What is SiC-coated graphite susceptor ?

Figure 1.SiC-coated graphite susceptor

1. Epitaxial layer and its equipment

During the wafer manufacturing process, we need to further build an epitaxial layer on some wafer substrates to facilitate the manufacture of devices. Epitaxy refers to the process of growing a new single crystal on a single crystal substrate that has been carefully processed by cutting, grinding, and polishing. The new single crystal can be the same material as the substrate, or a different material (homoepitaxial or heteroepitaxial). Since the new single crystal layer grows along the substrate crystal phase, it is called an epitaxial layer, and the device manufacturing is carried out on the epitaxial layer. 

For example, a GaAs epitaxial layer is prepared on a silicon substrate for LED light-emitting devices; a SiC epitaxial layer is grown on a conductive SiC substrate for the construction of SBD, MOSFET and other devices in power applications; a GaN epitaxial layer is constructed on a semi-insulating SiC substrate to further manufacture devices such as HEMT in radio frequency applications such as communications. Parameters such as the thickness of SiC epitaxial materials and background carrier concentration directly determine the various electrical properties of SiC devices. In this process, we cannot do without chemical vapor deposition (CVD) equipment.

Figure 2. Epitaxial film growth modes

2. Importance of SiC coated graphite susceptor in CVD equipment

In CVD equipment, we cannot place the substrate directly on the metal or simply on a base for epitaxial deposition, because it involves many factors such as gas flow direction (horizontal, vertical), temperature, pressure, fixation and contaminants. Therefore, we need to use a susceptor(wafer carrier) to place the substrate on a tray and use CVD technology to perform epitaxial deposition on it. This susceptor is the SiC-coated graphite susceptor (also called a tray).

2.1 Application of SiC coated graphite susceptor in MOCVD equipment

The SiC-coated graphite susceptor plays a key role in metal organic chemical vapor deposition (MOCVD) equipment to support and heat single crystal substrates. The thermal stability and thermal uniformity of this susceptor are crucial to the quality of epitaxial materials, so it is regarded as an indispensable core component in MOCVD equipment. Metal organic chemical vapor deposition (MOCVD) technology is currently widely used in the epitaxial growth of GaN thin films in blue LEDs because it has the advantages of simple operation, controllable growth rate and high purity.

As one of the core components in MOCVD equipment, Vetek semiconductor graphite susceptor is responsible for supporting and heating single crystal substrates, which directly affects the uniformity and purity of thin film materials, and thus is related to the preparation quality of epitaxial wafers. As the number of uses increases and the working environment changes, the graphite susceptor is prone to wear and is therefore classified as a consumable.

2.2. Characteristics of SIC coated graphite susceptor

To meet the needs of MOCVD equipment, the coating required for the graphite susceptor must have specific characteristics to meet the following standards:

✔  Good coverage: The SiC coating must completely cover the susceptor and have a high degree of density to prevent damage in a corrosive gas environment.

✔  High bonding strength: The coating should be firmly bonded to the susceptor and not easy to fall off after multiple high-temperature and low-temperature cycles.

✔  Good chemical stability: The coating must have good chemical stability to avoid failure in high temperature and corrosive atmospheres.

2.3 Difficulties and challenges in matching graphite and silicon carbide materials

Silicon carbide (SiC) performs well in GaN epitaxial atmospheres due to its advantages such as corrosion resistance, high thermal conductivity, thermal shock resistance and good chemical stability. Its thermal expansion coefficient is similar to that of graphite, making it the preferred material for graphite susceptor coatings.

However, after all, graphite and silicon carbide are two different materials, and there will still be situations where the coating has a short service life, is easy to fall off, and increases costs due to different thermal expansion coefficients. 

3. SiC Coating technology

3.1. Common types of SiC

At present, common types of SiC include 3C, 4H and 6H, and different types of SiC are suitable for different purposes. For example, 4H-SiC is suitable for manufacturing high-power devices, 6H-SiC is relatively stable and can be used for optoelectronic devices, and 3C-SiC can be used to prepare GaN epitaxial layers and manufacture SiC-GaN RF devices due to its similar structure to GaN. 3C-SiC is also commonly referred to as β-SiC, which is mainly used for thin films and coating materials. Therefore, β-SiC is currently one of the main materials for coatings.

3.2 . Silicon carbide coating preparation method

There are many options for the preparation of silicon carbide coatings, including gel-sol method, spraying method, ion beam spraying method, chemical vapor reaction method (CVR) and chemical vapor deposition method (CVD). Among them, chemical vapor deposition method (CVD) is currently the main technology for preparing SiC coatings. This method deposits SiC coatings on the surface of the substrate through gas phase reaction, which has the advantages of close bonding between the coating and the substrate, improving the oxidation resistance and ablation resistance of the substrate material.

The high-temperature sintering method, by placing the graphite substrate in the embedding powder and sintering it at high temperature under an inert atmosphere, finally forms a SiC coating on the surface of the substrate, which is called the embedding method. Although this method is simple and the coating is tightly bonded to the substrate, the uniformity of the coating in the thickness direction is poor, and holes are prone to appear, which reduces the oxidation resistance.

✔  The spraying method involves spraying liquid raw materials on the surface of the graphite substrate, and then solidifying the raw materials at a specific temperature to form a coating. Although this method is low-cost, the coating is weakly bonded to the substrate, and the coating has poor uniformity, thin thickness, and poor oxidation resistance, and usually requires additional treatment.

✔  Ion beam spraying technology uses an ion beam gun to spray molten or partially molten material onto the surface of a graphite substrate, which then solidifies and bonds to form a coating. Although the operation is simple and can produce a relatively dense silicon carbide coating, the coating is easy to break and has poor oxidation resistance. It is usually used to prepare high-quality SiC composite coatings.

✔ Sol-gel method, this method involves preparing a uniform and transparent sol solution, applying it to the surface of the substrate, and then drying and sintering to form a coating. Although the operation is simple and the cost is low, the prepared coating has low thermal shock resistance and is prone to cracking, so its application range is limited.

✔ Chemical vapor reaction technology (CVR): CVR uses Si and SiO2 powder to generate SiO vapor, and forms a SiC coating by chemical reaction on the surface of the carbon material substrate. Although a tightly bonded coating can be prepared, a higher reaction temperature is required and the cost is high.

✔  Chemical vapor deposition (CVD): CVD is currently the most widely used technology for preparing SiC coatings, and SiC coatings are formed by gas phase reactions on the surface of the substrate. The coating prepared by this method is closely bonded to the substrate, which improves the substrate's oxidation resistance and ablation resistance, but requires a long deposition time, and the reaction gas may be toxic.

Figure 3.Chemical vapor depostion diagram

4. Market competition and Vetek Semiconductor’s technological innovation

In the SiC coated graphite substrate market, foreign manufacturers started earlier, with obvious leading advantages and a higher market share. Internationally, Xycard in the Netherlands, SGL in Germany, Toyo Tanso in Japan, and MEMC in the United States are mainstream suppliers, and they basically monopolize the international market. However, China has now broken through the core technology of uniformly growing SiC coatings on the surface of graphite substrates, and its quality has been verified by domestic and foreign customers. At the same time, it also has certain competitive advantages in price, which can meet the requirements of MOCVD equipment for the use of SiC coated graphite substrates. 

Vetek semiconductor has been engaged in research and development in the field of SiC coatings for more than 20 years. Therefore, we have launched the same buffer layer technology as SGL. Through special processing technology, a buffer layer can be added between graphite and silicon carbide to increase the service life by more than two times.