Vacuum Evaporation

Vacuum Evaporation by electrical resistance heated sources is often a preferred entrance level technique for the deposition of thin films. The process can be easy to master. If smaller dimensioned sources are chosen, the less expensive laboratory sized coating plants can be used. It is possible to ramp the process up to a large production scale by the use of the bigger sources and multiples of them - sometimes in extremely large capacity high vacuum chambers. The technique may not be the best for some thin film materials and applications; electron beam evaporation or sputtering are just two alternative techniques, but resistance heated evaporation sources often produce good cost effective results, and are in regular use. Resistively heated evaporation sources are made from high purity, minimum 99.95%, refractory metals which have high melting points and low vapour pressures. The most commonly used refractory materials are tungsten, molybdenum and tantalum. There are a number of evaporation source designs - multi-strand filaments, wire baskets, boats and crucibles. An electric power source with low voltage tappings and high currents will be needed. The boat evaporation sources are usually more demanding in their power requirements. Multi-strand (often three strand) tungsten filaments are typically used to evaporate metals such as aluminium or platinum. Tungsten wire baskets can be employed to evaporate chips or pellets of metals or dielectrics which either sublime or do not wet the basket when melting. They can also hold and heat crucibles containing the evaporant. Boat sources are also fabricated from tungsten, molybdenum and tantalum Most elements and compounds can be evaporated from the standard boat sources, but where the material "wets" the complete boat (such as nickel) an aluminium oxide coated source may be considered. For highly dense, uniform, chromium films, chromium plated tungsten rods are available and baffle sources for materials such as silicon monoxide and zinc sulphide.

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