Ultrathin crystal for optical sensing and optical-grade polishing method for end face of ultrathin crystal

Abstract

The invention discloses an ultra-thin crystal for optical sensing and an end face optical-grade polishing method thereof.The polishing method comprises the steps that a gasket and an ultra-thin crystal to be machined are fixed to a base of a laser cutting machine, cutting is conducted after the cutting size is set, and the cut crystal and the gasket are obtained; cleaning the surfaces of the cut crystal and the gasket, uniformly coating the gasket with an adhesive, and then transferring the crystal to the surface of the gasket so that the crystal and the gasket are completely overlapped and cured to obtain a combination of the crystal and the gasket; a combination of the crystal and the gasket is fixed to an end face polishing and grinding clamp, polishing and grinding are conducted through a chemical mechanical polishing and grinding method, the two end faces of the combination reach the optical level flatness, and the edge perpendicularity is good; and heating the polished and latticed crystal, then detaching the polished and latticed crystal from the end face polishing clamp, and cleaning surface residues to obtain the ultrathin crystal with the end face subjected to optical-grade polishing. The method has the advantages of high polishing precision, few polishing defects, simple and controllable polishing process, low cost, high efficiency and the like.

Description

Technical Field

[0001] The invention relates to the field of crystal processing, and in particular to an ultra-thin crystal for optical sensing and an optical-grade polishing method for an end face thereof. Background Art

[0002] Crystal materials have very important application value in the field of optical sensing, especially lithium niobate, lithium tantalate, single crystal silicon, silicon carbide, quartz glass, etc. They have rich photoelectric effects such as photorefractive effect, nonlinear effect, electro-optic effect, acousto-optic effect, piezoelectric effect and thermoelectric effect. The performance can be greatly controlled by methods such as crystal composition, element doping and valence state control. Devices made based on these crystal materials can obtain a bandwidth advantage of tens of G and are favored in the field of high-speed communications.

[0003] With the development of photonic chip technology, integrated optical devices require thinner and more flexible crystal materials. Thin-film crystal materials have brought new opportunities for the development of high-speed optical communications and integrated optical circuits. In 2022, a joint team led by Tian Yonghui of Lanzhou University and Su Yikai of Shanghai Jiaotong University deposited a layer of silicon nitride film on the surface of a thin-film lithium niobate wafer, and used a mature CMOS-compatible process to etch the silicon nitride layer to form a silicon nitride-lithium niobate heterogeneous ridge waveguide, realizing high-performance mode and polarization multiplexing devices; in the same year, a 2×2 Fabry-Perot resonant cavity thin-film lithium niobate electro-optic modulator prepared by Zhejiang University set a new record, with excess loss as low as 0.9dB and extinction ratio of 21 dB; in 2024, the lithium niobate microwave photonic chip developed by Wang Cheng's team at the City University of Hong Kong has lower energy consumption and nearly 1,000 times higher computing speed than silicon-based and indium phosphide. The wide range of applications of this chip covers 5G / 6G wireless communication systems, artificial intelligence, computer vision, and image / video processing. In the future, highly integrated multifunctional optoelectronic crystal chips will move towards industrialization, and on-chip integrated optical circuits will usher in an opportunity for rapid development.

[0004] The crystal materials used in optical devices need to have a certain surface precision to meet the optical requirements of the device, so the crystal materials need to be finely ground before preparing the optical device. Different from simple mechanical grinding or chemical corrosion, the use of chemical mechanical polishing technology to polish the surface of optical crystals with high precision, combined with different polishing pads, polishing liquids, polishing pressure, rotation speed and other process conditions, can produce a surface roughness of the nm level, and can well adapt to the surface polishing of hard and brittle materials such as lithium niobate and lithium tantalate, and the prepared crystal surface has high flatness.

[0005] At present, most of the crystal polishing technologies are aimed at large-area high-precision polishing of optical crystals, focusing on solving the problems of precision, defects and large-area polishing of hard and brittle materials in the polishing process. Few specialize in high-precision polishing of fine areas of ultra-thin optical crystals. In integrated optical devices, the processing of fine areas of ultra-thin optical crystal end faces is a very important link, but high-precision polishing of fine areas of ultra-thin hard and brittle materials is more challenging than conventional surface polishing. Ultra-thin hard and brittle materials are prone to edge collapse, brittle fracture, scratches, abrasive embedding, edge gelatinization, etc. during the polishing process. For high-precision optical devices, any damage and defects will greatly affect the performance of the device. Therefore, achieving high-precision grinding and polishing of the side of ultra-thin hard and brittle materials is a difficult problem that the industry urgently needs to solve. Summary of the invention

[0006] In view of this, the object of the present invention is to provide an ultra-thin crystal for optical sensing and an optical-grade polishing method for the end face thereof to solve the above-mentioned problems.

[0007] The embodiment of the present invention provides a method for optical-grade polishing of an end face of an ultra-thin crystal for optical sensing, which comprises the following steps: S1, fixing the gasket and the ultra-thin crystal to be processed on the base of the laser cutting machine respectively, adjusting the crystal axis direction of the ultra-thin crystal and the gasket to be consistent with the target cutting direction, setting the cutting size and performing cutting to obtain the cut crystal and gasket; S2, after cleaning the surfaces of the cut crystal and gasket, evenly apply adhesive on the gasket, and then transfer the crystal to the surface of the gasket so that the two are completely overlapped and solidified to obtain a combination of the crystal and the gasket; S3, fix the combination of the crystal and the gasket on the end face polishing fixture, and use chemical mechanical polishing to perform rough polishing and fine polishing to make the two end faces reach optical grade flatness and good edge verticality; wherein, three types of polishing particles with large, medium and small particle sizes are used in the polishing process for three-step polishing, and each step gradually reduces the load and the flow rate of the polishing liquid to reduce the edge rounding caused by the scratches on the crystal surface by the abrasive particles and the corrosion of the crystal edge by the polishing liquid during the polishing process; S4, remove the qualified crystal from the end face polishing fixture, clean the surface residue, and obtain an ultra-thin crystal with optical grade polishing on the end face.

[0008] Preferably, the ultra-thin crystal is glass, silicon crystal, lithium tantalate crystal, lithium niobate crystal, quartz crystal, barium titanate crystal or gallium arsenide crystal.

[0009] Preferably, the ultra-thin crystal is unsupported and has a thickness of 10-100 μm, preferably 1-50 μm.

[0010] Preferably, the gasket is made of glass, silicon crystal, lithium tantalate crystal, lithium niobate crystal, quartz crystal, barium titanate crystal or gallium arsenide crystal, and the gasket thickness is 0.3-1 mm.

[0011] Preferably, the gasket and the crystal may be made of the same material or different materials.

[0012] Preferably, step S2 is specifically as follows: place dust-free paper on the operating table, place the cut crystal on the dust-free paper, use another dust-free paper soaked in organic cleaning liquid to wipe the two surfaces of the crystal in a single direction for multiple times, clean the surface of the gasket and evenly apply the adhesive, and then transfer the crystal to the surface of the gasket so that the two completely overlap and solidify to obtain a combination of the crystal and the gasket.

[0013] Preferably, the adhesive for fixing the crystal and the gasket is one of UV curing glue, epoxy resin glue, hot melt paraffin, hot melt rosin, hot melt EVA or hot melt EVOH.

[0014] Preferably, the crystal may be fixed with gaskets on both sides, or may be supported on one side; the gasket and the crystal may be fixed with detachable hot melt adhesive or non-detachable ultraviolet glue or epoxy glue.

[0015] Preferably, in step S3, before polishing, the crystal supported by the gasket is fixed on a support fixture of suitable thickness so that the polishing end face protrudes 0.5-2mm; polishing is divided into three steps: the first step is to use a silica or alumina polishing liquid with a particle size of 30μm to perform rough polishing on a metal rough polishing wheel, and the rough polishing load is 30-200g / mm 2 , the polishing liquid flow rate is 10-30ml / min, the grinding wheel speed is 30-80r / min, the polishing time is 15-30min or grinding to the specified size; the second step is to replace the polishing liquid in the first step with a silica or alumina polishing liquid of about 1μm, the polishing liquid drop rate is adjusted to 5-10ml / min, and the polishing time is 15-30min; the third step is the fine polishing load of 0.2-15g / mm 2 The flow rate of the polishing liquid is 0.5-5ml / min, the particle size of the abrasive in the polishing liquid is 10-50nm, the rotation speed is 10-30r / min, and the polishing time is 2.5-5h.

[0016] The embodiment of the present invention further provides an ultra-thin crystal material for optical sensing, which is obtained by processing using the above-mentioned polishing method.

[0017] In summary, compared with the prior art, the present invention has the following advantages: (1) High polishing precision: It can successfully achieve high-precision polishing of the end faces of unsupported optical crystals, especially optical crystals with a thickness of less than 50 μm. After polishing, the light passing through the end faces has no obvious scattering, which meets the preparation requirements of optical devices; (2) Fewer polishing defects: The end faces of the polished ultra-thin crystals have fewer defects, such as cracks and scratches. The angles between the end faces and the surface are almost right angles when observed at the micrometer scale, and there are no large rounded corners with blurred edges. (3) The polishing process is simple and controllable, with low cost and high efficiency: The present invention not only reduces the time of fine polishing through optimized multi-step polishing, but also optimizes the optimal dripping rate of the polishing liquid through a large number of experiments, which is much smaller than the dripping rate of the conventional polishing process. The new process uses less fine polishing liquid, which is the main consumable cost in the polishing process. Therefore, the polishing cost of this process is low and the efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the ultrathin crystal prepared in Example 1 under a 50x microscope.

[0019] Figure 2 This is a schematic diagram of the ultrathin crystal prepared in Example 2 under a 20x microscope.

[0020] Figure 3 This is a schematic diagram of the ultrathin crystal prepared in comparative example 1 under a 50x microscope.

[0021] Figure 4 This is a schematic diagram of the ultrathin crystal prepared in comparative example 2 under a 20x microscope. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] Embodiment 1: 1) Use a laser cutting machine to cut a 20μm X-cut lithium tantalate crystal into a 10×15mm crystal, and at the same time use 0.5mm thick glass to cut two 10×15mm glass spacers; 2) Cleaning and protection of ultra-thin crystals: Place a piece of dust-free paper on the operating table, place the cut crystal on it, take another piece of dust-free paper soaked in acetone and carefully wipe the surface of the cut crystal 3-5 times in one direction, wait until the surface of the cut crystal and the dust-free paper are completely dry, pull up one end of the dust-free paper to turn the cut crystal over, and wipe it again 3-5 times according to the above method; clean the surface of the gasket, evenly apply purple glue, transfer the crystal to a gasket so that the crystal and the gasket completely overlap, and after curing, the cut crystal is firmly fixed on the surface of the gasket, and then the fixed crystal is placed on a constant temperature heating table, and after preheating, a thin layer of rosin is evenly applied, and after the rosin is completely melted, another gasket is placed, and after cooling and curing, a combination of protected crystals and gaskets is prepared; 3) Polishing: Fix the assembly on the fixture with rosin, leaving 1mm of the polished end surface exposed. Polishing is carried out according to the steps and parameters shown in Table 1: Among them, the polishing fluid for the first and second steps (model 15μm and 1μm) was provided by Guangdong Metallographic Testing Technology Co., Ltd., and the polishing fluid for the third step (model P00-2) was provided by Tianjin Ximei Semiconductor Materials Co., Ltd., and the abrasive particles were all silicon dioxide.

[0024] Table 1

[0025] 4) After polishing the assembly according to the above steps, use a 50x microscope to observe whether the surface is flat, whether the edges are broken, out of focus, or fuzzy. After polishing meets the technical requirements, remove the crystal by heating and clean it.

[0026] In this embodiment, a double-sided gasket is used to clamp the ultra-thin crystal to protect and fix the ultra-thin crystal. One side is fixed with non-detachable purple glue and the other side is fixed with detachable hot melt glue. This not only meets the operational requirements of the later crystal surface treatment (such as electrode plating), but also can firmly fix and support the ultra-thin crystal to prevent the crystal from breaking during the subsequent polishing and cleaning processes. During the polishing process, three steps of polishing are performed using large, medium and small particle sizes. Each step gradually reduces the load and the flow rate of the polishing liquid, thereby reducing the edge rounding caused by scratches on the crystal surface caused by the abrasive particles and corrosion of the crystal edge by the polishing liquid during the polishing process. The ultra-thin crystal finally obtained by polishing has a smooth surface without edge collapse, and the edges are clear and not blurred. Figure 1 shown.

[0027] Embodiment 2: 1) Use a laser cutter to cut a 10μm Z-cut lithium niobate crystal into a 10×15mm crystal, and use a 0.5mm thick lithium niobate wafer to cut two 10×15mm gaskets; 2) Cleaning and protection of ultra-thin crystals: Place a piece of dust-free paper on the operating table, place the cut crystal on it, take another piece of dust-free paper soaked in acetone and carefully wipe the surface of the cut crystal 3-5 times in one direction, wait until the surface of the cut crystal and the dust-free paper are completely dry, pull up one end of the dust-free paper to turn the cut crystal over, and wipe it 3-5 times according to the above method; clean the surface of the gasket and place it on a constant temperature table, evenly apply EVA hot melt adhesive, transfer the crystal to a gasket so that the crystal and the gasket completely overlap, then apply a layer of EVA hot melt adhesive on the crystal and cover it with another gasket so that the two layers of gaskets and the crystal completely overlap, cool naturally, and after solidification, the cut crystal is firmly fixed between the two gaskets to prepare a combination of protected crystals and gaskets; 3) Polishing: Fix the protected assembly on the fixture with rosin, leaving 0.5mm of the polished end surface exposed. Polishing is carried out according to the parameters and steps shown in Table 2.

[0028] Among them, the polishing fluid for the first and second steps (model 15μm and 1μm) was provided by Guangdong Metallographic Testing Technology Co., Ltd., and the polishing fluid for the third step (model P00-5) was provided by Tianjin Ximei Semiconductor Materials Co., Ltd., and the abrasive particles were all silicon dioxide.

[0029] Table 2

[0030] 4) After polishing the assembly according to the above steps, use a 20x microscope to observe whether the surface is flat, whether the edges are broken, out of focus, or fuzzy. After the polishing meets the technical requirements, remove the crystal and clean it.

[0031] In this embodiment, homologous gaskets are used to clamp the ultra-thin crystal on both sides to protect and fix the ultra-thin crystal. Both sides are fixed with detachable hot-melt adhesive. After end face polishing, a double-sided unsupported crystal is obtained, and both surfaces can be freely operated. In the polishing process, three-step polishing is performed using large, medium and small particle sizes of polishing particles. Compared with Example 1, the abrasive particle size in the third step becomes larger and the load becomes smaller. Large abrasive particles are prone to form indentations and scratches on the crystal surface. This adverse effect is offset by reducing the ballast load, and the target polishing accuracy is achieved by extending the polishing time. The surface of the ultra-thin crystal finally obtained by polishing is smooth without chipping, and the edges are clear without blurring. Figure 2 shown.

[0032] Comparative Example 1: 1) Use a laser cutter to cut a 50μm single crystal silicon crystal into 10×15mm crystals, and use a 0.5mm thick lithium niobate wafer to cut two 10×15mm gaskets; 2) Cleaning and protection of ultra-thin crystals: Place a piece of dust-free paper on the operating table, place the cut crystal on it, take another piece of dust-free paper soaked in acetone and carefully wipe the surface of the cut crystal 3-5 times in one direction, wait until the surface of the cut crystal and the dust-free paper are completely dry, pull up one end of the dust-free paper to turn the cut crystal over, and wipe it 3-5 times according to the above method; clean the surface of the gasket and place it on a constant temperature table, evenly apply EVA hot melt adhesive, transfer the crystal to a gasket so that the crystal and the gasket completely overlap, then apply a layer of EVA hot melt adhesive on the crystal and cover it with another gasket so that the two layers of gaskets and the crystal completely overlap, cool naturally, and after solidification, the cut crystal is firmly fixed between the two gaskets to prepare a well-protected assembly; 3) Polishing: Fix the protected assembly on the fixture with rosin, leaving 0.5mm of the polished end surface exposed. Polishing is carried out according to the parameters and steps shown in Table 3.

[0033] The polishing liquid used in the first step of polishing was provided by Guangdong Metallographic Testing Technology Co., Ltd. (model 8μm), and the polishing liquid used in the second step (model P00-2) was provided by Tianjin Ximei Semiconductor Materials Co., Ltd., and the abrasive particles were all silicon dioxide.

[0034] Table 3

[0035] 4) After polishing the combined crystal according to the above steps, use a 50x microscope to observe whether the surface is flat and whether the edges are broken, out of focus, or fuzzy.

[0036] In this embodiment, the ultra-thin crystal is clamped on both sides by homologous gaskets to protect and fix the ultra-thin crystal. Both sides are fixed with detachable hot melt adhesive. After the end faces are polished, a double-sided unsupported crystal is obtained, and free operation can be performed on both surfaces. However, during the polishing process, large and small particle size polishing particles are used for polishing. Compared with the first and second embodiments, slightly smaller polishing particles are used for rough polishing to avoid large and deep scratches formed by rough polishing. It takes a little more time to trim the polishing surface and then perform fine polishing. The time required for fine polishing is significantly longer. The surface after polishing can reach the required accuracy, but the efficiency is low. Figure 3 shown.

[0037] Comparative Example 2: 1) Use a laser cutting machine to cut a 100μm single crystal silicon crystal into a 10×15mm crystal; 2) Polishing: Fix the protected combined crystal on the fixture with rosin, leaving 0.5mm of the polished end surface exposed, and polishing is performed according to the parameters and steps shown in Table 4. The polishing liquid (model 15μm and 1μm) used in the first and second steps of polishing is provided by Guangdong Metallographic Detection Technology Co., Ltd., and the polishing liquid (model P00-5) used in the third step of polishing is provided by Tianjin Ximei Semiconductor Materials Co., Ltd., and the abrasive particles are all silicon dioxide.

[0038] Table 4

[0039] 3) After polishing the assembly according to the above steps, use a 20x microscope to observe whether the surface is flat and whether the edges are broken or out of focus.

[0040] In this embodiment, the crystal thickness (100 μm) is relatively thick and can be self-supporting during the polishing process. After the polishing is completed, the two surfaces can be freely operated. In the polishing process, three-step polishing is performed using three types of polishing particles with large, medium and small particle sizes. Compared with the first embodiment, the dripping rate of the fine polishing liquid in the third step is significantly increased. The surface of the ultra-thin crystal obtained by polishing is smooth without edge collapse, but the edge is out of focus and blurred, such as Figure 4 shown.

[0041] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for optical-grade polishing of the end face of an ultra-thin crystal for optical sensing, characterized in that: The following steps are involved: S1, fixing the gasket and the ultra-thin crystal to be processed on the base of the laser cutting machine respectively, adjusting the crystal axis direction of the ultra-thin crystal and the gasket to be consistent with the target cutting direction, setting the cutting size and performing cutting to obtain the cut crystal and gasket; S2, after cleaning the surfaces of the cut crystal and gasket, evenly apply adhesive on the gasket, and then transfer the crystal to the surface of the gasket so that the two are completely overlapped and solidified to obtain a combination of crystal and gasket; S3, fix the combination of crystal and gasket on the end face polishing fixture, and polish it by chemical mechanical polishing so that the two end faces reach optical grade flatness and good edge verticality; wherein, three types of polishing particles with large, medium and small particle sizes are used in the polishing process for three steps, and each step gradually reduces the load and the flow rate of the polishing liquid; S4, remove the qualified crystal from the end face polishing fixture, clean the surface residue, and obtain an ultra-thin crystal with optical grade polishing on the end face.

2. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing according to claim 1, characterized in that: The ultra-thin crystal is glass, silicon crystal, lithium tantalate crystal, lithium niobate crystal, quartz crystal, barium titanate crystal or gallium arsenide crystal.

3. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing according to claim 1, characterized in that: The ultra-thin crystal is unsupported and has a thickness of 10-100 μm, preferably 1-50 μm.

4. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing according to claim 1, characterized in that: The gasket is made of glass, silicon crystal, lithium tantalate crystal, lithium niobate crystal, quartz crystal, barium titanate crystal or gallium arsenide crystal, and has a thickness of 0.3-1 mm.

5. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing as claimed in claim 4, characterized in that: The gasket and the crystal are made of the same material or different materials.

6. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing according to claim 1, characterized in that: Step S2 is specifically as follows: place dust-free paper on the operating table, place the cut crystal on the dust-free paper, use another dust-free paper soaked in organic cleaning liquid to wipe the two surfaces of the crystal in a single direction for multiple times, clean the surface of the gasket and evenly apply adhesive, then transfer the crystal to the surface of the gasket so that the two completely overlap and solidify to obtain a combination of crystal and gasket.

7. The optical grade polishing method for the end surface of an ultra-thin crystal for optical sensing according to claim 6, characterized in that: The adhesive for fixing the crystal and the gasket is one of UV curing adhesive, epoxy resin adhesive, hot melt paraffin, hot melt rosin, hot melt EVA, hot melt PEG or hot melt EVOH.

8. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing according to claim 7, characterized in that: Gaskets are fixed on both sides of the crystal or on one side; the gasket and the crystal are fixed by detachable hot melt adhesive or non-detachable ultraviolet adhesive or epoxy adhesive.

9. The optical grade polishing method for the end face of an ultra-thin crystal for optical sensing according to claim 1, characterized in that: In step S3, before polishing, the crystal supported by the gasket is fixed on a support fixture of suitable thickness so that the polishing end surface protrudes 0.5-2 mm; Polishing is divided into three steps: the first step is to use silica or alumina polishing liquid with a particle size of 30μm to rough polish on the metal rough polishing wheel, and the rough polishing load is 30-200g / mm 2 , the polishing liquid flow rate is 10-30ml / min, the grinding wheel speed is 30-80r / min, the polishing time is 15-30min or polished to the specified size; the second step is to replace the polishing liquid in the first step with 1μm silicon dioxide or aluminum oxide polishing liquid, the polishing liquid drop rate is adjusted to 5-10ml / min, and the polishing time is 15-30min; the third step is the fine polishing load 0.2-15g / mm 2 The flow rate of the polishing liquid is 0.5-5ml / min, the particle size of the abrasive in the polishing liquid is 10-50nm, the rotation speed is 10-30r / min, and the polishing time is 2.5-5h.

10. An ultra-thin crystal for optical sensing, characterized in that: The method is obtained by the polishing method according to any one of claims 1 to 9.

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