Investigation of effect of excitation frequency on electron energy distribution functions in low pressure radio frequency bounded plasmas

Particle in cell (PIC) simulations are employed to investigate the effect of excitation frequency omega on electron energy distribution functions (EEDFs) in a low pressure radio frequency (rf) discharge. The discharge is maintained over a length of 0.10 m, bounded by two infinite parallel plates, with the coherent heating field localized at the center of the discharge over a distance of 0.05 m and applied perpendicularly along the y and z directions. On varying the excitation frequency f (omega/2 pi) in the range 0.01-50 MHz, it is observed that for f <= 5 MHz the EEDF shows a trend toward a convex (Druyvesteyn-like) distribution. For f > 5 MHz, the distribution resembles more like a Maxwellian with the familiar break energy visible in most of the distributions. A prominent "hot tail" is observed at f >= 20 MHz and the temperature of the tail is seen to decrease with further increase in frequency (e.g., at 30 MHz and 50 MHz). The mechanism for the generation of the "hot tail" is considered to be due to preferential transit time heating of energetic electrons as a function of omega, in the antenna heating field. There exists an optimum frequency for which high energy electrons are maximally heated. The occurrence of the Druyvesteyn-like distributions at lower omega may be explained by a balance between the heating of the electrons in the effective electric field and elastic cooling due to electron neutral collision frequency nu(en); the transition being dictated by omega similar to 2 pi nu(en). (C) 2011 American Institute of Physics. [doi: 10.1063/1.3605021]