A method for polishing silicon carbide substrate

Abstract

The present invention provides a method for polishing a silicon carbide substrate, which adopts electrochemical + chemical mechanical polishing to treat the silicon carbide surface, effectively improves the degree of the oxide layer on the silicon carbide surface through anodic electrochemical oxidation and cathode active oxygen oxidation, and improves the polishing effect of the silicon carbide.

Description

Technical Field

[0001] The invention belongs to the technical field of polishing processing and relates to an electrochemical mechanical polishing method for a silicon carbide wafer. Background Art

[0002] With the development of high-tech enterprises in semiconductors, new energy, high-end intelligent manufacturing, 5G information technology, and artificial intelligence (AI), the semiconductor industry is becoming an increasingly popular sector. The research and development of semiconductor materials preparation and processes are closely linked to important national strategic areas such as military, defense, and aerospace. The booming semiconductor market has led to an increasing demand for both quantity and quality of semiconductor materials. Among various semiconductor materials, single-crystal silicon carbide (SiC) wafers are one of the most representative third-generation semiconductor materials. SiC single crystals are widely used in many key sectors, including IT, consumer electronics, automotive, industrial, aerospace, smart grid, rail transportation, power electronics, and marine applications.

[0003] Silicon carbide has a low coefficient of thermal expansion, high hardness, and excellent strength. It performs stably under high temperature and high pressure. It also boasts high breakdown electric field strength and maximum current density, strong corrosion resistance, a high elastic modulus, and a long service life. It can be used in complex environments such as those subject to radiation, corrosive conditions, and high temperatures and pressures. SiC is also an ideal substrate material. Because SiC single crystals have excellent thermal expansion coefficients and thermal conductivity, they can be used as substrates for high-power lighting devices, such as LEDs, solving the heat dissipation issues associated with high-power devices in these applications. Furthermore, SiC single crystal component materials exhibit high dimensional stability, strong resistance to thermal shock, chemical stability, non-toxicity, and moderate density. Therefore, SiC components are also used as reflector materials in optical systems, such as those in ground-based and space applications, and are extensively used to develop lightweight, large-scale optical parts.

[0004] Currently, the general processing process for SiC single crystal wafers is as follows: cutting, rough grinding, fine grinding, rough polishing (mechanical polishing), and fine polishing (chemical mechanical polishing). Due to SiC's relatively high hardness, machining is very difficult. When the crystal diameter is relatively large (50mm or above), traditional techniques are no longer sufficient. Wire cutting technology is commonly used to achieve the required size and shape accuracy of the workpiece. The surface roughness of the SiC crystal after processing can reach the nanometer level.

[0005] A smooth surface free of scratches and subsurface damage is crucial for single-crystal SiC device applications. However, single-crystal SiC is difficult to polish due to its hard, brittle material properties and strong chemical inertness. However, the polishing effect can be improved by increasing the reaction rate through electrochemical anodization of the SiC. The electrochemical process must avoid excessive corrosion of the SiC specimen. In the electrochemical mechanical polishing process, the SiC surface is transformed into a loose oxide layer through anodization. This layer can then be easily removed using abrasives with a harder surface than SiC, such as CeO2 / SiO2, resulting in a surface free of scratches and subsurface damage.

[0006] Specifically:

[0007] CN110197789A Xi'an University of Technology discloses an ultrasonic-assisted electrochemical mechanical polishing processing device and method for SiC single crystal wafers. The SiC single crystal wafer and stainless steel electrodes are respectively connected to the positive and negative electrodes of a pulse power supply to form a closed circuit in the polishing liquid. The wafer acts as an anode for anodization to generate an oxide film, which is then mechanically removed by a polishing pad and abrasives. This method has a high polishing material removal efficiency, and mechanical polishing will not cause damage to the SiC single crystal substrate; it does not consume a large amount of electricity, is energy-saving and environmentally friendly. In addition, the processing device of the present invention is simple, the processing method is easy to implement, and is suitable for large-scale promotion and use.

[0008] CN114654380A discloses an electrochemical mechanical polishing method for silicon carbide wafers, which belongs to the field of polishing processing technology. First, a silicon carbide wafer is bonded to a copper polishing head connected to the positive pole of a power supply using conductive adhesive. A polyurethane polishing pad with through holes is fixed to a graphite disk connected to the negative pole of the power supply using resin screws. The polishing power supply is a DC regulated power supply. Driven by a motor, the polishing pad and wafer undergo relative motion. During polishing, the polishing pad and wafer are immersed in an electrolyte solution whose level is higher than the polishing pad surface. The polishing solution is dripped onto the polishing pad surface via a peristaltic pump. The present invention utilizes the anodic oxidation reaction of silicon carbide crystals in a NaNO3 electrolyte to achieve efficient polishing of silicon carbide wafers. The use of small-particle diamond abrasive in the polishing solution acts as a stress source for the oxidation process, accelerating the occurrence of anodic oxidation and thereby improving polishing efficiency. Furthermore, the present invention can significantly increase the polishing rate of silicon carbide wafers and ensure good surface quality during the fine polishing stage.

[0009] Although the above patents disclose means for electro-oxidation treatment of silicon carbide wafers, or use of pulse power to treat silicon carbide, the above patents focus on devices and do not disclose specific electrochemical polishing reagents or compositions. Summary of the Invention

[0010] To address the above problems, the present invention provides a partially neutral, oxidant-free electrochemical polishing solution and a polishing method thereof. The oxidation and reduction of silicon carbide wafers are achieved by pulse switching power supply, effectively obtaining OH- ions at the anode and obtaining hydrogen peroxide with strong oxidizing properties at the cathode, effectively oxidizing silicon carbide to obtain a silicon carbide-silicon oxide surface. Chemical mechanical polishing is then performed using the silicon oxide in the polishing solution, ultimately obtaining a silicon carbide wafer product with nanometer-scale roughness. The electrochemical polishing solution is green and pollution-free, the neutral solution is easy to store and use, has a simple composition, and is highly cost-effective. Specifically:

[0011] A method for polishing a silicon carbide substrate comprises the following steps:

[0012] (1) Surface pretreatment of silicon carbide substrate;

[0013] (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction;

[0014] (3) Nitrogen purge drying treatment;

[0015] Wherein the electrolytic polishing solution comprises the following components:

[0016] 20-30wt% 20-40nm alumina grinding media;

[0017] 5-7wt.%Na2SO4;

[0018] 0.2-0.3wt.% polyoxypropylene glycerol ether;

[0019] 0.1-0.2wt.% α-olefin sulfonate;

[0020] 2-3wt% propylene glycol;

[0021] Deionized water;

[0022] Oxygen intake 0.2-0.3m 3 / h.

[0023] The oxygen aeration device is an air bubble stone;

[0024] The distance between silicon carbide and graphite material is 3-5 cm.

[0025] The power supply is a pulse power supply.

[0026] The pulse power supply parameters are as follows: positive pulse voltage: 10-12V, positive pulse width: 400-500μs, positive duty cycle: 30-40%, negative pulse voltage: 20-30V, negative pulse width: 2-3s, negative duty cycle: 50-65%, pulse time is 5-30min, temperature: 10-15℃.

[0027] The surface of the silicon carbide substrate is pretreated, and the pretreatment includes directional cutting, grinding and mechanical polishing.

[0028] The polishing liquid is used for ultra-precision polishing of silicon carbide.

[0029] The surface roughness of the silicon carbide wafer after polishing is 0.05-0.1nm.

[0030] The removal rate of silicon carbide wafers in the polishing liquid is 200-400nm / h.

[0031] The theoretical basis for the electrochemical treatment and polishing of silicon carbide according to the present invention is as follows:

[0032] During the electrochemical polishing process, the principle that the metal dissolves faster at the micro-rough peaks on the surface of the anode workpiece in an appropriate electrolyte is used to gradually reduce the peak value and the surface roughness. In theory, the polishing accuracy of the ion level can be achieved.

[0033] The present invention uses a bidirectional pulse power supply, that is, silicon carbide switches between the anode and the cathode at any time.

[0034] When used as an anode, the rough tip of the silicon carbide is electrochemically oxidized to modify the SiC surface of the present invention into a loose SiO2 oxide layer. That is, electrochemical oxidation mainly occurs in this process, and the main equation may be: SiC + 4H2O - 8e - →SiO2+CO2+8H + ;

[0035] When used as cathode, due to the continuous introduction of oxygen into the polishing liquid, the polishing liquid is in an oxygen-saturated state, and highly active oxides are formed near the electrode. 2- and OH - , freshly prepared HO 2- and OH - Loses electrons to form •OH with strong oxidizing properties, and O2 gains electrons to produce O 2- , its oxidizing property is much higher than that of hydrogen peroxide, potassium permanganate or hypochlorous acid, and then chemically oxidizes silicon nitride. The possible equation is as follows:

[0036] O2+H2O+2e - →HO 2- +OH - ;

[0037] O2+H2O+4e - →4OH - ;

[0038] 2H2O+2e - →2OH - +H2;

[0039] SiC+8 OH - →SiO2+CO2+4H2O; that is, when used as an anode, it is electrochemical anodic oxidation, and when used as a cathode, it is chemically active group oxidation, which can effectively oxidize the rough silicon carbide surface into a low-hardness silicon oxide layer. Then, mechanical polishing is performed using a polishing pad and 20-30nm polishing silicon oxide medium particles in chemical mechanical polishing to achieve the requirements of no scratches and sub-surface damage, and obtain a low-roughness silicon carbide wafer surface.

[0040] Regarding the electrochemical polishing solution, comprising 20-30wt% 20-40nm aluminum oxide grinding media;

[0041] 5-7wt.%Na2SO4;

[0042] 0.2-0.3wt.% polyoxypropylene glycerol ether;

[0043] 0.1-0.2wt.% α-olefin sulfonate;

[0044] 2-3wt% propylene glycol;

[0045] Deionized water;

[0046] Oxygen intake 0.2-0.3m 3 / h.

[0047] First, the grinding medium is aluminum oxide, which has a harder surface than the silicon oxide oxidized on the surface of silicon carbide. Through the close contact between the wafer surface and the polishing pad, the aluminum oxide medium in the polishing liquid enters between the SiC surface and the polishing pad, thereby effectively removing the oxide on the surface of the silicon carbide by friction, and obtaining a low-roughness wafer surface in the continuous oxidation-polishing process.

[0048] The electrolyte solution is Na2SO4, which is mainly used to improve the conductivity of the solution. In the prior art, sodium chloride is used as the electrolyte solution, but the chloride ions therein will undergo electrolysis to form chlorine gas, which is not conducive to polishing. Therefore, it is strictly prohibited to use sodium chloride as the electrolyte in the electrolyte of the present invention, and attention should also be paid to the control of deionized water.

[0049] Regarding oxidants, the polishing liquid of the present invention does not contain any oxidant raw materials. In the prior art, hydrogen peroxide and sodium hypochlorite are generally used as oxidants. However, after being prepared into a polishing liquid, the oxidants are easily self-decomposed. Therefore, they are generally prepared and used immediately and are not easy to preserve. In comparison, the present invention does not contain any oxidants, nor does it contain any easily decomposable components. The oxide on the surface of silicon carbide is mainly obtained through chemical oxidation and electrochemical oxidation. In addition, the electrolyte of the present invention is neutral and non-alkaline or acidic. The solution environment is also very friendly to the polishing medium of aluminum oxide. Therefore, the electrolytic polishing liquid of the present invention will not deteriorate or fail even after being stored for 3-5 years.

[0050] α-Olefin sulfonate is a surfactant that helps increase the contact area between the polishing liquid and the hydrophobic silicon carbide surface, thereby effectively improving the polishing effect. However, it is easy to form bubbles, so a defoamer is needed to minimize the foaming level. Polyoxypropylene glycerol ether needs to be added. The water solubility of surfactants and defoamers is average, especially polyoxypropylene glycerol ether has poor water solubility. Therefore, propylene glycol needs to be added to the polishing liquid to effectively disperse the defoamer and surfactant.

[0051] In addition, oxygen is the main oxidizing component of the present invention and needs to be introduced in advance before polishing. Generally, the oxygen introduction amount is 0.2-0.3m 3 / h, and the pre-introduction time is 3-5min, in order to reach the saturation state of the dissolved liquid.

[0052] The electrolytic polishing of silicon carbide of the present invention belongs to ultra-fine. The polishing liquid is used for ultra-precision polishing of silicon carbide. The surface roughness of the silicon carbide wafer after polishing is 0.05-0.1nm, and the removal rate of the silicon carbide wafer in the polishing liquid is 200-400nm / h. That is, the silicon carbide surface needs to be pretreated before polishing. The pretreatment includes directional cutting, grinding and mechanical polishing. Figure 1 As shown in FIG, the roughness of the silicon carbide surface after surface pretreatment is Ra=2.73nm.

[0053] Beneficial technical effects:

[0054] (1) The present invention adopts electrochemical + chemical mechanical polishing to treat the surface of silicon carbide. Through anodic electrochemical oxidation and cathode active oxygen oxidation, the degree of the oxide layer on the surface of silicon carbide is effectively increased, thereby improving the polishing effect of silicon carbide.

[0055] (2) The polishing liquid used in the present invention is a neutral reagent with high storage stability and does not contain any oxidant. It is green and pollution-free, and has high cost performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Attachment Figure 1 AMF image of silicon carbide after mechanical polishing (without polishing of the present invention).

[0057] Attachment Figure 2 AMF diagram of silicon carbide treated in Example 2.

[0058] Attachment Figure 3 AMF diagram of silicon carbide treated in Comparative Example 1.

[0059] Attachment Figure 4 AMF diagram of silicon carbide treated in Comparative Example 2. DETAILED DESCRIPTION Example 1

[0060] A method for polishing a silicon carbide substrate comprises the following steps:

[0061] (1) Surface pretreatment of silicon carbide substrate;

[0062] (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction;

[0063] (3) Nitrogen purge drying treatment;

[0064] Wherein the electrolytic polishing solution comprises the following components:

[0065] 20wt% 20-40nm alumina grinding media;

[0066] 5wt.%Na2SO4;

[0067] 0.2wt.% polyoxypropylene glycerol ether;

[0068] 0.1wt.% α-olefin sulfonate;

[0069] 2wt% propylene glycol;

[0070] Deionized water;

[0071] Oxygen intake 0.2m 3 / h.

[0072] The oxygen aeration device is an air bubble stone;

[0073] The distance between silicon carbide and graphite material is 3 cm.

[0074] The power supply is a pulse power supply.

[0075] The pulse power supply parameters are as follows: positive pulse voltage: 10 V, positive pulse width: 400 μs, positive duty cycle: 30%, negative pulse voltage: 20 V, negative pulse width: 2 s, negative duty cycle: 50%, pulse time: 5 min, temperature: 10 °C. Example 2

[0076] A method for polishing a silicon carbide substrate comprises the following steps:

[0077] (1) Surface pretreatment of silicon carbide substrate;

[0078] (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction;

[0079] (3) Nitrogen purge drying treatment;

[0080] Wherein the electrolytic polishing solution comprises the following components:

[0081] 25wt% 20-40nm alumina grinding media;

[0082] 6wt.%Na2SO4;

[0083] 0.25wt.% polyoxypropylene glycerol ether;

[0084] 0.15wt.% α-olefin sulfonate;

[0085] 2.5wt% propylene glycol;

[0086] Deionized water;

[0087] Oxygen intake 0.25m 3 / h.

[0088] The oxygen aeration device is an air bubble stone;

[0089] The distance between silicon carbide and graphite material is 4 cm.

[0090] The power supply is a pulse power supply.

[0091] The pulse power supply parameters are as follows: positive pulse voltage: 11 V, positive pulse width: 450 μs, positive duty cycle: 35%, negative pulse voltage: 25 V, negative pulse width: 2.5 s, negative duty cycle: 558%, pulse time: 25 min, temperature: 12.5 °C. Example 3

[0092] A method for polishing a silicon carbide substrate comprises the following steps:

[0093] (1) Surface pretreatment of silicon carbide substrate;

[0094] (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction;

[0095] (3) Nitrogen purge drying treatment;

[0096] Wherein the electrolytic polishing solution comprises the following components:

[0097] 30wt% 20-40nm alumina grinding media;

[0098] 7wt.%Na2SO4;

[0099] 0.3wt.% polyoxypropylene glycerol ether;

[0100] 0.2wt.% α-olefin sulfonate;

[0101] 3wt% propylene glycol;

[0102] Deionized water;

[0103] Oxygen intake 0.3m 3 / h.

[0104] The oxygen aeration device is an air bubble stone;

[0105] The distance between silicon carbide and graphite material is 5 cm.

[0106] The power supply is a pulse power supply.

[0107] The pulse power supply parameters are as follows: positive pulse voltage: 12V, positive pulse width: 500μs, positive duty cycle: 40%, negative pulse voltage: 30V, negative pulse width: 3s, negative duty cycle: 65%, pulse time: 30min, temperature: 15℃.

[0108] Comparative Example 1

[0109] A method for polishing a silicon carbide substrate comprises the following steps:

[0110] (1) Surface pretreatment of silicon carbide substrate;

[0111] (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction;

[0112] (3) Nitrogen purge drying treatment;

[0113] Wherein the electrolytic polishing solution comprises the following components:

[0114] 25wt% 20-40nm alumina grinding media;

[0115] 6wt.%Na2SO4;

[0116] 0.25wt.% polyoxypropylene glycerol ether;

[0117] 0.15wt.% α-olefin sulfonate;

[0118] 2.5wt% propylene glycol;

[0119] Deionized water;

[0120] Oxygen intake 0.25m 3 / h.

[0121] The oxygen aeration device is an air bubble stone;

[0122] The distance between silicon carbide and graphite material is 4 cm.

[0123] The power supply is a direct current power supply, wherein silicon carbide is an anode.

[0124] Comparative Example 2

[0125] A method for polishing a silicon carbide substrate comprises the following steps:

[0126] (1) Surface pretreatment of silicon carbide substrate;

[0127] (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction;

[0128] (3) Nitrogen purge drying treatment;

[0129] Wherein the electrolytic polishing solution comprises the following components:

[0130] 25wt% 20-40nm alumina grinding media;

[0131] 6wt.%Na2SO4;

[0132] Oxygen intake 0.25m 3 / h.

[0133] The oxygen aeration device is an air bubble stone;

[0134] The distance between silicon carbide and graphite material is 4 cm.

[0135] The power supply is a pulse power supply.

[0136] The pulse power supply parameters are as follows: positive pulse voltage: 11V, positive pulse width: 450μs, positive duty cycle: 35%, negative pulse voltage: 20-30V, negative pulse width: 2-3s, negative duty cycle: 50-65%, pulse time is 5-30min, temperature: 10-15℃.

[0137] As attached Figure 2 , attached Figure 3 , attached Figure 4As shown, the surface roughness Ra obtained in Example 2 is 0.05nm, and the removal rate in the polishing liquid is 253nm / h; the surface roughness Ra of Comparative Example 1 is 1.13nm, and the removal rate in the polishing liquid is 198nm / h; the surface roughness Ra of Comparative Example 2 is 0.89nm, and the removal rate in the polishing liquid is 249nm / h. The oxidation degree of the comparative example is low, resulting in a low removal rate in the polishing liquid and a high roughness. Comparative Example 2 lacks the necessary additives. Although there is no obvious difference in the oxidation degree or removal rate, its roughness is significantly higher than that of Example 2.

[0138] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A method for electrochemical mechanical polishing of a silicon carbide substrate, characterized in that The steps include: (1) Surface pretreatment of silicon carbide substrate; (2) Using silicon carbide as the working electrode and graphite material as the counter electrode, place them in the electrolytic polishing solution to perform the electrolytic polishing reaction; (3) Nitrogen purge drying treatment; Wherein the electrolytic polishing solution comprises the following components: 20-30wt% 20-40nm alumina grinding media; 5-7wt.%Na2SO4; 0.2-0.3wt.% polyoxypropylene glycerol ether; 0.1-0.2wt.% α-olefin sulfonate; 2-3wt% propylene glycol; Deionized water; Oxygen intake 0.2-0.3m 3 / h; The electropolishing power supply is a bidirectional pulse power supply.

2. The electrochemical mechanical polishing method for a silicon carbide substrate according to claim 1, characterized in that The oxygen ventilation device is an air bubble stone.

3. The electrochemical mechanical polishing method for a silicon carbide substrate according to claim 1, characterized in that The distance between silicon carbide and graphite material is 3-5 cm.

4. The electrochemical mechanical polishing method for a silicon carbide substrate according to claim 1, wherein The pulse power supply parameters are as follows: positive pulse voltage: 10-12V, positive pulse width: 400-500μs, positive duty cycle: 30-40%, negative pulse voltage: 20-30V, negative pulse width: 2-3s, negative duty cycle: 50-65%, pulse time is 5-30min, temperature: 10-15℃.

5. The electrochemical mechanical polishing method for a silicon carbide substrate according to claim 1, wherein Pre-treatment includes directional cutting, grinding and mechanical polishing.

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