Excitation dynamics in electrically asymmetric capacitively coupled radio frequency discharges: experiment, simulation, and model

The symmetry of capacitively coupled radio frequency (CCRF) discharges can be controlled electrically by applying a fundamental frequency and its second harmonic with fixed but adjustable phase shift θ between the driving voltages to one electrode. In such a discharge a variable dc self-bias η is generated as an almost linear function of θ for 0° ⩽ θ ⩽ 90° via the Electrical Asymmetry Effect. The control parameter for η and the discharge symmetry is θ. Here electron dynamics in electrically asymmetric geometrically symmetric dual frequency discharges operated in argon at 13.56 and 27.12 MHz is investigated experimentally by a particle-in-cell simulation and by an analytical model. The electron dynamics is probed by the electron impact excitation rate of energetic electrons from the ground state into highly excited levels. At high pressures (collisional sheaths) the excitation dynamics is found to work differently compared with conventional CCRF discharges. Unlike in classical discharges the maxima of the time modulated excitation at the powered and grounded electrode within one low frequency period will be similar (symmetric excitation), if η is strong at θ ≈ 0°, 90°, and significantly different (asymmetric excitation), if η ≈ 0 V at θ ≈ 45°. At low pressures (collisionless sheaths) the excitation dynamics works similar to classical discharges, i.e. the excitation will be asymmetric, if η is strong, and symmetric, if η ≈ 0 V. This dynamics is understood in the frame of an analytical model, which provides a more detailed insight into electron heating in CCRF discharges and could be applied to other types of capacitive RF discharges as well.