Different modes of electron heating in dual-frequency capacitively coupled radio frequency discharges

A systematic study of different modes of electron heating in dual-frequency capacitively coupled radio frequency (CCRF) discharges is performed using a particle-in-cell simulation. Spatio-temporal distributions of the total excitation/ionization rates under variation of gas pressure, applied frequencies and gas species are discussed. Some results are compared qualitatively with an experiment (phase resolved optical emission spectroscopy) operated under conditions similar to a parameter set used in the simulation. Different modes of electron heating are identified and compared with alpha- and gamma-mode operation of single-frequency CCRF discharges. In this context the frequency coupling and its relation to the ion density profile in the sheath are discussed and quantified. In light gases the ion density in the sheath is time modulated. This temporal modulation is well described by an analytical model and is found to affect the excitation dynamics via the frequency coupling. It is shown that the frequency coupling strongly affects the generation of beams of highly energetic electrons by the expanding sheath and field reversals caused by the collapsing sheath. The role of secondary electrons at intermediate and high pressures is clarified and the transition from alpha- to gamma-mode operation is discussed. Depending on the gas and the corresponding cross sections for excitation/ionization the excitation does not generally probe the ionization as is usually assumed.