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

Hollow neutral pressure profiles, with significant on-axis reductions in ne utral pressure (up to 40%), are observed across the face of an inert wafer in discharges with uniform plasma density. These results show that signific ant neutral depletion, which may cause the nonuniform plasma process result s, can occur in large-area high-density plasma sources with a wafer present . The neutral depletion is explained by the ion pumping effect, wherein ele ctron impact ionization of neutral particles is followed by their rapid mov ement from the plasma to the chamber wall by the presheath electric field. Cooling of plasma electrons via inelastic neutral collisions is also observ ed at elevated fill pressure, and results in a reduction of the magnitude o f neutral depletion, thus demonstrating the linkage between plasma equilibr ium and neutral equilibrium conditions. Initial experiments have also been performed in O-2 discharges. Similar hollow neutral pressure profiles are o bserved, suggesting that similar effects occur in chemically reactive disch arges. (C) 2000 American Institute of Physics. [S1070-664X(00)00808-9].