Ion Energy and Density Profile Control by Focus Ring and Associated External Circuit on Capacitively Coupled Plasma

Capacitively coupled plasmas are widely used in semiconductor processes. However, control of the plasma to obtain uniform deposition and etching over a large process space is still an open problem. One promising approach is to utilize focus ring (FR) with a tunable external circuit consisting of a variable capacitor and an inductor to extend the uniformity to the wafer edge. Usually, the geometry of FR allows gross adjustments of the plasma processing uniformity at the wafer edge. The addition of an external circuit coupling the ring to ground may optimize the ion energy and density profile by changing the impedance between the FR and the ground and allow wafer edge tuning over a wide range of operating parameters. In this work, it is found that the adjustable circuit can control the edge ion energy through distributing the voltage and the energy loss between different sheaths. The FR and its external circuit also control the spatial distribution of the electric field and thereby reduce plasma edge effects and improve density uniformity. These results point to possible source designs for engineering the distribution of power dissipation across these sheaths in industrial plasma reactors.