Graded conductivity electrodes as a means to improve plasma uniformity in dual frequency capacitively coupled plasma sources

Dual frequency, capacitively coupled plasma (DF-CCP) tools are now being used for etching of 30 cm diameter wafers during microelectronics fabrication. These tools typically use a high frequency (HF, tens to hundreds of megahertz) to sustain the plasma and a low frequency (LF, a few to 10 MHz) for ion acceleration into the wafer. With an increase in both the HF and the wafer size, electromagnetic (EM) wave effects can significantly affect power deposition and the distribution of the plasma due to constructive interference at the centre of the wafer. Here we report on a computational demonstration of using graded conductivity electrodes (GCEs) to improve the plasma uniformity in a DF-CCP reactor. GCEs consist of a metal electrode covered by a dielectric which is in direct contact with the plasma. The conductivity of the dielectric decreases from edge to centre. So as the HF wave propagates inwards from the edge of the electrode, the penetration of the HF field into the dielectric increases. This increasing penetration counteracts the increase in the electric field resulting from the constructive interference of the EM wave at the centre of the reactor, and hence improves the uniformity of the resulting plasma. (fast track communication)