Ion Energy Distribution Skew Control Using Phase-Locked Harmonic RF Bias Drive

The energy distribution of ions accelerated through a radio frequency sheath and incident on a plasma-facing material significantly influences material interaction with the plasma and can impact manufacturing at the nanoscale. Ion energy distributions are controlled through appropriate mixing of drive frequencies, which has been shown to control distribution width. This paper presents a modification to multifrequency drive for ion energy control by exploiting a digital frequency and phase controller that enables modification of the higher order moments of the distribution, specifically, controlling the skew of the distribution. By modulating the sheath with two frequencies where one frequency is the harmonic of the other and controlling the relative phase of these two waveforms incident on the plasma-facing surface, skew control is achieved. A simple empirical model is presented to describe this method, as well as experimental validation of the model and demonstration of skew control in a parallel plate capacitively coupled reactor.