Characteristics of capacitively coupled helium plasma driven by various frequencies under constant power conditions

The influence of excitation frequency (13.56-96 MHz) on the characteristics of capacitively coupled helium plasma is investigated by means of Langumir probe and CCD camera. Measurements are performed in helium pressure of 10.66 and 33.3 Pascal (Pa) under fixed dissipated power of 10 W. With increasing the driving frequency, the RF/HF voltage and dc-self bias markedly decrease. Meanwhile, the plasma density and electron temperature peak in the frequency range 27-56 MHz, beyond which they decrease as exciting frequency increase. A different feature of the electron energy probability function EEPF is observed with exciting frequency; Maxwellian type EEPF at low frequency of 13.56 MHz evolves into a bi-Maxwellian type with a hump/beamlike in the frequency range 27-56 and eventually comes back to Maxwellian distribution at frequency ≥76 MHz. The observed results are explained in terms of electromagnetic wave effect and capacitive to inductive heating transition induced by exciting frequency.