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

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 ∼1.5 × 1016 m-3 at 2.40 Pa, corresponding to an O- to electron density ratio (electronegativity α) of ∼2. Outside the area of dense negative ions, and in regions of the magnetic trap accessible to our probe we measure α < 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.