Film Deposition by Evaporation and Condensation in High Vacuum

2 FILM DEPOSITION BY EVAPORATION AND CONDENSATION IN HIGH VACUUM 

Thin films of materials of such different properties as metals, metal-halides, metal-oxides and sulphides can be obtained in the crystalline or amorphous state by condensation of the vapour on a glass substrate. The mechanism involved in forming the film may be a pure physical e.g. simple condensation or may also involve chemical reactions. 

Evaporated films were probably first made by Faraday [246] in 1857 when he exploded metal wires in an inert atmosphere. The deposition of metal films in a vacuum by resistance heating of platinum wires was performed by Nahrwold [247] in 1887 and only a year later this technique was used by Kundt [248] to produce metal films for measuring the refractive indices of metals. In the following period, evaporated thin films had only academic interest, although the vacuum evaporation of metal films by Pohl and Pringsheim in 1912 [249] was performed under better developed technological conditions. 

The techniques of forming thin films by condensation at very low pressures have been developed in parallel with industrial development of techniques of producing high vacua in large volumes, using pumps of very high speed and suitable cleanliness. 

Films obtained by condensation alone are usually made at pressures between 10 6 and 108 mbar and down to the lowest values that modem vacuum technology can attain. This is combined with fast deposition rates to produce pure deposits in which only few foreign gas atoms are incorporated. 

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