Measurement of radial neutral pressure and plasma density profiles in various plasma conditions in large-area high-density plasma sources

Abstract

Hollow neutral pressure profiles, with significant on-axis reductions in neutral pressure (up to 40%), are observed across the face of an inert wafer in discharges with uniform plasma density. These results show that significant neutral depletion, which may cause the nonuniform plasma process results, can occur in large-area high-density plasma sources with a wafer present. The neutral depletion is explained by the ion pumping effect, wherein electron impact ionization of neutral particles is followed by their rapid movement from the plasma to the chamber wall by the presheath electric field. Cooling of plasma electrons via inelastic neutral collisions is also observed at elevated fill pressure, and results in a reduction of the magnitude of neutral depletion, thus demonstrating the linkage between plasma equilibrium and neutral equilibrium conditions. Initial experiments have also been performed in O2 discharges. Similar hollow neutral pressure profiles are observed, suggesting that similar effects occur in chemically reactive discharges.