Effects of phase regulation on ion energy distribution in RF bias sputtering

Ion energy distributions (IEDs) at a radio frequency (RF) driven substrate electrode were measured in an rf (13.56 MHz) bias sputtering system with a phase shifting unit for regulating the two rf voltages applied to the target (upper) and the substrate (lower) electrodes. Without the regulation, the IED was of a centrally peaked shape, not of a bimodal shape which is a characteristic of ions accelerated across an RF sheath, while the IED with the regulation showed a strongly asymmetric bimodal saddle shape. Moreover, it was found that the energy spread and asymmetry of IEDs and the ion current varied with the phase angle difference, and were correlated with the measured sheath thickness variation. These results were discussed with a simple collisionless oscillating sheath model.