On the formation and loss of S₂ molecules in a reactive ion etching reactor operating with SF₆

Laser-induced-fluorescence (LIF) spectroscopy was used to study, with spatial and temporal resolution, the processes by which diatomic sulfur S ₂ is formed and lost in SF₆ plasmas. We present results concerning the relative S₂ number density in steady-state or pulsed discharges in a reactive ion etching (RIE) reactor operated at different SF ₆ gas pressures and RF powers, in the presence or absence of a silicon wafer. It is found that S₂ is formed mainly on surfaces under conditions when the F-atom density is high, but that volume formation can also occur when the F-atom population is depleted (namely, when Si is present). It is also shown that loss of S₂ is mainly due to diffusion out of the inter-electrode space to the main reactor volume, excluding electron-impact dissociation processes. It is apparent that, in a RIE reactor, the only electron process pertinent to the balance of S₂ density is the formation of its precursors (probably S atoms and SF molecules) by fragmentation of the SF₆ gas. The remaining reactions controlling the density of S ₂ are neutral-neutral interactions in the volume and on surfaces.