Sputtering of Alloys and Compounds

The general theory discussed in Sect. 12.2 explains that most sputtering phenomena in elemental materials are based on a collision cascade picture The incident ion initiates collisions in a volume (the collision cascade) surrounding the ion track. The energy of the incident ion is shared among those atoms within that volume and then dissipated. Only collisions that occur near the surface of the material can knock atoms out of the material. Most sputtered atoms emerge only from the first few atomic layers. The more collisions that occur near the surface, 

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R. Behrisch, ed., "Sputtering by Particle Bombardment II Sputtering of Alloys and Compound, Electron and Neutron Sputtering, Surface Topography," in Topics in Applied Physics, Vol. 52, Springer-Vedag, Berlin, 1983. 

The technique may be said to combine the advantages of vacuum evaporation and sputtering, so that excellent qualities of adhesion are obtained without a limitation of maximum thickness of the coating—while at the same time the rate of deposition can be comparatively high. Many metals, alloys, and compounds may be deposited, on both metallic and non-metallic articles. However, its use at present is mainly for functional and protective applications, particularly where high resistance to corrosion is required. Thus, as examples, aluminium may be deposited on various types of steel and on titanium for uses in the aerospace and defence industries—and can be regarded as a less hazardous replacement for cadmium electroplating. 

The main features remain the same for composite materials such as binary alloys. There are additional complications, however, because there are two kinds of atoms in the material. The two species may not be sputtered at an equal rate because of differences in energy sharing (in the collision cascade), ejection probabilities, or binding energies. Indeed, preferential sputtering of one species over the other has been observed in many alloys and compounds. 

The sputtering yield for alloys and compounds can generally not be ascertained from the values of the pure metals, however, estimation is usually possible. 

Films of IVA-VIA compounds have been prepared by the aqueous reactions of group IV nitrates with thio- or selenourea, in basic solution. More recently, bulk crystals, especially of the alloys, have been made by direct reaction. Control of stoichiometry is always difficult. At present, molecular beam epitaxy (precise evaporation of the elements) has become preeminent, because alloys of PbTe with both SnTe and EuTe can be made. It is surprising that a rare earth atom can be substituted into such a lattice, and even more surprising that its electronic behavior appears to be that of a substituent with a valence of + 2. Sn02, while differing widely from the lead salts , is also a IV-VI compound that can be prepared as films by spray pyrolysis of the chloride, or by reactive evaporation or sputtering. 

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