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

Summary form only given. Plasma uniformity has been recognized as a significant parameter in large sized high density plasma processing tools. In this paper we show experimental and modeling results which indicate that significant neutral uniformity variations can also occur in high density plasma processing tools. The experiments are carried out in both inductively coupled plasma (ICP) and helicon plasma sources. A movable static pressure gauge is used to obtain the static radial and axial neutral pressure distribution both with and without a discharge present. Without a wafer present in the reactor, significant (/spl sim/20-40%) reductions in neutral pressure are observed in these sources during steady-state plasma operations. This spatially averaged neutral depletion is accompanied by hollow neutral pressure profile. The degree of on-axis neutral depletion is found to be determined by both plasma density and neutral fill pressure. We attribute these variations to the "plasma pumping" effect, wherein electron impact ionization of neutral particles is followed by their rapid removal from the plasma by the presheath electric field. A one-dimensional neutral diffusion model that incorporates this mechanism provides reasonable agreement with our results. This net loss of neutral particles can result in a large (/spl sim/50%) neutral density variation across 300 mm wafers. Neutral recycling from the wafer can create a new particle source in the center region and partially mitigate these effects. Results which demonstrate this effect, along with associated modeling results will also be shown.