An epitaxial layer is a specific single-crystal thin film grown on a wafer substrate through an epitaxial process; the substrate wafer and the epitaxial film are collectively referred to as an epitaxial wafer. Among these, growing a silicon carbide epitaxial layer on a conductive silicon carbide substrate produces a homogeneous silicon carbide epitaxial wafer, which can be further fabricated into power devices such as Schottky diodes, MOSFETs, and IGBTs. The most widely used substrate type is the 4H-SiC substrate.
Since virtually all devices are fabricated on epitaxial layers, the quality of the epitaxial layer has a significant impact on device performance. However, the quality of the epitaxial layer is influenced by crystal and substrate processing, placing it at a critical juncture in the industry and playing a pivotal role in its development.
The primary methods for preparing silicon carbide epitaxial layers include: vapor deposition; liquid phase epitaxy (LPE); molecular beam epitaxy (MBE); and chemical vapor deposition (CVD). Among these, chemical vapor deposition (CVD) is the most widely used method for 4H-SiC homogeneous epitaxy. 4H-SiC CVD epitaxy typically employs CVD equipment, which ensures the continuity of the 4H-SiC crystal structure under high growth temperatures.
In CVD equipment, the substrate cannot be placed directly on a metal surface or simply placed on a base for epitaxial deposition, as this involves various influencing factors such as gas flow direction (horizontal, vertical), temperature, pressure, fixation, and contamination from shedding particles. Therefore, a support base is required; the substrate is placed on a tray, and epitaxial deposition is then performed on the substrate using CVD technology. This support base is a SiC-coated graphite base.
As a core component, the graphite substrate features high specific strength and specific modulus, as well as good thermal shock and corrosion resistance. However, during production, the graphite is subject to corrosion and powder shedding due to corrosive gases and residual organometallic compounds, significantly reducing the substrate’s service life. At the same time, the shed graphite powder can contaminate the chips. In the production of silicon carbide epitaxial wafers, it has become increasingly difficult to meet the ever-stricter requirements for graphite materials, severely limiting their development and practical application. Consequently, coating technology has emerged as a solution.
Advantages of SiC Coatings in the Semiconductor Industry
The physical and chemical properties of coatings must meet strict requirements for high-temperature and corrosion resistance, as these directly impact product yield and lifespan. SiC material possesses high strength, high hardness, a low coefficient of thermal expansion, and excellent thermal conductivity, making it a critical high-temperature structural and semiconductor material. When applied to graphite substrates, its advantages include:
1) SiC is corrosion-resistant and can fully encapsulate the graphite substrate. Its high density prevents damage from corrosive gases.
2) SiC has high thermal conductivity and strong bonding strength with the graphite substrate, ensuring the coating remains intact even after repeated high-low temperature cycles.
3) SiC possesses excellent chemical stability, preventing coating failure in high-temperature, corrosive atmospheres.
Furthermore, epitaxial furnaces made of different materials require graphite trays with specific performance specifications. The thermal expansion coefficient of the graphite material must be compatible with the growth temperature of the epitaxial furnace. For example, since the growth temperature for silicon carbide epitaxy is high, trays with a high degree of thermal expansion coefficient matching are required. The thermal expansion coefficient of SiC differs very little from that of graphite, making it the preferred material for coating the surface of graphite substrates.
SiC exists in multiple crystal forms, with the most common currently being 3C, 4H, and 6H types, each serving different applications. For instance, 4H-SiC is used to manufacture high-power devices; 6H-SiC is the most stable and is used to manufacture optoelectronic devices; and 3C-SiC, due to its structural similarity to GaN, is used to produce GaN epitaxial layers for the manufacture of SiC-GaN RF devices. 3C-SiC is also commonly referred to as β-SiC. A key application of β-SiC is as a thin-film and coating material; consequently, β-SiC is currently the primary material used for coatings.
As a common consumable in semiconductor production, SiC coatings are primarily used in processes such as substrate preparation, epitaxy, oxidation and diffusion, etching, and ion implantation. The physical and chemical properties of the coating must meet strict requirements for high-temperature resistance and corrosion resistance, as these directly impact product yield and lifespan. Consequently, the preparation of SiC coatings is critical.
Process for Preparing Silicon Carbide Coatings
Currently, the primary methods for preparing SiC coatings include the sol-gel method, the embedding method, brush coating, plasma spraying, chemical vapor reaction (CVR), and chemical vapor deposition (CVD).
Embedding Method
This method is a type of high-temperature solid-state sintering. It primarily uses a mixture of silicon (Si) powder and carbon (C) powder as the embedding powder. A graphite substrate is placed within the embedding powder and sintered at high temperatures in an inert gas atmosphere, ultimately forming a SiC coating on the surface of the graphite substrate. This method features a simple process and good adhesion between the coating and the substrate; however, the coating exhibits poor uniformity along the thickness direction and is prone to developing numerous pores, resulting in poor oxidation resistance.
Brush Coating Method
The brush coating method primarily involves applying liquid raw material to the surface of the graphite substrate, followed by curing the material at a specific temperature to form the coating. This method is simple and cost-effective; however, the resulting coating exhibits weak adhesion to the substrate, poor uniformity, and is relatively thin with low oxidation resistance, necessitating supplementary methods.
Plasma Spraying Method
The plasma spraying method involves using a plasma gun to spray molten or semi-molten raw material onto the surface of the graphite substrate, which then solidifies and bonds to form a coating. This method is simple to operate and can produce relatively dense silicon carbide coatings; however, the resulting coatings are often too thin, leading to poor oxidation resistance. Therefore, it is generally used in the preparation of SiC composite coatings to improve coating quality.
Gel-Sol Method
The gel-sol method primarily involves preparing a uniform, transparent sol solution to coat the substrate surface. After drying into a gel, the material is sintered to form the coating. This method is simple to operate and relatively low-cost; however, the resulting coatings suffer from drawbacks such as low thermal shock resistance and a tendency to crack, limiting their widespread application.
Chemical Vapor Reaction (CVR) Method
CVR primarily utilizes Si and SiO₂ powders to generate SiO₂ vapor at high temperatures, triggering a series of chemical reactions on the surface of the Si substrate to form a SiC coating. The SiC coating produced by this method bonds tightly to the substrate; however, the reaction temperature is high, and the cost is also relatively high.
Chemical Vapor Deposition (CVD)
CVD is currently the primary technology for preparing SiC coatings on substrate surfaces. The main process involves a series of physicochemical reactions between gaseous reactants and the substrate surface, ultimately resulting in the deposition of a SiC coating. SiC coatings produced by CVD technology bond tightly to the substrate surface and can effectively improve the substrate material’s oxidation resistance and ablation resistance; however, this method requires a relatively long deposition time, and the reaction gases contain certain toxic components.
Trends in SiC Coatings
SiC coatings are primarily used as consumables in semiconductor manufacturing, with key performance metrics including coating uniformity, coefficient of thermal expansion, and thermal conductivity. While there is significant domestic demand for SiC coatings in China, the technology remains immature. Overseas, only one U.S. company operates in this field, and its products are relatively expensive.
Crystal boats are consumables used in epitaxial production that can replace SiC-coated graphite, though the market volume is small. Next is TaC-coated graphite, which offers better chemical corrosion resistance than bare graphite or SiC-coated graphite, but it is prohibitively expensive and still in the early stages of development. Currently, domestic companies such as Liufang semiconductor, Semicera, and Vetek Semiconductor are engaged in R&D in this area.
Post time: May-20-2026