Electron density and ion energy dependence on driving frequency in capacitively coupled argon plasmas

The dependence of the electron density and the mean ion energy relative to the ground electrode on the driving frequency (13.56?156?MHz) is investigated in capacitively coupled argon plasmas. It is found that at constant rf power, the electron density increases with the driving frequency then starts to level off after reaching some transition frequency. When the driving frequency exceeds this transition frequency, almost all rf power is spent on electron heating and the electron density remains unchanged even when the frequency continues to increase. The measured data is compared with the results from an inhomogeneous plasma model. The variations of the measured electron density and mean ion energy are found to be consistent with the scaling between the electron heating and the ion acceleration power and their dependence on driving frequency, as obtained earlier by Godyak.