Plasma chemistry in high-density glow discharges used in materials processing

Motivation for the modelling of low pressure (1-100 mTorr), high density (10{dollar}sp{lcub}11{rcub}{dollar}-10{dollar}sp{lcub}12{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar}) plasma chemistry is based on experimental results in tungsten etching, which suggested that there are two limiting regimes which control the etch rate: an ion flux-limited regime and a neutral reactant-limited regime. A plasma chemistry model of oxygen was developed, with electron-neutral collision processes and heavy particle collisions included, and rate coefficients expressed as a function of electron temperature when appropriate. We found that the fractional dissociation in these high density sources is much higher than in traditional high pressure, low density parallel plate reactors. The ratio of negative ion to electron density decreases with increasing power density and decreasing pressure, and rarely exceeds unity.; General equations for particle and energy balances were developed and extended to more complicated systems, such as Ar/O{dollar}sb2{dollar}. Estimates were made of cross sectional data when they were not available in the literature. This leads to a high degree of uncertainty in the rate coefficients. Therefore, a sensitivity analysis is necessary to determine the reactions that are most affected by the uncertainties. The sensitivity analysis was developed for the argon/oxygen and chlorine systems. One key result of the analysis is that the surface recombination coefficient plays an important role, affecting both the degree of dissociation and the negative ion density. Since Cl{dollar}sb2{dollar} is a diatomic gas, formulation of the Cl{dollar}sb2{dollar} gas phase chemistry is straightforward and similar to that of O{dollar}sb2{dollar}. Results for a pure Cl{dollar}sb2{dollar} discharge without the presence of a silicon wafer were obtained and compared to O{dollar}sb2{dollar} discharges. The two systems showed significantly different trends, especially in the variation with pressure of total positive ion density and of the ratio of negative ion to electron density. The model was extended to couple gas phase and surface reactions in order to predict etch rate behavior as a function of operating conditions in Cl{dollar}sb2{dollar} etching of silicon, including the effects of SiCl, etch products (x = 0-4). We also considered the cases of nonreactive and reactive walls, which account for the wall surface being inert to silicon-containing species, or acting as a reactive site to form etch products. Two limiting mechanisms were observed: the ion flux-limited regime and the neutral reactant-limited regime. The reactive and nonreactive walls showed significantly different results, with a decrease in the absolute atomic silicon density, and a weaker dependence of etch rate on flowrate for the reactive wall.