Radical dynamics in unstable CF4 inductive discharges

Abstract

We have observed instabilities in low-pressure planar inductive discharges in CF4 gas, in many ways similar to those seen in other attaching gases. The plasma emitted light exhibits oscillations, with the intensity modulated from 15% to 100%. The instability regions were investigated as a function of pressure, CF4 flow rate and radio-frequency power. Two matching configurations were used. When the terminus of the planar spiral antenna was directly connected to ground, we observed low-frequency oscillations between 0.5 Hz and 15 Hz. When a 400 pF capacitance was added between the terminus and ground (i.e., the capacitive coupling was reduced), the oscillation frequencies ranged from 100 Hz to 1.5 kHz. Oscillations in the concentrations of unsaturated radicals (CF and CF2) were measured during the instability, using time-resolved laser induced fluorescence. The radical dynamics were significant at frequencies of about 1 kHz. Mechanisms are proposed for these variations, including gas temperature changes and chemical reactions. A global model originally developed for SF6 chemistry was adapted to this system. The appropriate rate coefficients for CF4 were incorporated and an expression for the positive ion flux in the low electronegativity case was developed. Gas heating effects were also included.