PONDEROMOTIVE FORCE AND ION ENERGY-DISTRIBUTIONS IN AN RF SHEATH

The incident-ion energy distribution in a collisionless rf sheath is derived analytically for a general electric-field configuration in the high-frequency regime. The analysis is based on a two-time-scale asymptotic expansion of the ion equation of motion, where the ratio of the ion transit frequency omega(tr) to the rf frequency-omega is assumed to be small. It is shown that the "ponderomotive force" due to the rf modulation of the electric field exerts a retarding effect on the ion motion, counteracting the dc-bias field. The results obtained here are applicable to rf-discharge-based process tools used in microelectronics fabrication, where the conditions of low collisionality and high rf frequency are usually satisfied.