Ion distribution functions behind an RF sheath

Abstract

Measurements of ion energy distribution functions were made at the grounded electrode of a parallel plate radiofrequency discharge by means of a gridded energy analyser in discharges in Ar, He, and O2 at pressures of the order of 0.001-0.1 mbar. In this regime the transition from the collisionless space charge sheath (in front of the electrode) to a sheath dominated by collisions, mainly charge exchange collisions, appears. Although the Debye length is small compared to the mean free path for charge exchange, for high sheath voltages the sheath thickness increases and may exceed the mean free path. Measured distributions in Ar are compared to energy distribution functions calculated in a self-consistent sheath model, including charge exchange collisions. The agreement of theory and experiment is very good.