Anisotropy in EEDF of capacitive RF discharge

Angle-resolved measurements of the EEDF in the midplane of capacitive, symmetrically driven, parallel plate RF discharges in argon and helium have been made using a small flat rotatable probe. Using a specially designed multifrequency probe filter, EEDF measurements were carried out at 13.56, 27.12 and 54.24 MHz. Measurements were made starting at the lowest gas pressure where a discharge could be maintained and up to a fraction of a Torr where the EEDF's were isotropic. Ripples on the tail of the EEDF were observed with the surface of the flat probe parallel to (and facing) an RF electrode, however, the degree of EEDF anisotropy even at the minimum gas pressure was not very large. This is in contradiction with results obtained in Ref. 2 under similar discharge conditions using a large flat probe. The anisotropy grew with increasing RF discharge voltage and disappeared at elevated gas pressures. At the same gas pressure, the anisotropy in argon was found to be less than that in helium. The dependence of the EEDF anisotropy on frequency and RF voltage, the difference in the degree of anisotropy for argon and helium, and the reason for the discrepancy between the results and those in Ref.2 will be discussed in the presentation