Effect of driving frequency on the electron energy distribution function and
electron-sheath interaction in a low pressure capacitively coupled plasmas
By using a self-consistent particle-in-cell simulation we investigated the effect of
driving frequency (27.12–70 MHz) on the electron energy distribution function
(EEDF) and electron-sheath interaction in a low pressure (5 mTorr) capacitively
coupled Ar discharge for a fixed discharge voltage. We observed a mode transition
with driving frequency, changing the shape of EEDF from a strongly bi-Maxwellian
at a driving frequency of 27.12 MHz, to a convex type distribution at an intermediate
frequency, 50 MHz, and finally becomes a weak bi-Maxwellian at a higher driving
frequency i.e. above 50 MHz. The transition is caused by the electric field transients
which is of the order of electron plasma frequency caused by the energetic ‘beams’ of
electrons ejected from near the sheath edge. Below the transition frequency, 50 MHz,
these high energy electrons redistributes their energy with low energy electrons
thereby increasing the effective electron temperature in the plasma, whereas, the
plasma density remains nearly-constant. Above the transition frequency high-energy
electrons are confined between opposite sheaths which increases the ionization
probability and therefore the plasma density increases drastically.