Deposition of Materials

Deposition of Elements. Elements are typically deposited by vaporization of the elemental material directly. The purity of the deposited film depends on the purity of the starting material and contaminant gases in the ambient.

Deposition of Alloys. Alloys can be deposited directly by the vaporization of the alloy material if the vapor pressures of the constituent are comparable. However, if the vapor pressures differ, the composition of the film will change as the deposition proceeds and the composition of the melt changes.

Rod-Fed Source. One technique for depositing a constant composition alloy film is to use a rod-fed electron beam evaporation source and to keep the temperature and volume of the molten pool constant. After an equilibration time, the composition of the vapor flux is the same as the composition of the material being melted, even though the composition of the molten pool is not the same as that of the bulk material (Ref 18, 19). Using this technique, alloys whose constituents vary in vapor pressure by as much as 103 are being deposited on aircraft engine turbine blades.

Sequential Layers. Alloy films can be formed by depositing alternating layers of the different materials from different sources. The layers are then diffused to form the alloy film. The alloy composition then depends on the relative amounts of materials in the films.

Multiple Sources. Alloy films can be deposited using multiple separate vaporization sources with individual deposition rate controllers. In this case, the vapor flux distribution from each source must be taken into account. The multiple source technique can also be used to deposit layered composite films (Ref 2, 20). Sources with overlapping flux distributions can be used to form films having a range of compositions over the substrate surface.

Flash Evaporation. A constant-composition alloy film can be deposited using flash evaporation techniques, where a small amount of the alloy material is periodically completely vaporized (Ref 2, 21, 22, 23). Flash evaporation uses a very hot surface and requires dropping a pellet or periodically touching a wire tip to the surface, so that the molten pellet or tip is completely vaporized.

Deposition of Compounds. When compounds are vaporized, some of the lighter fragments, such as oxygen, may be lost by scattering in the gas phase and by not reacting with the deposited material when it reaches the substrate. In the vaporization of SiO2, this results in an oxygen-deficient SiO2_x film that is yellowish in color. The composition of the deposited material is determined by the degree of dissociation, the loss of materials in the mass transport process, and the reaction coefficient of the reactive species at the film surface. The lost oxygen can be replaced by reactive deposition in an oxygen environment (see the section "Reactive Evaporation" in this article), or postdeposition heat treatments in oxygen (Ref 24).

In some cases, the state of reaction can be increased by concurrent bombardment with a reactive species from a plasma (activated reactive ion plating) (Ref 25) or an ion source (reactive ion beam assisted deposition). For example, SiO, which is easily evaporated, can be bombarded with oxygen ions to give SiO18, which is of interest as a transparent, insulating, permeation-barrier coating on polymers for the packaging industry.

Graded Interfaces. When layered structures are deposited, the interface between the layers can be graded from one composition to the other by beginning the second deposition before the first is completed. This forms a "pseudodiffusion" interface between the two layers and prevents possible contamination/reaction of the first layer by the ambient environment before the second layer begins depositing. Grading the interface between deposited films often provides better adhesion than abruptly changing from one material to another.

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