Polishing method of polycrystalline silicon carbide substrate

Abstract

The invention relates to a polishing method for a polycrystalline silicon carbide substrate, and belongs to the field of polishing of polycrystalline silicon carbide substrates. The method comprises the following steps: (1) roughly polishing a silicon carbide wafer until Ra is not greater than 0.3 nm by adopting a polyurethane polishing pad and an oxidant-free nano-diamond polishing solution; the Shore D hardness of the polyurethane polishing pad is 70 to 75; and (2) a damping cloth polishing pad is adopted, and pure silica sol without an oxidizing agent serves as a polishing solution to conduct fine polishing till Ra is not larger than 0.2 nm. According to the method, the polyurethane high-hardness polishing pad is matched with the nano-diamond polishing solution to mechanically polish the polycrystalline silicon carbide, so that orange peel defects on the surface of the polycrystalline silicon carbide are avoided, and the surface roughness is reduced; furthermore, a damping cloth polishing pad is matched with an oxidant-free nano silica sol polishing solution for fine polishing, so that the roughness of the surface of the wafer can be further reduced.

Description

Technical Field

[0001] The invention relates to a polishing method for a polycrystalline silicon carbide substrate, and belongs to the polishing field of polycrystalline silicon carbide substrates. Background Art

[0002] Silicon carbide substrate is a single crystal silicon carbide material formed by sublimation and redeposition process. The substrate is a wide bandgap semiconductor material with the characteristics of high thermal stability, high breakdown electric field, high electron mobility and high mechanical strength. The high thermal stability is mainly manifested in the high thermal conductivity and small thermal expansion coefficient of the silicon carbide substrate. It can withstand high temperature environment and is suitable for high-power and high-frequency radio frequency devices. The silicon carbide substrate has high hardness and high mechanical strength. It can withstand high-voltage environment and high load of more than 1200 volts, so it has obvious advantages in harsh environments. However, the growth technology of SiC crystals is difficult and costly, which makes the final cost of SiC wafers high, limiting the large-scale application of silicon carbide substrates. Therefore, reducing the cost of silicon carbide substrates is of great significance to the development of the entire industry.

[0003] Research has found that the thickness of the silicon carbide single crystal that works in the silicon carbide substrate is in the range of 0.5 to 100 μm, and the remaining thickness mainly plays a role in mechanical support. Therefore, in order to solve the problem of high cost of single-crystal silicon carbide substrates, polycrystalline silicon carbide substrates and single-crystal silicon carbide thin layers are bonded to form a composite structure substrate. As the supporting layer of the composite structure substrate, the polycrystalline silicon carbide substrate has lower resistivity and lower cost. However, before bonding, the surface roughness of the polycrystalline silicon carbide substrate needs to be processed to Ra < 0.2nm.

[0004] In the prior art, chemical mechanical polishing is generally used for processing polycrystalline silicon carbide substrates, and strong oxidants such as potassium permanganate, potassium nitrate, hydrogen peroxide, sodium hypochlorite, and strong alkali are used as oxidants. However, polycrystalline silicon carbide itself contains countless original grains, and the structures between different grains and grain boundaries are inconsistent, and their corrosion resistance is also different. During the strong oxidative polishing process, the surface of polycrystalline silicon carbide is prone to orange peel defects due to different corrosion rates, resulting in the heterogeneity of the final polycrystalline silicon carbide surface. Summary of the invention

[0005] The purpose of the present invention is to provide a polishing method for a polycrystalline silicon carbide substrate to solve the problem of inhomogeneity of the polycrystalline silicon carbide substrate after polishing in the prior art.

[0006] In order to achieve the above purpose, the technical solution of the present invention is: A method for polishing a polycrystalline silicon carbide substrate comprises the following steps: (1) Rough polishing a silicon carbide wafer using a polyurethane polishing pad and an oxidant-free nano-diamond polishing liquid until Ra is no more than 0.3 nm; the Shore D hardness of the polyurethane polishing pad is 70-75; (2) Use damping cloth polishing pad and oxidant-free pure silica sol as polishing liquid for fine polishing until Ra is no more than 0.2nm.

[0007] The polishing method of the polycrystalline silicon carbide substrate of the present invention is an improved invention. The present invention adopts a polyurethane high-hardness polishing pad in combination with a nano-diamond polishing liquid to mechanically polish polycrystalline silicon carbide, which not only avoids orange peel defects caused by inconsistent corrosion rates on the surface of polycrystalline silicon carbide, improves the homogeneity of the wafer surface, but also makes the surface roughness of the wafer Ra < 0.3nm; further, a damping cloth polishing pad is used in combination with an oxidant-free nano-silica sol polishing liquid for fine polishing, which can further significantly reduce the roughness of the wafer surface, so that the surface roughness of the wafer Ra < 0.2nm.

[0008] Preferably, the diamond particle size in the nano-diamond polishing liquid is 80-100 nm, pH=8-9, and the diamond content in the diamond polishing liquid is 5-8%. Polishing within this particle size range can increase the contact area between the abrasive and polycrystalline silicon carbide, further improving the polishing efficiency. Nano-scale diamond can maintain good dispersibility under weak alkaline conditions.

[0009] Preferably, the particle size of silicon dioxide in the oxidant-free pure silica sol is 50-80 nm, pH=9-10.5, and the content of silicon dioxide is 39-41%.

[0010] Preferably, the rough polishing time is 1.5 to 3 hours, and the fine polishing time is 2 to 2.5 hours. Performing rough polishing and fine polishing within this time range can not only ensure the quality of the polycrystalline silicon carbide substrate, but also improve the polishing efficiency, save time cost, and further save production cost.

[0011] Preferably, the polishing pressure of the rough polishing is 200-300 g / cm 2 , the rotation speed is 100-150rpm, and the circulation flow rate of the polishing liquid is 200-250g / min. After the circulation flow rate of the polishing liquid is matched with the nano-diamond polishing liquid, the polishing effect of the polishing liquid can be better exerted.

[0012] In order to further improve the efficiency of fine polishing, preferably, the polishing pressure of the fine polishing is 100-150 g / cm 2 , the rotation speed is 100~150rpm, and the polishing liquid circulation flow rate is 200~250g / min.

[0013] Preferably, Ra of the silicon carbide wafer before rough polishing is less than 2 nm. DETAILED DESCRIPTION

[0014] The polycrystalline silicon carbide substrate of the prior art is polished by chemical mechanical polishing. The strong oxidizing property of the polishing liquid causes orange peel-like defects on the surface of the polycrystalline silicon carbide, resulting in heterogeneity problems. The present invention uses oxidant-free nanodiamond as an abrasive in the polishing liquid and polishes the polycrystalline silicon carbide with a high-hardness polyurethane polishing pad, which not only solves the problem of orange peel-like defects, but also reduces the surface roughness of the wafer to below 0.3nm, further expanding the application scenarios of polycrystalline silicon carbide.

[0015] The nano-diamond polishing liquid used for rough polishing in the following embodiments is free of oxidant, wherein the particle size of the nano-diamond in the polishing liquid is 80-100nm, the content of the nano-diamond is 5-8%, and the pH is 8-9; the polishing liquid used for fine polishing is an oxidant-free pure silica sol, wherein the particle size of the silicon dioxide particles is 50-80nm, the silicon dioxide content is 39-41%, and the pH is 9-10.5.

[0016] The nano-diamond polishing liquid and oxidant-free pure silica sol involved in the following embodiments are both commercially available products. The nano-diamond polishing liquid can be selected from the diamond polishing liquid with the item number NDS / 100 or NDS / 80 of Henan United Precision Materials Co., Ltd., or the diamond polishing liquid with the item number DND-100-W of Beijing Guoruisheng; the oxidant-free pure silica sol polishing liquid can be selected from Shanghai Yingzhi Abrasive Materials Co., Ltd. with the item number YZ8040 or YZ7540K, or SD-12040 of Huihe Yongsheng Nanotechnology.

[0017] The technical solution of the present invention is further described below in conjunction with specific implementation methods.

[0018] 1. The specific embodiment of the polishing method of the polycrystalline silicon carbide substrate of the present invention is as follows: Example 1 The polishing method of the polycrystalline silicon carbide substrate of this embodiment adopts the following steps: (1) Rough polishing: A polyurethane polishing pad with a Shore D hardness of 70-75 was selected, with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 20 mm. A nanodiamond solution with a product number of NDS / 100 from Henan United Precision Materials Co., Ltd. was selected as the polishing liquid, wherein the particle size of the nanodiamond was 100 nm, the abrasive content was 5%, and the pH value was 8-9. A single-sided polishing machine was used to polish polycrystalline silicon carbide with a surface roughness of Ra=1.6-1.8 nm. The disk diameter was 400 mm, the polishing liquid circulation rate was 200 g / min, and the polishing pressure was 300 g / cm 2, the polishing disc speed is 100rpm, the final polishing efficiency is about 4μm / h, and after polishing for 1.5h, there is no orange peel defect on the surface of polycrystalline silicon carbide, and the surface roughness Ra=0.25-0.30nm; (2) Fine polishing: select a damping cloth polishing pad with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 20 mm; select YZ8040 oxidant-free pure silica sol produced by Shanghai Yingzhi Abrasive Materials Co., Ltd. as the polishing liquid, wherein the particle size of the silica particles is 80 nm, the silica content is 40%, and the pH value is 9-10; use a single-sided polishing machine to polish the polycrystalline silicon carbide after rough polishing in step (1), with a disk diameter of 400 mm, a polishing liquid circulation flow rate of 200 g / min, and a polishing pressure of 100 g / cm 2 , the polishing disk speed is 100rpm, there is no removal, and the surface roughness of the chip Ra=0.1-0.15nm after polishing for 2h.

[0019] Example 2 The polishing method of the polycrystalline silicon carbide substrate of this embodiment adopts the following steps: (1) Rough polishing: A polyurethane polishing pad with a Shore D hardness of 70-75 was selected, with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 10 mm. The nanodiamond solution with the product number NDS / 80 of Henan United Precision Materials Co., Ltd. was selected as the polishing liquid, wherein the particle size of the nanodiamond was 80 nm, the abrasive content was 5%, and the pH value was adjusted to 8-9 by triethanolamine. The polycrystalline silicon carbide with a surface roughness of Ra = 1.6-1.8 nm was polished using a single-sided polishing machine, with a disc diameter of 400 mm, a polishing liquid circulation rate of 200 g / min, and a polishing pressure of 200 g / cm 2 , the polishing disc speed is 100rpm, the final polishing efficiency is about 2μm / h, and after polishing for 3h, there is no orange peel defect on the surface of polycrystalline silicon carbide, and the surface roughness Ra=0.20-0.25nm; (2) Fine polishing: select a damping cloth polishing pad with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 10 mm; select YZ7540K oxidant-free pure silica sol produced by Shanghai Yingzhi Abrasive Materials Co., Ltd. as the polishing liquid, wherein the average particle size of the silica particles is 75 nm, the silica content is 40%, and the pH value is 9-10; use a single-sided polishing machine to polish the polycrystalline silicon carbide after rough polishing in step (1), with a disk diameter of 400 mm, a polishing liquid circulation flow rate of 200 g / min, and a polishing pressure of 150 g / cm 2 , the polishing disk speed is 100rpm, there is no removal, and the surface roughness of the chip is Ra=0.15-0.20nm after polishing for 2h.

[0020] 2. Comparison Comparative Example 1 The polishing method of the polycrystalline silicon carbide substrate of this comparative example adopts the following steps: (1) Rough polishing: Select a polishing pad suba800 with a Shore A hardness of 78-80, with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 20 mm; select a nanodiamond solution with a product number of NDS / 80 from Henan United Precision Materials Co., Ltd. as the polishing liquid, where the particle size of the nanodiamond is 80 nm, the abrasive content is 5%, and the pH is 8-9; use a single-sided polishing machine to polish polycrystalline silicon carbide, with a disc diameter of 400 mm, a polishing liquid circulation rate of 200 g / min, and a polishing pressure of 200 g / cm 2 , the polishing disc speed is 100rpm, the final polishing efficiency is about 1μm / h, and after polishing for 3h, the surface of polycrystalline silicon carbide shows orange peel defects, and the surface roughness Ra=0.30-0.35nm; (2) Fine polishing: select a damping cloth polishing pad with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 10 mm; select YZ7540K oxidant-free pure silica sol produced by Shanghai Yingzhi Abrasive Materials Co., Ltd. as the polishing liquid, wherein the particle size of the silica particles is 75 nm, the silica content is 40%, and the pH value is 9-10; use a single-sided polishing machine to polish the polycrystalline silicon carbide after rough polishing in step (1), with a disk diameter of 400 mm, a polishing liquid circulation flow rate of 200 g / min, and a polishing pressure of 150 g / cm 2 , the polishing disk speed is 100rpm, there is no removal, and the surface roughness of the chip is Ra=0.20-0.25nm after polishing for 2h.

[0021] The polishing pad used for rough polishing in this comparative example is a soft polishing pad compared to Example 1. Under the same other conditions, the polishing efficiency is reduced from 4μm / h to 1μm / h, and after polishing for 3 hours, the surface of the polycrystalline silicon carbide shows orange peel-like defects and the surface roughness increases. This is because the hardness of the surface of the polycrystalline silicon carbide material itself varies, and the softer the material, the greater the microscopic deformation of the polishing pad. When polishing with abrasives, the soft polishing pad will have a stronger polishing effect on the area with low hardness on the surface of the material, and the polishing effect is poor in the area with high hardness, which further increases the unevenness of the material surface, resulting in orange peel-like defects and increased surface roughness.

[0022] Comparative Example 2 The polishing method of the polycrystalline silicon carbide substrate of this comparative example adopts the following steps: (1) Rough polishing: A polyurethane polishing pad with a Shore D hardness of 70-75 was selected, with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 10 mm. A nanodiamond solution with the product number NDS / 80 from Henan United Precision Materials Co., Ltd. was selected as the polishing liquid, and potassium permanganate was added to the polishing liquid. The mass fraction of potassium permanganate in the polishing liquid was 0.3%, and the pH was adjusted to 8-9 with potassium hydroxide. A single-sided polishing machine was used to polish polycrystalline silicon carbide. The disk diameter was 400 mm, the polishing liquid circulation rate was 200 g / min, and the polishing pressure was 200 g / cm 2 , the polishing disc speed is 100rpm, the final polishing efficiency is about 3μm / h, and after polishing for 3h, the surface of polycrystalline silicon carbide shows orange peel defects, and the surface roughness Ra=0.35-0.45nm; (2) Fine polishing: select a damping cloth polishing pad with a groove width of 2 mm, a groove depth of 1 mm, and a groove spacing of 10 mm; select YZ7540K oxidant-free pure silica sol produced by Shanghai Yingzhi Abrasive Materials Co., Ltd. as the polishing liquid, wherein the particle size of the silica particles is 75 nm, the silica content is 40%, and the pH value is 9-10; use a single-sided polishing machine to polish the polycrystalline silicon carbide after rough polishing in step (1), with a disk diameter of 400 mm, a polishing liquid circulation flow rate of 200 g / min, and a polishing pressure of 150 g / cm 2 , the polishing disk speed is 100rpm, there is no removal, and the surface roughness of the chip Ra = 0.25-0.30nm after polishing for 2h.

[0023] Compared with Example 2, the polishing liquid selected for rough polishing is a diamond polishing liquid containing potassium permanganate oxidant, and chemical mechanical polishing is adopted. The polishing efficiency is increased to 3μm / h, but the surface of the wafer after polishing shows orange peel-like defects and the roughness is also increased.

Claims

1. A method for polishing a polycrystalline silicon carbide substrate, characterized in that: The following steps are involved: (1) Rough polishing a silicon carbide wafer using a polyurethane polishing pad and an oxidant-free nano-diamond polishing liquid until Ra is no more than 0.3 nm; the Shore D hardness of the polyurethane polishing pad is 70-75; (2) Use damping cloth polishing pad and oxidant-free pure silica sol as polishing liquid for fine polishing until Ra is no more than 0.2nm.

2. The method for polishing a polycrystalline silicon carbide substrate according to claim 1, characterized in that: The particle size of diamond in the nano-diamond polishing liquid is 80-100 nm, the pH value is 8-9, and the content of diamond in the diamond polishing liquid is 5-8%.

3. The method for polishing a polycrystalline silicon carbide substrate according to claim 1, wherein: The particle size of silicon dioxide in the oxidant-free pure silica sol is 50-80 nm, pH=9-10.5, and the content of silicon dioxide is 39-41%.

4. The polishing method for a polycrystalline silicon carbide substrate according to any one of claims 1 to 3, characterized in that: The time for the rough polishing is 1.5 to 3 hours, and the time for the fine polishing is 2 to 2.5 hours.

5. The polishing method for a polycrystalline silicon carbide substrate according to any one of claims 1 to 3, characterized in that: The polishing pressure of the rough polishing is 200-300 g / cm 2 , the rotation speed is 100~150rpm, and the polishing liquid circulation flow rate is 200~250g / min.

6. The method for polishing a polycrystalline silicon carbide substrate according to claim 5, characterized in that: The polishing pressure of the fine polishing is 100-150 g / cm 2 , the rotation speed is 100~150rpm, and the polishing liquid circulation flow rate is 200~250g / min.

7. The polishing method for a polycrystalline silicon carbide substrate according to any one of claims 1 to 3, characterized in that: The Ra of the silicon carbide wafer before rough polishing is less than 2nm.