Electron heating mode transition observed in a very high frequency capacitive discharge

The effect of excitation frequency in the 13.56-60 MHz range on the electron energy distribution function (EDF) of capacitively coupled plasma is investigated. Under a fixed rf voltage (50-130 V peak-to-peak) and argon pressure (100 mTorr), a remarkable change in the EDF is observed: a Druyvesteyn type at low frequencies (≅13.56 MHz) evolves into a bi-Maxwellian type in a very high frequency (VHF) above 30 MHz. The transition frequency decreases with increasing the rf voltage. The observed frequency effect on the EDF is tentatively explained in terms of the transition of electron heating mode from the collisional ohmic heating at low frequencies into the plasma surface heating in the VHF range