Gain and loss mechanisms for neutral species in low pressure fluorocarbon plasmas by infrared spectroscopy

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This article examines the chemical reaction pathways of stable neutral species in fluorocarbon plasmas. Octafluorocyclobutane (c-C4F8) inductively coupled plasma discharges were found to primarily produce stable and metastable products downstream from the discharge, including c-C4F8, C2F4, C2F6, CF4, C3F8, C4F10, C3F6, and CF2. A novel analysis technique allows the estimation of gain and loss rates for neutral species in the steady state as functions of residence time, pressure, and discharge power. The gain and loss rates show that CF4, C2F6, C3F8, and C4F10 share related gain mechanisms, speculated to occur at the surface. Further analysis confirms that CF2 is predominantly produced at the chamber walls through electron impact dissociation of C2F4 and lost through gas-phase addition reactions to form C2F4. Additionally, time-resolved FTIR spectra provide a second-order rate coefficient of 1.8 × 10−14 cm3/s for the gas-phase addition of CF2 to form C2F4. Finally, C2F4, which is much more abundant than CF2 in the discharge, is shown to be dominantly produced through electron impact dissociation of c-C4F8 and lost through either surface or gas-phase addition reactions.