A two-region model of a radiofrequency low-pressure, high-density plasma

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Abstract
A two-region model of a low-pressure, high-density RF excited discharge was developed. A well-mixed bulk plasma model was coupled to a collisionless sheath to predict the species density, the time-dependent electron-velocity distribution function and the ion bombardment flux and energy. The model was applied to a chlorine discharge sustained in a transformer-coupled plasma reactor. The discharge was found to be moderately electronegative; the negative ion to electron density ratio increased with increasing pressure, decreasing power and/or increasing wall recombination probability gamma of the Cl atoms, Under these conditions the dominant ion was Cl2+. On the other hand, low pressure, high power and/or small gamma resulted in a large degree of gas dissociation; the dominant ion was then Cl+. The ion flux to the walls increased linearly with power and decreased with pressure. The model predictions were in reasonable quantitative agreement with experimental measurements. The model is most useful for sorting out the complex chemistry of plasmas and for rapid evaluation of expected reactor performance.

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