Method for polishing gallium oxide crystal

Abstract

The invention relates to the technical field of semiconductor material processing, in particular to a method for polishing a gallium oxide crystal, which comprises the following steps: a) a surface pretreatment step: treating the surface of the gallium oxide crystal by adopting CF / Oplasma to form a GaF and Ga (OH) mixed modified layer; b) a main polishing step: carrying out main polishing on the pretreated gallium oxide crystal by using a polishing solution containing a hydrogen peroxide system activated by bicarbonate in a low-temperature environment in combination with ultrasonic assistance; c) a precision polishing step: in a lower temperature environment, using a polishing solution with lower pressure and an improved formula to carry out precision polishing on the gallium oxide crystal; and d) a cleaning and post-processing step: a multi-stage cleaning process is adopted to remove polishing residues and repair surface defects, the material removal rate is increased by about 2.3 times (reaching 1.2 [mu] m / h), and the surface roughness Ra is reduced by about 2 times (reaching 0.09 nm).

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor material processing, and in particular to a method for polishing gallium oxide crystals, and is particularly suitable for high-quality surface polishing of β-Ga2O3 ultra-wide bandgap semiconductor crystals. Background Art

[0002] Gallium oxide (β-Ga2O3), a new generation of ultra-wide bandgap semiconductor materials, demonstrates broad application prospects in high-temperature, high-frequency, and high-power electronic devices, as well as deep ultraviolet optoelectronics and sensors, thanks to its excellent electronic properties. This material boasts an ultra-wide bandgap of approximately 4.8-4.9 eV and a breakdown electric field of up to 8 MV / cm, far superior to traditional semiconductor materials such as Si and GaAs. Its excellent high-temperature stability also enables reliable operation in harsh environments.

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

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

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

[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 have a difficult-to-balance contradiction 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 unique to β-Ga2O3 crystals; (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 ultra-precision polishing effect with atomic-level flatness; (5) the process parameters are difficult to accurately control, and the polishing quality of different crystal surfaces 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 technical difficulties and achieve ultra-precision polishing effects 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 object of the present invention is to provide a method for polishing gallium oxide crystals. The method adopts a multiple synergistic mechanism and organically combines technical means such as surface fluorination plasma pretreatment, active oxygen free radical-enhanced polishing liquid, and ultrasonic-assisted low-temperature precision polishing to effectively solve key technical problems such as easy cleavage, anisotropy, subsurface damage, and low material removal rate in the polishing of β-Ga2O3 crystals, thereby achieving an ultra-precision polishing effect with high efficiency, low damage, and atomic-level flatness.

[0011] To achieve the above object, the present invention provides a method for polishing gallium oxide crystals, which is characterized by comprising the following steps: a) Surface pretreatment step: using CF4 / O2 plasma to treat the surface of the gallium oxide crystal to form a mixed modified layer of GaF3 and Ga(OH)3; b) Main polishing step: using a polishing liquid containing a bicarbonate-activated hydrogen peroxide system in a low-temperature environment, combined with ultrasonic assistance, to perform the main polishing of the pre-treated gallium oxide crystal; c) Precision polishing step: Precision polishing of gallium oxide crystals at a lower temperature using lower pressure and a polishing fluid with an improved formula; d) Cleaning and post-processing steps, using a multi-stage cleaning process 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 is 45 sccm, O2 gas flow rate is 15 sccm, RF power is 150 W, bias power is 50 W, treatment pressure is 10-20 Pa, treatment temperature is 25°C, and treatment time is adjusted according to different crystal planes, where the (100) plane is 60 seconds, the (010) plane is 90 seconds, and the (-201) plane is 120 seconds.

[0013] Preferably, the polishing liquid used in the main polishing step is composed of: 5 wt % colloidal silica (50-70 nm), 6 wt % hydrogen peroxide (H2O2), 0.8 wt % sodium bicarbonate (NaHCO3), 0.5 wt % phosphoric acid (H3PO4) for adjusting 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 balance of deionized water to 100%.

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

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

[0016] Preferably, the process parameters of the precision polishing step are: carrier speed 40 rpm, polishing disk speed 40 rpm, polishing pressure 1 kPa, polishing temperature 0°C, ultrasonic parameters 40 kHz, 50 W, intermittent mode (on 10 seconds, off 5 seconds), and polishing time is 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) deionized water ultrasonic cleaning: 40 kHz, 2 minutes; d) anhydrous ethanol cleaning: 1 minute; e) nitrogen blow drying; f) selective oxygen atmosphere annealing: 600°C, 30 minutes, oxygen flow rate 100 sccm, and natural cooling to room temperature.

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

[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, and then 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 through surface fluorination plasma pretreatment, which significantly reduces the surface hardness of β-Ga2O3 (by about 80%), effectively inhibits cleavage, and creates favorable conditions for subsequent polishing; 2. Using a bicarbonate-activated hydrogen peroxide (BAP) system to generate a variety of active oxygen free radicals (•CO3⁻, •O2⁻, ¹O2) synergistically, significantly increasing the dissolution rate of the modified layer; 3. Combining ultrasonic assistance with a low-temperature environment (0-5°C) can enhance chemical reaction and mechanical removal efficiency at the micro level and inhibit cleavage at the macro level; 4. Based on the anisotropy of β-Ga2O3, differentiated polishing parameters are designed to solve the problem of inconsistent polishing quality on different crystal surfaces; 5. Achieve a highly stable and reliable process through multi-parameter real-time monitoring and adaptive control system; 6. Compared with the existing technology, the present invention increases the material removal rate by about 2.3 times (reaching 1.2 μm / h), reduces the surface roughness Ra by about 2 times (reaching 0.09 nm), reduces the subsurface damage depth by about 10-20 times (less than 10 nm), and shortens the polishing time by about 4-7 times (only 1.5-2 hours). DETAILED DESCRIPTION

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

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

[0024] To address these challenges, the present invention proposes a new, multi-synergistic polishing method, comprising four key stages: surface pretreatment, main polishing, precision polishing, and post-cleaning. Surface pretreatment involves forming a thickness-controlled GaF3 / Ga(OH)3 modified layer using CF4 / O2 plasma, reducing surface hardness and preventing cleavage. The main polishing stage utilizes a bicarbonate-activated hydrogen peroxide (BAP) system to generate a variety of reactive oxygen radicals that synergistically dissolve the modified layer. Ultrasonic assistance and a low-temperature environment are combined to improve efficiency and inhibit cleavage. The precision polishing stage utilizes milder conditions to achieve atomic-level surface flatness. Finally, multi-stage cleaning and selective annealing are used to remove residues and repair minor surface defects. Example

[0025] In a basic embodiment of the present invention, a method for polishing a β-Ga2O3 (100) surface crystal is provided, which specifically comprises the following steps: First, surface pretreatment was performed. The β-Ga2O3 (100) surface wafer was ultrasonically cleaned using anhydrous ethanol and deionized water for 5 minutes each, dried with nitrogen, and 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, treatment pressure 15 Pa, treatment temperature 25°C, and treatment time 60 seconds. After treatment, the wafer was immersed in a 0.5% citric acid solution for 30 seconds for neutralization and then rinsed thoroughly 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 step was performed. A BAP-SiO2 polishing slurry was prepared with the following composition: 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 (100%) deionized water. Polishing was performed using a custom ultrasonic-assisted CMP system with the following process parameters: slurry flow rate of 100 mL / min, carrier speed of 60 rpm, polishing pad speed of 70 rpm, polishing pressure of 3 kPa, polishing temperature of 5°C, ultrasonic parameters of 40 kHz and 100 W, an IC1000+Suba400 composite polishing pad, and a polishing time of 45 minutes.

[0027] Then, precision polishing was performed. A precision polishing slurry was prepared, consisting of 2 wt% nanoscale colloidal silica (25 nm), 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 deionized water to 100%. The precision polishing process parameters were: carrier speed 40 rpm, polishing disk speed 40 rpm, polishing pressure 1 kPa, polishing temperature 0°C, ultrasonic parameters 40 kHz, 50 W (intermittent mode, on 10 seconds, off 5 seconds), a SubaXIV ultrasoft polishing pad, and a polishing time of 25 minutes.

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

[0029] After the above process, the surface roughness Ra of the β-Ga2O3 (100) surface 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-maximum width) is 14 arc seconds. Example

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

[0031] During the surface pretreatment phase, the same cleaning method as in Example 1 was used. The plasma source parameters were set as follows: 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 15 Pa, treatment temperature of 25°C, and treatment time extended to 90 seconds to form a thicker modified layer (approximately 95 nm). The remaining neutralization treatment was the same as in Example 1.

[0032] During the main polishing phase, the same polishing slurry 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 all other parameters remained unchanged. This is because the mechanical strength of the (010) plane is lower than that of the (100) plane, requiring a lower pressure to avoid cleavage damage.

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

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

[0035] After treatment, the surface roughness Ra of the β-Ga2O3 (010) plane 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-maximum width) was 13 arc seconds. Example

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

[0037] During the surface pretreatment phase, the same cleaning method as in the previous example was used. The plasma source parameters were set as follows: 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 15 Pa, and treatment temperature of 25°C. The treatment time was further extended to 120 seconds, resulting in 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 liquid remained unchanged, and the process parameters were adjusted to: polishing pressure 2.5 kPa, polishing time 50 minutes, and other parameters remained unchanged.

[0039] During the precision polishing stage, the composition of the polishing liquid remained unchanged, the polishing pressure was 1 kPa, the polishing temperature was 0°C, the polishing time was 25 min, and other parameters remained unchanged.

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

[0041] After treatment, the surface roughness Ra of the β-Ga2O3(-201) surface 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-maximum width) was 15 arc seconds. Example

[0042] This example investigates the effect of sodium bicarbonate concentration on polishing. The treatment method for β-Ga2O3 (100) face crystals is essentially the same as in Example 1, except that the sodium bicarbonate concentration in the main polishing solution is adjusted to 0.6 wt%, while keeping the 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 example investigates the effect of hydrogen peroxide concentration on polishing. The treatment method for the (100) face of β-Ga2O3 crystals is essentially the same as in Example 1, except that the hydrogen peroxide concentration in the main polishing solution is adjusted to 7 wt%, while keeping the 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 show that appropriately increasing the hydrogen peroxide concentration can increase the material removal rate, but at the same time, subsurface damage also increases slightly, requiring a balance between efficiency and quality. Example

[0046] This example investigates the effect of polishing temperature on the polishing effect. The treatment method for the β-Ga2O3 (100) face crystal is essentially 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 show that appropriately lowering the polishing temperature can help suppress cleavage and improve surface quality, but excessively high temperatures increase the risk of cleavage. Example

[0048] This example investigates the effect of ultrasonic power on polishing. The treatment method for the (100) face of β-Ga2O3 crystals is essentially the same as in Example 1, except that the ultrasonic power during the main polishing phase 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 show that an ultrasonic power of 100 W, compared to 80 W, is more effective in promoting contact between the polishing fluid and the surface, increasing the material removal rate without increasing subsurface damage. Example

[0050] This example investigates the effect of sorbitol concentration during the precision polishing stage on the final surface quality. The treatment method for the (100) face of β-Ga2O3 crystals is essentially the same as in Example 1, except that the sorbitol concentration in the precision polishing solution is adjusted to 1.5 wt%, while keeping all 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 in Example 1, and the subsurface damage depth was approximately 7 nm. The results show that appropriately increasing the sorbitol concentration can help improve the surface modification effect and further reduce the surface roughness. Example

[0052] This example investigates the effect of plasma pretreatment time on polishing performance. The treatment method for β-Ga2O3 (100) face crystals is essentially 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 show that appropriately extending the plasma treatment time can form a thicker modified layer and improve the material removal rate, but excessive treatment time may lead to unstable interface between the modified layer and the substrate. Example

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

[0055] After treatment, the surface roughness Ra of a 4-inch β-Ga2O3 wafer was less than 0.12 nm across the entire surface area, with in-plane uniformity exceeding 95%. The material removal rate was 1.10 μm / h, and the subsurface damage depth was approximately 10 nm. This demonstrates the scalability of the proposed method and its suitability for industrial processing of large-scale wafers.

[0056] Comparative Example 1 In order to verify the necessity of the surface fluorination plasma pretreatment, this comparative example omitted the plasma pretreatment step and directly performed the main polishing process. Other conditions were exactly the same as those in Example 1.

[0057] After treatment, the surface roughness Ra of the β-Ga2O3 (100) face crystal was 0.25 nm, the material removal rate was only 0.45 μm / h, the subsurface damage depth reached approximately 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. Other conditions were exactly the same as in Example 1.

[0059] After treatment, the surface roughness Ra of the β-Ga2O3 (100) face 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 a variety of reactive oxygen free radicals, significantly increasing the dissolution rate of the modified layer, which is one of the key mechanisms of the present invention's efficient polishing.

[0060] Comparative Example 3 In order to verify the importance of ultrasonic assistance, this comparative example removed the ultrasonic assistance system during the polishing process. Other conditions were exactly the same as those in Example 1.

[0061] After treatment, the surface roughness Ra of the β-Ga2O3 (100) face 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 contact between the polishing liquid and the surface, accelerate chemical reactions, and provide micromechanical effects, which are important factors in improving polishing efficiency and quality.

[0062] Comparative Example 4 In order to verify the importance of low temperature environment, the temperature of main polishing and precision polishing was adjusted to 25°C (room temperature) in this comparative example. Other conditions were exactly the same as those in Example 1.

[0063] After treatment, obvious cleavage cracks appeared on the surface of the β-Ga2O3 (100) face crystal, with a surface roughness Ra of 0.32nm, 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 In order to verify the necessity of the crystal plane-guided polishing parameters, this comparative example uses exactly the same polishing parameters (using the parameters of Example 1) for different crystal planes (100 plane, 010 plane and -201 plane).

[0065] After treatment, the surface roughness Ra of the (100) surface was 0.10 nm. However, due to excessive pressure, the (010) surface exhibited significant cleavage, resulting in a surface roughness Ra of 0.28 nm. Due to insufficient plasma treatment time, the material removal rate of the (-201) surface was only 0.65 μm / h, resulting in a surface roughness Ra of 0.18 nm. This indicates that the use of differentiated polishing parameters to address the anisotropy of different β-Ga2O3 crystal faces is crucial for ensuring high-quality surfaces on all crystal faces.

[0066] The following table summarizes the main polishing results of the above examples and comparative examples: Sample number Surface roughness Ra (nm) Material removal rate (μm / h) Subsurface damage depth (nm) Remark 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 Ultrasonic power 80 W 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 chip Comparative Example 1 0.25 0.45 35 No plasma pretreatment Comparative Example 2 0.18 0.65 20 No sodium bicarbonate Comparative Example 3 0.16 0.75 15 No ultrasound 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 Use the same parameters for different crystal planes Traditional CMP 0.18 0.4 About 200 Literature reported value Acidic CMP 0.21 0.5 About 100 Literature reported value Low-pressure CMP 0.15 0.4 About 50 Literature reported value Through the detailed comparative analysis of the above examples and comparative examples, it can be clearly seen that the method of the present invention has significant advantages. Based on the experimental results, the optimal process parameter combination is: (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, treatment time adjusted according to the crystal plane ((100) plane 60 seconds, (010) plane 90 seconds, (-201) plane 120 seconds); (2) Main polishing: colloidal silica (50-70 nm) 5 wt%, hydrogen peroxide 6 wt%, sodium bicarbonate 0.8 wt%, phosphoric acid 0.5 wt%, polishing temperature 5°C, ultrasonic parameters 40 kHz, 100 W, polishing pressure adjusted according to the crystal plane ((100) plane 3 kPa, (010) plane 2 kPa, (-201) plane 2.5 kPa); (3) Precision polishing: nano-scale colloidal silica (20-30 nm) 2 wt%, hydrogen peroxide 3 wt%, sodium bicarbonate 0.5 wt%, phosphoric acid 0.3 wt%, sorbitol 1-1.5 wt%, 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, successfully solves the key technical difficulties in β-Ga2O3 crystal polishing, and 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 the present invention can be summarized as the following synergistic mechanisms: (1) Surface fluorination plasma modification mechanism: F radicals generated by CF4 / O2 plasma react with the surface of β-Ga2O3 to form GaF3. At the same time, the active oxygen generated by O2 plasma causes part of the surface layer to form Ga(OH)3. The two components work synergistically to form a mixed modified layer, which significantly reduces the hardness and creates favorable conditions for subsequent polishing. (2) Cooperative dissolution mechanism of active oxygen free radicals: In the bicarbonate-activated peroxide system (BAP), HCO3⁻ reacts with H2O2 to produce active oxygen free radicals such as carbonate free radicals (•CO3⁻), superoxide free radicals (•O2⁻), and singlet oxygen (¹O2). Each of them has its own characteristics: •CO3⁻ has strong oxidizing ability and can oxidize Ga³⁺ in GaF3; •O2⁻ has moderate oxidizing ability and good stability; ¹O2 is high-energy oxygen and can activate the surface Ga-O bond. These active oxygen free radicals work synergistically to significantly increase the dissolution rate of the modified layer; (3) Ultrasonic wave promotes chemical reaction mechanism: Ultrasonic wave produces cavitation effect in the polishing liquid, forming a high temperature and high pressure microenvironment, accelerating the chemical reaction. At the same time, the shock wave generated by the collapse of microbubbles provides microscopic mechanical removal effect; (4) Low temperature inhibition of cleavage mechanism: The low temperature environment of 0-5°C reduces the thermal vibration of β-Ga2O3 surface atoms, reduces the probability of bond breakage under stress, and effectively inhibits the cleavage phenomenon; (5) Mechanism of chemical reactivity differences between crystal planes: Different crystal planes exhibit different chemical reactivity due to differences in Ga-O bond arrangement and density. For example, the octahedral Ga-O bonds on the (100) plane are more susceptible to hydrolysis, while the (010) plane has higher thermal conductivity and different mechanical properties, requiring differentiated polishing parameters.

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

[0070] In summary, the present invention provides a new method for ultra-precision polishing of gallium oxide crystals with multiple synergistic effects. Through the organic combination of surface fluorination plasma pretreatment, bicarbonate-activated hydrogen peroxide system, ultrasonic-assisted low-temperature CMP and crystal plane-guided polishing strategy, it successfully solves the key technical difficulties in polishing 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 the present invention has a simple operation process, stable and reliable process control, is suitable for industrial production, and has broad application prospects and good economic benefits. Furthermore, the method of the present invention uses biocompatible materials such as chitosan oligosaccharides and sorbitol to replace traditional corrosive chemicals, which conforms to the development trend of green environmental protection.

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

Claims

1. A method for polishing gallium oxide crystals, characterized in that: The following steps are involved: a) Surface pretreatment step: using CF4 / O2 plasma to treat the surface of the gallium oxide crystal to form a mixed modified layer of GaF3 and Ga(OH)3; b) Main polishing step: using a polishing liquid containing a bicarbonate-activated hydrogen peroxide system in a low-temperature environment, combined with ultrasonic assistance, to perform the main polishing of the pre-treated gallium oxide crystal; c) Precision polishing step: Precision polishing of gallium oxide crystals at a lower temperature using lower pressure and a polishing fluid with an improved formula; d) Cleaning and post-processing steps, using a multi-stage cleaning process to remove polishing residues and repair surface defects.

2. The method according to claim 1, characterized in that 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, RF 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 planes, where the (100) plane is 60 seconds, the (010) plane is 90 seconds, and the (-201) plane is 120 seconds.

3. The method according to claim 1, characterized in that The polishing liquid used in the main polishing step is composed of: Colloidal silica (50-70 nm): 5 wt%; Hydrogen peroxide (H2O2): 6 wt%; Sodium bicarbonate (NaHCO3): 0.8 wt%; Phosphoric acid (H3PO4): 0.5 wt%, used to adjust the pH to 4.2 ± 0.1; Polyvinylpyrrolidone (PVP, K30): 0.2 wt% as a dispersant; Nonionic surfactant (Triton X-100): 0.05 wt%; Citric acid: 0.3 wt%, as a complexing agent; Deionized water: balance to 100%.

4. The method according to claim 1, wherein The process parameters of the main polishing step are as follows: polishing liquid flow rate 100 mL / min, carrier speed 60 rpm, polishing disk speed 70 rpm, polishing temperature 5°C, ultrasonic parameters 40 kHz, 100 W, and polishing pressure adjusted according to different crystal planes, where the (100) plane is 3 kPa, the (010) plane is 2 kPa, and the (-201) plane is 2.5 kPa.

5. The method according to claim 1, wherein The polishing liquid used in the precision polishing step is composed of: Nanoscale colloidal silica (20-30 nm): 2 wt%; Hydrogen peroxide (H2O2): 3 wt%; Sodium bicarbonate (NaHCO3): 0.5 wt%; Phosphoric acid (H3PO4): 0.3 wt%, used to adjust the pH to 4.5 ± 0.1; Chitosan oligosaccharide: 0.1 wt% as an environmentally friendly complexing agent; Sorbitol: 1 wt% as a surface modifier; Deionized water: balance to 100%.

6. The method according to claim 1, characterized in that The process parameters of the precision polishing step are: carrier speed 40 rpm, polishing disk speed 40 rpm, polishing pressure 1 kPa, polishing temperature 0°C, ultrasonic parameters 40 kHz, 50 W, intermittent mode (on 10 seconds, off 5 seconds), and polishing time is 20-30 minutes.

7. The method according to claim 1, characterized in that The cleaning and post-processing 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 in deionized water: 40 kHz, 2 minutes; d) Cleaning with anhydrous ethanol: 1 minute; e) Dry with nitrogen; f) Selective oxygen atmosphere annealing: 600°C, 30 min, oxygen flow rate 100 sccm, and naturally cool to room temperature.

8. The method according to claim 1, characterized in that The system also includes real-time monitoring and adaptive control steps, which are implemented by the following systems: a) Acoustic monitoring system: Acoustic emission sensors are used to monitor microscopic fracture signals during the polishing process; b) Temperature monitoring system: Use infrared thermal imager to monitor the polishing interface temperature in real time; c) Automatic pH adjustment system for polishing liquid: maintains pH stability through microfluidic system; d) Rotational torque monitoring system: inferring material removal status through the change of rotational torque; Based on the monitoring data, the polishing pressure (±0.5 kPa), ultrasonic power (±20 W), polishing liquid flow rate (±20 mL / min), and polishing disk speed (±10 rpm) were adjusted in real time.

9. The method according to claim 1, characterized in that The CF4 / O2 plasma treatment uses an inductively coupled plasma (ICP) source. After the pretreatment step, a neutralization treatment is performed: soaking in 0.5% citric acid solution for 30 seconds and then rinsing with deionized water.

10. The method according to claim 1, characterized in that 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.