Production and destruction of CFx radicals in radio-frequency fluorocarbon plasmas

Spacially resolved densities of CF, CF2, and CF3 radicals in capacitively coupled 13.56 MHz radio-frequency (rf) discharges in CF4 and CHF3 were determined by means of infrared absorption spectroscopy employing a tunable diode laser spectrometer. It was established that the stationary CF2 density and density profile in a CF4 plasma depend strongly on the electrode material. This is attributed to different sticking coefficients of CF2 on different surfaces. Furthermore, it was found that the densities of all CFx radicals increase near the electrodes at high gas pressures and rf powers in a CHF3 plasma. This leads to the conclusion that production of CFx radicals takes place in the sheath region close to the electrodes. It is proposed that collisions between ions and source gas molecules are responsible for this production of CFx radicals. In the presence of a destruction process in the plasma glow (e.g., by three-body recombination with other radicals) and the absence of a fast surface loss process this results in the observed increase of CFx densities near the electrodes. Ln order to study the radical kinetics time dependent measurements were performed during power modulation of the plasma. It was found that the decay time of the CF2 density in the afterglow of a CF4 plasma is much shorter than the corresponding decay time in a CHF3 discharge. This suggests that the surface loss is relatively less important in the latter case, in agreement with measurements of spatial density distributions. This is explained by the presence of a (CFx)(n) layer, which is readily deposited on the electrodes in a CHF3 discharge, and by low sticking probabilities of CF and CF2 radicals on such a layer. (C) 1996 American Vacuum Society.