A method for polishing processing of gallium oxide crystal

Abstract

The present application relates to the technical field of semiconductor material processing, and particularly relates to a method for polishing gallium oxide crystal, which comprises the following steps: a) a surface pretreatment step, wherein a CF4 / O2 plasma is used to treat the surface of the gallium oxide crystal to form a mixed modified layer of GaF3 and Ga(OH)3; b) a main polishing step, wherein in a low-temperature environment, a polishing liquid containing a hydrogen peroxide system activated by bicarbonate is used in combination with ultrasonic assistance to perform main polishing on the pretreated gallium oxide crystal; c) a precision polishing step, wherein in a lower-temperature environment, a polishing liquid with lower pressure and an improved formula is used to perform precision polishing on the gallium oxide crystal; and d) a cleaning and post-treatment step, wherein a multistage cleaning process is used to remove polishing residues and repair surface defects, so that the material removal rate is increased by about 2.3 times (to 1.2 micrometers / hour), and the surface roughness Ra is reduced by about 2 times (to 0.09 nanometers).

Description

Technical Field

[0001] This invention relates to the field of semiconductor material processing technology, specifically to a method for polishing gallium oxide crystals, particularly a high-quality surface polishing technique suitable for β-Ga2O3 ultrawide bandgap semiconductor crystals. Background Technology

[0002] Gallium oxide (β-Ga2O3), as a new generation of ultrawide bandgap semiconductor material, has shown broad application prospects in high-temperature, high-frequency, high-power electronic devices, as well as deep-ultraviolet optoelectronics and sensors, thanks to its excellent electronic properties. This material possesses an ultrawide bandgap of approximately 4.8-4.9 eV and a breakdown electric field as high as 8 MV / cm, far superior to traditional semiconductor materials such as Si and GaAs. Furthermore, its excellent high-temperature stability enables it to operate reliably in harsh environments.

[0003] Existing polishing methods for β-Ga2O3 mainly include mechanical polishing (MP), chemical mechanical polishing (CMP), and their variants. Each of these methods has its limitations: Mechanical polishing: Although the equipment is simple, it can cause severe surface damage and cleavage pits, with a surface roughness Ra as high as 52 nm.

[0004] Traditional CMP: can obtain a relatively smooth surface, but has a low material removal rate, is sensitive to process parameters, and may introduce subsurface damage.

[0005] Acidic CMP: Compared with traditional alkaline CMP, it improves polishing efficiency by about 20%, but still faces polishing slurry aging and environmental issues.

[0006] Low-pressure CMP: Using a low pressure of 1 kPa can avoid subsurface damage, but the material removal rate is further reduced (~0.4 μm / h).

[0007] These technologies present a difficult trade-off between efficiency, surface quality, and damage control.

[0008] The above-mentioned existing technical methods all have the following common problems: (1) They cannot effectively solve the surface cracking and damage problems caused by the easy cleavage characteristics along the (100) plane of β-Ga2O3 crystal; (2) The polishing efficiency is low and the material removal rate is insufficient; (3) There is still an obvious subsurface damage layer on the surface after polishing; (4) It is difficult to achieve an ultra-precision polishing effect with atomic-level flatness; (5) The process parameters are difficult to control precisely, and the polishing quality of different crystal planes is inconsistent.

[0009] Therefore, there is an urgent need to develop a new method for polishing gallium oxide crystals that can simultaneously solve the above-mentioned technical problems and achieve ultra-precision polishing with high efficiency, low damage, and atomic-level flatness. Summary of the Invention

[0010] In view of the above-mentioned problems existing in the prior art, the purpose of the present invention is to provide a method for polishing gallium oxide crystals. This method adopts a multi-synergistic mechanism, and through the organic combination of surface fluorination plasma pretreatment, active oxygen free radical enhanced polishing slurry and ultrasonic-assisted low-temperature precision polishing, it effectively solves the key technical problems in β-Ga2O3 crystal polishing such as easy cleavage, anisotropy, subsurface damage and low material removal rate, and achieves a high-efficiency, low-damage and atomically flat ultra-precision polishing effect.

[0011] To achieve the above objectives, the present invention provides a method for polishing gallium oxide crystals, characterized by comprising the following steps: a) Surface pretreatment step: The surface of gallium oxide crystal is treated with CF4 / O2 plasma to form a mixed modified layer of GaF3 and Ga(OH)3; b) The main polishing step involves using a polishing slurry containing a hydrogen peroxide system activated by bicarbonate in a low-temperature environment, combined with ultrasonic assistance, to perform the main polishing on the pretreated gallium oxide crystal. c) Precision polishing step: Gallium oxide crystals are precision polished in a lower temperature environment using a lower pressure and a modified polishing slurry. d) Cleaning and post-treatment steps: A multi-stage cleaning process is used to remove polishing residues and repair surface defects.

[0012] Preferably, in the surface pretreatment step, the parameters of the CF4 / O2 plasma treatment are: CF4 gas flow rate of 45 sccm, O2 gas flow rate of 15 sccm, radio frequency power of 150 W, bias power of 50 W, treatment pressure of 10-20 Pa, treatment temperature of 25°C, and treatment time adjusted according to different crystal faces, wherein the (100) face is 60 seconds, the (010) face is 90 seconds, and the (-201) face is 120 seconds.

[0013] Preferably, the polishing solution used in the main polishing step comprises: 5 wt% colloidal silica (50-70 nm), 6 wt% hydrogen peroxide (H2O2), 0.8 wt% sodium bicarbonate (NaHCO3), 0.5 wt% phosphoric acid (H3PO4) to adjust the pH to 4.2±0.1, 0.2 wt% polyvinylpyrrolidone (PVP, K30) as a dispersant, 0.05 wt% nonionic surfactant (Triton X-100), 0.3 wt% citric acid as a complexing agent, and the remainder deionized water to bring the total to 100%.

[0014] Preferably, the process parameters of the main polishing step are: polishing fluid flow rate 100 mL / min, carrier rotation speed 60 rpm, polishing disc rotation speed 70 rpm, polishing temperature 5°C, ultrasonic parameters 40 kHz and 100 W, and polishing pressure adjusted according to different crystal faces, wherein the (100) face is 3 kPa, the (010) face is 2 kPa, and the (-201) face is 2.5 kPa.

[0015] Preferably, the polishing solution used in the precision polishing step comprises: 2 wt% nano-colloidal silica (20-30nm), 3 wt% hydrogen peroxide (H2O2), 0.5 wt% sodium bicarbonate (NaHCO3), 0.3 wt% phosphoric acid (H3PO4) to adjust the pH value to 4.5±0.1, 0.1 wt% chitosan oligosaccharide as an environmentally friendly complexing agent, 1 wt% sorbitol as a surface modifier, and the remainder deionized water to bring the total to 100%.

[0016] Preferably, the process parameters for the precision polishing step are: carrier rotation speed 40 rpm, polishing disc rotation speed 40 rpm, polishing pressure 1 kPa, polishing temperature 0°C, ultrasonic parameters 40 kHz, 50 W, intermittent mode operation (on for 10 seconds, off for 5 seconds), and polishing time 20-30 minutes.

[0017] Preferably, the cleaning and post-treatment steps include: a) SC-1 cleaning: NH4OH:H2O2:H2O = 1:1:5, 50°C, 5 minutes; b) acid cleaning: diluted citric acid (1 wt%), room temperature, 5 minutes; c) ultrasonic cleaning with deionized water: 40 kHz, 2 minutes; d) anhydrous ethanol cleaning: 1 minute; e) nitrogen drying; f) selective oxygen atmosphere annealing: 600°C, 30 minutes, oxygen flow rate 100 sccm, naturally cooled to room temperature.

[0018] Preferably, the present invention further includes a real-time monitoring and adaptive control step, which is implemented through the following systems: a) an acoustic monitoring system: monitoring microscopic fracture signals during the polishing process using an acoustic emission sensor; b) a temperature monitoring system: monitoring the polishing interface temperature in real time using an infrared thermal imager; c) an automatic pH adjustment system for the polishing slurry: maintaining pH stability through a microfluidic system; d) a rotational torque monitoring system: inferring material removal status through changes in rotational torque; and based on the monitoring data, adjusting the polishing pressure (±0.5 kPa), ultrasonic power (±20 W), polishing slurry flow rate (±20 mL / min), and polishing disc rotation speed (±10 rpm) in real time.

[0019] Preferably, the CF4 / O2 plasma treatment uses an inductively coupled plasma (ICP) source, and after the pretreatment step, a neutralization treatment is performed: soaking in a 0.5% citric acid solution for 30 seconds, followed by rinsing with deionized water.

[0020] Preferably, the main polishing step uses a porous polyurethane polishing pad (IC1000 + Suba400 composite pad), and the precision polishing step uses an ultra-soft polyurethane polishing pad (SubaXIV). The method achieves a material removal rate of 1.2 μm / h, a surface roughness Ra of 0.09 nm, and a subsurface damage depth of less than 10 nm.

[0021] The present invention has the following beneficial effects: 1. A GaF3 / Ga(OH)3 modified layer is formed by surface fluorination plasma pretreatment, which significantly reduces the surface hardness of β-Ga2O3 (by about 80%), effectively suppresses cleavage, and creates favorable conditions for subsequent polishing. 2. The use of a bicarbonate-activated hydrogen peroxide (BAP) system generates a variety of reactive oxygen free radicals (•CO3⁻, •O2⁻, ¹O2) that work synergistically to significantly improve the dissolution rate of the modified layer; 3. Combining ultrasonic assistance with a low-temperature environment (0-5°C) enhances the efficiency of chemical reactions and mechanical removal at the microscopic level, and suppresses cleavage phenomena at the macroscopic level; 4. Based on the anisotropy of β-Ga2O3, differentiated polishing parameters guided by crystal planes are designed to solve the problem of inconsistent polishing quality of different crystal planes; 5. Achieving a highly stable and reliable process through a multi-parameter real-time monitoring and adaptive control system; 6. Compared with the prior art, the present invention increases the material removal rate by about 2.3 times (to 1.2 μm / h), reduces the surface roughness Ra by about 2 times (to 0.09 nm), reduces the subsurface damage depth by about 10-20 times (to less than 10 nm), and shortens the polishing time by about 4-7 times (only 1.5-2 hours). Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0023] The core innovation of this invention lies in solving the key technical challenges of polishing β-Ga2O3 crystals through multiple synergistic mechanisms. β-Ga2O3, as a novel ultrawide bandgap semiconductor material, faces many challenges in the polishing process: its easy cleavage along the (100) plane leads to the formation of cracks during polishing; strong anisotropy results in significant differences in polishing quality between different crystal planes; traditional polishing methods often introduce subsurface damage with a depth of 100-200 nm; and the stable oxidation state results in extremely low material removal rates in conventional CMP processes.

[0024] To address the aforementioned challenges, this invention proposes a novel polishing method with multiple synergistic effects, comprising four key stages: surface pretreatment, main polishing, precision polishing, and post-cleaning treatment. Surface pretreatment utilizes CF4 / O2 plasma to form a GaF3 / Ga(OH)3 modified layer of controllable thickness, reducing surface hardness and preventing cleavage. The main polishing stage employs a bicarbonate-activated hydrogen peroxide (BAP) system to generate various reactive oxygen species that synergistically dissolve the modified layer. This, combined with ultrasonic assistance and a low-temperature environment, improves efficiency and suppresses cleavage. The precision polishing stage uses gentler conditions to achieve atomic-level surface smoothness. Finally, multi-stage cleaning and selective annealing remove residues and repair minor surface defects. Example

[0025] In a basic embodiment of the present invention, a method for polishing β-Ga2O3(100) plane crystals is provided, specifically including the following steps: First, surface pretreatment was performed. The β-Ga₂O₃(100) facet wafer was ultrasonically cleaned with anhydrous ethanol and deionized water for 5 minutes each, dried with nitrogen, and then placed in an inductively coupled plasma (ICP) chamber. The plasma source parameters were set as follows: CF₄ gas flow rate 45 sccm, O₂ gas flow rate 15 sccm, RF power 150 W, bias power 50 W, processing pressure 15 Pa, processing temperature 25°C, and processing time 60 seconds. After treatment, the wafer was immersed in a 0.5% citric acid solution for 30 seconds for neutralization, and then thoroughly rinsed with deionized water. This pretreatment step formed a GaF₃ / Ga(OH)₃ mixed modified layer with a thickness of approximately 75 nm, and the hardness was reduced by approximately 80% compared to the untreated surface.

[0026] Next, the main polishing process was performed. A BAP-SiO2 polishing slurry was prepared, consisting of: 5 wt% colloidal silica (60 nm), 6 wt% hydrogen peroxide (H2O2), 0.8 wt% sodium bicarbonate (NaHCO3), 0.5 wt% phosphoric acid (H3PO4) to adjust the pH to 4.2, 0.2 wt% polyvinylpyrrolidone (PVP, K30), 0.05 wt% nonionic surfactant (Triton X-100), 0.3 wt% citric acid, and the balance being deionized water to 100%. Polishing was performed using a custom-designed ultrasonic-assisted CMP system with the following parameters: polishing slurry flow rate 100 mL / min, carrier rotation speed 60 rpm, polishing disc rotation speed 70 rpm, polishing pressure 3 kPa, polishing temperature 5°C, ultrasonic parameters 40 kHz, 100 W, using an IC1000+Suba400 composite polishing pad, and a polishing time of 45 minutes.

[0027] Next, precision polishing is performed. The precision polishing solution is prepared as follows: 2 wt% nano-colloidal silica (25nm), 3 wt% hydrogen peroxide (H2O2), 0.5 wt% sodium bicarbonate (NaHCO3), 0.3 wt% phosphoric acid (H3PO4) to adjust the pH to 4.5, 0.1 wt% chitosan oligosaccharide, 1 wt% sorbitol, and the balance being deionized water to 100%. The precision polishing process parameters are: carrier speed 40 rpm, polishing disc speed 40 rpm, polishing pressure 1 kPa, polishing temperature 0°C, ultrasonic parameters 40kHz, 50 W (intermittent mode, on for 10 seconds, off for 5 seconds), using a SubaXIV ultra-soft polishing pad, and polishing time 25 minutes.

[0028] Finally, post-cleaning treatment was performed. The following treatments were carried out in sequence: SC-1 cleaning (NH4OH:H2O2:H2O = 1:1:5), 50°C, 5 minutes; cleaning with diluted citric acid (1 wt%), room temperature, 5 minutes; ultrasonic cleaning with deionized water, 40 kHz, 2 minutes; cleaning with anhydrous ethanol, 1 minute; nitrogen drying; annealing in an oxygen atmosphere, 600°C, 30 minutes, oxygen flow rate 100 sccm, and natural cooling to room temperature.

[0029] After the above process, the surface roughness Ra of the β-Ga2O3(100) crystal reaches 0.10 nm, the material removal rate is 1.15 μm / h, the subsurface damage depth is about 8 nm, and the lattice integrity (XRD half width at half maximum) is 14 arcseconds. Example

[0030] In another embodiment of the present invention, a method for polishing β-Ga2O3 (010) facet crystals is provided. Since the (010) facet is more fragile than the (100) facet, the processing parameters are appropriately adjusted in this embodiment.

[0031] In the surface pretreatment stage, after using the same cleaning method as in Example 1, the plasma source parameters were set as follows: CF4 gas flow rate 45 sccm, O2 gas flow rate 15 sccm, RF power 150 W, bias power 50 W, processing pressure 15 Pa, processing temperature 25°C, and processing time extended to 90 seconds to form a thicker (approximately 95 nm) modified layer. The remaining neutralization treatment was the same as in Example 1.

[0032] In the main polishing stage, the polishing slurry with the same composition as in Example 1 was used, but the process parameters were adjusted as follows: the polishing pressure was reduced to 2 kPa, the polishing time was extended to 60 minutes, and other parameters remained unchanged. This is because the mechanical strength of the (010) surface is lower than that of the (100) surface, and a lower pressure is required to avoid cleavage damage.

[0033] In the precision polishing stage, the polishing fluid composition is the same as in Example 1, but the polishing pressure is further reduced to 0.8 kPa, the polishing temperature is maintained at 0°C, the polishing time is 30 minutes, and other parameters remain unchanged.

[0034] The post-cleaning treatment stage is exactly the same as in Example 1.

[0035] After processing, the surface roughness Ra of the β-Ga2O3(010) crystal reached 0.09 nm, the material removal rate was 0.95 μm / h, the subsurface damage depth was about 7 nm, and the lattice integrity (XRD half width at half maximum) was 13 arcseconds. Example

[0036] In another embodiment of the present invention, a method for polishing β-Ga2O3(-201) surface crystals is provided.

[0037] In the surface pretreatment stage, after using the same cleaning method as in the previous embodiment, the plasma source parameters were set as follows: CF4 gas flow rate 45 sccm, O2 gas flow rate 15 sccm, RF power 150 W, bias power 50 W, processing pressure 15 Pa, processing temperature 25°C, and the processing time was further extended to 120 seconds, forming a modified layer with a thickness of approximately 110 nm. The neutralization treatment remained unchanged.

[0038] During the main polishing stage, the composition of the polishing slurry remains unchanged, and the process parameters are adjusted to: polishing pressure 2.5 kPa, polishing time 50 minutes, and other parameters remain unchanged.

[0039] During the precision polishing stage, the composition of the polishing slurry remains unchanged, the polishing pressure is 1 kPa, the polishing temperature is 0°C, the polishing time is 25 minutes, and other parameters remain unchanged.

[0040] The post-cleaning treatment stage remains unchanged.

[0041] After processing, the surface roughness Ra of the β-Ga2O3(-201) crystal reached 0.11 nm, the material removal rate was 1.05 μm / h, the subsurface damage depth was about 9 nm, and the lattice integrity (XRD half width at half maximum) was 15 arcseconds. Example

[0042] This embodiment investigated the effect of sodium bicarbonate concentration on polishing effect. The treatment method for β-Ga2O3(100) facet crystals was basically the same as in Example 1, but the sodium bicarbonate concentration in the main polishing solution was adjusted to 0.6 wt%, while keeping other components and process parameters unchanged.

[0043] After treatment, the surface roughness Ra was 0.11 nm, the material removal rate was 1.05 μm / h, and the subsurface damage depth was approximately 9 nm. Compared with Example 1, the material removal rate was slightly lower, indicating that a sodium bicarbonate concentration of 0.8 wt% is more conducive to the generation of reactive oxygen free radicals, thereby increasing the dissolution rate of the modified layer. Example

[0044] This embodiment investigates the effect of hydrogen peroxide concentration on polishing effect. The treatment method for β-Ga2O3(100) crystals is basically the same as in Example 1, but the hydrogen peroxide concentration in the main polishing solution is adjusted to 7 wt%, while keeping other components and process parameters unchanged.

[0045] After treatment, the surface roughness Ra was 0.10 nm, the material removal rate was 1.22 μm / h, and the subsurface damage depth was approximately 10 nm. The results indicate that appropriately increasing the hydrogen peroxide concentration can increase the material removal rate, but it also slightly increases the subsurface damage, requiring a balance between efficiency and quality. Example

[0046] This embodiment investigates the effect of polishing temperature on polishing effect. The treatment method for β-Ga2O3(100) plane crystals is basically the same as in Example 1, but the temperature of the main polishing stage is adjusted to 3°C, while keeping other components and process parameters unchanged.

[0047] After treatment, the surface roughness Ra was 0.12 nm, the material removal rate was 1.25 μm / h, and the subsurface damage depth was approximately 11 nm. The results indicate that appropriately lowering the polishing temperature helps suppress cleavage and improve surface quality, but excessively high temperatures increase the risk of cleavage. Example

[0048] This embodiment investigates the effect of ultrasonic power on polishing effect. The treatment method for β-Ga2O3(100) facet crystals is basically the same as in Example 1, but the ultrasonic power in the main polishing stage is adjusted to 80 W, while keeping other components and process parameters unchanged.

[0049] After treatment, the surface roughness Ra was 0.11 nm, the material removal rate was 1.05 μm / h, and the subsurface damage depth was approximately 8 nm. The results indicate that 100 W of ultrasonic power, compared to 80 W, can more effectively promote the contact between the polishing slurry and the surface, improving the material removal rate without increasing subsurface damage. Example

[0050] This embodiment investigates the effect of sorbitol concentration during the precision polishing stage on the final surface quality. The treatment method for β-Ga2O3(100) facet crystals is basically the same as in Example 1, but the sorbitol concentration in the precision polishing solution is adjusted to 1.5 wt%, while keeping other components and process parameters unchanged.

[0051] After treatment, the surface roughness Ra was 0.09 nm, the material removal rate was similar to that of Example 1, and the subsurface damage depth was approximately 7 nm. The results indicate that appropriately increasing the sorbitol concentration helps improve the surface modification effect and further reduce the surface roughness. Example

[0052] This embodiment investigates the effect of plasma pretreatment time on polishing effect. The treatment method for β-Ga2O3(100) crystal is basically the same as in Example 1, but the plasma treatment time is extended to 75 seconds while keeping other parameters unchanged.

[0053] After a complete polishing process, the surface roughness Ra was 0.10 nm, the material removal rate was 1.20 μm / h, and the subsurface damage depth was approximately 8 nm. The results indicate that appropriately extending the plasma treatment time can form a thicker modified layer and improve the material removal rate; however, excessively long treatment times may lead to instability at the interface between the modified layer and the substrate. Example

[0054] This embodiment optimizes the polishing process for large-size (4-inch) β-Ga2O3 wafers. The processing method is basically the same as in Example 1, but the following adjustments were made: the plasma treatment adopts a zoned scanning method to ensure large-area uniformity; the polishing slurry flow rate in the main polishing stage is increased to 150 mL / min, and the polishing time is extended to 60 minutes; the precision polishing time is extended to 35 minutes. Simultaneously, a real-time monitoring and adaptive control system was added, including an acoustic monitoring system, a temperature monitoring system, an automatic pH adjustment system for the polishing slurry, and a rotational torque monitoring system, to adjust process parameters in real time based on monitoring data.

[0055] After processing, the surface roughness Ra of the 4-inch β-Ga2O3 wafer is less than 0.12 nm throughout the entire surface area, the in-plane uniformity reaches over 95%, the material removal rate is 1.10 μm / h, and the subsurface damage depth is approximately 10 nm. This indicates that the method of the present invention has good scalability and is suitable for the industrial processing of large-size wafers.

[0056] Comparative Example 1 To verify the necessity of surface fluorination plasma pretreatment, this comparative example omitted the plasma pretreatment step and proceeded directly to the main polishing process. All other conditions were identical to those in Example 1.

[0057] After treatment, the surface roughness Ra of the β-Ga2O3(100) crystal was 0.25 nm, the material removal rate was only 0.45 μm / h, the subsurface damage depth reached about 35 nm, and obvious cleavage pits and scratches were present on the surface. This indicates that surface fluorination plasma pretreatment is a key step in reducing the surface hardness of β-Ga2O3, preventing cleavage damage, and improving the material removal rate.

[0058] Comparative Example 2 To verify the importance of the bicarbonate-activated hydrogen peroxide (BAP) system, this comparative example removed the sodium bicarbonate component from the main polishing solution and used only hydrogen peroxide as the oxidant. All other conditions were identical to those in Example 1.

[0059] After treatment, the surface roughness Ra of the β-Ga2O3(100) crystal was 0.18 nm, the material removal rate was only 0.65 μm / h, and the subsurface damage depth was approximately 20 nm. This indicates that the synergistic effect of bicarbonate and hydrogen peroxide can generate various reactive oxygen free radicals, significantly improving the dissolution rate of the modified layer, which is one of the key mechanisms for efficient polishing in this invention.

[0060] Comparative Example 3 To verify the importance of ultrasonic assistance, the ultrasonic assistance system was removed during the polishing process in this comparative example. All other conditions were exactly the same as in Example 1.

[0061] After treatment, the surface roughness Ra of the β-Ga2O3(100) crystal was 0.16 nm, the material removal rate was 0.75 μm / h, and the subsurface damage depth was approximately 15 nm. This indicates that ultrasonic assistance can promote the contact between the polishing slurry and the surface, accelerate the chemical reaction, and provide micromechanical effects, which is an important factor in improving polishing efficiency and quality.

[0062] Comparative Example 4 To verify the importance of a low-temperature environment, the temperatures for both primary polishing and precision polishing in this comparative example were adjusted to 25°C (room temperature). All other conditions were exactly the same as in Example 1.

[0063] After treatment, obvious cleavage cracks appeared on the surface of the β-Ga2O3(100) crystal, with a surface roughness Ra of 0.32 nm, a material removal rate of 1.30 μm / h, and a subsurface damage depth exceeding 50 nm. This indicates that a low-temperature environment (0-5°C) is crucial for suppressing the cleavage phenomenon of β-Ga2O3. Although the material removal rate is slightly improved at room temperature, the surface quality is significantly reduced, making it unsuitable for high-quality polishing requirements.

[0064] Comparative Example 5 To verify the necessity of crystal plane-guided polishing parameters, this comparative example uses the exact same polishing parameters (the parameters of Example 1) on different crystal planes (100 plane, 010 plane and -201 plane).

[0065] After processing, the surface roughness Ra of the (100) facet was 0.10 nm; however, the (010) facet exhibited significant cleavage due to excessive pressure, resulting in a surface roughness Ra of 0.28 nm; and the (-201) facet, due to insufficient plasma treatment time, had a material removal rate of only 0.65 μm / h, with a surface roughness Ra of 0.18 nm. This indicates that employing differentiated polishing parameters is crucial for addressing the anisotropy of different crystal facets of β-Ga2O3, and is key to ensuring high-quality surfaces for each facet.

[0066] The table below summarizes the main polishing results of the above embodiments and comparative examples: Example 1 0.1 1.15 8 (100) Standard process Example 2 0.09 0.95 7 (010) Surface optimization process Example 3 0.11 1.05 9 (-201) Surface optimization process Example 4 0.11 1.05 9 Sodium bicarbonate concentration 0.6 wt% Example 5 0.1 1.22 10 Hydrogen peroxide concentration 7 wt% Example 6 0.12 1.25 11 Polishing temperature 3°C Example 7 0.11 1.05 8 80 W ultrasonic power Example 8 0.09 1.15 7 Sorbitol concentration 1.5 wt% Example 9 0.1 1.2 8 Plasma treatment for 75 seconds Example 10 <0.12 1.1 10 4-inch large-size chip Comparative Example 1 0.25 0.45 35 Plasma-free pretreatment Comparative Example 2 0.18 0.65 20 Sodium bicarbonate-free Comparative Example 3 0.16 0.75 15 No ultrasonic assistance Comparative Example 4 0.32 1.3 >50 Room temperature polishing Comparative Example 5 0.10-0.28 0.65-1.15 August 30 Using the same parameters for different crystal planes Traditional CMP 0.18 0.4 Approximately 200 Literature report value Acidic CMP 0.21 0.5 Approximately 100 Literature report value Low pressure CMP 0.15 0.4 Approximately 50 Literature report value Through detailed comparative analysis of the above embodiments and comparative examples, the significant advantages of the method of the present invention can be clearly seen. Based on the experimental results, the optimal combination of process parameters is as follows: (1) Surface pretreatment: CF4 flow rate 45 sccm, O2 flow rate 15 sccm, RF power 150 W, bias power 50 W, treatment pressure 15 Pa, treatment temperature 25°C, and treatment time adjusted according to the crystal face ((100) face 60 seconds, (010) face 90 seconds, (-201) face 120 seconds); (2) Main polishing: 5 wt% colloidal silica (50-70 nm), 6 wt% hydrogen peroxide, 0.8 wt% sodium bicarbonate, 0.5 wt% phosphoric acid, polishing temperature 5°C, ultrasonic parameters 40 kHz, 100 W, polishing pressure adjusted according to crystal plane (3 kPa for (100) plane, 2 kPa for (010) plane, 2.5 kPa for (-201) plane); (3) Precision polishing: 2 wt% of nano-grade colloidal silica (20-30 nm), 3 wt% of hydrogen peroxide, 0.5 wt% of sodium bicarbonate, 0.3 wt% of phosphoric acid, 1-1.5 wt% of sorbitol, polishing temperature 0°C, ultrasonic parameters 40 kHz, 50 W (intermittent mode), polishing pressure 0.8-1 kPa.

[0067] This multi-synergistic polishing method, through the organic combination of innovative technologies such as surface selective fluorination modification, active oxygen free radical enhanced oxidation, ultrasonic-assisted low-pressure polishing, crystal plane guided polishing, and temperature gradient control, has successfully solved key technical problems in the polishing of β-Ga2O3 crystals. It achieves ultra-precision polishing effects with high efficiency, low damage, and atomic-level flatness, providing key technical support for the research and development and industrialization of β-Ga2O3-based high-performance electronic and optoelectronic devices.

[0068] The working principle of this invention can be summarized by the following synergistic mechanisms: (1) Surface fluorination plasma modification mechanism: F free radicals generated by CF4 / O2 plasma react with the β-Ga2O3 surface to form GaF3, while active oxygen generated by O2 plasma causes Ga(OH)3 to form on part of the surface layer. The two components work together to form a mixed modified layer, which significantly reduces the hardness and creates favorable conditions for subsequent polishing. (2) Synergistic dissolution mechanism of reactive oxygen species: In the bicarbonate-activated hydrogen peroxide (BAP) system, HCO3⁻ reacts with H2O2 to generate reactive oxygen species such as carbonate radicals (•CO3⁻), superoxide radicals (•O2⁻), and singlet oxygen (¹O2). Each of these reactive oxygen species has its own characteristics: •CO3⁻ has strong oxidizing power and can oxidize Ga³⁺ in GaF3; •O2⁻ has moderate oxidizing power and good stability; ¹O2 is a high-energy oxygen that can activate surface Ga-O bonds. The synergistic effect of these reactive oxygen species significantly increases the dissolution rate of the modified layer. (3) Mechanism of ultrasonic waves promoting chemical reaction: Ultrasonic waves generate cavitation effect in polishing fluid, forming a high temperature and high pressure microenvironment, which accelerates chemical reaction. At the same time, the shock wave generated by the rupture of microbubbles provides micro-mechanical removal. (4) Low temperature suppression mechanism for cleavage: The low temperature environment of 0-5°C reduces the thermal vibration of atoms on the β-Ga2O3 surface, reduces the probability of bond breakage under stress, and effectively suppresses cleavage phenomenon; (5) Mechanism of differences in chemical reaction of crystal facets: Different crystal facets exhibit different chemical reactivity due to differences in the arrangement and density of Ga-O bonds. For example, the octahedral Ga-O bonds in the (100) facet are more easily hydrolyzed, while the (010) facet has higher thermal conductivity and different mechanical properties, requiring the use of different polishing parameters.

[0069] The organic combination of these mechanisms forms the multi-synergistic polishing technology system of this invention, which is significantly superior to existing technologies. Compared with traditional polishing methods, this invention has the following significant advantages: material removal rate is increased by about 2.3 times; surface roughness is reduced by about 2 times; polishing time is shortened by about 4-7 times; subsurface damage depth is reduced by about 10-20 times; and lattice integrity is improved by about 1.5 times.

[0070] In summary, this invention provides a novel method for ultra-precision polishing of gallium oxide crystals with multiple synergistic effects. By organically combining surface fluorinated plasma pretreatment, bicarbonate-activated hydrogen peroxide system, ultrasonic-assisted low-temperature CMP, and crystal plane-guided polishing strategy, it successfully solves key technical challenges in the polishing of β-Ga2O3 crystals, providing important technical support for the research and development and industrialization of high-performance β-Ga2O3-based electronic and optoelectronic devices.

[0071] The method of this invention is simple to operate, with stable and reliable process control, making it suitable for industrial production and possessing broad application prospects and good economic benefits. Furthermore, the method utilizes biocompatible materials such as chitosan oligosaccharides and sorbitol to replace traditional corrosive chemicals, aligning with the trend of green and environmentally friendly development.

[0072] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for polishing gallium oxide crystals, characterized in that, Includes the following steps: a) Surface pretreatment step: The surface of gallium oxide crystal is treated with CF4 / O2 plasma to form a mixed modified layer of GaF3 and Ga(OH)3. The parameters of the CF4 / O2 plasma treatment are: CF4 gas flow rate of 45 sccm, O2 gas flow rate of 15 sccm, RF power of 150 W, bias power of 50 W, treatment pressure of 10-20 Pa, treatment temperature of 25℃, and treatment time is adjusted according to different crystal faces, of which (100) face is 60 seconds, (010) face is 90 seconds, and (-201) face is 120 seconds. b) The main polishing step involves using a polishing slurry containing a hydrogen peroxide system activated by bicarbonate in a low-temperature environment, combined with ultrasonic assistance, to perform the main polishing on the pretreated gallium oxide crystal. The polishing slurry used in the main polishing step consists of: 5 wt% colloidal silica with a particle size of 50-70 nm, 6 wt% hydrogen peroxide, 0.8 wt% sodium bicarbonate, 0.5 wt% phosphoric acid (used to adjust the pH value to 4.2±0.1), 0.2 wt% polyvinylpyrrolidone, 0.05 wt% nonionic surfactant, 0.3 wt% citric acid, and the remainder being deionized water to 100%. The process parameters for the main polishing step are: polishing slurry flow rate 100 mL / min, carrier rotation speed 60 rpm, polishing disc rotation speed 70 rpm, polishing temperature 5℃, ultrasonic parameters 40 kHz and 100 W, and polishing pressure adjusted according to the different crystal faces, with 3 kPa for the (100) face, 2 kPa for the (010) face, and 2.5 kPa for the (-201) face. c) Precision polishing step: Under a lower temperature environment, gallium oxide crystals are precision polished using a polishing slurry with lower pressure and a modified formula. The polishing slurry used in the precision polishing step consists of: 2 wt% nano-sized colloidal silica with a particle size of 20-30 nm, 3 wt% hydrogen peroxide, 0.5 wt% sodium bicarbonate, 0.3 wt% phosphoric acid (used to adjust the pH to 4.5±0.1), 0.1 wt% chitosan oligosaccharide, 1 wt% sorbitol, and the balance being deionized water to 100%. The process parameters for the precision polishing step are: carrier rotation speed 40 rpm, polishing disc rotation speed 40 rpm, polishing pressure 1 kPa, polishing temperature 0℃, ultrasonic parameters of 40 kHz and 50 W, operating in an intermittent mode of 10 seconds on and 5 seconds off, with a polishing time of 20-30 minutes. d) Cleaning and post-treatment steps: A multi-stage cleaning process is used to remove polishing residues and repair surface defects. The cleaning and post-treatment steps include: SC-1 cleaning, NH4OH:H2O2:H2O = 1:1:5, 50℃, 5 minutes; acid cleaning, diluted citric acid 1 wt%, room temperature, 5 minutes; ultrasonic cleaning with deionized water, 40 kHz, 2 minutes; cleaning with anhydrous ethanol, 1 minute; nitrogen drying; selective oxygen atmosphere annealing, 600℃, 30 minutes, oxygen flow rate 100 sccm, and natural cooling to room temperature.

2. The method according to claim 1, characterized in that, It also includes real-time monitoring and adaptive control steps, which are implemented through the following system: a) Acoustic monitoring system: Monitors microscopic fracture signals during the polishing process using acoustic emission sensors; b) Temperature monitoring system: Uses an infrared thermal imager to monitor the temperature of the polishing interface in real time; c) Automatic pH adjustment system for polishing fluid: Maintains pH stability through a microfluidic system; d) Rotational torque monitoring system: Infers material removal status by analyzing changes in rotational torque; Based on monitoring data, the polishing pressure was adjusted in real time by ±0.5 kPa, ultrasonic power by ±20 W, polishing fluid flow rate by ±20 mL / min, and polishing disc rotation speed by ±10 rpm.

3. The method according to claim 1, characterized in that, The CF4 / O2 plasma treatment uses an inductively coupled plasma source. After the surface pretreatment step is completed, a neutralization treatment is performed: soaking in a 0.5% citric acid solution for 30 seconds, followed by rinsing with deionized water.

4. The method according to claim 1, characterized in that, The main polishing step uses a porous polyurethane polishing pad, which is a composite pad of IC1000 and Suba400; the precision polishing step uses an ultra-soft polyurethane polishing pad, which is SubaXIV.