Experimental and calculated radial pressure gradients in reactive sputter processes

Abstract

Partial pressure gradients in reactive dc-magnetron sputtering are investigated. The combination of diffusion of the reactive gas (e.g., O2) in a sputtering gas (e.g., Ar) which is in a dynamic equilibrium and the high pumping speed of the substrate due to gettering of the reactive gas causes large partial pressure variations of the reactive gas along the surface of the substrate. Numerical calculations for an Al/O2 process show quantitative data on the influence of the deposition rate, sputter pressure, target-to-substrate distance, and substrate size. The outcome of the calculations is in good agreement with the measurements. The variations in the partial pressure of the reactive gas can be quite substantial. For example, at an Ar pressure of 8 mTorr, a target-to-substrate distance of 6 cm, and a deposition rate of 20 Å/s, the O2 partial pressure at 20 cm from the substrate center is 2×10−4 Torr (2.6×10−2 Pa) whenever the partial pressure in the center is 4.4×10−5 Torr (5.9×10−3 Pa).