Materials for Vaporization

Material placed in the vaporization source is called a charge and can be in the form of powder, chunks, wire, slugs, and so on. The desired purity of the source material depends on the application and the effect of purity on film properties and process reproducibility. It is possible to obtain some material with extremely high purity (>99.999%), although the cost increases rapidly with the purity level. Often impurities such as oxygen, nitrogen, carbon, and hydrogen are not specified by the supplier. They can be present in significant quantities as oxidized surfaces of reactive metals, hydrogen incorporated into electrorefined chromium, carbon monoxide in nickel purified by the carbonyl process, and helium in natural quartz. Generally, it is better to specify vacuum-melted materials from the supplier when possible. Very reactive metals should be nitrogen-packed in glass ampules to prevent oxidation, and the ampules should be opened and handled in an inert gas dry box where the reactive gas content is kept low by the use of getter materials such as liquid NaK [K:Na (20-50%)].

Careful specification of purity, allowable impurities, fabrication method, postfabrication treatments, and packaging of the source materials purchased is important to obtaining a reproducible process. Using inexpensive material or material of unknown origin often creates problems. Source material should be carefully cleaned and handled, because on heating, the impurities and surface contaminants are the first materials to be vaporized. The source and source material can be outgassed and premelted prior to film deposition.

Materials Utilization. Often material utilization in an evaporation process is poor unless proper fixturing and tooling is used to intercept the maximum amount of the flux. This can be accomplished by having the substrates as close as possible to the vaporization source, though this can result in excessive heating of the substrate during deposition. Deposition on large numbers of parts or over large areas can be done using large chambers with many (or large) vaporization sources.

Excess deposited material builds up on walls and fixtures and can flake off, producing particulate contamination in the processing chamber. The deposit buildup on surfaces also generates high surface areas that can absorb contaminants and are slow to desorb the contaminants. This results in changes in the pumping characteristics of the deposition system with use. It is desirable to have an easy way to collect and remove the excess evaporated material, and it can also be important to collect the excess material for reclamation. Excess evaporant can be collected by surrounding the source with a removable collector that defines the flux of evaporant to the substrate and collects excess evaporant. The chamber can have a removable liner to collect excess material, and the fixtures can be surrounded with panels or foil that can be easily removed.

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