Experimental study of intermediate-Pressure RF-CCP in Ar-O₂ mixtures

Radio frequency (RF) capacitively coupled plasma (CCP) sources at pressures below a few 100 mTorr are widely used for industrial etching thin films [1] and have been extensively studied experimentally and modelled. However, RF-CCP’s at higher pressures (up to 10 Torr), widely used for thin film deposition, have been much less studied, because many of the diagnostic and modelling techniques developed at low pressure become problematic. We present an experimental study of a highly-symmetric CCP (Ø 50cm gap 2,.5cm), excited at 13.56 MHz (100-900W), at 1-6 Torr pressure in gas mixtures varying from pure Ar to pure O₂. We used cavity ringdown spectroscopy (CRDS) at 630 nm [2] to measure oxygen atom densities and temperatures, as well as the O^- negative ion density and ozone generation in the afterglow. Additionally we estimated electron density using a microwave hairpin probe and determined the RF current, voltage and phase at the electrodes.

Even small (5%) addition of O₂ to Ar strongly reduces the electron density and drastically changes the discharge impedance. Further oxygen addition (up to 100%) continues these trends, but more weakly. At high pressures the plasma becomes much more resistive. This can be attributed to the strong inelastic electron collision processes in oxygen, as well as the generation of negative ions. In some conditions (certain pressure and power), the O atom density passes through a maximum as the Ar % is increased, due to the competition between decreased O₂ feedstock and increased electron density/dissociation rate. While in most conditions, the O density decreases monotonically with Ar addition, although the O₂ dissociation fraction increases consistently, approaching 70%.