Mass Spectrometry of Background Gases in Glow-Discharge Sputtering of Tantalum Thin Films

Abstract

The composition of the ambient atmosphere during the sputtering of tantalum films in argon glow discharges has been studied by means of a mass spectrometer adapted both for residual analysis of vacua between 10−9−10−5 Torr, and process monitoring at pressures of (10−50)×10−5 Torr. The partial pressures of a number of residuals were found to increase immediately upon initiation of the discharge in both an oil diffusion and a turbomolecular pump system. The total background gas levels during the discharge were in the 1×10−5−5×10−4 Torr range, with the major component being hydrogen, and smaller contributions from water vapor and methane. The pressure increase of these species resulted from ion bombardment decomposition of adsorbed surface layers, and exhibited two maxima. The first is attributed to desorption from the electrodes, and the second to desorption from the bell jar wall. The spectrometer was used for detailed studies of the gettering of deliberate additions of N2, CH4 and H2 during sputtering in argon. Considerable differences were found in the effect of the glow discharge on those gases, with the gettering of CH4 and H2 exhibiting a complex time dependence. Tantalum films sputtered without deliberate gas additions, were found to be of the beta-tantalum form even when the discharge-generated background was reduced to ∼1×10−6 Torr by long presputtering.