Neutral depletion and transport mechanisms in large-area high density plasma sources

Plasma uniformity has been recognized as a significant parameter in large-s ized high density plasma processing tools, but neutral uniformity issues ha ve received less attention. In this article we show experimental and modeli ng results which indicate that significant neutral uniformity variations ca n occur in high density plasma processing tools. The experiments are carrie d out in both inductively coupled plasma and helicon plasma sources. A mova ble static pressure gauge is used to obtain the static radial neutral press ure distribution both with and without a discharge present. Without a wafer present in the reactor, significant (similar to 20%-40%) reductions in neu tral pressure are observed in these sources during steady-state plasma oper ations. This spatially averaged neutral depletion is accompanied by hollow neutral pressure profiles. The degree of on-axis neutral depletion depends upon both plasma density and neutral fill pressure. We show that the "plasm a pumping" effect, wherein electron impact ionization of neutral particles is followed by their rapid removal from the plasma by the pre-sheath electr ic field, can reproduce the experimental results. This effect has the poten tial to result in large (similar to 50%) neutral density variation across 3 00 mm wafers in high density plasma sources. (C) 1999 American Institute of Physics. [S0021-8979(99)06721-3].