PRODUCTION AND DESTRUCTION OF CFX RADICALS IN RADIOFREQUENCY FLUOROCARBON PLASMAS

Spacially resolved densities of CF, CF2, and CF3 radicals in capacitiv ely 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 stati onary CF2 density and density profile in a CF4 plasma depend strongly on the electrode material. This is attributed to different sticking co efficients of CF2 on different surfaces. Furthermore, it was found tha t the densities of all CFx radicals increase near the electrodes at hi gh gas pressures and rf powers in a CHF3 plasma. This leads to the con clusion that production of CFx radicals takes place in the sheath regi on close to the electrodes. It is proposed that collisions between ion s 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 absenc e of a fast surface loss process this results in the observed increase of CFx densities near the electrodes. Ln order to study the radical k inetics time dependent measurements were performed during power modula tion of the plasma. It was found that the decay time of the CF2 densit y in the afterglow of a CF4 plasma is much shorter than the correspond ing decay time in a CHF3 discharge. This suggests that the surface los s is relatively less important in the latter case, in agreement with m easurements of spatial density distributions. This is explained by the presence of a (CFx)(n) layer, which is readily deposited on the elect rodes in a CHF3 discharge, and by low sticking probabilities of CF and CF2 radicals on such a layer. (C) 1996 American Vacuum Society.