Reactive Evaporation

Films of compounds can be deposited by reactive deposition, in which the elemental constituents of the compound are vaporized and codeposited, or in which the deposited material reacts with a reactive gaseous ambient on the surface of the depositing film. Most reactive deposition involves a condensible species (that is, titanium, zirconium, or aluminum) and a gaseous species (O2, N2), although some compounds of two or more condensible species can be deposited (for example, titanium and carbon). When codepositing two reactive species, obtaining the correct composition is often difficult, because the relative fluxes must be carefully controlled and generally the substrate temperature must be high to ensure reaction (for example, TiC from titanium plus carbon).

In reactive evaporation the reactive gas is in its natural (that is, molecular) state (Ref 94, 95, 96). This technique generally requires that there be many more reactive gas molecules striking the depositing film than are needed for forming the compound, because the reaction coefficient is generally much less than one.

Activated Reactive Evaporation. If the reactive gas is "activated" by forming new molecular species (that is, ions, radicals, and excited species), the reactive deposition process is termed activated reactive evaporation (ARE) (Ref 97, 98, 99, 100). Because the reactive gas is activated, the gas density needed for reaction is less than that needed for reactive evaporation. The reactive gas can be activated by a plasma (Fig. 9), thermal decomposition, photoadsorption, or some other means. The use of atomic or radical beams of the reactive gas, directed toward the depositing material during deposition, allows reactive deposition to take place at a relatively low chamber pressure. An example of activated reactive evaporation that does not use a plasma is the deposition of a thin layer of titanium nitride to reduce secondary electron emission, by evaporating titanium in ammonia where the ammonia is decomposed by the hot evaporating filament to provide reactive nitrogen radicals (Ref 101).

Evaporation Process

Fig. 9 Schematic of the activated reactive evaporation process using an electron-beam gun as an evaporation source. Source: Ref 99

In reactive evaporation, the interface can be graded by controlling the availability of the reactive gas. For example, in depositing titanium nitride the material can be graded from titanium to titanium nitride by controlling the availability of the nitrogen. Reactive evaporation can be used to form compounds when the evaporant is another compound. For example, cubic boron nitride (CBN) has been formed by the evaporation of H3BO3 in a plasma of NH3 (Ref 102).

0 0

Post a comment