Thinning method of large-size lithium niobate wafer

Abstract

The invention relates to the technical field of lithium niobate wafers, in particular to a method for thinning a large-size lithium niobate wafer, which comprises the following steps: surface pretreatment: immersing the large-size lithium niobate wafer in a mixed acid solution for soaking treatment, removing the residual acid solution on the surface after soaking is completed, and then carrying out vacuum drying treatment. According to the method, the surface quality is high, scratches, damaged layers and roughness on the surface of the wafer are effectively reduced through the synergistic effect of surface pretreatment for removing impurities, temporary bonding for providing stable support, rough thinning, fine thinning and chemical mechanical polishing, the surface roughness Ra of the wafer can reach 0.2 nm or below, the flatness error is controlled within 1 [mu] m, and the product quality is improved. And the harsh requirement of a high-end optoelectronic device on the surface quality of the wafer is met.

Description

Technical Field

[0001] This invention relates to the field of lithium niobate wafer technology, and more specifically to a method for thinning large-size lithium niobate wafers. Background Technology

[0002] Lithium niobate crystals, with their excellent piezoelectric, electro-optic, acousto-optic, and nonlinear optical properties, have wide applications in many fields such as optical communication, optical signal processing, and microwave devices. With the continuous advancement of related technologies, the demand for large-size lithium niobate wafers is increasing, and in applications, they often need to be thinned to specific thicknesses to meet different performance requirements.

[0003] However, the thinning of large-size lithium niobate wafers currently faces numerous challenges. On the one hand, while traditional mechanical grinding methods are highly efficient, they easily generate scratches, damaged layers, and stress concentrations on the wafer surface, severely affecting the surface quality and subsequent performance of the wafer. For example, during high-speed grinding, the intense friction between the grinding particles and the wafer surface can lead to localized temperature increases, resulting in thermal stress that may cause cracks or even breakage. On the other hand, while chemical etching methods can achieve relatively uniform material removal, they suffer from difficulties in precisely controlling the etching rate, environmental pollution, and the introduction of impurities. Furthermore, for large-size lithium niobate wafers, ensuring the uniformity of the overall wafer thickness during the thinning process is a pressing issue. Existing technologies have limitations in addressing these problems and cannot simultaneously meet the requirements of thinning efficiency, surface quality, thickness uniformity, and environmental protection. Therefore, developing an efficient, high-quality, and environmentally friendly method for thinning large-size lithium niobate wafers has significant practical importance and market demand. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for thinning large-size lithium niobate wafers. This method addresses the problems of large surface damage, poor thickness uniformity, easy introduction of impurities, and environmental unfriendliness in existing thinning methods. By optimizing the thinning process and parameters, a large-size lithium niobate wafer with high surface quality, uniform thickness, and stable performance can be obtained, meeting the stringent requirements of high-end optoelectronic devices for large-size lithium niobate wafers.

[0005] The present invention solves the technical problem by adopting the following technical solution: a method for thinning large-size lithium niobate wafers, comprising the following steps: Surface pretreatment: Large-sized lithium niobate wafers are immersed in a mixed acid solution prepared by hydrofluoric acid, nitric acid, and deionized water in a volume ratio of 1:(2-3):(20-30) at 20-25°C for 10-15 minutes. The mixed acid solution reacts chemically with impurities on the surface of the lithium niobate crystals to remove the oxide layer, organic matter, and other impurities. After immersion, the wafers are transferred to pure water at 30-35°C for rinsing for 5-8 minutes to remove residual acid. Subsequently, the wafers are placed in a vacuum drying oven at a vacuum degree of 5×10⁻⁶. -3 - 1×10 -2 Dry the wafer at 60-80°C for 15-20 minutes to ensure the wafer surface is dry and clean, providing a good foundation for subsequent thinning processes.

[0006] Temporary bonding: A 20-30 μm thick layer of silicone binder is uniformly applied to the surface of the pre-treated large-size lithium niobate wafer to be thinned, and then bonded to a glass substrate with a diameter 20-30 mm larger than the lithium niobate wafer. This bonding is achieved using a vacuum adsorption device at a vacuum level of 1×10⁻⁶. -3 - 5×10 -3 Under the condition of Pa, the wafer and the carrier are tightly bonded to ensure a strong bond. Then, the bonded wafer-carrier assembly is placed in a constant temperature oven and cured at 80-100℃ for 2-3 hours to enhance the bonding strength of the adhesive, provide stable mechanical support during the thinning process, and prevent the wafer from warping or breaking during the thinning process.

[0007] Coarse thinning: The bonded wafer-carrier assembly is fixed on the worktable of a high-precision double-sided polishing machine, using a silicon carbide polishing disc with a grit size of 800-1200 mesh. The disc material undergoes special heat treatment to improve its hardness and wear resistance. The polishing pressure is controlled at 0.05-0.1 MPa, and an intelligent pressure control system ensures that the pressure is evenly distributed on the wafer surface. The polishing disc rotation speed is set to 20-30 r / min, using a water-based polishing slurry containing 10-15 wt% silicon carbide abrasive, 3-5 wt% dispersant, and 2-4 wt% lubricant, continuously supplied at a flow rate of 5-8 L / min. During the polishing process, the wafer thickness change is monitored in real time. When the wafer thickness is reduced to 1.2-1.5 times the target thickness, coarse thinning is stopped. This stage can quickly remove most of the excess material, improving thinning efficiency.

[0008] Fine thinning: Replace the silicon carbide grinding disc with a grit size of 2000-3000 mesh to further improve grinding precision. Adjust the grinding pressure to 0.02-0.05 MPa, set the grinding disc speed to 15-25 r / min, and use a newly formulated water-based grinding slurry containing 8-12 wt% silicon carbide abrasive, 4-6 wt% lubricant, and 1-3 wt% surfactant at a flow rate of 3-6 L / min for fine grinding. Simultaneously, use a high-precision laser thickness gauge to monitor the wafer thickness in real time, and automatically adjust the grinding parameters based on the thickness feedback data to ensure wafer thickness uniformity. When the wafer thickness reaches 1.05-1.1 times the target thickness, fine thinning is stopped. This stage mainly aims to further improve the wafer surface flatness and thickness uniformity, and reduce surface damage.

[0009] Chemical Mechanical Polishing (CMP): The thinned wafer-carrier assembly is placed in a CMP device, using silica sol as the polishing slurry. The silica particles have a diameter of 50-80 nm and a concentration of 6-9 wt%, with 0.8-1.2 wt% sodium hydroxide added as a catalyst to promote the chemical etching reaction. The polishing pressure is 0.01-0.03 MPa, monitored and adjusted in real time by a high-precision pressure sensor. The polishing pad rotation speed is 30-45 r / min, and the polishing time is 30-45 min. During the polishing process, through the mechanical friction between the polishing pad and the wafer surface and the chemical etching effect of the polishing slurry, trace amounts of material are removed, further reducing the surface roughness of the wafer, improving surface flatness, and achieving the target wafer thickness, while simultaneously eliminating the surface damage layer generated during the grinding process.

[0010] Debonding and Cleaning: The CMP-completed wafer-carrier assembly is immersed in a debonding solution prepared by mixing ethanol and acetone in a volume ratio of 1:(1-2) at 35-45°C for 30-45 minutes to dissolve the silicone binder and separate the wafer from the carrier. The separated wafer is then ultrasonically cleaned in deionized water at a frequency of 40-50 kHz for 10-15 minutes, utilizing the cavitation effect of ultrasound to remove residual binder and impurities from the surface. Next, the wafer is immersed in a cleaning solution prepared by mixing dilute hydrochloric acid (3-5% by mass) and deionized water in a volume ratio of 1:(10-15) for 5-8 minutes to remove any remaining metal ions. Finally, it is rinsed 3-5 times with deionized water, each rinse lasting 3-5 minutes, to ensure a clean and uncontaminated wafer surface, resulting in a large-size lithium niobate thinned wafer with high surface quality and uniform thickness.

[0011] Compared with the prior art, the present invention has the following beneficial effects: This invention features high surface quality. Impurities are removed through surface pretreatment, and temporary bonding provides stable support. The synergistic effect of coarse thinning, fine thinning, and chemical mechanical polishing effectively reduces scratches, damage layers, and roughness on the wafer surface, enabling the wafer surface roughness Ra to reach below 0.2 nm and the flatness error to be controlled within 1 μm, thus meeting the stringent requirements of high-end optoelectronic devices for wafer surface quality.

[0012] With good thickness uniformity, the wafer thickness is monitored in real time using a high-precision laser thickness gauge during both rough and fine thinning processes. The uniformity of the thickness of the entire large-size lithium niobate wafer is ensured through an intelligent pressure control system and automatic parameter adjustment function. The thickness deviation can be controlled within ±2μm, which improves the consistency and yield of the wafer.

[0013] Environmentally friendly and pollution-free, the chemical reagents used in this invention are all common and relatively environmentally friendly materials. The waste liquid and waste gas generated during the thinning process have minimal environmental pollution after proper treatment. For example, the ethanol and acetone in the debonding solution can be recycled and reused, and the concentration of dilute hydrochloric acid in the cleaning solution is low, making it easy to neutralize and meet environmental protection requirements.

[0014] With strong process adaptability, this thinning method is applicable to large-size lithium niobate wafers of different sizes. By adjusting the process parameters, it can meet the requirements of various application scenarios for the thickness and performance of lithium niobate wafers, and has broad application prospects and market value. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to specific examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] This embodiment of a method for thinning a large-size lithium niobate wafer includes the following steps: Step 1, Surface pretreatment: Large-sized lithium niobate wafers are immersed in a mixed acid solution for soaking treatment. After soaking, the residual acid solution on the surface is removed, and then vacuum drying is performed. Step 2, Temporary Bonding: A layer of silicone adhesive with a thickness of 20-30μm is uniformly applied to the thinning surface of the pretreated large-size lithium niobate wafer, and then bonded to a glass substrate with a diameter 20-30mm larger than the lithium niobate wafer. Using a vacuum adsorption device, at a vacuum degree of 1×10 -3 -5×10 -3Under the condition of Pa, the wafer and the carrier are tightly bonded to ensure a strong bond. Then, the bonded wafer-carrier assembly is placed in a constant temperature oven and cured at 80-100℃ for 2-3 hours. Step 3, coarse thinning: The bonded wafer-carrier assembly is fixed on the worktable of a high-precision double-sided polishing machine. During the polishing process, the thickness change of the wafer is monitored in real time. When the wafer thickness is reduced to 1.2-1.5 times the target thickness, coarse thinning is stopped. Step four, refine and thin; Step 5, Chemical Mechanical Polishing: The thinned wafer-carrier assembly is placed in a CMP device for polishing. Step six: Debonding and cleaning.

[0017] The mixed acid solution described in this embodiment is prepared by mixing hydrofluoric acid, nitric acid, and deionized water in a volume ratio of 1:(2-3):(20-30); The soaking treatment involves immersing the sample at a temperature of 20-25℃ for 10-15 minutes. To remove residual acid from the surface, the wafer is transferred to pure water at 30-35°C and rinsed for 5-8 minutes to remove residual acid. Vacuum drying involves placing the wafers in a vacuum drying oven at a vacuum level of 5 × 10⁻⁶. -3 -1×10 -2 Dry at 60-80℃ for 15-20 minutes.

[0018] In step three of this embodiment, a silicon carbide polishing disc with a particle size of 800-1200 mesh is used, and the polishing pressure is controlled at 0.05-0.1MPa. Through an intelligent pressure control system, the pressure is ensured to be evenly distributed on the wafer surface. The polishing disc rotation speed is set to 20-30r / min. A water-based polishing fluid containing 10-15wt% silicon carbide abrasive, 3-5wt% dispersant and 2-4wt% lubricant is continuously supplied at a flow rate of 5-8L / min. The water-based polishing fluid lubricant is made of nano-tungsten disulfide and water in a weight ratio of 2:7, and the dispersant is sodium tripolyphosphate.

[0019] In this embodiment, the wafer thinning process involves replacing the silicon carbide grinding disc with a particle size of 2000-3000 mesh to improve grinding accuracy. The grinding pressure is adjusted to 0.02-0.05 MPa, and the grinding disc rotation speed is set to 15-25 r / min. A newly formulated water-based grinding slurry containing 8-12 wt% silicon carbide abrasive, 4-6 wt% lubricant, and 1-3 wt% surfactant is used for fine grinding at a flow rate of 3-6 L / min. Simultaneously, a high-precision laser thickness gauge is used to monitor the wafer thickness in real time, and the grinding parameters are automatically adjusted based on the thickness feedback data to ensure wafer thickness uniformity. When the wafer thickness reaches 1.05-1.1 times the target thickness, the fine thinning process is stopped. The lubricant is molybdenum selenide; the water-based grinding fluid of the surfactant is prepared by mixing sodium dodecylbenzenesulfonate and water in a weight ratio of 2:5.

[0020] In step five of this embodiment, silica sol is used as the polishing liquid, wherein the silica particles have a particle size of 50-80nm and a concentration of 6-9wt%, and 0.8-1.2wt% sodium hydroxide is added as a catalyst; The polishing pressure is 0.01 - 0.03 MPa, which is monitored and adjusted in real time by a high-precision pressure sensor. The polishing pad rotation speed is 30 - 45 r / min, and the polishing time is 30 - 45 min.

[0021] In step six of this embodiment, the CMP-completed wafer-carrier assembly is immersed in a debonding solution made of ethanol and acetone in a volume ratio of 1:(1-2) and immersed at a temperature of 35-45°C for 30-45 minutes to dissolve the silicone binder and achieve separation of the wafer and the carrier. After separation, the wafers are first ultrasonically cleaned in deionized water to remove residual adhesives and impurities from the surface using the cavitation effect of ultrasound. Then, the wafers are immersed in a cleaning solution prepared by dilute hydrochloric acid and deionized water at a volume ratio of 1:(10-15) for 5-8 minutes to remove residual metal ions. Finally, the wafers are rinsed 3-5 times with deionized water, each rinse lasting 3-5 minutes, to ensure that the wafer surface is clean and free of contamination, resulting in large-size lithium niobate thinned wafers with high surface quality and uniform thickness.

[0022] The mass fraction of dilute hydrochloric acid in this embodiment is 3-5%.

[0023] In this embodiment, the ultrasonic cleaning frequency is 40-50kHz, and the cleaning time is 10-15 minutes.

[0024] Example 1 1. Surface Pretreatment: Large-sized lithium niobate wafers with a diameter of 200mm are immersed in a mixed acid solution of hydrofluoric acid, nitric acid, and deionized water in a volume ratio of 1:2:22 at 23°C for 13 minutes, rinsed in pure water at 33°C for 7 minutes, and then subjected to a vacuum of 6×10⁻⁶. -3 Dry in a vacuum drying oven at 75°C for 16 minutes.

[0025] 2. Temporary bonding: Apply a 28μm thick silicone adhesive and bond it to a 230mm diameter glass substrate under a vacuum of 2×10⁻⁶. -3 The material is bonded under Pa and cured in a 90℃ constant temperature oven for 2.8 hours.

[0026] 3. Coarse thinning: A 1200-mesh silicon carbide polishing disc is used, with a polishing pressure of 0.09 MPa and a polishing disc rotation speed of 28 r / min. A water-based polishing slurry containing 13 wt% silicon carbide abrasive, 4.5 wt% dispersant and 3.5 wt% lubricant is supplied at a flow rate of 7 L / min. Coarse thinning is stopped when the wafer thickness is reduced to 1.4 times the target thickness (180 μm) (i.e., 252 μm).

[0027] 4. Fine thinning: Replace with a 3000-mesh silicon carbide polishing disc, polishing pressure 0.04MPa, polishing disc speed 22r / min, water-based polishing slurry containing 11wt% silicon carbide abrasive, 5.5wt% lubricant and 2.5wt% surfactant, fine polishing at a flow rate of 5L / min. When the wafer thickness reaches 1.06 times the target thickness (i.e. 190.8μm), stop fine thinning.

[0028] 5. Chemical Mechanical Polishing (CMP): Use silica sol polishing slurry with silica particles of 70nm diameter and 7wt% concentration, add 1.1wt% sodium hydroxide, polishing pressure 0.025MPa, polishing pad rotation speed 40r / min, polishing time 40min.

[0029] 6. Debonding and Cleaning: Immerse in a debonding solution of ethanol and acetone (volume ratio 1:1.2) at 42°C for 35 minutes, followed by ultrasonic cleaning in deionized water at 48 kHz for 13 minutes. Then, immerse in a cleaning solution of 3.5% dilute hydrochloric acid and deionized water (volume ratio 1:13) for 7 minutes, followed by rinsing with deionized water five times for 3 minutes each time. Testing showed that the surface roughness Ra of the thinned wafer was 0.15 nm, and the thickness deviation was ±1.8 μm, meeting the requirements for manufacturing high-performance optoelectronic devices.

[0030] Comparative Example 1. Unlike Example 1, no coarse thinning process was used.

[0031] Comparative Example 2. Unlike Example 3, no silicone adhesive was used in the temporary bonding process.

[0032] Comparative Example 3. Unlike Example 3, the fine thinning process did not use silicon carbide abrasive, 5.5 wt% lubricant and 2.5 wt% surfactant water-based polishing fluid, and the fine polishing was carried out at a flow rate of 5 L / min.

[0033] Comparative Example 4. Unlike Example 3, chemical mechanical polishing was not used.

[0034] Comparative Example 5. Unlike Example 3, the debonding and cleaning process did not involve immersion in a debonding solution with a volume ratio of 1:1.2 of ethanol and acetone.

[0035] The product performance tests for Examples 1-3 and Comparative Examples 1-5 are as follows:

[0036] As can be seen from Example 1 and Comparative Examples 1-5, the surface roughness and stress deformation value of the product in Example 1 of the present invention are excellent. However, in Comparative Examples 1-5, the performance of the products showed a significant trend of deterioration because the coarse thinning process and chemical mechanical polishing were not used. Furthermore, the performance of the products also showed a trend of deterioration because the silicone adhesive coating process and the debonding solution with a volume ratio of 1:1.2 of ethanol and acetone were not used. Only by using the process method of the present invention can the product achieve the most significant effect.

[0037] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0038] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for thinning large-size lithium niobate wafers, characterized in that, Includes the following steps: Step 1, Surface pretreatment: Large-sized lithium niobate wafers are immersed in a mixed acid solution for soaking treatment. After soaking, the residual acid solution on the surface is removed, and then vacuum drying is performed. Step 2, Temporary Bonding: A layer of silicone adhesive with a thickness of 20-30μm is uniformly applied to the thinning surface of the pretreated large-size lithium niobate wafer, and then bonded to a glass substrate with a diameter 20-30mm larger than the lithium niobate wafer. Using a vacuum adsorption device, at a vacuum degree of 1×10 -3 -5×10 -3 Under the condition of Pa, the wafer and the carrier are tightly bonded to ensure a strong bond. Then, the bonded wafer-carrier assembly is placed in a constant temperature oven and cured at 80-100℃ for 2-3 hours. Step 3, coarse thinning: The bonded wafer-carrier assembly is fixed on the worktable of a high-precision double-sided polishing machine. During the polishing process, the thickness change of the wafer is monitored in real time. When the wafer thickness is reduced to 1.2-1.5 times the target thickness, coarse thinning is stopped. Step four, refine and thin; Step 5, Chemical Mechanical Polishing: The thinned wafer-carrier assembly is placed in a CMP device for polishing. Step six: Debonding and cleaning.

2. The method for thinning large-size lithium niobate wafers according to claim 1, characterized in that, The mixed acid solution is prepared by mixing hydrofluoric acid, nitric acid and deionized water in a volume ratio of 1:(2-3):(20-30); The soaking treatment involves immersing the sample at a temperature of 20-25℃ for 10-15 minutes. To remove residual acid from the surface, the wafer is transferred to pure water at 30-35°C and rinsed for 5-8 minutes to remove residual acid. Vacuum drying involves placing the wafers in a vacuum drying oven at a vacuum level of 5 × 10⁻⁶. -3 -1×10 -2 Dry at 60-80℃ for 15-20 minutes.

3. The method for thinning large-size lithium niobate wafers according to claim 1, characterized in that, In step three, a silicon carbide polishing disc with a particle size of 800-1200 mesh is used, and the polishing pressure is controlled at 0.05-0.1MPa. Through an intelligent pressure control system, the pressure is ensured to be evenly distributed on the wafer surface. The polishing disc rotation speed is set to 20-30r / min. A water-based polishing fluid containing 10-15wt% silicon carbide abrasive, 3-5wt% dispersant and 2-4wt% lubricant is continuously supplied at a flow rate of 5-8L / min. The water-based polishing fluid lubricant is made of nano-tungsten disulfide and water in a weight ratio of 2:7, and the dispersant is sodium tripolyphosphate.

4. The method for thinning large-size lithium niobate wafers according to claim 3, characterized in that, During the fine thinning process, a silicon carbide grinding disc with a particle size of 2000-3000 mesh is used to improve grinding accuracy. The grinding pressure is adjusted to 0.02-0.05 MPa, and the grinding disc speed is set to 15-25 r / min. A newly formulated water-based grinding fluid containing 8-12 wt% silicon carbide abrasive, 4-6 wt% lubricant, and 1-3 wt% surfactant is used for fine grinding at a flow rate of 3-6 L / min. At the same time, a high-precision laser thickness gauge is used to monitor the wafer thickness in real time, and the grinding parameters are automatically adjusted according to the thickness feedback data to ensure wafer thickness uniformity. When the wafer thickness reaches 1.05-1.1 times the target thickness, the fine thinning is stopped. The lubricant is molybdenum selenide; the water-based grinding fluid of the surfactant is prepared by mixing sodium dodecylbenzenesulfonate and water in a weight ratio of 2:

5.

5. The method for thinning large-size lithium niobate wafers according to claim 1, characterized in that, In step five, silica sol is used as the polishing liquid, wherein the silica particles have a particle size of 50-80nm and a concentration of 6-9wt%, and 0.8-1.2wt% sodium hydroxide is added as a catalyst; The polishing pressure is 0.01 - 0.03 MPa, which is monitored and adjusted in real time by a high-precision pressure sensor. The polishing pad rotation speed is 30 - 45 r / min, and the polishing time is 30 - 45 min.

6. The method for thinning large-size lithium niobate wafers according to claim 1, characterized in that, In step six, the CMP-completed wafer-carrier assembly is immersed in a debonding solution made of ethanol and acetone in a volume ratio of 1:(1-2) at 35-45°C for 30-45 minutes to dissolve the silicone binder and separate the wafer from the carrier. After separation, the wafers are first ultrasonically cleaned in deionized water to remove residual adhesives and impurities from the surface using the cavitation effect of ultrasound. Then, the wafers are immersed in a cleaning solution prepared by dilute hydrochloric acid and deionized water at a volume ratio of 1:(10-15) for 5-8 minutes to remove residual metal ions. Finally, the wafers are rinsed 3-5 times with deionized water, each rinse lasting 3-5 minutes, to ensure that the wafer surface is clean and free of contamination, resulting in large-size lithium niobate thinned wafers with high surface quality and uniform thickness.

7. The method for thinning a large-size lithium niobate wafer according to claim 6, characterized in that, The mass fraction of dilute hydrochloric acid is 3-5%.

8. The method for thinning a large-size lithium niobate wafer according to claim 6, characterized in that, The ultrasonic cleaning frequency is 40-50kHz, and the cleaning time is 10-15 minutes.

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