Effect of changing the electrode gap on the spatial and electrical properties of O2/CF4 plasmas

Abstract

Planar laser-induced fluorescence (PLIF) measurements were made to determine two-dimensional spatial maps of CF2 density as a chemical marker of plasma uniformity in 9% O2/91% CF4 chamber-cleaning plasmas. Broadband optical emission and discharge current and voltage measurements were also made and compared to the PLIF results. Measurements were made in a capacitively coupled Gaseous Electronics Conference Reference Cell as pressure was varied from 13 (100 mTorr) to 133 Pa (1000 mTorr) and electrode gap was varied from 2.25 to 0.5 cm. Smaller gaps resulted in increased radial uniformity and extended the pressure range over which CF2 density and electrical properties remain insensitive to pressure. These effects are explained by a decrease in the bulk plasma resistance at narrower electrode gaps, which changes the path that rf current takes through the discharge, thus affecting where electron heating and CF2 production occur. These results provide insight into the optimization of chamber-cleaning processes and reactors as well as provide necessary data for validation of plasma simulations.