Chemical vapor deposition(CVD) reaction mechanism

As mentioned before chemical vapor deposition is based on chemical reactions, the selection of appropriate reaction materials and deposition reactions helps to obtain high performance materials.

a) Pyrolysis reaction

The simplest and most direct way of CVD deposition reaction is pyrolysis reaction, the principle is mainly solid compounds will decompose into solid target products and gaseous by-products when heated up to a certain temperature. The operation procedure is generally to introduce the reaction gas into a single-temperature tube furnace under vacuum or inert atmosphere, raise the furnace temperature to the decomposition temperature of the compound to decompose, and deposit the target product on the substrate. The key to thermal decomposition reaction lies in the selection of suitable volatile source and decomposition temperature, especially need to pay special attention to the decomposition products of raw materials at different temperatures. At present, the commonly used raw materials are hydride, carbonyl compounds and metal-organic compounds, etc., because the dissociation energy of their chemical bonds are generally small, easy to decompose, the decomposition temperature is relatively low, especially after the decomposition of hydride by-products are not corrosive hydrogen gas. Thermal decomposition reaction is mainly applicable to the preparation of metals, semiconductors, insulators and other materials.

(1) hydride decomposition of polysilicon and amorphous silicon: SiH4 (g) → Si (s) + 2H2 (g) 650 ℃

(2) carbonyl chloride decomposition deposition of precious metals or transition metals: Ni (CO) 4 (s) → Ni (s) + 4CO (g) 140-240 ℃

(3) metal organic decomposition and deposition of Al2O3: 2Al (OC3H7) 3 (s) → Al2O3 (s) +6C3H6 (g) +3H2O (g) 420 ℃

b) Chemical synthesis reaction

The most widely used CVD deposition reaction is the chemical synthesis reaction, which mainly involves a variety of reactive gases on the surface of the substrate to react with each other to generate a solid film deposition process, so it is called the chemical synthesis reaction, CVD deposition reaction mostly belongs to this category. Generally, a variety of reactive gases are introduced into a single-temperature zone tube furnace under vacuum or inert atmosphere, and the furnace temperature is raised to a suitable temperature so that the synthesis reaction occurs on the substrate to obtain the target product. The key to the chemical synthesis reaction is the choice of reaction products, the principle of avoiding the generation of by-products as far as possible. Because the use of thermal decomposition deposition target product of the raw material selection range is relatively narrow, and theoretically any kind of inorganic materials can be obtained through a variety of raw materials of the chemical synthesis reaction. Therefore, compared with the thermal decomposition reaction, the chemical synthesis reaction is the most widely used, and its main application is to prepare a variety of polycrystalline and glassy deposition layer, insulating film, etc., such as SiO2, Al2O3, Si3N4.

(1) silicon tetrachloride epitaxial growth of silicon epitaxial wafer: SiCl4 (s) + 2H2 (g) → Si (s) + 4HCl (g) 1150-1200 ℃

(2) Semiconductor SiO2 mask process: SiH4 (s) + 2O2 (g) → SiO2 (s) + 2H2O (g) 325-475 ℃

(3) Si3N4 and other insulating film deposition: 3SiCl4 (s) +4NH3 (g) → Si3N4 (s) +12HCl (g) 850-900 ℃

c) Chemical transport reaction

Chemical transport reaction takes the target product as a volatile source, deposits the target product with the help of equilibrium reaction, which reacts with it to generate gaseous compounds with the help of gas, and the generated gaseous compounds are transported by the carrier gas to the deposition area where the temperature is different from that of the volatile area to undergo a reverse reaction, and generates the source material on the substrate. The key to the chemical transport reaction lies in the selection of the transport reaction system and its conditions (temperature, amount of transport agent, etc.), which involves part of the knowledge related to chemical thermodynamics. Generally, the temperature at which gaseous compounds are generated tends to be higher than that at which the re-reaction is deposited.

Purification of rare metals and growth of single crystals such as ZnSe: ZnSe (s) + I2 (g) ZnI2 (g) + 1/2 Se2 (g)

ZnS (s) + I2 (g) ZnI2 (g) + 1/2 S2 (g)