The Evaporation Coating

The evaporation coating

The evaporation of a substance by heat to deposit it on a solid surface is called evaporative coating. This method was first proposed by M. Faraday in 1857 and has become one of the most commonly used coating technologies in modern times.
Evaporating materials such as metals and compounds are placed in the crucible or hung on the hot wire as the evaporation source, and the workpiece, such as metal, ceramic, plastic, and other substrates are placed in front of the crucible. After the system is pumped to a high vacuum, the crucible is heated to evaporate the substance in it. Atoms or molecules of evaporating material are deposited on the substrate surface by condensation. The film thickness is determined by the evaporation rate and time of the source (or by the amount of charge) and by the distance between the source and the substrate. For large area coating, rotating substrate or multiple evaporation sources are often used to ensure the uniformity of film thickness. The distance from the evaporation source to the substrate should be less than the average free path of the vapor molecules in the residual gas, so as to avoid the collision of the vapor molecules with the residual gas molecules to causing chemical reactions.

There are three types of evaporation sources. (1) Resistance heating source: refractory metal such as tungsten, tantalum made of boat foil or filamentous, through the current, heating in the above or placed in the crucible of evaporating substances resistance heating source is mainly used for evaporation Cd, Pb, Ag, Al, Cu, Cr, Au, Ni, and other materials; (2) high-frequency induction heating source: the heating crucible and evaporating material with high-frequency induction current; (3) Electron beam heating source: suitable for high evaporation temperature of the material, that is, electron beam bombardment of the material to make it evaporate.
Compared with other vacuum coating methods, evaporation coating has a higher deposition rate and can be used to produce simple and refractory compound films.

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