Metastable CF and CF2 molecules in CF4 inductively-coupled plasmas

Abstract

The radicals CF and CF2, which are important intermediates in fluorocarbon plasma chemistry, both have low-lying metastable levels (4CF at 3.54 eV and 3CF2 at 2.46 eV). Recent calculations (Rozum et al 2006 J. Phys. Chem. Ref. Data in press) indicate that electron-impact excitation of the ground-state radicals into these states could be fast. A recent study of inductively-coupled plasmas (ICP) in low-pressure CF4 (Booth et al 2005 Plasma Sources Sci. Technol. 14 273) indicated the presence of a fast electron-impact induced loss process for ground-state CF and CF2 molecules, which could be attributed to this process. In the current study 4CF and 3CF2 were detected in the afterglow of ICP in pure CF4 at pressures between 3 and 33 mTorr, from their weak forbidden optical emission back to their respective ground-states. From the lifetimes of these optical emission signals, determined as a function of gas pressure, the quenching coefficients at the chamber walls and the metastable destruction rates by gas-phase processes (giving unknown products) were estimated. Another prominent and long-lived feature of the afterglow is strong emission from the d state of C2 molecules: the emitting C2 molecules may be produced by chemiluminescent reactions or by excitation transfer from 3CF2.