Excitation dynamics in electrically asymmetric capacitively coupled radio frequency discharges

The symmetry of capacitively coupled radio frequency discharges can be controlled electrically by applying a fundamental frequency and its second harmonic with adjustable phase shift $\theta $ between the driving voltages to one electrode. A variable DC self bias $\eta $ is generated as a function of $\theta $ via the Electrical Asymmetry Effect. Here excitation dynamics in electrically asymmetric geom. symmetric dual frequency discharges operated in argon at 13.56 + 27.12 MHz is investigated experimentally, by a PIC simulation, and by an analytical model. At low pressures (collisionless sheaths) the excitation dynamics works similar to classical discharges: The maxima of the time modulated excitation at the powered and grounded electrode within one low frequency period will be different (asymmetric excitation), if $\eta $ is strong at $\theta \approx $0\r{ } 90\r{ } and similar (symmetric excitation), if $\eta \approx $0 V at $\theta \approx $45\r{ }. At high pressures (collisional sheaths) the excitation dynamics is found to work differently. The excitation will be symmetric, if $\eta $ is strong, and asymmetric, if $\eta \approx $0 V. These phenomena are understood by an analytical model.