Growth of strontium tetraborate crystals

Abstract

The invention provides a method for growing a strontium tetraborate (SrB4O7) crystal. The method includes lowering a seed crystal into a melt having a mixture of sources including Sr, B, O, and Cl. The method also includes heating and melting the mixture to a temperature sufficient to form strontium tetraborate crystals.

Description

[0001] Cross-reference of related applications

[0002] This application relates to U.S. Provisional Patent Application No. 63 / 656,193, filed June 5, 2024, and U.S. Provisional Patent Application No. 63 / 537,558, filed September 11, 2023, the disclosures of which are each incorporated herein by reference in their entirety. Technical Field

[0003] This disclosure relates to crystal growth, and more specifically, to the growth of the single-crystal material strontium tetraborate (SrB4O7), for use in metrology and inspection systems in semiconductor manufacturing (including those systems for inspecting and / or measuring photomasks, photomasks, and semiconductor wafers) as linear or nonlinear optical components, such as mirrors, lenses, prisms, beam splitters, windows, lamp units, quasi-phase matching, and other frequency conversion designs. Background Technology

[0004] As the size of semiconductor devices shrinks, the size of even the smallest particle or pattern defect that can cause device failure also shrinks. Therefore, there is a need to detect smaller particles and defects on both patterned and unpatterned semiconductor wafers and photomasks. The intensity of light scattered by particles smaller than the wavelength of that light is typically scaled to a high power of the particle's size (e.g., the total scattered intensity of light from isolated small spherical particles is proportional to the sixth power of the sphere's diameter and inversely proportional to the fourth power of the wavelength). Due to the increased intensity of scattered light, shorter wavelengths will generally provide better sensitivity for detecting small particles and defects compared to longer wavelengths. Therefore, high-speed inspection in the semiconductor industry is often performed in machines that utilize ultraviolet (UV) light. One method for generating UV light involves using a nonlinear crystal that is transparent in UV light to convert frequencies from longer wavelengths to UV wavelengths.

[0005] Because the intensity of light scattered from small particles and defects is typically very low, high illumination intensity is required to generate a signal that can be detected in a very short time. In addition to requiring high UV transmittance, this necessitates the use of windows, lenses, and other optical components with high damage thresholds.

[0006] As examples, linear and nonlinear optical crystals are used in microscopes, telescopes, virtual reality systems, lasers, and semiconductor systems. Optical crystals are widely used in semiconductor testing and metrology systems, including prisms, lenses, laser crystals and windows, and other components.

[0007] Few optical crystals are transparent in the deep ultraviolet (DUV) and vacuum ultraviolet (VUV) wavelengths, which range from approximately 200 nm to 280 nm and approximately 100 nm to 200 nm, respectively. The most commonly used optical crystals for linear optics applications are calcium fluoride and magnesium fluoride, which transmit light with wavelengths as short as approximately 130 nm. However, most fluorides are hygroscopic, or absorb moisture from the atmosphere. This water will absorb UV light, and the absorbed water can cause stress in the crystal, which may lead to changes in the shape of the optics, thus degrading performance.

[0008] Nonlinear optical crystals capable of frequency conversion for wavelengths shorter than approximately 190 nm are not commercially available. Several materials (such as potassium beryllium fluoroborate KBe₂BO₃F₂ (KBBF)) and other materials in ABe₂BO₃F₂ (A = Na, K, Rb, Cs, Tl, NH₄) can transmit wavelengths shorter than 190 nm and can be phase-matched. However, for wavelengths shorter than 190 nm, the transmission of KBBF decreases significantly. In addition, large-scale growth of these materials has not yet been achieved, and further research is needed on their damage threshold and lifetime.

[0009] Strontium tetraborate (SrB4O7) is a material that overcomes many of the aforementioned obstacles. SBO is transmissive for wavelengths as short as approximately 125 nm (see Optical Materials, Vol. 4, p. 669 (1995), YS Oseledchik, AL Prosvirnin, AI Pisarevskiy, VV Starshenko, VV Osadchuk, SP Belokrys, NV Svitanko, AS Korol, SA Krikunov, and AF Selevich, “New nonlinear optical crystals: strontium and lead tetraborate”). (Tetraborates), which are incorporated herein by reference in their entirety, are non-hygroscopic and have a strength of 16.4 J / cm² at 266 nm. 2The damage threshold of SBO is higher than that of CaF2 (Optics Express, Vol. 28, No. 20, p. 29239 (2020), Tanaka et al., “High surface laser-induced damage threshold of SBO single crystals under 266-nm (DUV) laser irradiation”) and other nonlinear crystals (e.g., CLBO). Although SBO cannot be phase-matched in DUV or VUV due to its small birefringence, it has a large d 33 Nonlinear coefficients, and therefore quasi-phase matching is possible. See U.S. Provisional Patent Application No. 63 / 038,134, filed June 12, 2020, entitled "177 nm and 133 nm CW Lasers Using Stacked Strontium Tetraborate Plates," and U.S. Provisional Patent Application No. 63 / 076,391, filed September 10, 2020, entitled "152 nm and 177 nm CW Lasers Using Stacked Strontium Tetraborate Plates," and U.S. Provisional Patent Application No. 63 / 076,391, filed September 10, 2020, entitled "Frequency Conversion Using Stacked Strontium Tetraborate Plates." U.S. Patent No. 11,237,455 entitled “TetraboratePlates”, U.S. Provisional Patent Application No. 63 / 282,706 entitled “Frequency Conversion Using Interdigitated Nonlinear Crystal Gratings” filed on November 24, 2021, and U.S. Patent Application No. KLA P6074 entitled “193 nm Laser Using Strontium Tetraborate For Frequency Conversion” are incorporated herein by reference in their entirety.

[0010] SBO growth has been demonstrated using the Chuklaski method, micro-pulling, hydrothermal method, and top-seeding solution growth. The Chuklaski method produces crystals with antiparallel domains of random thickness along the z-axis. This is likely attributed to the incomplete formation of the boron ion network during growth due to variations in microscopic growth parameters, leading to seed-inverted domains (Crystals, Vol. 2, pp. 1393–1409 (2012), “Applications of Random Nonlinear Photonic Crystals Based on Strontium Tetraborate” by Aleksandrovsky (AS), Vyunishev (AM), and Zaitsev (AI). Micro-pulling does not produce crystals large enough for most optical applications. Hydrothermal methods have shown promise for SBO growth, but large crystals have not yet been observed, and further work is needed to develop this technique. The top-seeded solution method has yielded the largest and purest SBO single crystal to date, with a mass of 300 g, as demonstrated by Y. Tanaka et al. in "Kyropoulos growth of a 300 g SrB4O7 single crystal using atwin-type stirring blade" in the Japanese Journal of Applied Physics, Vol. 61, p. 07503 (2022), which is incorporated herein by reference in its entirety. The melt used by Y. Tanaka et al. was a B-rich self-fluxing agent consisting of 67.3% B2O3 and 32.7% SrCO3. This high B2O3 concentration in the melt resulted in approximately 1 The high viscosity has been observed during the growth of other borate crystals, attributable to high B₂O₃ concentrations, for example, those with 87.5 wt% B₂O₃ and 6.3 wt%. The viscosity of LBO (J. CrystalGrowth, Vol. 158 (1996), "Growth of lithium triborate crystals II. Experimental results" by Sergei Parfeniuk C.) is relevant. The high viscosity of the melt reduces fluid velocity and mixing during crystal growth, resulting in higher defect density and therefore greater light absorption and scattering. High viscosity also necessitates a slow crystal growth rate to ensure adequate mixing occurs within the melt. To cut, polish, and etch SBO for linear and nonlinear optical components, a sufficient number of SBO ingots several centimeters in size must be grown, requiring higher growth rates and lower defect densities.

[0011] Solution growth, or crystal growth from a non-stoichiometric melt containing a solvent or flux, is a method for growing larger crystals at lower defect densities or lower temperatures. Finding a compatible flux from the virtually countless available options can be difficult; the flux must have appropriate melting and boiling points, viscosity, and must not be incorporated into the target crystal during growth. For example, before the discovery of Na₂O as a flux for producing high-quality crystals, a range of different fluxes were used to grow β-BBO of varying masses. Although Tanaka et al. used a slightly borate self-fluxing flux to grow SBO from a highly viscous melt, to our knowledge, fluxes using other compounds for growing SBO have not yet been attempted or explored. A fluxing compound, SrCl2, has been used as a flux in crystal growth, including lead tungstate (PbWO4) grown in a non-seeding solution, as demonstrated by Arora (SK) and Chudasam (B.) in *Crystal Growth and Design*, Vol. 7, No. 2 (2007), "Flux Growth and Optoelectronic Study of PbWO4 Single Crystals". SrCl2 has also been used as a flux in SrAl2O4:Eu in AC-LEDs, as demonstrated by Li (B.) et al. in *Journal of Alloys and Compounds*, Vol. 651 (2015), "SrCl2 as a Flux in the Growth of SrAl2O4:Eu in AC-LEDs". 2+ , Dy 3+Effects of SrCl2 on the structural and luminescent properties of SrAl2O4:Eu 2+ , Dy 3+ In the study of phosphors for AC-LEDs, SrAl2O4:Eu is used as a phosphorus. 2+ , Dy 3+ Fluxes used in high-temperature solid-state reaction growth. The growth methods, crystal structures, crystal quality, crystal sizes, and applications of these two materials differ from those of high-quality SBOs required for VUV optics and frequency conversion.

[0012] An SBO growth method to overcome the above limitations will be well received in relevant industries. Summary of the Invention

[0013] According to one or more embodiments of this disclosure, this disclosure generally relates to a method for growing SBO, wherein the melt or solution method includes strontium chloride (SrCl2) as a flux and components of the melt or solution. These SBO crystals are grown using the bubble-growing method, high-temperature solution top-seeding method, hydrothermal seeding method, or other seeding or non-seeding flux, solution, or melt method.

[0014] According to one or more embodiments of this disclosure, an optical system is disclosed. In one illustrative embodiment, the optical system includes one or more linear and nonlinear optical components, wherein at least a portion of the one or more linear and nonlinear optical components is formed of strontium tetraborate.

[0015] According to one or more additional and / or alternative embodiments of this disclosure, an optical system is disclosed. In one illustrative embodiment, the optical system includes a stage for supporting a sample. In another illustrative embodiment, the optical system includes an illumination source. In yet another illustrative embodiment, the optical system includes one or more linear optical components configured to direct illumination from the illumination source to the sample, wherein at least a portion of said one or more linear and nonlinear optical components is formed of strontium tetraborate.

[0016] According to one or more embodiments of this disclosure, a method for crystal growth is disclosed. In one illustrative embodiment, the crystal growth method includes a furnace for growing the crystal. In another illustrative embodiment, the crystal growth method includes a solution comprising sources of strontium, oxygen, boron, and strontium chloride for the growth of strontium tetraborate.

[0017] It should be understood that the foregoing general description and the following detailed description are merely illustrative and explanatory and do not necessarily limit the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention. Attached Figure Description

[0018] By referring to the accompanying drawings, those skilled in the art can better understand the many advantages of this disclosure, in which:

[0019] Figure 1 This is a simplified block diagram illustrating an exemplary testing or measurement system according to one aspect of this disclosure;

[0020] Figure 2 This describes an ultraviolet lamp that incorporates SrBO as one or more optical components according to one or more embodiments of the present disclosure; and

[0021] Figure 3 This is a simplified diagram illustrating an exemplary furnace for growing SBO according to one aspect of this disclosure. Detailed Implementation

[0022] This disclosure relates to improvements in the growth quality and growth rate of SBO crystals for semiconductor inspection systems. The following description is presented to enable those skilled in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms (e.g., “top,” “left,” “right,” “horizontal,” and “down”) are intended to provide relative positions for descriptive purposes and are not intended to specify an absolute reference frame. Various modifications to the described embodiments will be apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments. Therefore, this disclosure is not intended to be limited to the specific embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0023] Various aspects of this disclosure relate to the following references, each of which is incorporated herein by reference in its entirety: U.S. Provisional Patent Application No. 63 / 521,880, filed June 20, 2023, entitled “Growth of Quasi-Phase Matched Strontium Tetraborate and Lithium Triborate Crystals for Frequency Conversion”; U.S. Provisional Patent Application No. 63 / 038,134, filed June 12, 2020, entitled “177 nm and 133 nm CW Lasers Using Stacked Strontium Tetraborate Plates”; and U.S. Provisional Patent Application No. 152 nm and 177 nm CW Lasers Using Stacked Strontium Tetraborate Plates, filed September 10, 2020. U.S. Provisional Patent Application No. 63 / 076,391 entitled “CW Lasers Using Stacked Strontium Tetraborate Plates”; U.S. Patent Application No. 17 / 991,198 entitled “193 nm Laser Using Strontium Tetraborate For Frequency Conversion” filed November 21, 2022; U.S. Patents 11,237,455 and 11,567,391 by Chuang et al.; 6,201,601 by Vaez-Iravani et al.; 6,271,916 by Marxer et al.; 7,525,649 by Leong et al.; 7,817,260 by Chuang et al.; Armstrong et al. g) U.S. Patents 8,298,335 and 8,824,514; Genis's U.S. Patent 8,976,343; Dribinski's U.S. Patent 9,023,152; Dribinski et al.'s U.S. Patents 9,461,435 and 9,059,560; Zhuang et al.'s U.S. Patents 9,293,882 and 9,660,409; Zhuang et al.'s U.S. Patents 9,250,178, 9,459,215, 9,509,112, 10,044,166 and 10,283,366;U.S. Patent Application No. 2014 / 0305367, filed by Delinsky et al.

[0024] It should be noted that SBO exhibits unique optical and mechanical properties. The transparency range of SBO is from 130 nm to 3200 nm. See "New nonlinear optical crystals: strontium and lead tetraborates," by Yu. S. Oseredik, A. L. Prosvernin, A. I. Pisarevsky, V. V. Starshenko, V. V. Osadchuk, S. P. Belokris, V. V. Svetlanko, A. S. Koroly, S. A. Krikunov, and A. V. Serevich, in their entirety, which is incorporated herein by reference. This broad range of transparency covers the VUV, DUV, visible, and near-infrared (IR) wavelength ranges. The VUV and DUV ranges are of particular interest for semiconductor testing and metrology. It should also be noted that the transmittance is high, for example, from about 250 nm to about 2500 nm. SBO exhibits a transmittance exceeding 80% at wavelengths over 200 nm. This high transmittance makes SBO a promising candidate for frequency generation, particularly in the UV wavelength range. If SBO is grown under optimal conditions, even better transmittance profiles can be obtained: transmittance can reach over 80% for wavelengths longer than 200 nm and over 50% for wavelengths from 130 nm to 200 nm. The dielectric and optical properties of Strontium tetraborate glasses are described by M.V. Shankar and K.B. Barma in "Dielectric and Optical Properties of Strontium Tetraborate Glass," Journal of Materials Science Letters, Vol. 15 (1996), pp. 858–860, which is incorporated herein by reference in its entirety.

[0025] One material that has recently generated increasing interest in DUV frequency generation is SBO. SBO possesses the Pmn21 space group and mm2 point group, which indicates d 33 Nonlinear coefficients may be used for quasi-phase matching. For harmonics from 800 nm to 400 nm, this d... 33The nonlinear coefficient has been measured to be 1.5 pm / V (Optics Letters, Vol. 29, No. 4 (2004), "Application of the nonlinear crystal SrB4O7 for ultrafast diagnostics converting to wavelengths as short as 125 nm" by Petrov, V. et al.). Furthermore, SBO exhibits DUV transparency for wavelengths as short as 125 nm and frequency conversion to this wavelength has been demonstrated. SBO exhibits a UV-induced damage threshold of 16.4 J / cm² at 266 nm, significantly higher than that of CaF₂ (11.4 J / cm²) and silicon dioxide (4.8 J / cm²) ("High surface laser-induced damage threshold of SBO single crystals under 266-nm (DUV) laser irradiation," Tan et al., *Optics Letters*, Vol. 28, No. 20, p. 29239 (2020)). Although biaxial, SBO is nearly isotropic ("New nonlinear optical crystals: strontium and lead tetraborates," Yu. S. Oseledik et al.), and therefore birefringent phase matching is impossible for frequency conversion in DUV. Due to the high d... 33The nonlinear coefficient, SBO, is a candidate for quasi-phase matching, where the fundamental and second harmonics are parallel to each other and parallel to the c-axis polarization. Phase mismatch caused by the different refractive indices of the fundamental and higher harmonics is compensated by alternately flipping the c-axis orientation of the material by 180 degrees, thereby mitigating the phase difference between the harmonics by the different signs of the nonlinear coefficient. See U.S. Patent Application No. 63 / 521,880, entitled “Growth of Quasi-Phase Matched Strontium Tetraborate and Lithium Triborate Crystals for Frequency Conversion,” which is incorporated herein by reference in its entirety.

[0026] Because SBO has low birefringence, it is a good candidate material for a variety of linear optical components, such as windows, lenses, coatings, bulbs, and other components. See U.S. Patent 11,255,797 B2, entitled "Strontium tetraborate as optical glass material," which is incorporated herein by reference in its entirety.

[0027] Figure 1 An exemplary inspection system 100 configured to inspect or measure a sample 108 according to one or more embodiments of the present disclosure is shown. The inspection system 100 may be configured as an inspection system or metrology system for inspecting and / or measuring the sample 108. The inspection system 100 may also be configured to cut, drill, or ablate material from the sample 108, or expose a pattern onto a photoresist on the sample 108.

[0028] Sample 108 may comprise any sample known in the art, such as, but not limited to, wafers, photomasks, or the like. In one embodiment, sample 108 may be mounted on stage assembly 112 to facilitate movement of sample 108. Stage assembly 112 may comprise any stage assembly known in the art, including, but not limited to, XY stages, R-θ stages, and the like. In another embodiment, stage assembly 112 is capable of adjusting the height of sample 108 during inspection to maintain focus on sample 108. In yet another embodiment, a lens (e.g., objective lens 150) may be moved up and down during inspection to maintain focus on sample 108.

[0029] The testing system 100 includes a generator that outputs a frequency ω. OUT and the corresponding wavelength L in the range of approximately 120 nm to approximately 200 nm. OUTThe laser 200-0 has an illumination source 102. Illumination source 102 may include additional light sources, such as lasers operating at longer or shorter wavelengths or broadband light sources. The laser 200-0 may be incorporated into the growth SBO. The inspection system 100 includes components configured to adjust the light intensity. OUT And guide it to sample 108 and can be configured by one or more optical components, such as beam splitters, mirrors, lenses, apertures, and waveplates, by growing SBO. The optical components can be configured to illuminate areas, lines, or points on sample 108. In one embodiment, beam splitters or mirrors 134, 137, and 138 and lens 152 are configured to illuminate sample 108 from below so that transmitted light L... INT The light passes through the sample to achieve the inspection or measurement of sample 108. In another embodiment, beam splitters or mirrors 134 and 135, mirror 136 and lens 151 are configured to use light at an oblique incident angle L. Obl Illuminate sample 108, for example, at an angle of incidence greater than 60° relative to the normal of the sample surface. In this embodiment, the specularly reflected light L Spec It can be blocked or discarded rather than collected. In yet another embodiment, the optics 103 are configured to direct the illumination light L IN Guided to the top surface of sample 108.

[0030] When the sample 108 is illuminated in one or more of the above modes, the optical device 103 is also configured to collect light reflected, scattered, diffracted, transmitted and / or emitted from the sample 108. R / S / T And light L R / S / T The sensor 106 is guided and focused onto the detector assembly 104. It should be noted herein that the sensor 106 and detector assembly 104 may comprise any sensor 106 known in the art. The sensor may include, but is not limited to, charge-coupled device (CCD) detectors, complementary metal-oxide-semiconductor (CMOS) detectors, time-delay integration (TDI) detectors, photomultiplier tubes (PMTs), avalanche photodiodes (APDs), line sensors, electron-bombarded line sensors, or the like. The detector assembly 104 is communicatively coupled to the computing system 114.

[0031] The computing system 114 is configured to store and / or analyze data from the detector assembly 104 under the control of program instructions 118 stored on the carrier medium 116. The computing system 114 may be further configured to control other components of the inspection system 100, such as the stage 112, the illumination source 102, and the optics 103.

[0032] In one embodiment, the optics 103 includes an illumination tube lens 132. The illumination tube lens 132 can be configured to image an illumination pupil aperture 131 onto a pupil within an objective lens 150. For example, the illumination tube lens 132 can be configured such that the illumination pupil aperture 131 and the pupil within the objective lens 150 are conjugate to each other. In one embodiment, the illumination pupil aperture 131 can be configured by switching different apertures to positions of the illumination pupil aperture 131. In another embodiment, the illumination pupil aperture 131 can be configured by adjusting the diameter or shape of the opening of the illumination pupil aperture 131. In this regard, depending on the characterization (e.g., measurement or inspection) performed under the control of the controller 114, the sample 108 can be illuminated from different ranges of angles. The illumination pupil aperture 131 may also include a polarizing element to control the illumination light L. IN The polarization state.

[0033] In one embodiment, one or more optical elements 103 include a collection tube lens 122. For example, the collection tube lens 122 may be configured to image the pupil within the objective lens 150 onto the collection pupil aperture 121. For example, the collection tube lens 122 may be configured such that the collection pupil aperture 121 and the pupil within the objective lens 150 are conjugate to each other. In one embodiment, the collection pupil aperture 121 can be configured by switching different apertures to positions of the collection pupil aperture 121. In another embodiment, the collection pupil aperture 121 can be configured by adjusting the diameter or shape of the opening of the collection pupil aperture 121. In this regard, under the control of the controller 114, different ranges of illumination angles reflected or scattered from the sample 108 can be guided to the detector assembly 104. The collection pupil aperture 121 may also include a polarizing element to allow selection of light L R / S / T Specific polarized light is transmitted to sensor 106.

[0034] In another embodiment, the illumination pupil aperture 131 and / or the collection pupil aperture 121 may include programmable apertures. Programmable apertures are typically discussed in U.S. Patent No. 9,255,887, entitled "2D programmable aperture mechanism," issued to Brunner on February 9, 2016; and U.S. Patent No. 9,645,287, entitled "Flexible optical aperture mechanisms," issued to Brunner on May 9, 2017, both of which are incorporated herein by reference in their entirety. Methods for selecting aperture configurations for inspection are typically 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," issued to Kolchin et al. on July 18, 2017; and U.S. Patent No. 9,726,617, entitled "Apparatus and methods for finding a best aperture and mode to enhance defect detection," issued to Kolchin et al. on August 8, 2017. Both patents are incorporated herein by reference in their entirety.

[0035] Figure 1 The various optical elements and operating modes depicted are merely illustrative of 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 inspection system 100 can be implemented. Figure 1 The modes and subsets or supersets of optics depicted herein can be incorporated as needed for specific applications. The relevant references listed above and other references cited herein disclose many other important details that can be incorporated into systems of the laser 200-0 of this invention.

[0036] Figure 2This illustration shows a simplified schematic view of an ultraviolet lamp incorporating SBO as one or more optical components according to one or more embodiments of the present disclosure. The ultraviolet lamp may be a laser-driven light source. In this example, a laser 211 emits a laser beam 212, which is guided by a mirror 213 and focused by lenses 214 and 225 to generate plasma 202 within the lamp unit 201. The plasma 202 emits broadband ultraviolet light 205 over a wide wavelength range including DUV and / or VUV wavelengths. One or more windows 203 may be placed in the wall of the lamp unit 201 to allow the broadband ultraviolet light 205 to pass through the lamp unit 201. In one embodiment, the lamp unit 201 may be made of SBO. In this embodiment, SBO can be used to form a transparent bulb containing a gas for generating plasma 202. In another embodiment, one or more windows 203 may be formed of SBO. In yet another embodiment, both the lamp unit 201 and the one or more windows 203 may be made of SBO. The overall luminous flux of an ultraviolet lamp can be improved by appropriately using SBO as the optical glass material for one or more optical components. The lifetime of the ultraviolet lamp and critical optical components can also be improved by using SBO. Any of the aforementioned optical components can be manufactured using SBO crystals or glass, and the scope of this disclosure is by no means limited to SBO-based windows or plasma units. Specifically, as previously discussed herein, any number of linear optical components of this disclosure can be formed from SBO and can be implemented in any optical context, which may include, but is not limited to, semiconductor testing or metrology.

[0037] Figure 3 This description describes a simplified furnace 301 used for growing SBO from seed crystal 305 using the top-seeding bubble growth method. Furnace 301 may include a five-, six-, or more-zone resistance heating furnace, comprising a platinum crucible 303 having a diameter of approximately 150 mm and a height of approximately 150 mm. Depending on the final size of the grown crystal, the platinum crucible 303 may be larger or smaller, and should be approximately twice the diameter of the desired final crystal size. Seed crystal 305 is secured to alumina tube 302 to prevent it from falling into melt 304, and coolant or air is also provided to seed crystal 305 to prevent melting during growth.

[0038] Melt 304 comprises strontium carbonate (SrCO3), boron trioxide (B2O3), and strontium chloride (SrCl2). L-shaped or double-type stirrers can be used to promote melt mixing. The temperature of melt 304 should be maintained close to 1000°C. B2O3 has an approximately 9.8% viscosity at close to 1000°C. The high viscosity (see "Viscosity and Density of Boron Trioxide" by A. Napolitano et al., Journal of the American Ceramic Society, Vol. 48, No. 12 (1965)) leads to the self-fluxing of B2O3 in the work of Yamaguchi Tanaka et al., producing approximately 1 The total solution viscosity is unfavorable for mixing and therefore unfavorable for crystal growth. Strontium chloride is a salt with a melting point of 874°C and a boiling point of 1250°C. SBO has a melting point of approximately 1015°C and a recrystallization temperature of approximately 950°C; therefore, SrCl2 is an ideal flux for top-seeding solution growth of SBO because SrCl2 recrystallizes at a lower temperature than SBO. Furthermore, SrCl2 has a water solubility of 53.8 g / 100 mL at 20°C, and therefore excess SrCl2 can be easily removed from the SBO crystals by rinsing with water after growth is complete. The viscosity of SrCl2 was measured to be 2.25 m³ / mL between 880°C and 1050°C. up to 3.75 m (J. Chem. Eng. Data, Vol. 27, p. 387 (1982), "Viscosity of Molten Alkaline-Earth Chlorides" by K. Tørklep and Harald A. Øye), which is three orders of magnitude lower than the viscosity of B₂O₃. This lower viscosity improves single-crystal growth by replenishing the depleted melt near the surface of the growing crystal and improving temperature uniformity, while inhibiting the growth of other crystalline phases. Cl - The ions have different numbers of valence shell electrons and larger ionic radii than the other atoms constituting SBO, thus reducing Cl substitution and interstitial defects. This is due to the Sr from the ionic liquid of molten SrCl2. 2+ The number of valence electrons and Sr from dissolved SrCO3 2+ The number of valence electrons in SrCl2 is the same, therefore Sr from SrCl2 2+ It can significantly participate in crystal growth.

[0039] An exemplary chemical reaction involving raw materials B2O3 and SrCO3 for the formation of SBO is...

[0040] 2B2O3 + SrCO3 -> SrB4O7 + CO2.

[0041] Other chemical reactions involving the same or different stoichiometry of reactants may occur. Other reactants may be involved, such as SrCl2. Sr from molten SrCl2 in solution... 2+ The ions will react with Sr from dissolved SrCO3 3+ The ions cannot be distinguished. Other phases in the SrO-B2O3 system that crystallize at compositions and temperatures close to those used to form SrB4O7 have Sr:B ratios greater than 1:4 (J. Crystal Growth, Vol. 254 (2003), "New Sr4B Borate Crystals" by D.P. Kudrjavtcev et al.). 14 O 25 Growth of a newstrontium borate crystal Sr4B 14 O 25 This leads to a preference for using stoichiometric Sr:B feedstocks to form SrB4O7. A B-rich starting stoichiometry may be preferred to avoid the formation of other Sr-BO phases, as observed in both top-seeded solution growth and hydrothermal methods. However, a higher B2O3 percentage will increase the melt viscosity.

[0042] Another exemplary chemical reaction involving the raw materials H3BO3 and SrCO3 for the formation of SBO.

[0043] 4H3BO3 + SrCO3 -> SrB4O7 + CO2 + 6H2O.

[0044] Other chemical reactions involving the same or different stoichiometry of reactants may occur. Other reactants may be involved. In this reaction, the addition of SrCl2 flux makes the Sr:B ratio in the solution equal to or greater than 1:4.

[0045] Seed crystal 305 may include a single crystal with a fixed orientation, or a patterned crystal (such as the patterned crystal found in U.S. Provisional Patent Application KLA No. 63 / 521,880 entitled “Growth of Quasi-Phase Matched Strontium Tetraborate and Lithium Triborate Crystals for Frequency Conversion”, which is incorporated herein by reference in its entirety).

[0046] Optical glasses made of SBO can be formed by melting single-crystal SBO grown using SrCl2 incorporated into the melt. For more details on the manufacture of SBO glasses, see U.S. Patent 1,255,797 to Zhuang et al. entitled "Strontium tetraborate as optical glass material". This patent is incorporated herein by reference. SBO optical coatings used, for example, in anti-reflective coatings, can be deposited from these grown SBO single crystals via sputtering, electron beam evaporation, thermal evaporation, pulsed laser deposition, molecular beam epitaxy, or other thin film deposition methods known to those skilled in the art. These high damage threshold, DUV and VUV transmission optical glasses and coatings can be used in semiconductor testing and metrology systems, including mirrors, lenses, laser crystals, windows and lamps, and other components.

[0047] Although various temperatures, pressures, melt stoichiometry, furnace types, crucible sizes, and crucible and crystal rotation rates are used to describe this crystal growth method herein, other temperatures, pressures, melt stoichiometry, furnace types, crucible sizes, and crucible and crystal rotation rates are considered to be within the scope of this invention unless otherwise specified in the appended claims.

[0048] Nonlinear crystals with transmittance as low as approximately 120 nm are not commercially available. Specifically, there is no prior art for the large-scale production of SBO crystals larger than 350 g with high purity, high damage threshold, high nonlinear coefficient, and high transparency in the sub-200 nm region. Embodiments of this method provide a lower viscosity melt for SBO growth, thereby enabling the formation of larger, higher purity ingots. This growth method also uses a non-toxic melt composition and is therefore easy and inexpensive to implement and maintain.

[0049] Those skilled in the art will readily appreciate that, in addition to applications in semiconductor testing and metrology, the laser crystal growth method of the present invention described herein has many possible applications. For example, in a photolithography system configured to expose a pattern to a photoresist coated on a substrate (e.g., a semiconductor wafer), a laser incorporating an SBO crystal grown using this method and operating at wavelengths close to 193.4 nm can be used. In another example, a laser incorporating an SBO crystal grown using this method and operating at wavelengths between about 120 nm and about 200 nm can be used in a system configured to cut or ablate biological tissue. Although this disclosure has been described with respect to certain specific embodiments, those skilled in the art will understand that the inventive features of this disclosure are also applicable to other embodiments, all of which are intended to fall within the scope of this disclosure.

Claims

1. A method for growing strontium tetraborate (SrB4O7) crystals, comprising: The seed crystal is lowered into the melt, wherein the melt comprises: Forming a mixture of sources including Sr, B, O, and Cl; and The mixture is heated and melted to a temperature sufficient to form strontium tetraborate crystals.

2. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture of Sr, B, O and Cl is heated and melted to a temperature of approximately 960°C to 1030°C, and then cooled to a temperature of approximately 950°C to form the strontium tetraborate crystals.

3. The method for growing SrB4O7 crystals according to claim 1, wherein the source of Cl is SrCl2.

4. The method for growing SrB4O7 crystals according to claim 1, wherein the source of Sr, B, O and Cl includes at least one of B2O3, SrCO3 and SrCl2.

5. The method for growing SrB4O7 crystals according to claim 3, wherein the melt comprises approximately 0.5 mol% to 34 mol% SrCl2.

6. The method for growing SrB4O7 crystals according to claim 4, wherein the melt comprises approximately 0.5 mol% to 34 mol% SrCO3.

7. The method for growing SrB4O7 crystals according to claim 4, wherein the melt comprises approximately 66 mol% to 90 mol% B2O3.

8. The method for growing SrB4O7 crystals according to claim 3, wherein the melt comprises approximately 2 mol% to 15 mol% SrCl2.

9. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture further comprises a source of H2O.

10. The method for growing SrB4O7 crystals according to claim 1, further comprising growing the strontium tetraborate crystals by a top-seeding solution method.

11. The method for growing SrB4O7 crystals according to claim 1, further comprising growing the strontium tetraborate crystals by a fluxing method.

12. The method for growing SrB4O7 crystals according to claim 1, further comprising growing the strontium tetraborate crystals by a melt method.

13. The method for growing SrB4O7 crystals according to claim 1, comprising alternating crystal plates to achieve quasi-phase matching seed crystals.

14. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture of Sr, B, O and Cl is heated and melted to a temperature of approximately 960°C to 1030°C, and then cooled to a temperature of approximately 900°C to form the strontium tetraborate crystals.

15. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture of Sr, B, O and Cl is heated and melted to a temperature of approximately 960°C to 1030°C, and then cooled to a temperature of approximately 875°C to form the strontium tetraborate crystals.

16. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture of Sr, B, O and Cl is heated and melted to a temperature of approximately 1001°C to 1030°C, and then cooled to a temperature of approximately 1000°C to form the strontium tetraborate crystals.

17. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture of Sr, B, O and Cl is heated and melted to a temperature of approximately 1016°C to 1030°C, and then cooled to a temperature of approximately 1015°C to form the strontium tetraborate crystals.

18. The method for growing SrB4O7 crystals according to claim 1, wherein the mixture of Sr, B, O and Cl is heated and melted to a temperature of approximately 960°C to 1030°C, and then cooled to a temperature of approximately 875°C to 1015°C to form the strontium tetraborate crystals.

19. A frequency conversion device comprising a crystal grown according to the method of claim 1.

20. A linear optical device comprising a crystal grown according to the method of claim 1.

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