Thin Films Deposited by Bias Sputtering

Abstract

The application of a suitable negative bias to a film that is being deposited by dc sputtering produces a film of substantially lower resistivity (than without bias). The action of the bias is shown to result from positive ion bombardment of the film during deposition, leading to the selective removal of adsorbed impurities. An analytical expression describing the process is derived and found to give good agreement for tantalum films sputtered in argon containing oxygen as a contaminant. The effect of bias on contaminants originating both at the cathode and in the gas is compared. Comparison of bias sputtering and asymmetric ac sputtering in terms of an ion bombardment model shows that bias sputtering leads to purer films because ion clean-up is in operation during the entire deposition time only for the case of bias sputtering. Good adhesion of bias-sputtered Ta films to glass substrates is obtained by initially applying a small positive bias to the substrate.