Now we know that diamond is a metastable phase at normal temperature and pressure, in which four valence electrons of carbon atoms hybridize by sp3 to form a tetrahedral coordination bond structure. Graphite, on the other hand, is a more stable allotrope, and its carbon atoms are hybridized by sp2 to form a triple coordinate bond structure. The formation of graphite is better than that of diamond in thermokinetics, which means that the sp2 hybrid bond in metastable phase can only be formed in non-equilibrium process. Diamond-like carbon films are metastable materials, and the growth surface needs to be bombarded with charged ions in the preparation method. Since Aisenberg and Chabot deposited DLC films by carbon ion beam, many new methods and technologies for preparing DLC films by physical vapor deposition, chemical vapor deposition and liquid phase method have been successfully developed.

Among them, there are two methods: gas phase method and deposition method:

Gas-phase method is a method that directly uses gas, or changes substances into gas by various means, making them undergo physical changes or chemical reactions in the gas state, and finally agglomerating and growing to form nanoparticles during cooling. Precipitation methods can be divided into direct precipitation method, * * precipitation method and homogeneous precipitation method, all of which are prepared by liquid phase reaction that produces precipitation.

(1) physical vapor deposition

Physical vapor deposition is referred to as PVD for short, and its core technology refers to the vapor deposition process in which at least one deposition element is atomized (atomized) under all vacuum conditions. This technology is a technology to modify the surface of materials. At first, it was also the most successful field in semiconductor industry, aerospace and other special fields. In the early 1980 s, it was used as a new surface strengthening coating technology in machinery industry. This technology focuses on the surface strengthening of cutting tools to improve the performance of mechanical friction pairs. Its characteristic is that thin films can be deposited on various substrates, the interface between thin film substrates is improved and the deposition rate is high. Physical vapor deposition of diamond-like carbon generally uses high purity graphite as carbon source, and methane gas can also be used as carbon source. Specific methods mainly include ion beam deposition, sputtering deposition, vacuum cathode arc deposition, pulsed laser deposition and so on.

In classification, PVD (Physical Vapor Deposition) coating technology is mainly divided into three categories: vacuum evaporation coating, vacuum sputtering coating and vacuum ion coating. Corresponding to the three categories of PVD technology, there are three kinds of vacuum coating equipment: vacuum evaporation coating machine, vacuum sputtering coating machine and vacuum ion coating machine.

In recent ten years, vacuum ion plating technology has developed fastest and has become one of the most advanced surface treatment methods. We usually say PVD coating refers to vacuum ion coating; PVD coater usually refers to vacuum ion coater.

(2) chemical vapor deposition

Chemical vapor deposition (CVD) is a technology that uses various energy sources, such as heating, plasma excitation or light radiation, to make gaseous or vaporous chemicals react chemically at the gas phase or gas-solid interface in the reactor to form solid deposits.

The English word chemical vapor deposition originally means chemical vapor deposition (CVD), because many reaction substances are liquid or solid under normal circumstances, and then participate in the reaction after vaporizing into steam. The ancient primitive form of chemical vapor deposition can be traced back to the black carbon layer smoked on the cave walls or rocks when ancient people warmed or barbecued.

As the primary stage of the development of modern CVD technology, it mainly focuses on the application of tool coatings in the 1950s. Since 1960s and 1970s, due to the development and production of semiconductor and integrated circuit technology, CVD technology has been developed more rapidly and extensively.

Chemical vapor deposition (CVD) is the most widely used technology in modern semiconductor industry to deposit a variety of materials, including a variety of insulating materials, most metal materials and metal alloy materials. Theoretically, chemical vapor deposition refers to a chemical process in which gaseous substances react to form solid substances and deposit them on the substrate. This method is often used to prepare hydrogen-containing carbon films. Its basic principle is to decompose hydrocarbons such as benzene, methane and acetylene into ch ions in plasma generated by glow discharge or other conditions, and at the same time apply negative bias to the substrate. Under the action of negative bias, these ionic groups containing hydrocarbons are deposited on the substrate to form carbon films. A good example is the deposited silicon nitride film (Si3N4), which is formed by the reaction of silane and nitrogen.

The researchers found that in order to meet the needs of CVD technology, the selection of raw materials, products and reaction types should usually meet the following requirements: at room temperature or not too high temperature, the reactants are preferably gaseous or liquid or solid substances with high vapor pressure and high purity; The deposition reaction is easy to generate the required material deposits, while other by-products are volatile and remain in the gas phase for discharge or easy separation; The reaction is easy to control.

In fact, for chemical vapor deposition, the reaction is very complicated and there are many factors that must be considered. The deposition parameters vary greatly: the pressure in the reaction chamber, the temperature of the wafer, the flow rate of the gas, the distance of the gas passing through the wafer, the chemical composition of the gas, the ratio of one gas to another gas, the role of the reaction intermediate products, and whether other external energy sources outside the reaction chamber are needed to accelerate or induce the desired reaction. Additional energy sources, such as plasma energy, will of course produce a whole set of new variables, such as the ratio of ions to neutral gas flow, ion energy and RF bias on the wafer.

Then, the variables in the deposited film are considered, such as the thickness uniformity in the whole wafer and the coverage characteristics on the pattern (the latter refers to the coverage across the pattern steps), the chemical composition (chemical composition and distribution state) of the film, crystal orientation and defect density, etc. Of course, the deposition rate is also an important factor, because it determines the yield of chemical vapor deposition reaction, and high deposition rate is often considered as a compromise with the high quality of thin films. The film produced by the reaction will not only be deposited on the wafer, but also on other parts of the reaction chamber. The frequency and thoroughness of cleaning the reaction chamber are also very important.

At present, farmland heating is a development direction of CVD reactive deposition temperature. Metal-organic chemical vapor deposition (MOCVD) is a medium-temperature chemical vapor deposition technology, which uses metal-organic substances as deposition reactants and realizes chemical vapor deposition through decomposition of metal-organic substances at low temperature.

Plasma enhanced chemical vapor deposition (PECVD) developed in recent years is also a good method, which was first used in the processing of semiconductor materials, that is, depositing SiO2 _ 2 on the substrate of semiconductor materials by using silicon. PECVD reduces the deposition temperature from 1000℃ to below 600℃, and the lowest is only about 300℃. In addition to semiconductor materials, plasma enhanced chemical vapor deposition technology has been successfully applied to tools, molds and other fields.