates," published on February 1, 2022; U.S. Patent No. 11,567,391, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings," published on January 31, 2023; and U.S. Patent No. 11,899,338, entitled "Deep Ultraviolet Laser Using Strontium Tetraborate for Frequence Conversion," published on February 13, 2024, all of which are incorporated herein by reference. This single crystal is polished or etched into thin slabs of a thickness suitable for pseudo-phase matching, each slab is oriented so that its c-axis is reversed relative to the others, and there are gaps between the slabs or they are in optical contact.Another method for creating pseudo-phase-matched SBOs is the Czochralski method (Aleksandrovsky, AS, Vyunishev, AM, and Zaitsev, AI, "Applications of Random Nonlinear Photonic Crystals Based on Strontium Tetraborate," Crystals, 2, 1393-1409 (2012), the entire document is cited and incorporated herein). As discovered by Aleksandrovsky et al., the c-axis of the SBO spontaneously switches during Czochralski growth. This study found that controlling the periodicity of each layer during the growth process is difficult, and generation using randomly spaced layers was considered. Random pseudo-phase matching is less efficient than true pseudo-phase matching (except in the statistically rare case where the thickness of each randomly grown layer is exactly an odd multiple of the coherence length) and gives unpredictable results that differ from crystal to crystal, making it undesirable for commercial lasers.

[0027] High-quality, large-volume SBO growth using the Kyropoulos process with twin-type stirring blades has been demonstrated in Tanaka, Y. et al., "Kyropoulos growth of a 300 g SBO single crystal using a twin-type stirring blade," Japanese Journal of Applied Physics 61, 075503 (2022), and this entire document is cited and incorporated into this application. Under atmospheric pressure, the SBO phase exists in a small region in the phase diagram, so careful control is required to grow stoichiometric SrB4O7. A 6-zone resistance heating furnace can be used for Kyropoulos growth, and growth is carried out at 995-1005°C (e.g., 1002°C). With twin-type stirring blades and a Pt crucible rotating at 15 rpm, growth rates of 0.1-5 mm / day have been recorded. When a polished, optically contacted pseudo-phase-matched SBO seed crystal is used as the starting material and Kilopoulos growth is performed in a similar manner, the crystal pattern of the seed crystal propagates, resulting in a larger pseudo-phase-matched SBO crystal. By dicing and stacking this crystal, the number of pseudo-phase-matched layers can be increased.

[0028] LBO can be grown as high-quality, large boules using the high-temperature solution top seeding method. The crystals can be grown using the flux method with boron trioxide (B2O3) as the self-flux, but by adding molybdenum trioxide (MoO3) to reduce the viscosity of the flux, crystals reaching a maximum weight of 2 kg have been reported in the literature. Details of this growth method are described in the literature, for example, Hu, ZG et al., "Large LBO Crystal Growth at 2 kg-level," J.Cryst.Growth, 335 (2011), and this entire document is incorporated herein by reference.

[0029] Figure 1 shows a schematic block diagram of an optical system 100 according to one or more embodiments of the present disclosure. The optical system 100 can be configured as an inspection system or measurement system for inspecting a sample 108 and / or obtaining optical measurement values ​​from the sample 108. The optical system 100 may include a semiconductor manufacturing system. For example, the optical system 100 may include a manufacturing system that can be configured to cut, perforate, or ablate material from the sample 108, or to expose a pattern onto a photoresist on the sample 108.

[0030] The sample 108 may include, but is not limited to, any sample known in the art, such as a wafer, reticle, or photomask. In an embodiment, the sample 108 may be placed on a stage assembly 112 to facilitate the movement of the sample 108. The stage assembly 112 may include, but is not limited to, any stage assembly known in the art, such as an XY stage or an R-θ stage. In an embodiment, the stage assembly 112 is adjustable in height to maintain focus on the sample 108 during inspection. In an embodiment, a lens, such as an objective lens 150, may be moved up and down to maintain focus on the sample 108 during inspection.

[0031] In one embodiment, the optical system 100 has an output frequency ω OUT It has an output light L with corresponding wavelengths in the range of approximately 120 nm to approximately 200 nm. OUT The system includes an illumination source 102 incorporating a laser 200-0 that generates the material. Details of the laser 200-0 can be seen in the description of Figure 2 and Table 1. The laser 200-0 incorporates at least one of SBO and LBO pseudo-phase-matched crystals grown using the method described herein. The illumination source 102 may include additional light sources such as lasers operating at longer or shorter wavelengths, or broadband light sources.

[0032] In an embodiment, the optical system 100 includes, but is not limited to, one or more optical components such as a beam splitter, mirror, lens, aperture, and waveplate, which contribute to the output light L OUT The optical components are configured to condition and direct the light onto the sample 108. The optical components may be configured to illuminate a region, line, or spot on the sample 108. In an embodiment, the beam splitter or mirror 134, mirrors 137 and 138, and lens 152 are configured to illuminate the light L INT The beam splitter or mirrors 134 and 135, mirror 136, and lens 151 are configured to illuminate the sample 108 from below, enabling inspection or measurement of the sample 108 by allowing the sample to pass through. In the embodiment, the beam splitter or mirrors 134 and 135, mirror 136, and lens 151 are configured to illuminate at an oblique angle of incidence L, for example, at an angle of incidence greater than 60° with respect to the normal to the sample surface. obl In this embodiment, the sample 108 is illuminated. Spec It may be blocked or discarded without being collected. In the embodiment, the optical component 103 emits illumination light L IN It is collectively configured to orient the upper surface of sample 108.

[0033] Furthermore, when sample 108 is illuminated in one or more of the modes described above, the optical system 103 receives light L reflected, scattered, diffracted, transmitted, and / or emitted from sample 108. R / S / T Collect this light L R / S / T The detector assembly 104 is configured to direct and focus the light onto the sensor 106. It should be noted that the sensor 106 and the detector assembly 104 can include any sensor 106 known in the art. For example, the sensor 106 can include, but is not limited to, a charge-coupled device (CCD) detector, a complementary metal-oxide-semiconductor (CMOS) detector, a time-delay integral (TDI) detector, a photomultiplier tube (PMT), an avalanche photodiode (APD), a line sensor, an electron shock line sensor, etc. The detector assembly 104 can be communicatively coupled to the controller 114.

[0034] The controller 114 can be configured to store and / or analyze data from the detector assembly 104 under the control of program instructions 118 stored in the carrier medium 116. The controller 114 can be further configured to control other elements of the inspection system 100, such as the stage assembly 112, the illumination source 102, and the optical system 103.

[0035] In an embodiment, the optical system 103 includes an illumination tube lens 133. The illumination tube lens 132 can be configured to image the illumination pupil aperture 131 onto the pupil in the objective lens 150. For example, the illumination tube lens 132 can be configured such that the illumination pupil aperture 131 and the pupil in the objective lens 150 are conjugate to each other. In an embodiment, the illumination pupil aperture 131 may be configurable by switching different apertures to the position of the illumination pupil aperture 131. In an embodiment, the illumination pupil aperture 131 may be configurable by adjusting the diameter or shape of the opening of the illumination pupil aperture 131. In this regard, the sample 108 may be illuminated in different angular ranges depending on the characterization (e.g., measurement or inspection) performed under the control of the controller 114. The illumination pupil aperture 131 is illuminated by illumination light L IN It may include a polarizing element for controlling the polarization state.

[0036] In an embodiment, one or more optical elements 103 include a focusing tube lens 122. For example, the focusing tube lens 122 can be configured to image the pupil in the objective lens 150 onto the focusing pupil aperture 121. For example, the focusing tube lens 122 can be configured such that the focusing pupil aperture 121 and the pupil in the objective lens 150 are conjugate to each other. In an embodiment, the focusing pupil aperture 121 may be configurable by switching different apertures to the position of the focusing pupil aperture 121. In an embodiment, the focusing pupil aperture 121 may be configurable by adjusting the diameter or shape of the aperture of the focusing pupil aperture 121. In this regard, illumination at different angular ranges reflected or scattered from the sample 108 can be directed to the detector assembly 104 under the control of the controller 114. Also, the focusing pupil aperture 121 allows light L R / S / T A polarizing element can be included so that a specific polarization can be selected to transmit to the sensor 106.

[0037] In embodiments, the illumination pupil aperture 131 and / or the focusing pupil aperture 121 may include a programmable aperture. Programmable apertures are generally disclosed in U.S. Patent No. 9,255,887, entitled "2D programmable aperture mechanism," published to Brunner on February 9, 2016, and U.S. Patent No. 9,645,287, entitled "Flexible optical aperture mechanisms," published to Brunner on May 9, 2017, both of which are incorporated herein by reference. Methods for selecting aperture configurations for inspection are generally described in U.S. Patent No. 9,709,510, entitled "Determining a configuration for an optical element positioned in a collection aperture during wafer inspection," published to Kolchin et al. on July 18, 2017, and in U.S. Patent No. 9,726,617, entitled "Apparatus and methods for finding a best aperture and mode to enhance defect detection," published to Kolchin et al. on August 8, 2017. Both of these documents are incorporated herein by reference.

[0038] The various optical elements and operating modes shown in Figure 1 are examples illustrating how the laser 200-0 can be used in the inspection system 100 and are not intended to limit the scope of this disclosure. A practical optical system 100 may implement a subset or superset of the modes and optical elements shown in Figure 1. Additional optical elements and subsystems may be incorporated as needed for a particular application. The related literature cited above and other literature cited herein disclose many other important details of systems in which the laser 200-0 can be incorporated.

[0039] Figure 2 is a schematic block diagram showing a laser assembly 200 configured to generate wavelengths in the range of approximately 120 nm to approximately 200 nm (e.g., approximately 193 nm) according to one embodiment of the present disclosure.

[0040] In one embodiment, the laser assembly 200 includes a first fundamental laser 210 and two frequency conversion (multiplication) stages (i.e., one intermediate frequency conversion (multiplication) stage 220 and a final frequency conversion (multiplication) stage 230), which are coordinately configured to produce laser output light 239 having a wavelength in the range of about 120 nm to about 200 nm. The first fundamental laser 210 has a first fundamental wavelength in the range of about 720 nm to about 800 nm and a corresponding first fundamental frequency ω y The first frequency conversion (doubling) stage 220 is configured to generate a first fundamental wave light 211 having the first fundamental frequency ω. y The second harmonic frequency ω is equal to twice that of . x It generates a second harmonic light 212 having the first fundamental frequency ω. The final (second) frequency conversion (doubling) stage 230 receives the second harmonic light (intermediate frequency light) 212 and generates a first fundamental frequency ω. y Output frequency ω is equal to 4 times OUT It generates laser output light 239 having the following properties.

[0041] Referring to Figure 2, the first fundamental wave laser 210 has a first fundamental frequency ω y The system is configured using any suitable technique for generating the first fundamental wave light 211 (or "fundamental wave") in the system. In an embodiment, the first fundamental wave laser 210 has a first fundamental frequency ω corresponding to a wavelength in the range of about 720 nm to about 800 nm (e.g., a wavelength of about 774 nm). yThe laser is configured to generate a first fundamental wave light 211. In some embodiments, the first fundamental wave laser 210 is implemented using a titanium-sapphire (Ti-sapphire) laser oscillating medium. To adequately generate light with a wavelength of about 193 nm for inspecting semiconductor wafers, reticles, or photomasks, it is intended herein that the first fundamental wave laser 210 should generate a fundamental wave light 211 of tens or hundreds of watts. Other applications may not require such high power, or may require higher power. Depending on the pulse width and repetition rate requirements of the laser 200, the first fundamental wave laser may be configured as a Q-switched laser, a mode-locked laser, or a CW laser.

[0042] The first frequency conversion (doubling) stage 220 can be configured to generate second harmonic light 212 from the first fundamental wave light 211. In an embodiment, the first frequency conversion (doubling) stage 220 incorporates a lithium triborate (LBO) nonlinear crystal configured for critical phase matching between the first fundamental frequency and the second harmonic frequency. The first frequency conversion (doubling) stage 220 may optionally include other components, such as a prism, for separating the second harmonic light 212 from the unconsumed fundamental wave light. The first frequency conversion (doubling) stage 220 may include a cavity that resonates at the first fundamental frequency to increase conversion efficiency.

[0043] The final frequency conversion (doubling) stage 230 can be configured to generate laser output light 239 from second harmonic light 212. The final frequency conversion (doubling) stage 230 can incorporate a nonlinear crystal 400B configured to double the frequency of the second harmonic light 212 and output light 435-OUT containing light of the frequency of the laser output light 239 and unconsumed second harmonic light. The nonlinear crystal 400B can include a stack of SBO or LBO plates. Note here, for illustrative purposes, Figure 2 shows four such plates 435B-1, 435B-2, 435B-3 and 435B-4 stacked vertically. However, note here that in actual embodiments, there may be tens, hundreds or thousands of stacked plates. Figure 2 shows the plates in contact with each other. The thickness of each plate is selected to allow pseudo-phase matching for the doubling of the frequency of the second harmonic light 212. Adjacent plates (plates 435B-1 and 435B-2, etc.) have their crystal c-axes oriented in opposite directions relative to each other. These and other important aspects of the nonlinear crystal are described in detail below in relation to Figure 4B.

[0044] The final frequency conversion (doubling) stage 230 may optionally include other optical components, such as prisms, to separate the laser output light 239 from the unconsumed fundamental and second harmonic light. The final frequency conversion (doubling) stage 230 may include a cavity for recirculating the second high frequency to increase conversion efficiency.

[0045] In an embodiment, a single cavity may include both a first frequency doubling stage 220 and a final frequency doubling stage 230. In an embodiment, the first fundamental laser 210 includes a laser having an output frequency of about 1000 nm, such as 1064 nm or 1030 nm. In an embodiment, the first frequency conversion stage 220 includes a sum frequency generator, sum / difference frequency generator, optical parametric oscillator, or optical parametric amplifier stage. In an embodiment, the final frequency conversion stage 230 includes a sum frequency generator having a frequency conversion crystal 400B, as further described in Figure 4B. Frequency conversion paths are generally described in U.S. Patent No. 11,543,732, entitled "Frequency Conversion Using Stacking strontium tetraborate Plates," published on January 3, 2023; U.S. Patent No. 11,237,455, entitled "Frequency Conversion Using Stacked strontium tetraborate Plates," published on February 1, 2022; U.S. Patent No. 11,567,391, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings," published on January 31, 2023; and U.S. Patent No. 11,899,338, entitled "Deep Ultraviolet Laser Using strontium tetraborate for Frequency Conversion," published on February 13, 2024. All of these documents are incorporated herein by reference.

[0046] Figure 3 is a schematic block diagram showing a furnace 301 for growing SBO from seed crystal 305 using the top-seed Kylopoulos method.

[0047] In embodiments, the furnace 301 may include a resistance heating furnace having 5, 6 or more zones, including a platinum crucible 303 with a diameter of approximately 150 mm and a height of approximately 150 mm. The platinum crucible 303 may be larger or smaller depending on the final size of the crystal to be grown, and should be about twice the diameter of the desired final crystal size. The seed crystal 305 can be fixed to an alumina tube 302 to prevent the seed crystal 305 from falling into the molten metal 304 and to supply a coolant or gas to the seed crystal 305 to prevent melting during growth. In embodiments, the molten metal 304 consists of strontium carbonate (SrCO3) and boron trioxide powder in a composition containing about 67% boron trioxide (B2O3) as a self-flux in the molten metal. L-shaped or twin-shaped stirring blades may be used to promote mixing of the molten metal. The temperature of the melt 304 should be maintained at approximately 1000°C, which may be measured by one or more thermocouples 306, and the rotation of the crucible should be approximately 10-20 rpm. Note that the seed crystal 305 will be further described herein with respect to Figure 4A.

[0048] Another method that can be used to grow periodically polarized SBO crystals from periodically polarized seed crystals is hydrothermal growth. Hydrothermal growth of single-crystal SBO was demonstrated by McMillen, C. in his doctoral dissertation, "Hydrothermal Crystal Growth of Oxides for Optical Applications" (May 2007, Chapter 4), which is incorporated herein by reference. SBO growth was demonstrated with a Sr:B ratio of 1:14.6. The mineralizer used was 1M sodium hydroxide (NaOH) at 565°C and 20kpsi, with boron being (NH4)2B 10 O 16It was introduced at high density via 8H2O. Alternatively, mineralizers containing one or more of the following may be used: NaOH, sodium chloride (NaCl), strontium chloride (SrCl2), potassium chloride (KCl), strontium hydroxide (Sr(OH)2), cesium fluoride (CsF), lithium chloride (LiCl), potassium hydroxide (KOH), LiOH, and water (H2O) (see Carla Heyward's PhD dissertation, "Hydrothermal Crystal Growth of Metal Borates for Optical Applications," August 2013, which is incorporated herein by reference). Sr(OH)2 was used as the strontium source. The SBO crystals in this dissertation were grown without seeds in silver ampoules with an outer diameter of 0.25 inches and reached a maximum size of 1.5 mm. A temperature gradient of 10–120°C was maintained throughout the autoclave and could be measured by an externally mounted thermocouple. The hydrothermal method overcomes the problem of high viscosity of the B2O3 melt used when growing borate crystals by the Kilopoulos process and avoids inclusions caused by additives such as MoO3 used to reduce viscosity. For the hydrothermal growth of large crystals, the ideal solubility value is 1-5% by weight. As raw materials for SBO hydrothermal growth, glassy crystals containing strontium and borate, as well as other raw material sources such as Sr(OH)2, B2O3, H3BO3, and SrCO3, can be used.

[0049] Another method that can be used to grow periodically polarized SBO crystals from periodically polarized seed crystals is edge-defined film-feed growth (EFG) or micro-pull-down. In EFG, a stoichiometric melt of SBO can be contained in a crucible made of platinum or another inert material, with one end of a die or tube immersed in the melt. Capillary action draws the melt up to the slit, die, or the unimmersed end of the tube, where it comes into contact with the surface of the periodically polarized SBO seed crystal. The seed crystal is slowly pulled away from the slit, die, or tube, and as the melt adjacent to the seed crystal cools, it crystallizes with the same periodic polarization pattern as the seed crystal. Capillary force continues to supply the melt while the seed crystal is being pulled away. In this way, the shape of the crystal can be controlled by the shape of the die or tube. This method is currently used in the production of high-quality sapphire crystals. The micro-pull-down method can use a periodically polarized SBO or LBO seed crystal positioned below a nozzle connected to a crucible of stoichiometric melt at a controlled temperature. The nozzle and crucible may be made of platinum or another inert material. A seed crystal, which is colder than the molten material, comes into contact with a molten droplet at the tip of the nozzle, and the seed crystal is pulled down from the nozzle at a controlled rate. As the molten material in contact with the seed crystal moves away from the nozzle, it cools and crystallizes with the same periodic polarization pattern as the seed crystal. The nozzle is replenished with molten material. The micro-pull-down method has been demonstrated by Maeda, K. et al. in "Fabrication of Quasi-Phase-Matching Structure during Paraelectric Borate Crystal Growth" Applied Physics Express 6,015501 (2013) using a polarized lithium tetraborate seed crystal, and this entire document is incorporated herein by reference. Single-crystal SBO was grown using the micro-pull-down method by Machida, T. et al. in "Growth of Transparent SrB4O7 Crystal Fiber by the μ-PD Method," Trans. Mater. Res. Soc. Jpn., 42, 123-126 (2017), and this entire document is cited and incorporated into this application.

[0050] Periodically polarized LBO crystals can be grown by a high-temperature solution top seeding method using a periodically polarized LBO seed crystal, as discussed herein in relation to Figure 4A. The crystals can be grown by a flux method using B2O3 as the self-flux, or by adding MoO3 to reduce the viscosity of the flux. Details of this growth method can be found in the literature, for example, Hu, ZG et al., "Large LBO Crystal Growth at 2 kg-level," J.Cryst.Growth, 335 (2011), and this entire document is incorporated herein by reference.

[0051] In the embodiment, other crystal growth methods such as the Czochralski method, other pulling methods, or other flux methods or melt growth methods can be used.

[0052] Figure 4B is a schematic block diagram showing a periodically polarized crystal of SBO or LBO. In particular, Figure 4B shows that the nonlinear crystal 400B has a frequency ω x An embodiment is shown that includes four stacked SBO or LBO plates 435B-1 to 435B-8 configured to double the frequency of an input light 401 having a periodic structure. Figure 4B illustrates a nonlinear crystal 400B including eight stacked SBO or LBO crystal plates 435B-1 to 435B-8 having a periodic structure, but the total number of SBO or LBO plates may be as few as 2, more than 10, or more than 100. The number of plates may be odd or even. The thickness of each of the SBO or LBO plates 435B-1 to 435B-4 may be from several hundred nanometers to several hundred microns. Specifically, the thickness Λ of the SBO or LBO plate in the propagation direction of light 401A within the crystal plate is given by the following formula. Λ=mL c (Formula 1) Here, m is an odd number (for example, 1, 3, 5, 7...), and L c This is the pseudo-phase-matching (QPM) critical length.

number

number

[0053] In an unrestricted example, the thickness and orientation of multiple crystal plates are configured to achieve phase matching for generating a wavelength of 193 nm. In an additional unrestricted example, the thickness and orientation of multiple crystal plates are configured to achieve phase matching for generating a wavelength of 172–178 nm. In an additional unrestricted example, the thickness and orientation of multiple crystal plates are configured to achieve phase matching for generating a wavelength of 147–153 nm. In an additional unrestricted example, the thickness and orientation of multiple crystal plates are configured to achieve phase matching for generating a wavelength of 129–134 nm.

[0054] In an unrestricted example, the crystal plate thickness is an odd multiple of at least one of the following: 700–860 nm, 435–620 nm, 510–690 nm, 200–380 nm, 200–320 nm, or 80–175 nm. In an additional unrestricted example, the crystal plate thickness is an odd multiple of at least one of the following: 700–920 nm, 420–646 nm, and 460–730 nm.

[0055] In the case of frequency doubling of input light 401 having a wavelength of 386.8 nm, the pseudo-phase matching critical length L of SBOc The critical length L is approximately 0.85 μm (thickness of 0.8 μm to 0.9 μm, etc.). In LiB3O5, the critical length L is for type I pseudo-phase matching (where 386.8 nm light is polarized along the a crystallographic axis and 193.4 nm light is polarized along the c crystallographic axis). c The thickness is approximately 0.9 μm (e.g., 0.85 μm to 0.95 μm). In type II pseudo-phase matching (where half of the 386.8 nm light is polarized along the a crystallographic axis, half is polarized along the c crystallographic axis, and the 193.4 nm light is polarized along the c crystallographic axis), the critical length L c The thickness is approximately 0.72 μm (e.g., 0.7 μm to 0.8 μm). To achieve a slab thickness that is convenient for handling and processing, a reasonable value of m can range from 1 to approximately 999. An exemplary QPM critical length for generating light having a wavelength of 193.4 nm by frequency-duplicated light having a wavelength of 386.8 nm was calculated from the associated refractive index of SBO using the Sellmeyer model described in "Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV" Opt. Express 23, 10091 (2015) by P. Trabs, F. Noack, ASAleksandrovsky, AIZaitsev, NVRadionov, and V. Petrov, and the associated refractive index of LBO using the Sellmeyer model described in "Temperature-tuned 90° phase-matching properties of LiB3O5" IEEE J.Quant.Electr.30(12), 2950-2952 (1994) by K. Kato. These entire documents are incorporated herein by reference. The accuracy of these Sellmeyer models is uncertain. Furthermore, variations in the impurity concentration in the SBO or LBO crystal, or the presence of defects within the crystal, may slightly alter the refractive index of the crystal. Here, those skilled in the art will be able to understand how to calculate the QPM critical length for specific input and output frequencies using the above formula, given the precise refractive index of the crystal.

[0056] Referring to Figure 4B, the frequency ω can be composed of one or more wavelengths depending on whether second harmonic generation or sum frequency generation is preferred. X The input light 401 is incident on the input surface 435-IN of the nonlinear crystal 400B. The SBO or LBO plates 435B-1 to 435B-8 have an input surface 435-IN and an output surface 435-OUT with a frequency ω X Angle θ with respect to the propagation direction of the input light 401 B They are stacked in optical contact with each other so as to be oriented at a certain angle. When the plate thickness is not exactly as intended due to manufacturing variations, the orientation of the nonlinear crystal 400B can be fine-tuned to adjust the path length Λ of light within the SBO plate (i.e., fine-tune the incident angle θ) in order to more precisely achieve QPM. In addition, by adjusting the temperature of the slab laminate, the temperature-dependent refractive index can be changed to compensate for the effects of thickness variations. Light 403 emitted from the SBO or LBO plate laminate is 2ω in the case of second harmonic generation. X It includes the second harmonic of the input light at the frequency of ω, and in the case of sum frequency generation, it includes the sum of the two fundamental frequencies, x This includes unconsumed input light of a certain frequency. Since the seed crystal slabs are in optical contact, losses due to reflection or scattering between each slab are minimized. The grown crystal portion is continuous, and therefore the refractive indices are matched, so there are no losses due to reflection or scattering at the interface.

[0057] In this embodiment, as shown in Figure 4B, the angle θ BThe angle is approximately the Brewster angle to minimize reflection loss without using an anti-reflective coating. In SBO, the Brewster angle is approximately equal to 60.3° with respect to the surface normal N when the wavelength is around 386 nm and the polarization is parallel to the c-axis of the SBO crystal, and approximately equal to 61.9° with respect to the surface normal N when the wavelength is around 193 nm and the polarization direction is the same. The polarization direction of the input light 401 to the periodically polarized crystal of SBO is indicated by the dashed arrow 402. Since reflection loss is small at any angle within a few degrees of the Brewster angle (e.g., within ±2°), reflection loss becomes extremely small for both input and output light when the incident angle is around 61°.

[0058] In the embodiment, frequency ω X The input light 401 can be prism-coupled to the laminate. In one such embodiment, excess material generated during the growth process at both ends of the laminate can be removed so that the light is coupled at the Brewster angle, then passes through the non-polarized material before reaching the polarized material.

[0059] In the embodiment, to minimize the reflection of optical frequencies involved in the conversion, an anti-reflective coating for the input light 401 and output light 403 can be applied to the input surface 435-IN and / or the output surface 435-OUT.

[0060] Referring to Figure 4A, in order to generate a periodic structure for pseudo-phase matching with respect to the seed crystal 400A, SBO or LBO plates 435B-1 to 435B-4 are arranged with one rotated relative to the other so that their corresponding c-crystal axes are inverted relative to each other (shown in the two insets in Figure 4A for the SBO case). The surface normal N of the SBO plate with thickness Λ (where Λ is the pole spacing in the crystal along the propagation direction of light 401A in the crystal) and the propagation direction of light 401A in the SBO plate are shown in the two insets. This physical arrangement of crystal plates enables pseudo-phase matching. This configuration can be considered similar to using PPLN (periodically polarized lithium niobate) for pseudo-phase matching, but differs in that lithium niobate is a ferroelectric crystal and can be periodically polarized by the application of an electric field. In contrast, since SBO and LBO are not known to be ferroelectric along the c-axis, the crystal plates must be physically arranged to generate a periodic structure for pseudo-phase matching.

[0061] In this embodiment, the crystal axes of the SBO plates 435B-1 to 435B-4 are oriented such that light 401 propagating within the SBO plate propagates almost perpendicular to the c-axis, and the polarization direction (electric field direction) of the light 401A is almost parallel to the c-axis. This is because the maximum nonlinear coefficient d in the SBO is present. 33 This is used to maximize the conversion efficiency. In a preferred embodiment using LBO, the maximum nonlinear coefficient d 31To access the LBO, there are additional constraints on it: in the case of type I phase matching, the polarization direction of the fundamental wave of light should be approximately parallel to the a-axis; in the case of type II phase matching, it should be parallel to the c-axis and a-axis; and the polarization of the generated harmonics should be approximately parallel to the c-axis in plates 435B-1 to 435B-4 in both type I and type II phase matching cases. Therefore, the LBO crystal plate must be cut perpendicular to the b-axis in the ca-plane. Since the SBO crystal has polarizations where both the fundamental frequency and the generated frequency are approximately parallel to the c-axis, it can be cut perpendicular to the a-axis in the cb-plane, perpendicular to the b-axis in the ac-plane, or at any angle as long as the c-axis is in the plane. For example, in an embodiment, the crystal axes of SBO plates 435B-1 to 435B-4 can be oriented such that the propagation direction of light 401 is approximately parallel to the a-axis of the SBO crystal. In this embodiment, the crystal axis can be oriented so that light 401A propagates parallel to the b axis, or propagates at a certain angle within the ab-plane of the crystal. In other words, the crystal axis shown in the two insets in Figure 4A can be rotated around the c axis. When the incident surface of the SBO plate 435B-1 is oriented at a Brewster angle with respect to the incident light 401, the propagation direction of light 401 within plates 435B-1 to 435B-4 is approximately 29.7° with respect to the surface normal N, when the fundamental wavelength is approximately 386 nm.

[0062] Herein, it is intended that there are many methods for manufacturing and assembling the nonlinear seed crystal 400A. When only a small number of plates are needed for the laser (e.g., when high conversion efficiency is not required), it may be convenient to polish the plates to the desired thickness and then optically contact them in the appropriate orientation. When hundreds (or more) plates are needed to achieve the required conversion efficiency, other manufacturing methods may be more convenient. For example, U.S. Patent No. 11,567,391, filed December 18, 2021, entitled "Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings," generally discusses interdigitated nonlinear crystal diffraction gratings and methods for manufacturing them, and these documents in their entirety are incorporated herein by reference. SBO and LBO slabs can be etched in the correct orientation, removed from the substrate, and optically contacted.

[0063] Another method for producing seed crystals is described by Maeda, K. et al., “Fabrication of Quasi-Phase-Matching Structure during Paraelectric Borate Crystal Growth,” Applied Physics Express 6,015501 (2013), which is incorporated herein by reference. This method uses a thin crystal having two distinct adjacent crystal domains. A platinum (Pt) wire melts a small portion of the crystal in one domain that is close to the size of the Pt wire, is drawn slowly until it comes into contact with the second domain, and then returns to the first domain. The molten material “behind” the wire recrystallizes in the orientation of the second domain, creating a finger of the second domain within the first domain. By repeating this process, a periodic spacing of fingers with the correct spacing for pseudo-phase matching can be produced. This crystal can then be used as a seed crystal in pulling, solution growth, melt growth, or hydrothermal synthesis to grow a larger crystal.

[0064] Figure 5 shows the final crystal 500 grown from seed crystal 400A. In Figure 5, only four slabs are shown for the seed crystal, but 2 to several thousand slabs can be included in the seed crystal. After growth, the grown crystal 500 can be cut into more desired configurations by dicing the grown crystals parallel to the minimum dimensions of each slab and stacking the slabs. In embodiments, the grown crystal 500 can be diced and stacked in other configurations. The grown crystal 500 can be diced and stacked to produce a crystal with more slabs 501-A, 501-B, 501-C, 501-D, which can then be used as seed crystal 400A for further growth. Thus, the seed crystal can be iteratively grown, diced, and stacked to produce a grown crystal with several thousand periods for pseudo-phase matching.

[0065] Figure 6 is a flowchart of method 600 for growing a periodically polarized nonlinear crystal. In step 602, a seed crystal can be placed in the melt to form a seed crystal melt mixture, the seed crystal comprising at least one of strontium (SBO) or lithium triborate (LBO), and the melt comprising at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. As shown in Figure 5, in step 604, the seed crystal melt mixture can be heated and cooled to a predetermined temperature until a periodically polarized nonlinear crystal is formed.

[0066] Table 1 shows the laser output light L having wavelengths in the range of approximately 125 nm to 140 nm (e.g., approximately 133 nm) and 147 nm to 155 nm (e.g., approximately 152 nm) according to exemplary embodiments of the present disclosure. OUTThe table includes exemplary coherence lengths required in a pseudo-phase-matched SBO or LBO crystal for the wavelengths produced by the laser assemblies 200-0 and 200A in Figures 1 and 2 to produce laser output light 161 having wavelengths of approximately 170 nm to 180 nm (e.g., approximately 177 nm) and approximately 190 nm to 195 nm (e.g., approximately 193 nm). For each type of fundamental laser, an exemplary fundamental wavelength is shown along with the wavelengths corresponding to the harmonics. The exact wavelength of a fundamental laser depends on many factors, including the exact composition of the laser oscillating medium, the operating temperature of the oscillating medium, and the design of the optical cavity. Two lasers using the same laser line of a given laser oscillating medium may operate at wavelengths that differ by a fraction of a nm or several nm due to the factors mentioned above and other factors. Those skilled in the art will understand how to select appropriate first and second fundamental wavelengths to produce the desired output wavelength from any fundamental wavelength close to the fundamental wavelengths listed in the table. Similarly, if the desired output wavelength differs by a few nanometers from 133 nm, 152 nm, 177 nm, or 193 nm, the desired output wavelength can also be achieved by appropriately adjusting the wavelength of the first or second fundamental wavelength. [Table 1]

[0067] This disclosure describes various fundamental wavelengths that facilitate the generation of laser output light of a desired wavelength of approximately 120–200 nm, but other wavelengths within a few nanometers or tens of nanometers of this desired wavelength can also be generated by changing the wavelength of the first fundamental wave laser (i.e., laser assembly 200A). Unless otherwise specified in the appended claims, such lasers and systems utilizing such lasers are considered to be within the scope of this disclosure.

[0068] Lasers with wavelengths below 200 nm are not commercially available at sufficient power levels, or they are unreliable or have high operating costs. Periodically polarized SBO and LBO crystals are not commercially available. In particular, there are no conventional techniques for growing periodically polarized crystals from periodically polarized seeds with high purity, high damage threshold, high nonlinear coefficients, and high transmittance in the sub-200 nm wavelength range.

[0069] Lasers with wavelengths shorter than 200 nm are not commercially available at sufficient power levels, or they are unreliable or have high operating costs. Aside from excimer lasers, no prior art exists that generates optical output of 1 W or more in the wavelength range of approximately 120 nm to 200 nm with a lifetime of several months. Embodiments of this disclosure generate wavelengths of 120 to 200 nm, and therefore improve the sensitivity to detecting minute particles and defects compared to longer wavelengths. Because the lasers of this disclosure do not use toxic or corrosive gases, they are easy to operate and maintain, and inexpensive.

[0070] Those skilled in the art will readily understand that the laser crystals described herein are available for many applications in addition to their use in semiconductor inspection and measurement. For example, a laser operating at a wavelength around 193.4 nm can be used in a lithography system configured to expose a pattern onto a photoresist coated on a substrate such as a semiconductor wafer. In another example, a laser operating at a wavelength of approximately 120 nm to 200 nm can be used in a system for cutting or ablating biological tissue. The lasers described herein can be configured to generate very short pulses at the output wavelength, enabling selective removal of material by ablation rather than heating, thereby reducing damage to surrounding materials. For example, such lasers can be used in laser ophthalmic surgery or laser vision correction. While this disclosure has described certain embodiments, it will be apparent to those skilled in the art that the features of this disclosure are applicable to other embodiments, and that all of these are intended to be within the scope of this disclosure.

[0071] All methods described herein may include storing the results of one or more steps of an embodiment of the method in memory. The results may include any of the results described herein and may be stored in any manner known in the art. The memory may include any memory described herein or any other suitable storage medium known in the art. After the results are stored, they may be accessed in memory, used by any of the embodiments of the method or system described herein, formatted for display to a user, used by another software module, method or system, etc. Furthermore, the results may be stored “permanently,” “semi-permanently,” “temporarily,” or over a period of time. For example, the memory may be random-access memory (RAM), and the results do not necessarily have to persist indefinitely in memory.

[0072] Each embodiment of the above-described method is further intended to include any other step of any other method described herein. In addition, each embodiment of the above-described method can be carried out by any of the systems described herein.

[0073] Those skilled in the art will recognize that the components, actions, devices, objects, and accompanying discussions described herein are used as examples for conceptual clarity, and that various structural modifications are intended. Therefore, when used herein, the specific examples and accompanying discussions described are intended to represent their more general class. In general, the use of any particular example is intended to represent its class, and the exclusion of specific components, actions, devices, and objects should not be interpreted as limitation.

[0074] As used herein, terms indicating direction, such as “top,” “bottom,” “up,” “down,” “upper,” “upward,” “lower,” and “downward,” are intended to indicate relative positions for illustrative purposes and not to specify an absolute reference system. Various modifications to the embodiments described herein will be apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments.

[0075] With regard to the use of substantially any plural and / or singular terms herein, those skilled in the art can convert from plural to singular and / or singular to plural as appropriate to the context and / or use. Various singular / plural substitutions are not explicitly stated herein for the sake of clarity.

[0076] The subject matter described herein sometimes refers to different components that are included in or connected to other components. It should be understood that the architectures described herein are merely examples, and in practice, numerous other architectures can be implemented to achieve the same function. Conceptually, any arrangement of components that achieve the same function is effectively “associated” in such a way that the desired function is achieved. Thus, two components combined herein to achieve a particular function can be recognized as “associated” with each other, independently of architecture or intermediate components, in such a way that the desired function is achieved. Similarly, any two such associated components can also be considered “connected” or “joined” with each other to realize a desired function, and any two components that can be associated in such a way can also be considered “joinable” with each other to achieve a desired function. Specific examples of joinable components include, but are not limited to, physically joinable and / or physically interacting components and / or wirelessly interactable and / or wirelessly interacting components and / or logically interacting and / or logically interactable components.

[0077] Furthermore, it should be understood that this disclosure is defined by the attached claims. In general, it will be understood by those skilled in the art that the terms used herein and in particular in the attached claims (e.g., the text of the attached claims) are intended to be "open" terms (e.g., the term "contains" should be interpreted as "contains but not limited to," the term "has" should be interpreted as "has at least," and the term "contains" should be interpreted as "contains but not limited to," etc.). It will further be understood by those skilled in the art that if a specific number of claims to be introduced is intended, such intention will be explicitly stated in that claim, and if there is no such statement, such intention does not exist. For example, for the sake of understanding, the attached claims below may include introducing the claims using the introductory phrases "at least one" and "one or more." However, the use of such phrases should not be interpreted as meaning that the introduction of a claim by the indefinite article "a" or "an" limits any particular claim containing such introduced claim to a disclosure containing only one such claim. The same applies to the use of clear articles used to introduce the description of a claim, even when the same claim includes introductory phrases such as "one or more" or "at least one" and indefinite articles such as "a" or "an" (for example, "a" and / or "an" should typically be interpreted as meaning "at least one" or "one or more"). Furthermore, even when the specific number of items described in the introduced claim is explicitly listed, a person skilled in the art will recognize that such a description should typically be interpreted as meaning at least the number listed (for example, a bare list of "two lists" without other modifiers typically means at least two lists, or two or more lists).Furthermore, in cases where a conventional expression similar to "at least one of A, B, and C, etc." is used, such configurations are generally intended in a way that a person skilled in the art would understand the conventional expression to be (for example, "a system having at least one of A, B, and C" is not limited to but includes systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or systems having both A, B, and C). In cases where a conventional expression similar to "at least one of A, B, or C, etc." is used, such configurations are generally intended in a way that a person skilled in the art would understand the conventional expression to be (for example, "a system having at least one of A, B, or C" is not limited to but includes systems having only A, only B, only C, both A and B, both A and C, and / or systems having both A, B, and C). It will be further understood by those skilled in the art that virtually any separate word and / or phrase presenting two or more alternative terms should be understood, wherever it may be in the description, claims, or drawings, as construing the possibility of including one of those terms, either of those terms, or both of those terms. For example, the phrase “A or B” would be understood to include the possibilities of “A” or “B” or “A and B.”

[0078] The present disclosure and its numerous accompanying advantages are to be understood by the foregoing description, and it will be clear that various modifications can be made to the form, structure, and arrangement of the components without departing from the disclosed subject matter or sacrificing all of the substantial advantages disclosed. The described forms are merely illustrative, and the following claims are intended to imply and encompass such modifications. Furthermore, it will be understood that the present disclosure is defined by the appended claims.

Claims

1. A method for growing a periodically polarized nonlinear crystal, The method involves placing a periodically polarized seed crystal in a melt to form a seed crystal melt mixture, wherein the seed crystal comprises at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and the melt comprises at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. The seed crystal melt mixture is heated and cooled to one or more predetermined temperatures until the periodically polarized nonlinear crystal is formed. A method characterized by including the following.

2. A method according to claim 1, characterized in that the molten material is contained in a platinum crucible of a furnace and the seed crystal is fixed in an alumina tube.

3. A method according to claim 2, characterized in that the melt contains strontium carbonate and boron trioxide.

4. A method according to claim 3, characterized in that the predetermined temperature is 995°C to 1005°C.

5. A method according to claim 3, characterized in that the boron trioxide acts as a self-flux and constitutes 67% of the melt.

6. A method according to claim 1, characterized in that the melt contains strontium hydroxide.

7. The method according to claim 1, wherein the melt is A method characterized by comprising at least one of boron oxide autoflux or molybdenum trioxide.

8. The method according to claim 1, Polishing thin slabs of crystal, By bringing polished thin slabs into contact with each other in alternating c-axis orientations, the periodically polarized crystals are formed. It further includes, The method is characterized in that the thin slab of the crystal contains at least one of strontium tetraborate or lithium triborate.

9. A periodically polarized nonlinear seed crystal, It includes a plurality of crystal plates arranged in a stacked configuration, wherein the plurality of crystal plates include at least a first crystal plate and a second crystal plate, and the first crystal plate is adjacent to the second crystal plate. The plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) plates or one or more lithium triborate (LBO) plates. The crystal is characterized in that the plurality of crystal plates are configured to form a periodic structure, and the periodic structure achieves pseudo-phase matching (QPM) of light.

10. A crystal according to claim 9, characterized in that the first crystal axis of the first crystal plate is inverted with respect to the second crystal axis of the second crystal plate.

11. A crystal according to claim 9, characterized in that the thickness and orientation of the plurality of crystal plates are configured to achieve phase matching for generating a wavelength of 193 nm.

12. A crystal according to claim 9, characterized in that the thickness and orientation of the plurality of crystal plates are configured to achieve phase matching for generating wavelengths of 172 to 178 nm.

13. A crystal according to claim 9, characterized in that the thickness and orientation of the plurality of crystal plates are configured to achieve phase matching for generating wavelengths of 147 to 153 nm.

14. A crystal according to claim 9, characterized in that the thickness and orientation of the plurality of crystal plates are configured to achieve phase matching for generating wavelengths of 129 to 134 nm.

15. The crystal according to claim 9, wherein the thickness of the crystal plate is At least one odd multiple of 700-860 nm, 435-620 nm, 510-690 nm, 200-380 nm, 200-320 nm, or 80-175 nm, A crystal characterized in that the c-crystal axis of the first crystal plate is inverted with respect to the c-crystal axis of the second crystal plate.

16. The crystal according to claim 9, wherein the thickness of the crystal plate is At least one odd multiple of 700-920 nm, 420-646 nm, and 460-730 nm, A crystal characterized in that the c-crystal axis of the first crystal plate is inverted with respect to the c-crystal axis of the second crystal plate.

17. An optical system, A lighting source configured to generate illumination with wavelengths of 120 nm to 200 nm, An optical subsystem configured to direct the illumination from the illumination source onto the sample, Equipped with, The aforementioned light source is A first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency and a fundamental wavelength of 720 nm to 800 nm, The device includes two or more frequency doubling stages, wherein the two or more frequency doubling stages include at least an intermediate frequency doubling stage and a final frequency doubling stage, the intermediate frequency doubling stage is configured to receive the first fundamental frequency and generate second harmonic light having a second harmonic frequency, the final frequency doubling stage is configured to generate laser output light from the second harmonic light, and the final frequency doubling stage includes a nonlinear crystal configured to double the frequency of the second harmonic light. The nonlinear crystal includes a plurality of crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, and the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates. The optical system is characterized in that the plurality of crystal plates are coordinately configured to form a periodic structure that achieves pseudo-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

18. It is a laser assembly, A first fundamental laser configured to generate a fundamental laser beam having a corresponding fundamental frequency and a fundamental wavelength of 720 nm to 800 nm, The device includes two or more frequency doubling stages, wherein the two or more frequency doubling stages include at least an intermediate frequency doubling stage and a final frequency doubling stage, the intermediate frequency doubling stage is configured to receive the first fundamental frequency and generate second harmonic light having a second harmonic frequency, the final frequency doubling stage is configured to generate laser output light from the second harmonic light, and the final frequency doubling stage includes a nonlinear crystal configured to double the frequency of the second harmonic light. The nonlinear crystal includes a plurality of crystal plates arranged in a stacked configuration such that each first SBO crystal plate is adjacent to at least one second crystal plate, and the plurality of crystal plates include at least one of one or more strontium tetraborate (SBO) crystal plates or one or more lithium triborate (LBO) crystal plates. The laser assembly is characterized in that the plurality of crystal plates are coordinately configured to form a periodic structure that achieves pseudo-phase matching (QPM) of the first fundamental frequency and the second harmonic frequency.

19. A method for growing a periodically polarized nonlinear crystal, The method involves contacting a periodically polarized seed crystal with a molten mixture from a Pt nozzle connected to a platinum (Pt) crucible containing the molten mixture, wherein the periodically polarized seed crystal comprises at least one of strontium tetraborate (SBO) or lithium triborate (LBO), and the molten mixture comprises at least one of a mixture of Sr, B, and O or a mixture of Li, B, and O. The periodically polarized seed crystal is separated from the Pt nozzle at a predetermined speed while maintaining contact with the melt until the periodically polarized nonlinear crystal is formed. A method characterized by including the following.

20. A method according to claim 19, characterized in that the molten metal is contained in a platinum crucible of a furnace and the molten metal is maintained at a predetermined temperature.

21. A method according to claim 20, characterized in that the predetermined temperature is 995°C to 1005°C.

22. A method according to claim 19, characterized in that the melt contains stoichiometric amounts of Sr, B, and O.

23. A method according to claim 19, characterized in that the melt contains stoichiometric amounts of Li, B, and O.

24. A method according to claim 19, characterized in that the nozzle includes a capillary tube, a slit, or a die, and transports the molten material from the Pt crucible to the seed crystal by capillary action.

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