ION DISTRIBUTION-FUNCTIONS BEHIND AN RF SHEATH

Measurements of ion energy distribution functions were made at the gro unded electrode of a parallel plate radiofrequency discharge by means of a gridded energy analyser in discharges in Ar, He, and O2 at pressu res 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, a ppears. Although the Debye length is small compared to the mean free p ath for charge exchange, for high sheath voltages the sheath thickness increases and may exceed the mean free path. Measured distributions i n Ar are compared to energy distribution functions calculated in a sel f-consistent sheath model, including charge exchange collisions. The a greement of theory and experiment is very good.