In the process of wafer manufacturing, some wafer substrates need to be further constructed epitaxial layer to facilitate the manufacturing of devices. Typically, LED light-emitting devices need to prepare GaAs epitaxial layer on silicon substrate. The SiC epitaxial layer grown on the conductive SiC substrate is used to construct devices such as SBD and MOSFET for high voltage, high current and other power applications. GaN epitaphic layer is constructed on semi-insulating SiC substrate to further construct HEMT and other devices for RF applications such as communication. This process cannot be separated from CVD equipment.
In CVD equipment, the substrate cannot be directly placed on the metal or simply placed on a base for epitaxial deposition, because it involves the gas flow direction (horizontal, vertical), temperature, pressure, fixation, removal of pollutants and other factors. Therefore, a base is used, and then the substrate is placed on the disk, and then the CVD technology is used to epitaxial deposition on the substrate. This base is the SiC coated graphite base (also known as the tray).
SiC coated graphite substrates are commonly used in metal-organic chemical vapor deposition (MOCVD) equipment for supporting and heating components of single-crystal substrates. The thermal stability, thermal uniformity and other performance parameters of SiC coated graphite base play a decisive role in the quality of epitaxial material growth, so it is the core key component of MOCVD equipment.
Metal organic chemical vapor deposition (MOCVD) technology is the mainstream technology for the epitaxial growth of GaN film in blue LED, which has the advantages of simple operation, controllable growth rate and high purity of GaN film grown. The bearing base used for the epitaxial growth of GaN film, as an important part of the reaction cavity of MOCVD equipment, needs to have the advantages of high temperature resistance, uniform thermal conductivity, good chemical stability, strong thermal shock resistance and so on. Graphite material can meet the above conditions.
As one of the core components of MOCVD equipment, graphite base is the carrier and heating element of the substrate, which directly determines the uniformity and purity of the film material. Therefore, its quality directly affects the preparation of the epitaxial sheet. At the same time, with the increase of use times and the change of working conditions, it is very easy to lose, which is a consumable material.
Although graphite has excellent thermal conductivity and stability, it has a good advantage as a base component of MOCVD equipment, but in the production process, graphite will be due to the residue of corrosive gas and metal organic matter, so that it will corroded powder, and the service life of graphite base will be greatly reduced. At the same time, the falling graphite powder can cause contamination of the chip.
The emergence of coating technology, which can provide surface powder fixation, enhance thermal conductivity and equalize heat distribution, has become the main technology to solve this problem. In the use environment of graphite base in MOCVD equipment.
The surface coating of graphite base should meet the following characteristics:
(1) The graphite base can be fully wrapped, and the density is good, otherwise the graphite base is easy to be corroded in the corrosive gas.
(2) High bonding strength with graphite base, to ensure that the coating is not easy to fall off after many cycles of high temperature and low temperature.
(3) It has good chemical stability to avoid the failure of the coating in a high temperature and corrosive atmosphere.
SiC has the advantages of corrosion resistance, high thermal conductivity, thermal shock resistance and high chemical stability, and can work well in GaN epitaxial atmosphere. In addition to this, the thermal expansion coefficient of SiC differs very little from that of graphite, so SiC is the preferred material for surface coating of graphite base.
At present, the common SiC are mainly 3C, 4H and 6H, and the SiC uses of different crystal forms are different. For example, 4H-SiC can manufacture high-power devices; 6H-SiC is the most stable and can be used to manufacture optoelectronic devices; Because of its similar structure to GaN, 3C-SiC can be used to produce GaN epitaxial layer and manufacture SiC-GaN RF devices. 3C-SiC is also commonly known as β-SiC. An important use of β-SiC is as a film and coating material. Therefore, β-SiC is the main material for coating at present.
Silicon carbide coating preparation method
At present, the preparation methods of SiC coating mainly include gel-sol method, embedding method, brush coating method, plasma spraying method, chemical gas phase reaction (CVR) method and chemical vapor deposition (CVD) method.
Method of embedding
This method is a kind of high temperature solid phase sintering, mainly using Si powder and C powder mixed as the embedding powder, the graphite matrix is placed in the embedding powder, sintered at high temperature in inert gas, and finally SiC coating is obtained on the surface of the graphite matrix. The process is simple, and the bonding between the coating and the matrix is good, but the uniformity of the coating along the thickness direction is poor, and it is easy to produce more holes, resulting in poor oxidation resistance.
Brush coating method
Brush coating method is mainly to brush the liquid raw material on the surface of the graphite matrix, and then the raw material is solidified at a certain temperature to prepare the coating. The process is simple and the cost is low, but the brush coating prepared by the coating and the matrix is weak, the coating uniformity is poor, the coating is thin and the oxidation resistance is low, so other methods are needed to assist.
Plasma spraying method
Plasma spraying method is mainly a plasma gun will melt or semi-melt state of the raw material spray on the surface of the graphite matrix, and then solidify and bond to form a coating. This method is simple to operate and can prepare relatively dense silicon carbide coating, but the silicon carbide coating prepared by this method is often too weak to lead to strong oxidation resistance, so it is generally used for the preparation of SiC composite coating to improve the quality of the coating.
Gel-sol method is mainly to prepare uniform and transparent sol solution to cover the surface of the matrix, dry into gel and then sintering to obtain the coating. This method is simple to operate and has low cost, but the coating can not be widely used because of its low thermal shock resistance and easy cracking.
Chemical gas phase reaction (CVR)
CVR mainly generates SiO steam by using Si and SiO2 powder at high temperature, and a series of chemical reactions occur on the surface of C material matrix to generate SiC coating. The SiC coating prepared by this method is closely bonded to the substrate, but the reaction temperature is high and the cost is also high.
Chemical Vapor Deposition (CVD)
CVD is the main technology to prepare SiC coating on the substrate surface at present. The main process is that a series of physical and chemical reactions occur on the substrate surface of the gas reacant raw material, and finally the SiC coating is prepared by deposition on the substrate surface. The SiC coating prepared by CVD technology is closely bound to the surface of the matrix, which can effectively improve the oxidation resistance and ablation resistance of the matrix material. However, the deposition time of this method is long, and there are certain toxic gases in the reaction gas.
Market scenario for Sic-coated Graphite Substrates
When foreign manufacturers start earlier, they have obvious leading advantages and high market share. Internationally, the mainstream suppliers of SiC coated graphite base include Xycard of the Netherlands, SGL of Germany, Toyo Carbon of Japan, MEMC of the United States and other enterprises, basically occupying the international market. Although our country has made a breakthrough in the key core technology of uniform growth of SiC coating on the surface of graphite matrix, high-quality graphite matrix still depends on Germany SGL, Japan Toyo carbon and other enterprises. The service life of graphite matrix provided by domestic enterprises is affected by quality problems such as thermal conductivity, elastic modulus, rigid modulus and lattice defects. It can not meet the requirements of MOCVD equipment for SiC coated graphite base.
China's semiconductor industry is developing rapidly. With the gradual improvement of the localization rate of MOCVD epitaxy equipment and the expansion of other process applications, the market of SiC coated graphite base products is expected to grow rapidly in the future. According to the preliminary estimate of the industry, the domestic graphite base market will exceed 500 million yuan in the next few years.
As the core component of compound semiconductor industrialization equipment, it is of great strategic significance to ensure the development of semiconductor industry in our country to master the key core technology of SiC coated graphite base and realize the localization of the whole industrial chain of raw materials-process-equipment. The field of domestic SiC coated graphite base is in the ascendancy, and the product quality will reach the international advanced level in the near future.