Plasma uniformity control in capacitive RF discharges by an individually RF driven sidewall electrode

Abstract

A novel method of uniformity control in low pressure capacitively coupled radio-frequency plasmas by an individually driven sidewall electrode is proposed and verified by two-dimensional kinetic simulations. At the sidewall electrode, energetic electron beams are generated by sheath expansion heating and propagate radially towards the reactor center. Upon reaching the edge of the planar powered electrode, where the wafer is located in plasma processing applications, they interact with the time-modulated RF sheath at this electrode. Depending on the amplitudes of and the phase shift between the voltage waveforms applied to the sidewall and the wafer electrode at the same frequency, they reach the wafer electrode edge at different times within one period of the local sheath oscillation. If the radially propagating beam electrons arrive at the wafer electrode edge, when the local sheath is thick, they will be accelerated upwards and cause significant local ionization peaks at the wafer electrode edge. This effect is demonstrated to improve the radial plasma uniformity at low pressures drastically, if a center high plasma density profile is present in the absence of an RF driven sidewall electrode. If the beam electrons arrive at the wafer electrode edge, when the local sheath is collapsed, they will continue propagating radially towards the center and the effect on the uniformity will be reduced. Thus, the voltage amplitude applied to the sidewall electrode and its phase relative to the RF voltage applied to the planar powered electrode are found to be effective parameters to electrically control the radial plasma uniformity above the wafer.