A nano-alumina polishing solution for silicon carbide substrate wafer and its preparation method and application

Abstract

The application relates to a preparation method and application of a nano-alumina polishing solution for a silicon carbide substrate sheet. The nano-alumina polishing solution comprises 0.5wt%-6.0wt% of alpha-Al2O3 abrasive, 0.5wt%-5.0wt% of an oxidizing agent, 0.1wt%-1.0wt% of an auxiliary oxidizing agent, 0.05wt%-0.50wt% of a pH regulator and 0.10wt%-0.50wt% of a dispersing agent, and the rest is water; the alpha-Al2O3 abrasive has a D10 of 0.100-0.140 microns, a D50 of 0.180-0.230 microns, a D90 of 0.240-0.300 microns, a D100 of 0.340-0.420 microns and a diameter interval of 0.6-0.9. The preparation method of the alpha-Al2O3 abrasive is as follows: alpha-Al2O3 powder is prepared into alpha-Al2O3 slurry, potassium citrate is added, sanding is carried out, and the alpha-Al2O3 slurry after sanding is subjected to particle grading to obtain the alpha-Al2O3 abrasive. Compared with the prior art, the alpha-Al2O3 abrasive after grading is selected as the abrasive particle for silicon carbide polishing, and the potassium permanganate with stronger oxidizing property is selected as the oxidizing agent, the prepared nano-alumina polishing solution is used in chemical mechanical polishing of the silicon carbide substrate sheet, the silicon surface material removal rate of the silicon carbide substrate sheet is improved, the surface roughness is reduced, and the surface quality of the atom level smoothness is obtained.

Description

Technical Field

[0001] This invention relates to the field of material polishing technology, and in particular to a nano-alumina polishing slurry for silicon carbide substrates, its preparation method, and its application. Background Technology

[0002] Silicon carbide, as a representative of third-generation semiconductor materials, possesses superior physical, chemical, and electrical properties, and is widely used in the electric vehicle industry, new energy, 5G communications, and defense. Furthermore, based on its electrical conductivity, silicon carbide is divided into conductive silicon carbide and semi-insulating silicon carbide. Conductive silicon carbide wafers are primarily used in the manufacture of high-temperature, high-voltage, and electrical-resistant power devices, with main applications including new energy vehicles, rail transportation, photovoltaic power generation, aerospace, and smart grids.

[0003] Silicon carbide (C), a third-generation semiconductor material, has superior physical, chemical, and electrical properties compared to first-generation semiconductor materials (Si) and second-generation semiconductor materials (GaAs, InP, etc.). Silicon carbide possesses excellent physical properties such as high hardness (Mohs hardness 9.5, second only to diamond's 10), high strength (high bending and tensile strength, reaching 300-500 MPa; high crack resistance, reaching 200-1000 MPa), high modulus (up to 400-500 GPa), high thermal conductivity (4.9 W / cm·K), and high temperature resistance (melting point up to 2730℃); excellent chemical properties such as high corrosion resistance (resistant to strong acid corrosion, strong alkali corrosion, and hydrothermal corrosion) and high chemical stability; and excellent electrical properties such as high bandgap (3.3 eV), high breakdown electric field (3.0 MV / cm), low electron mobility (<400 cm2·V-1), low hole mobility (<90 cm2·V-1), and high saturation migration velocity (2.0 × 10 7 cm / s). In addition, silicon carbide is in high demand in the manufacture of radio frequency devices and power devices due to its advantages such as lower impedance, which can reduce product size and improve conversion efficiency; higher frequency, which can reduce energy loss; and ability to withstand more extreme working environments. It is widely used in electric vehicle industry, new energy, 5G communication, defense and other fields.

[0004] Silicon carbide has various crystal forms, such as 3C, 4H, 6H, 9R, and 15R, where (C), (H), and (R) represent cubic, hexagonal, and rhombus, respectively. However, different electronic potentials in different silicon carbide crystal forms lead to different electronic band structures, resulting in significant differences in their optical and electrical properties. Compared to crystalline silicon carbide such as 3C-SiC and 6H-SiC, 4H-SiC has a higher bandgap and a higher breakdown electric field, making it suitable for manufacturing high-frequency, high-temperature, and high-power devices. Due to the rise of new energy vehicles, the demand for conductive silicon carbide wafers is increasing. As a crucial step in the silicon carbide device manufacturing process, the surface roughness and other key parameters of the silicon carbide substrate directly affect the performance of the device. Therefore, the polishing process must meet the requirements of subsequent processes regarding surface roughness. Chemical mechanical polishing (CMP), as a comprehensive planarization technique, has been widely used in the polishing process of silicon carbide substrates.

[0005] Because silicon carbide has a high Mohs hardness, silicon dioxide and cerium dioxide, which are commonly used as polishing abrasives for silicon carbide, have a Mohs hardness of only 7. They can still be used to produce qualified 4H-SiC substrates in chemical mechanical polishing (CMP), but the low MRR means that the CMP step requires extremely high time costs, which is increasingly unable to meet practical requirements.

[0006] Patent publication number CN111073520A discloses a polishing powder for polishing silicon carbide wafers, its preparation method, and a polishing slurry. The preparation method of the polishing powder for silicon carbide wafers includes the following steps: S1, preparing liquid A, a micron-sized α-alumina dispersion, and liquid B, an aqueous solution of potassium hydroxide containing sodium aluminate; S2, at a certain reaction temperature, adding liquid B and hydrochloric acid to liquid A, and continuing stirring for a period of time after adding liquid B and hydrochloric acid to obtain a slurry of aluminum hydroxide coated with micron-sized α-alumina, followed by aging treatment; S3, adding nano-α-alumina powder to the aged slurry while stirring, and continuing stirring for a period of time after adding nano-α-alumina powder to obtain a slurry of nano-α-alumina coated with aluminum hydroxide coated with micron-sized α-alumina; S4, separating and drying the above slurry, then calcining at high temperature, and finally undergoing post-treatment to obtain polishing powder. This process is complex, has high preparation costs, and a short service life. Summary of the Invention

[0007] The purpose of this invention is to overcome the defects of the prior art by providing a nano-alumina polishing slurry for silicon carbide substrates, its preparation method, and its application. Graded α-Al2O3 is selected as the abrasive for silicon carbide polishing, and potassium permanganate, which has stronger oxidizing properties, is selected as the oxidant. The prepared nano-alumina polishing slurry is used in the chemical mechanical polishing of silicon carbide substrates, thereby increasing the removal rate of silicon surface material from the silicon carbide substrate while reducing its surface roughness, resulting in an atomically smooth surface quality.

[0008] The objective of this invention can be achieved through the following technical solutions:

[0009] One of the technical solutions of the present invention is to provide a nano-alumina polishing slurry for silicon carbide substrates, comprising 0.5wt%~6.0wt% α-Al2O3 abrasive, 0.5wt%~5.0wt% oxidant, 0.1wt%~1.0wt% co-oxidant, 0.05wt%~0.5wt% pH adjuster, 0.10wt%~0.50wt% dispersant, and the balance being water;

[0010] The α-Al2O3 abrasive has a diameter of 0.100~0.140μm for D10, 0.180~0.230μm for D50, 0.240~0.300μm for D90, 0.340~0.420μm for D100, and a radial distance of 0.6~0.9.

[0011] Furthermore, the preparation method of the α-Al2O3 abrasive is as follows: α-Al2O3 powder is prepared into α-Al2O3 slurry, potassium citrate is added and then the slurry is ground, and the ground α-Al2O3 slurry is homogenized to obtain α-Al2O3 abrasive.

[0012] Furthermore, the solid content of the α-Al2O3 slurry is 20%~50%;

[0013] The mass of potassium citrate is 1% to 5% of the mass of the α-Al2O3 powder.

[0014] Furthermore, the purity of the α-Al2O3 powder is greater than 99.99%.

[0015] Furthermore, the average particle size of α-Al2O3 in the milled α-Al2O3 slurry is 200~300nm.

[0016] Furthermore, the specific parameters of the abrasive grinding are as follows: zirconia bead size 0.2-0.1-0.6 mm, filling rate 70%-85%, rotation speed 1000-3500 rpm, linear velocity 5-10 m / s, flow rate 80-120 L / h, and time 80-150 min.

[0017] Further, the specific process of the homogenization treatment is as follows: the α-Al2O3 slurry after grinding is centrifuged at 1000~3000rpm for 1~10min, and after centrifugation, an upper slurry, a middle slurry and a lower slurry are obtained. The middle slurry is filtered, and the filtration product is the α-Al2O3 abrasive.

[0018] The average particle size of the upper slurry is less than 100 nm, the average particle size of the middle slurry is 100~250 nm, and the average particle size of the lower slurry is greater than 250 nm.

[0019] The filtration includes primary filtration and secondary filtration. The primary filtration uses a filter bag, and the secondary filtration uses a microporous filter cartridge.

[0020] Furthermore, the middle layer slurry is filtered, and the filtration product is the α-Al2O3 abrasive. The retained filter residue is discarded. The middle layer slurry used is the main part of the α-Al2O3 slurry. The particles in the upper layer slurry have a particle size of less than 100 nm, accounting for only a very small part of the α-Al2O3 slurry, less than 10%. Although it can be used for chemical mechanical polishing experiments on silicon carbide substrates, the utilization rate of α-Al2O3 slurry is low, resulting in serious waste, and the material removal rate after polishing is low, making it difficult to meet practical needs.

[0021] Furthermore, the upper slurry can be recycled and used for other fine processing of the silicon carbide substrate, such as fine polishing. Fine polishing refers to a chemical mechanical polishing method that requires higher surface quality for silicon carbide substrates. If the silicon surface of the silicon carbide substrate is chemically mechanically polished using the upper slurry, its surface quality is even higher, with a surface roughness of approximately 0.02 nm.

[0022] Furthermore, the lower slurry can be recycled for further operations, such as sand milling experiments.

[0023] Furthermore, the filter bag is made of non-woven fabric with a pore size of 0.5~1.0μm.

[0024] Furthermore, the microporous filter element is made of polypropylene with a pore size of 0.5~1.0μm.

[0025] Furthermore, the oxidant includes potassium permanganate, sodium permanganate, sodium persulfate, hydrogen peroxide, and ammonium hypochlorite, preferably potassium permanganate. Potassium permanganate, as a strong oxidant, can oxidize and corrode the surface of a silicon carbide substrate with high hardness (Mohs hardness of 9) to form a soft silicon dioxide layer with lower hardness (Mohs hardness of 6), thereby reducing the processing difficulty of the silicon carbide substrate surface.

[0026] The pH adjuster includes nitric acid, potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium tetraborate.

[0027] The dispersant includes sodium hexametaphosphate, sodium silicate, potassium citrate, sodium polyacrylate, sodium maleate-acrylic acid copolymer, and polyethylene glycol.

[0028] The co-oxidant includes calcium nitrate, which, in combination with the oxidant, helps to improve the service life of the nano-alumina polishing slurry.

[0029] The second technical solution of the present invention is to provide a method for preparing a nano-alumina polishing slurry for silicon carbide substrates, wherein α-Al2O3 abrasive, oxidant, pH adjuster, dispersant and water are stirred in a mass percentage and mixed evenly to obtain a nano-alumina polishing slurry.

[0030] The third technical solution of the present invention is to provide an application of a nano-alumina polishing slurry for silicon carbide substrates, wherein the nano-alumina polishing slurry is used to perform chemical mechanical polishing on silicon carbide substrates.

[0031] Furthermore, the specific parameters of the chemical mechanical polishing are as follows: polishing pressure of 10~15psi, lower plate rotation speed of 20~60rpm, polishing fluid flow rate of 0.5~1.5L / min, and polishing time of 0.5~5h.

[0032] The fourth technical solution of the present invention is to provide a silicon carbide substrate, which is obtained by chemical mechanical polishing using the aforementioned nano-alumina polishing liquid. After the chemical mechanical polishing is completed, the surface is cleaned to remove residual alumina particles, thereby obtaining the silicon carbide substrate.

[0033] Furthermore, the material removal rate (MRR) of the silicon carbide substrate is 1.000~1.500 μm / h, and the surface roughness (Ra) is 0.065~0.075 nm.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] (1) In this invention, α-Al2O3 powder is selected, and after being made into an alumina slurry, it is milled. The milled α-Al2O3 slurry is then homogenized to obtain α-Al2O3 abrasive with a narrower and more uniform particle size distribution. Using the homogenized α-Al2O3 slurry as the abrasive, oxidants, pH adjusters, dispersants, and other additives are added to prepare a nano-alumina polishing slurry for chemical mechanical polishing of silicon carbide substrates. Silicon carbide substrates with high MRR and low Ra can be obtained, with an MRR of 1.000~1.500 μm / h and an Ra of 0.065~0.075 nm.

[0036] (2) Usually, the alumina particles after preparation or sand milling are not of a single size, but rather have a wide particle size distribution. This invention homogenizes the α-Al2O3 particles to separate alumina particles with an average particle size of less than 100 nm and an average particle size of greater than 250 nm, leaving an α-Al2O3 slurry with a narrow particle size distribution and relatively uniform particle size. Using this slurry in the chemical mechanical polishing experiment of silicon carbide substrate can effectively avoid scratches caused by large particles on the silicon carbide substrate. Detailed Implementation

[0037] The present invention will now be described in detail with reference to specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments. All other embodiments obtained by those skilled in the art based on the given embodiments without creative effort are within the scope of protection of this application.

[0038] Unless otherwise specified, the reagents, methods, instruments and equipment used in this invention are conventional reagents, methods, instruments and equipment in the art.

[0039] In the following embodiments, the particle size distribution was tested using a Bettersize2600 laser particle size analyzer (model BT-802) provided by Bettersize Electronics Technology (Shanghai) Co., Ltd.

[0040] The surface morphology of alumina particles was tested using a scanning electron microscope (ZEISG Gemini SEM 300) provided by Carl Zeiss (Shanghai) Management Co., Ltd.

[0041] Surface roughness was tested using an atomic force microscope (DIMENSION EDGESystem) provided by Shanghai Shuoyi Testing Technology Co., Ltd.

[0042] Chemical mechanical polishing experiments were conducted on the polishing slurry prepared in this invention using a single-sided polishing machine (model ES32B) from Zhejiang Senyong Optoelectronic Equipment Co., Ltd. The polishing machine parameters were set as follows: polishing pressure 10.873 psi, lower plate rotation speed 40 rpm, polishing slurry flow rate 1 L / min, and polishing time 1 h. The polishing pad was a non-woven polishing pad (model NWS-85) from Quzhou Bolainarun Electronic Materials Co., Ltd. The silicon carbide substrate was a 6-inch 4H-SiC substrate from Shanxi Shuoke Crystal Co., Ltd.

[0043] A nano-alumina polishing slurry for silicon carbide substrates comprises 0.5wt%~6.0wt% α-Al2O3 abrasive, 0.5wt%~5.0wt% oxidant, 0.1wt%~1.0wt% co-oxidant, 0.05wt%~0.5wt% pH adjuster, 0.10wt%~0.50wt% dispersant, and the balance being water;

[0044] The α-Al2O3 abrasive has a diameter of 0.100~0.140μm for D10, 0.180~0.230μm for D50, 0.240~0.300μm for D90, 0.340~0.420μm for D100, and a radial distance of 0.6~0.9.

[0045] As one of the preferred embodiments, the preparation method of the α-Al2O3 abrasive is as follows: α-Al2O3 powder is prepared into α-Al2O3 slurry, potassium citrate is added and then the slurry is ground, and the ground α-Al2O3 slurry is homogenized to obtain α-Al2O3 abrasive.

[0046] As one of the preferred embodiments, the solid content of the α-Al2O3 slurry is 20%~50%;

[0047] The mass of potassium citrate is 1% to 5% of the mass of the α-Al2O3 powder.

[0048] As one of the preferred embodiments, the purity of the α-Al2O3 powder is greater than 99.99%.

[0049] As one of the preferred embodiments, the average particle size of α-Al2O3 in the milled α-Al2O3 slurry is 200~300nm.

[0050] As one of the preferred embodiments, the specific parameters of the abrasive grinding are: zirconia bead size 0.2-0.1-0.6 mm, filling rate 70%-85%, rotation speed 1000-3500 rpm, linear speed 5-10 m / s, flow rate 80-120 L / h, and time 80-150 min.

[0051] As one of the preferred embodiments, the specific process of the homogenization treatment is as follows: the α-Al2O3 slurry after grinding is centrifuged at 1000~3000rpm for 1~10min, and after centrifugation, an upper slurry, a middle slurry and a lower slurry are obtained. The middle slurry is filtered, and the filtration product is the α-Al2O3 abrasive.

[0052] The average particle size of the upper slurry is less than 100 nm, the average particle size of the middle slurry is 100~250 nm, and the average particle size of the lower slurry is greater than 250 nm.

[0053] The filtration includes primary filtration and secondary filtration. The primary filtration uses a filter bag, and the secondary filtration uses a microporous filter cartridge.

[0054] In one preferred embodiment, the middle layer slurry is filtered, and the filtration product is the α-Al2O3 abrasive. The retained filter residue is discarded. The middle layer slurry used is the main part of the α-Al2O3 slurry. The particles in the upper layer slurry have a particle size of less than 100 nm, accounting for only a very small part of the α-Al2O3 slurry, less than 10%. Although it can be used for chemical mechanical polishing experiments on silicon carbide substrates, the utilization rate of α-Al2O3 slurry is low, resulting in serious waste, and the material removal rate after polishing is low, which is difficult to meet practical needs.

[0055] As one preferred embodiment, the upper slurry is recyclable and can be used for other fine processing of the silicon carbide substrate, such as fine polishing, thus avoiding waste of raw materials. Fine polishing refers to a chemical mechanical polishing method that requires higher surface quality for silicon carbide substrates. If the silicon surface of the silicon carbide substrate is chemically mechanically polished using the upper slurry, its surface quality is higher, with a surface roughness of approximately 0.02 nm.

[0056] As one of the preferred embodiments, the lower slurry can be recycled for further operations, thereby avoiding waste of raw materials, such as in sand milling experiments.

[0057] In one preferred embodiment, the filter bag is made of non-woven fabric with a pore size of 0.5~1.0μm.

[0058] In one preferred embodiment, the microporous filter element is made of polypropylene with a pore size of 0.5~1.0μm.

[0059] As one of the preferred embodiments, the oxidant includes potassium permanganate, sodium permanganate, sodium persulfate, hydrogen peroxide, and ammonium hypochlorite, preferably potassium permanganate. Potassium permanganate, as a strong oxidant, can oxidize and corrode the surface of a silicon carbide substrate with high hardness (Mohs hardness of 9) to form a soft silicon dioxide layer with lower hardness (Mohs hardness of 6), thereby reducing the processing difficulty of the silicon carbide substrate surface.

[0060] The pH adjuster includes nitric acid, potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium tetraborate.

[0061] The dispersant includes sodium hexametaphosphate, sodium silicate, potassium citrate, sodium polyacrylate, sodium maleate-acrylic acid copolymer, and polyethylene glycol.

[0062] The co-oxidant includes calcium nitrate, which, in combination with the oxidant, helps to improve the service life of the nano-alumina polishing slurry.

[0063] A method for preparing nano-alumina polishing slurry for silicon carbide substrates involves stirring α-Al2O3 abrasive, oxidant, pH adjuster, dispersant, and water according to a mass percentage, and mixing them evenly to obtain the nano-alumina polishing slurry.

[0064] An application of a nano-alumina polishing slurry for silicon carbide substrates, wherein the nano-alumina polishing slurry is used to perform chemical mechanical polishing on silicon carbide substrates.

[0065] As one of the preferred embodiments, the specific parameters of the chemical mechanical polishing are: polishing pressure of 10~15psi, lower plate rotation speed of 20~60rpm, polishing fluid flow rate of 0.5~1.5L / min, and polishing time of 0.5~5h.

[0066] A silicon carbide substrate is obtained by chemical mechanical polishing with the aforementioned nano-alumina polishing slurry. After chemical mechanical polishing, the surface is cleaned to remove residual alumina particles, thus obtaining the silicon carbide substrate.

[0067] As one of the preferred embodiments, the material removal rate (MRR) of the silicon carbide substrate is 1.000~1.500 μm / h, and the surface roughness (Ra) is 0.065~0.075 nm.

[0068] Example 1

[0069] This embodiment provides a nano-alumina polishing slurry for silicon carbide substrates, its preparation method, and its application. The preparation method includes the following steps:

[0070] (1) Preparation of α-Al2O3 abrasive

[0071] High-purity α-Al₂O₃ powder was prepared into an α-Al₂O₃ slurry with a solid content of 30 wt%. Potassium citrate (2% by weight of the α-Al₂O₃ powder) was added to the α-Al₂O₃ slurry, and the mixture was stirred for 30 min. Then, it was sand-milled using the following parameters: zirconia bead size 0.2 mm, filling rate 75 wt%, rotation speed 3000 rpm, linear velocity 10.26 m / s, flow rate 120 L / h, and time 80 min. The sand mill was an SMP 0.5 model manufactured by Husong Intelligent Equipment (Taicang) Co., Ltd. The α-Al₂O₃ powder was sourced from 3000-mesh high-purity alumina powder provided by Yangzhou Dilan Chemical Materials Co., Ltd.

[0072] The α-Al₂O₃ slurry after sand milling was homogenized as follows: ① Eight 50mL centrifuge tubes were filled with the α-Al₂O₃ slurry after sand milling, and centrifuged in a benchtop centrifuge with the following parameters: 1500rpm for 5min. After centrifugation, the upper, middle, and lower layers of slurry were separated. The average particle size of the upper layer slurry was less than 100nm, the average particle size of the middle layer slurry was 100~250nm, and the average particle size of the lower layer slurry was greater than 250nm. The middle layer slurry was collected. ② The middle layer slurry after centrifugation and separation was filtered twice using a filter bag and then twice using a microporous filter cartridge. The filtered product was the α-Al₂O₃ abrasive. The benchtop centrifuge is model TDL-40B, manufactured by Shanghai Anting Scientific Instruments Co., Ltd.; the filter bag is model No. 1, 180*430, 0.5 microns, manufactured by Shanghai Fuxin Environmental Protection Technology Co., Ltd.; the microporous filter element is model K68B-2H-M10*10´´(1μm), manufactured by Hangzhou Kesma Membrane Technology Co., Ltd.

[0073] (2) Preparation of nano-alumina polishing slurry

[0074] Weigh 3760g of deionized water, and while stirring, add 60g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 150g of α-Al₂O₃ abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and dissolve completely. Adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain nano-alumina polishing slurry (potassium permanganate content 1.5wt%, α-Al₂O₃ abrasive content 0.75wt%, aluminum nitrate nonahydrate content 0.25wt%, calcium nitrate tetrahydrate content 0.25wt%, sodium silicate content 0.25wt%).

[0075] (3) Perform chemical mechanical polishing on silicon carbide substrates.

[0076] Chemical mechanical polishing (CMP) was performed on the Si surface of a silicon carbide substrate using 4 kg of nano-alumina polishing slurry. The polishing pressure was 10.873 psi, the grinding wheel speed was 40 rpm, the slurry flow rate was 1 L / min, and the polishing time was 1 hour. The polishing pad was the NWS-85 non-woven polishing pad from Quzhou Bolainarun Electronic Materials Co., Ltd. The silicon carbide substrate was a 6-inch 4H-SiC substrate from Shanxi Shuoke Crystal Co., Ltd.

[0077] 4 kg of nano-alumina polishing slurry was recycled and used continuously for 4 × 1 h. After each hour of polishing, the mass of the silicon carbide substrate was weighed, and the MRR values ​​were 1.121 μm / h, 1.040 μm / h, 1.029 μm / h, and 1.011 μm / h, respectively. The surface roughness Ra was measured to be 0.068 nm using atomic force microscopy.

[0078] The formula for calculating MRR is as follows:

[0079]

[0080] Where Δm is the mass difference of the silicon carbide substrate before and after polishing (g), ρ is the density of the silicon carbide substrate (3.2g / cm3), s is the area of ​​the Si surface of the silicon carbide substrate (Π×7.52, cm2), and t is the polishing time (h).

[0081] Example 2

[0082] The majority of the contents are the same as in Example 1, except for the preparation of the nano-alumina polishing slurry in step (2):

[0083] Weigh 3730g of deionized water, and while stirring, add 40g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 200g of the homogenized α-Al₂O₃ abrasive (20% solid content) and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain a nano-alumina polishing slurry (containing 1.0wt% potassium permanganate, 1wt% α-Al₂O₃ abrasive, 0.25wt% aluminum nitrate nonahydrate, 0.25wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate).

[0084] 4 kg of polishing slurry was recycled and polished continuously for 4 × 1 h. After polishing at the end of each hour, the mass of the substrate was weighed, and the MRRs were 1.618 μm / h, 1.487 μm / h, 1.335 μm / h, and 1.188 μm / h, respectively. The surface roughness Ra was measured to be 0.070 using atomic force microscopy.

[0085] Example 3

[0086] The majority of the contents are the same as in Example 1, except for the preparation of the nano-alumina polishing slurry in step (2):

[0087] Weigh 3490g of deionized water, and while stirring, add 80g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 400g of the homogenized α-Al2O3 abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain a nano-alumina polishing slurry (containing 2.0wt% potassium permanganate, 2.0wt% α-Al2O3 abrasive, 0.25wt% aluminum nitrate nonahydrate, 0.25wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate).

[0088] 4 kg of polishing slurry was recycled and polished continuously for 4 × 1 h. After polishing at the end of each hour, the mass of the substrate was weighed, and the MRRs were 1.699 μm / h, 1.542 μm / h, 1.406 μm / h, and 1.204 μm / h, respectively. The surface roughness Ra was measured to be 0.072 using atomic force microscopy.

[0089] Example 4

[0090] The majority of the contents are the same as in Example 1, except for the preparation of the nano-alumina polishing slurry in step (2):

[0091] Weigh 3250g of deionized water, and while stirring, add 120g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 600g of the homogenized α-Al2O3 abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain a nano-alumina polishing slurry (containing 3.0wt% potassium permanganate, 3.0wt% α-Al2O3 abrasive, 0.25wt% aluminum nitrate nonahydrate, 0.25wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate).

[0092] 4 kg of polishing slurry was recycled and polished continuously for 4 × 1 h. After polishing at the end of each hour, the mass of the substrate was weighed, and the MRRs were 1.524 μm / h, 1.423 μm / h, 1.231 μm / h, and 1.204 μm / h, respectively. The surface roughness Ra measured by atomic force microscopy was 0.073 nm.

[0093] Example 5

[0094] The majority of the contents are the same as in Example 1, except for the preparation of the nano-alumina polishing slurry in step (2):

[0095] Weigh 3760g of deionized water, and while stirring, add 160g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 800g of the homogenized α-Al2O3 abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 8 using 20wt% nitric acid. Stir evenly to obtain a nano-alumina polishing slurry (containing 4.0wt% potassium permanganate, 4.0wt% α-Al2O3 abrasive, 0.25wt% aluminum nitrate nonahydrate, 0.25wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate).

[0096] The polishing slurry was recycled in cycles, and polishing was performed continuously for 4 × 1 h. After each hour of polishing, the substrate mass was measured, and the MRR values ​​were 1.134 μm / h, 1.204 μm / h, 1.145 μm / h, and 1.201 μm / h, respectively. The surface roughness Ra was measured to be 0.069 nm using atomic force microscopy.

[0097] Example 6

[0098] The majority of the contents are the same as in Example 1, except for the preparation of the nano-alumina polishing slurry in step (2):

[0099] Weigh 2770g of deionized water, and while stirring, add 200g of potassium permanganate and 20g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 1000g of the homogenized α-Al2O3 abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain a nano-alumina polishing slurry (containing 5.0wt% potassium permanganate, 5.0wt% α-Al2O3 abrasive, 0.5wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate).

[0100] The polishing slurry was recycled in 4 kg cycles, and polishing was performed continuously for 4 × 1 h. After each hour of polishing, the substrate mass was measured, and the MRR values ​​were 1.156 μm / h, 1.211 μm / h, 1.102 μm / h, and 1.123 μm / h, respectively. The surface roughness Ra was measured to be 0.067 nm using atomic force microscopy.

[0101] Example 7

[0102] The majority of the contents are the same as in Example 1, except for the preparation of the nano-alumina polishing slurry in step (2):

[0103] Weigh 2710g of deionized water, and while stirring, add 60g of potassium permanganate and 20g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 1200g of the homogenized α-Al2O3 abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain nano-alumina polishing slurry (wherein the content of potassium permanganate is 1.5wt%, the content of α-Al2O3 abrasive is 6.0wt%, the content of calcium nitrate tetrahydrate is 0.5wt%, and the content of sodium silicate is 0.25wt%).

[0104] The polishing slurry was recycled in 4 kg cycles, and polishing was performed continuously for 4 × 1 h. After each hour of polishing, the substrate mass was measured, and the MRR values ​​were 1.312 μm / h, 1.256 μm / h, 1.234 μm / h, and 1.179 μm / h, respectively. The surface roughness Ra was measured to be 0.066 nm using atomic force microscopy.

[0105] Comparative Example 1

[0106] This comparative example provides a nano-silica sol polishing slurry for silicon carbide substrates, its preparation method, and its application. The preparation process includes the following steps:

[0107] Weigh 3796.66g of deionized water, and while stirring, add 40g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 133.34g of silica sol (30wt% solid content) and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate, and after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain a nano-silica sol polishing solution (containing 1.0wt% potassium permanganate, 1.0wt% silica, 0.25wt% aluminum nitrate nonahydrate, 0.25wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate). The silica sol model is WY-145, and the manufacturer is Quzhou Bolainarun Electronic Materials Co., Ltd.

[0108] Chemical mechanical polishing (CMP) was performed on the Si surface of a silicon carbide substrate using 4 kg of nano-silica sol polishing slurry. The polishing pressure was 10.873 psi, the grinding wheel speed was 40 rpm, the slurry flow rate was 1 L / min, and the polishing time was 1 hour. The polishing pad was the NWS-85 non-woven polishing pad from Quzhou Bolainarun Electronic Materials Co., Ltd. The silicon carbide substrate was a 6-inch 4H-SiC substrate from Shanxi Shuoke Crystal Co., Ltd.

[0109] 4 kg of nano-silica sol polishing slurry was recycled and used continuously for 4 × 1 h. After each hour of polishing, the mass of the silicon carbide substrate was weighed, and the MRR values ​​were 0.805 μm / h, 0.786 μm / h, 0.765 μm / h, and 0.742 μm / h, respectively. The surface roughness Ra was measured to be 0.126 nm using atomic force microscopy.

[0110] Comparative Example 2

[0111] This comparative example provides a nano-alumina polishing slurry for silicon carbide substrates, its preparation method, and its application. The preparation process includes the following steps:

[0112] (1) Preparation of α-Al2O3 abrasive

[0113] High-purity α-Al₂O₃ powder was prepared into an α-Al₂O₃ slurry with a solid content of 30%. Potassium citrate (2% by weight of the α-Al₂O₃ powder) was added to the α-Al₂O₃ slurry and stirred for 30 minutes. The mixture was then milled using a sand mill to obtain α-Al₂O₃ abrasive. No homogenization treatment was performed on the α-Al₂O₃ abrasive. The milling parameters were: zirconia bead size 0.2 mm, filling rate 75 wt%, rotation speed 3000 rpm, linear velocity 10.26 m / s, flow rate 120 L / h, and time 80 min. The sand mill model was SMP 0.5, manufactured by Husong Intelligent Equipment (Taicang) Co., Ltd.

[0114] (2) Preparation of nano-alumina polishing slurry

[0115] Weigh 3490g of deionized water, and while stirring, add 80g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 400g of heterogeneously treated α-Al2O3 abrasive and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain nano-alumina polishing slurry (polishing slurry: potassium permanganate content 2.0wt%, heterogeneously treated α-Al2O3 abrasive content 2wt%, aluminum nitrate nonahydrate content 0.25wt%, calcium nitrate tetrahydrate content 0.25wt%, sodium silicate content 0.25wt%).

[0116] Chemical mechanical polishing (CMP) was performed on the Si surface of a silicon carbide substrate using 4 kg of nano-silica sol polishing slurry. The polishing pressure was 10.873 psi, the grinding wheel speed was 40 rpm, the slurry flow rate was 1 L / min, and the polishing time was 1 hour. The polishing pad was the NWS-85 non-woven polishing pad from Quzhou Bolainarun Electronic Materials Co., Ltd. The silicon carbide substrate was a 6-inch 4H-SiC substrate from Shanxi Shuoke Crystal Co., Ltd.

[0117] The polishing slurry was recycled for 4 x 1 h, and the substrate mass was measured after each hour of polishing. The MRR values ​​were 1.231 μm / h, 1.112 μm / h, 1.076 μm / h, and 0.978 μm / h, respectively. The surface roughness Ra was measured to be 0.098 nm using atomic force microscopy.

[0118] Comparative Example 3:

[0119] This comparative example provides a nano-alumina polishing slurry for silicon carbide substrates, its preparation method, and its application. The preparation process includes the following steps:

[0120] (1) Preparation of α-Al2O3 abrasive

[0121] High-purity α-Al₂O₃ powder was prepared into an α-Al₂O₃ slurry with a solid content of 30 wt%. Potassium citrate (2% by weight of the α-Al₂O₃ powder) was added to the α-Al₂O₃ slurry, and the mixture was stirred for 30 min. The slurry was then milled using a sand mill to obtain the α-Al₂O₃ slurry. The α-Al₂O₃ slurry was subjected to gravity sedimentation for 3 days. A clear separation boundary was observed. The upper clear liquid was slowly collected using a disposable pipette, while the lower precipitate remained at the bottom. The middle slurry was used for subsequent experiments to obtain the α-Al₂O₃ abrasive. The milling parameters were: zirconia bead size 0.2 mm, filling rate 75 wt%, rotation speed 3000 rpm, linear velocity 10.26 m / s, flow rate 120 L / h, and time 80 min. The sand mill model was SMP 0.5, manufactured by Husong Intelligent Equipment (Taicang) Co., Ltd.

[0122] (2) Preparation of nano-alumina polishing slurry

[0123] Weigh 3760g of deionized water, and while stirring, add 60g of potassium permanganate, 10g of aluminum nitrate nonahydrate, and 10g of calcium nitrate tetrahydrate until completely dissolved. Adjust the pH of the solution to 7.5 using potassium hydroxide. Then, weigh 150g of α-Al₂O₃ abrasive that has undergone gravity sedimentation and add it to the stirring potassium permanganate solution. Add 10g of sodium silicate and, after complete dissolution, adjust the pH of the solution to 9 using 20wt% nitric acid. Stir evenly to obtain a nano-alumina polishing slurry (containing 2.0wt% potassium permanganate, 2wt% alumina, 0.25wt% aluminum nitrate nonahydrate, 0.25wt% calcium nitrate tetrahydrate, and 0.25wt% sodium silicate).

[0124] Chemical mechanical polishing (CMP) was performed on the Si surface of a silicon carbide substrate using 4 kg of nano-silica sol polishing slurry. The polishing pressure was 10.873 psi, the grinding wheel speed was 40 rpm, the slurry flow rate was 1 L / min, and the polishing time was 1 hour. The polishing pad was the NWS-85 non-woven polishing pad from Quzhou Bolainarun Electronic Materials Co., Ltd. The silicon carbide substrate was a 6-inch 4H-SiC substrate from Shanxi Shuoke Crystal Co., Ltd.

[0125] 4 kg of polishing slurry was recycled and polished continuously for 4 × 1 h. After polishing at the end of each hour, the mass of the substrate was weighed. The MRR values ​​were 1.078 μm / h, 1.112 μm / h, 1.026 μm / h, and 0.953 μm / h, respectively. The surface roughness Ra measured using atomic force microscopy was 0.086 nm.

[0126] Table 1 summarizes the removal rate and surface roughness data of silicon carbide substrates obtained by chemical mechanical polishing of the materials in each embodiment and comparative example.

[0127] Table 1. Comparison of material removal rate and surface roughness in each embodiment and comparative example.

[0128]

[0129] According to Table 1, through the comparison of the above comparative examples and embodiments, it can be found that using graded α-Al₂O₃ abrasive for chemical mechanical polishing of silicon carbide substrates can improve the material removal rate while reducing the surface roughness of the silicon carbide surface, achieving a smooth surface and improving surface quality. The polishing abrasives used in Examples 1 to 6 were all graded alumina abrasive slurries, and the Ra values ​​were all less than 0.1 nm, as shown by atomic force microscopy. Compared with Comparative Example 1, when the polishing abrasives were replaced with alumina instead of silicon oxide, the material removal rate of the silicon carbide substrate was significantly improved; compared with Comparative Example 2, when the polishing abrasives were replaced with graded α-Al₂O₃ abrasive instead of ungraded α-Al₂O₃ abrasive, the surface roughness of the silicon carbide substrate was significantly reduced.

[0130] Table 2 shows the particle size distribution of alumina slurry before sand milling, after sand milling, and after gravity sedimentation and centrifugal sedimentation of the sand-milled slurry. The particle size distribution is given by distance = (D90 - D10) / D50, representing the particle size range and the spacing or difference between different particle sizes in the sample. D50 means that particles larger than this value account for 50% of the total particle volume, and particles smaller than this value also account for 50%. D10 means that particles larger than this value account for 90% of the total particle volume, and particles smaller than this value account for 10%. D90 means that particles larger than this value account for 10% of the total particle volume, and particles smaller than this value account for 90%.

[0131] Table 2 Comparison of particle size distribution of alumina slurry before and after sand milling and after different grading methods

[0132]

[0133] The main difference between Example 1 and Comparative Example 3 is the different methods of sedimentation and classification of the alumina slurry after sand milling. Example 1 was subjected to centrifugal sedimentation treatment, while Comparative Example 3 was subjected to gravity sedimentation treatment. As shown in Table 2, the alumina slurry with a smaller diameter after centrifugal sedimentation treatment was 0.6, while that after gravity sedimentation treatment was 1.31. The alumina particles with smaller diameters are used as abrasives for chemical mechanical polishing of the silicon surface of silicon carbide substrates. This results in a smaller surface roughness of the silicon surface, indicating that the surface is smoother and has higher surface quality.

[0134] Both centrifugal sedimentation and gravity sedimentation apply force to separate particles of different sizes. The larger the particle size, the faster the sedimentation speed. Centrifugal force is stronger than gravity force and can achieve gradient separation of different particle sizes in a short time. The main purpose of sedimentation separation is to remove larger particles in the D90-D100 range, because the presence of these particles will cause scratches of varying degrees on the surface of silicon carbide substrates. After centrifugal separation, the D100 of alumina particles is smaller than that after gravity separation. Therefore, after chemical mechanical polishing of silicon carbide, its surface roughness value is lower and the surface quality is better.

[0135] The polishing slurry prepared by this invention can meet the polishing requirements of precision workpieces in some industries, and the adjustment of the dosage of these components is also within the scope of protection of this invention.

[0136] Although the present invention has been described in detail above with general descriptions, specific embodiments, and experiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing a nano-alumina polishing slurry for silicon carbide substrates, characterized in that, The α-Al2O3 abrasive, oxidant, co-oxidant, pH adjuster, dispersant, and water are stirred according to the mass percentage and mixed evenly to obtain a nano-alumina polishing slurry. The nano-alumina polishing slurry comprises 0.5wt%~6.0wt% α-Al2O3 abrasive, 0.5wt%~5.0wt% oxidant, 0.1wt%~1.0wt% co-oxidant, 0.05wt%~0.5wt% pH adjuster, 0.10wt%~0.50wt% dispersant, and the balance being water; The α-Al2O3 abrasive has a diameter of 0.100~0.140μm, a diameter of 0.180~0.230μm, a diameter of 0.240~0.300μm, a diameter of 0.340~0.420μm, and a diameter-to-diameter ratio of 0.6~0.

9. The oxidizing agents include potassium permanganate, sodium permanganate, sodium persulfate, hydrogen peroxide, and ammonium hypochlorite; The pH adjuster includes nitric acid, potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium tetraborate. The dispersant includes sodium hexametaphosphate, sodium silicate, potassium citrate, sodium polyacrylate, sodium maleate-acrylic acid copolymer, and polyethylene glycol; The oxidizing agent includes calcium nitrate; The preparation method of the α-Al2O3 abrasive is as follows: α-Al2O3 powder is prepared into α-Al2O3 slurry, potassium citrate is added and then the slurry is ground, and the ground α-Al2O3 slurry is homogenized to obtain α-Al2O3 abrasive. The specific parameters for the abrasive grinding are as follows: zirconia bead size 0.1~0.6mm, filling rate 70%~85%, rotation speed 1000~3500rpm, linear velocity 5~10m / s, flow rate 80~120L / h, and time 80~150min; The specific process of the homogenization treatment is as follows: the α-Al2O3 slurry after grinding is centrifuged at 1000~3000rpm for 1~10min. After centrifugation, an upper slurry, a middle slurry, and a lower slurry are obtained. The middle slurry is filtered, and the filtration product is the α-Al2O3 abrasive. The average particle size of the upper slurry is less than 100nm, the average particle size of the middle slurry is 100~250nm, and the average particle size of the lower slurry is greater than 250nm. The filtration includes primary filtration and secondary filtration. The primary filtration is filtration using a filter bag, and the secondary filtration is filtration using a microporous filter cartridge.

2. The method for preparing a nano-alumina polishing slurry for silicon carbide substrates according to claim 1, characterized in that, The solid content of the α-Al2O3 slurry is 20%~50%; The mass of potassium citrate is 1% to 5% of the mass of the α-Al2O3 powder.

3. An application of a nano-alumina polishing slurry for silicon carbide substrates, characterized in that, The nano-alumina polishing slurry prepared by the preparation method described in claim 1 or 2 is used to perform chemical mechanical polishing on a silicon carbide substrate.

4. The application of the nano-alumina polishing slurry for silicon carbide substrates according to claim 3, characterized in that, The specific parameters for the chemical mechanical polishing are as follows: polishing pressure of 10~15psi, lower plate rotation speed of 20~60rpm, polishing fluid flow rate of 0.5~1.5L / min, and polishing time of 0.5~5h.

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