7.2.3 Chemical composition
As outlined in Table 2, some deposition techniques are best suited to conducting materials, whereas others come into their own only for chemical compounds. In either case, chemical composition may be an important consideration. Impurities can interfere with the conduction properties of the material (notably in the accidental doping of semiconductors, but also for metals and insulators) or may result in residues when the material is later etched.
As a rule of thumb, processes that act in a solution or that generate liquid by-products tend to produce the lowest purity in the deposited material, as it is easy for contaminants to be incorporated into the growing film. For deposition processes using a gas or plasma, the lower the pressure the cleaner the chamber will be and the more slowly contaminants will rain down onto the wafer. This must be traded off against the deposition rate of the film – fast deposition at low pressure provides the best film purity (assuming, of course, that the source material is pure in the first place).
As well as impurities, in a material with multiple components (an alloy or a compound) obtaining the correct stoichiometry (the relative quantities of the different components) is important. There is no guarantee, even though the source material has the correct composition, that this will be maintained into the deposited film. This is especially true of chemical compounds such as oxides, so it is usual to deposit these in a reactive gas atmosphere that re-forms the compound as it deposits.