What is Chemical Vapor Deposition?

Chemical vapor deposition is a chemical technology. The technology is mainly a method of forming a thin film by using a chemical reaction on the surface of a substrate by using one or more gas phase compounds or simple substances containing thin film elements. Chemical vapor deposition is a new technology for preparing inorganic materials developed in recent decades. Chemical vapor deposition has been widely used to purify substances, develop new crystals, and deposit various single crystal, polycrystalline, or glassy inorganic thin film materials. These materials can be oxides, sulfides, nitrides, carbides, or binary or multiple inter-element compounds in III-V, II-IV, and IV-VI groups, and their physical functions can be passed through the gas The doping deposition process is precisely controlled. At present, chemical vapor deposition has become a new field in inorganic synthetic chemistry.

Modern science and technology requires the use of a large number of inorganic new materials with different functions. These functional materials must be high-purity, or doped materials that are intentionally doped with certain impurities in the high-purity materials. However, many of the preparation methods we were familiar with in the past, such as high temperature melting,

Chemical vapor deposition technology is a process that uses gaseous substances to generate chemical reactions and transport reactions on solids and generate solid deposits. It consists of three steps:

(1) Formation of volatile substances;

(2) Transfer the above substances to the deposition area;

(3) Produce chemical reactions on solids and produce solid substances.

The most basic chemical vapor deposition reactions include thermal decomposition reactions, chemical synthesis reactions, and chemical transport reactions. ^[1]

1) At medium or high temperature, a solid substance is formed on the substrate by a gas-phase chemical reaction between gaseous initial compounds.

2) can be

The most important component of a chemical vapor deposition device is a reactor. According to the differences in the structure of the reactor, we can divide the chemical vapor deposition technology into two types: open tube / sealed tube gas flow method:

1 Sealing method

This reaction method is to place a certain amount of reaction material and collectively on both sides of the reactor, evacuate the reactor to a vacuum, then inject part of the transport gas into it, and then seal it again, and then control the temperature of the two ends of the reactor to make it There are certain differences. Its advantages are: it can effectively avoid external pollution; the interior can be kept vacuum without continuous pumping. Its disadvantages are: the material is produced slowly; the pressure in the tube is not easy to grasp.

2 Open tube method

The characteristic of this preparation method is that the reaction gas mixture can be replenished at any time. The exhaust gas can also be discharged from the reaction device in time. In terms of heating methods, the open-tube airflow method should be divided into two types: hot wall and cold wall. The former heating will heat up the entire deposition chamber wall, so deposition will also occur on the tube wall. In the latter, only the body itself will be heated, and there is no such disadvantage. Cold wall heating generally uses induction heating, electric heating, infrared heating and so on. ^[1]

1 Chemical vapor deposition method to produce crystals and crystal thin films

Chemical vapor deposition can not only help to improve the properties of crystals or crystalline thin films, but also can produce many crystals that cannot be prepared by other means. The most common use of chemical vapor deposition is to generate a new epitaxial single crystal layer on a crystal substrate. It was initially used to prepare silicon, and then an epitaxial compound semiconductor layer was prepared. It is also more common in the preparation of metal single crystal films (such as the preparation of W, Mo, Pt, Ir, etc.) and individual compound single crystal films (such as nickel ferrite film, yttrium iron garnet film, cobalt ferrite film, etc.) ).

2 production of whiskers

Whiskers belong to a type of single crystal that is thought to develop. It has a great role in the category of conforming materials and can be used to produce some new composite materials. Chemical vapor deposition uses the hydrogen-reducing properties of metal halides when producing whiskers. Chemical vapor deposition can not only produce all kinds of metal whiskers, but also produce compound whiskers, such as alumina, silicon carbide, titanium carbide whiskers and so on.

3 Chemical vapor deposition technology for the production of polycrystalline / amorphous material films

Chemical vapor deposition has a wide range of applications in the semiconductor industry. For example, a polysilicon deposition layer is used as an edge dielectric isolation layer. In modern times, new types of amorphous materials are increasingly used in microelectronic components. Such materials include phosphosilicate glass, borosilicate glass, SiO ₂ and Si ₃ N ₄ and so on. In addition, there are also some materials that may be developed into switches and memory in the future, such as copper oxide-phosphorus pentoxide, copper oxide-vanadium pentoxide-phosphorus pentoxide, and vanadium pentoxide-phosphorus pentoxide. Production is performed using chemical vapor deposition. ^[1]

1 Production of several precious metal films by chemical vapor deposition

Precious metal films have attracted researchers' interest because of their good oxidation resistance, high electrical conductivity, strong catalytic activity, and extreme stability. Compared with other methods of generating precious metal films, chemical vapor deposition has more technical advantages, so most methods of preparing precious metal films will use this method. The types of depositor materials used for depositing noble metal films are relatively wide, but most of them are halides and organic compounds of noble metal elements, such as COCl ₂ , platinum chlorocarbonate, iridium chlorocarbonate, and DCPD compounds.

The Goto team has done a lot of work on precious metal films used as electrode materials. The substrate materials they use include sapphire, quartz glass, and yttria-stabilized zirconia (YSZ). Oxygen is passed into the device during deposition in order to eliminate the carbon generated by the raw materials due to thermal decomposition, and to prepare a noble metal film with more metallic luster. Iridium particles wrapped in a crystalline carbon layer. The iridium-carbon cluster film deposited on YSZ has excellent electrical properties and catalytic activity. At relatively low temperatures, the interfacial conductivity of the iridium-carbon cluster film can reach more than a hundred times that of pure iridium or pure platinum. The cluster membrane composed of precious metals and carbon is a cluster membrane with strong porous catalytic activity, and its use in electrode materials will have great potential in the future.

2 Chemical vapor deposition method for the production of high temperature coatings of precious metal iridium

Since the 1980s, NASA has tried to use chemical vapor deposition of metal organic compounds to produce composite nozzles using osmium-based iridium as a coating, and has achieved success. At this time, chemical vapor deposition was only available in the field of producing precious metal coatings. A certain degree of breakthrough.

NASA used C ₁₅ H ₂₁ IrO ₆ as the material for preparing the iridium coating, and deposited it using the thermal decomposition reaction of C ₁₅ H ₂₁ IrO ₆ . The deposition rate of iridium is very fast, it can reach 3 ~ 20m / h. The deposition thickness also reached 50 m, and the production efficiency of C ₁₅ H ₂₁ IrO ₆ was more than 70%.

3 chemical vapor deposition of palladium

Pd and its alloys have a strong adsorption effect on hydrogen and special selective permeability. It is an ideal material for storing or purifying hydrogen. At present, most of the use of Pd is to produce hydrogen purification equipment by palladium alloy or palladium coating. Some scholars also use chemical vapor deposition to make palladium into thin films or layers. The specific method is to use a metal-organic compound with a very low decomposition temperature as a material for preparing palladium, which specifically includes: allyl [-ketoimine] Pd (), Pd (-C ₃ H ₅ ) (-C ₅ Materials such as H ₅ ) and Pd (-C ₃ H ₅ ) (CF ₃ COCHCOCF ₃ ) can be used to produce high purity palladium films.

Chemical vapor deposition technology is an important material preparation method, which plays an important role in precious metal films and coatings. At present, China is still in the development stage of the aerospace field, and the use of chemical vapor deposition technology is still very much explored. Space requires us to invest more energy in research. ^[1]