Introduction

VACUUM DEPOSITION, or vacuum evaporation, is a physical vapor deposition process in which the atoms or molecules from a vaporization source reach the substrate without colliding with residual gas molecules. Generally, the vaporization source is one that vaporizes materials by thermal means (that is, evaporation or sublimation), but other vapor sources can be used. The advantage of vacuum evaporation is that films of a variety of materials can be deposited at high rates over large areas in a very pure form. Limitations of vacuum evaporation are that often the films do not have optimum properties and that there are relatively few deposition parameters that can be modified to improve the film properties.

Vacuum deposition requires a vacuum of better than 13 mPa (0.1 mtorr). At this pressure, there is still a large amount of concurrent bombardment of the substrate by potentially undesirable residual gases that can contaminate the film. If film contamination is a problem, a high (13 pPa, or 10"7 torr) or ultrahigh (0.13 pPa, or 10-9 torr) vacuum environment can be used to produce a film with the desired purity, depending on the deposition rate, reactivities of the residual gases and depositing species, and the tolerable impurity level in the deposit.

Vacuum deposition of a film was first reported by Faraday in 1857 using exploding wires. Nahrwold was the first to use thermal evaporation in a vacuum to produce a thin film, in 1887. Vacuum deposition was not routinely used until about 1929, when it was first used for optical coatings (Ref 1). Strong, for example, metallized the 100 in. mirror for the Mount Wilson Observatory in 1935. The subject of vacuum deposition was thoroughly investigated by Glang in 1970 (Ref 2), and most review articles and book chapters on the subject since that time have drawn heavily on his work.

In reactive evaporation, a partial pressure of reactive gas is used to deposit compounds of the vaporized material by the reaction of deposited atoms with ambient gases. Reactive evaporation was first reported by Auwarter in 1952 and Brinsmaid et al. in 1953. In 1971, Heitmann used reactive evaporation to deposit oxide films by evaporating the film material through a plasma of the reactive gas, and this technique is now generally called activated reactive evaporation. With this method, either a gas, such as oxygen or nitrogen, or a hydrocarbon is metered into the vacuum chamber, where it reacts with the vapor from a metallic evaporant to form a metal oxide, nitride, or carbide coating.

In gas evaporation, a high residual gas pressure causes the formation of ultrafine (100 nm, or 1000 A) particles by gas phase collision and nucleation. Gas evaporation is a term given to the deposition of ultrafine particles ("smokes"), which are formed by gas phase nucleation due to collision of the evaporated atoms with residual gas molecules. This typically requires an ambient gas pressure greater than about 1.3 mPa (10 torr). The formation of useful films of ultrafine particles formed by gas evaporation was reported by Pfund, who produced "zinc black" infrared absorbing films in 1933.

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