Electron energy distribution functions in processing plasmas

Abstract

The electron energy distribution function (EEDF) in a low pressure processing discharge is a good indicator of the state of the plasma. Chemical kinetics are especially sensitive to the EEDF and the electron population plays a central role in coupling power into the desired surface reactions. Process development and transfer will be aided by a knowledge of the EEDF and its sensitivity to various process parameters. The EEDF can be measured using an electrostatic probe to sample the electron population by drawing a small current from the plasma. Electron and ion density and a characteristic electron (Maxwellian) temperature can be deduced from probe data. Furthermore, the current-voltage characteristic of the probe represents integrated EEDF information which can be recovered by differentiation. The practical use of this probe technique is considered for a variety of plasmas (both DC and RF generated). The problems associated with processing plasmas such as deposition and RF excitation are discussed with particular emphasis on 13.56MHz capacitively coupled systems. Typical data are viewed in terms of energy resolution and accuracy and conditions necessary for reliable data acquisition are considered. © 1990 IUPAC