If a temporally symmetric voltage waveform is applied to a capacitively coupled radio frequency (CCRF) discharge, that contains one or more even harmonics of the fundamental frequency, the sheaths in front of the two electrodes will necessarily be asymmetric even in a geometrically symmetric discharge. Optimally this is achieved with a dual-frequency discharge driven at a phase locked fundamental frequency and its second harmonic, e.g. 13.56 MHz and 27.12 MHz. An analytical model, a hybrid fluid/Monte-Carlo kinetic model as well as a Particle in Cell (PIC) simulation show that this Electrical Asymmetry Effect (EAE) leads to the generation of a DC self bias as a function of the phase between the applied voltage harmonics in geometrically symmetric as well as asymmetric discharges. The DC self bias depends almost linearly on the phase angle and the role of the electrodes (powered and grounded) can be reversed. At low pressures the EAE is self-amplifying due to the conservation of ion flux in the sheaths. By tuning the phase, precise and convenient control of the ion energy at the electrodes can be achieved, while the ion flux remains constant. The maximum ion energy can typically be changed by a factor of about three at both electrodes. At the same time the ion flux is constant within 5%.
ASJC Scopus subject areas
- Physics and Astronomy(all)