The spatial distribution of negative oxygen ion densities in a dc reactive magnetron discharge

Abstract

Using Langmuir probe-assisted eclipse laser photodetachment, the spatial distribution of O- densities in the bulk plasma of magnetron sputter tool has been determined for a range of pressures, 0.79 to 2.40 Pa. The discharge was operated in dc (200 W) with a Ti target and a fixed oxygen-argon pressure ratio of 0.2, in poisoned mode. Measurements show significant O- densities occupying an annulus downstream from the magnetic trap in regions of most positive plasma potential. With increasing pressure the region of high O- density expands and the peak densities increase reaching similar to 1.5 x 10(16) m(-3) at 2.40 Pa, corresponding to an O- to electron density ratio (electronegativity alpha) of similar to 2. Outside the area of dense negative ions, and in regions of the magnetic trap accessible to our probe we measure alpha < 0.2. The results show that these reactive magnetron plasmas, utilized for oxide film production, to be highly electronegative in regions close to the substrate.