Evaporation coating: By heating and evaporating a certain substance to deposit it on the solid surface, it is called evaporation coating. This method was first proposed by M. Faraday in 1857, and it has become one of the
commonly used coating techniques in modern times. The structure of the evaporation coating equipment is shown in Figure 1.
Evaporated substances such as metals, compounds, etc. are placed in a crucible or hung on a hot wire as the evaporation source, and the workpiece to be plated, such as metal, ceramic, plastic and other substrates, is placed in front of the crucible. After the system is evacuated to a high vacuum, the crucible is heated to evaporate the contents. The atoms or molecules of the evaporated substance are deposited on the surface of the substrate in a condensed manner. The thickness of the film can range from hundreds of angstroms to several microns. The thickness of the film is determined by the evaporation rate and time of the evaporation source (or the loading amount), and is related to the distance between the source and the substrate. For large-area coatings, a rotating substrate or multiple evaporation sources are often used to ensure the uniformity of the film thickness. The distance from the evaporation source to the substrate should be less than the mean free path of vapor molecules in the residual gas to prevent the collision of vapor molecules with residual gas molecules from causing chemical effects. The average kinetic energy of vapor molecules is about 0.1 to 0.2 electron volts.
There are three types of evaporation sources.
①Resistance heating source: Use refractory metals such as tungsten and tantalum to make boat foil or filament, and apply electric current to heat the evaporated substance above it or in the crucible (Figure 1 [Schematic diagram of evaporation coating equipment] vacuum coating) Resistance heating source is mainly used to evaporate materials such as Cd, Pb, Ag, Al, Cu, Cr, Au, Ni;
②High-frequency induction heating source: use high-frequency induction current to heat the crucible and evaporation material;
③Electron beam heating source: applicable For materials with higher evaporation temperature (not lower than 2000 [618-1]), the material is vaporized by bombarding the material with electron beams.
Compared with other vacuum coating methods, evaporative coating has a higher deposition rate, and can be coated with elementary and non-thermally decomposed compound films.
In order to deposit a high-purity single crystal film, molecular beam epitaxy can be used. The molecular beam epitaxy device for growing doped GaAlAs single crystal layer is shown in Figure 2 [Schematic diagram of molecular beam epitaxy device vacuum coating]. The jet furnace is equipped with a molecular beam source. When it is heated to a certain temperature under ultra-high vacuum, the elements in the furnace are ejected to the substrate in a beam-like molecular stream. The substrate is heated to a certain temperature, the molecules deposited on the substrate can migrate, and the crystals are grown in the order of the substrate crystal lattice. Molecular beam epitaxy can be used to
obtain a high-purity compound single crystal film with the required stoichiometric ratio. The film grows the slowest The speed can be controlled at 1 single layer/sec. By controlling the baffle, the single crystal film with the required composition and structure can be made accurately. Molecular beam epitaxy is widely used to manufacture various optical integrated devices and various superlattice structure films.
Post time: Jul-31-2021