Laser-induced fluorescence detection of SiF₂ as a primary product of Si and SiO₂ reactive ion etching with CF₄ gas

The SiF₂ radical was detected in the gas phase during the etching of both Si and SiO₂ substrates under reactive ion etching conditions in steady state and in the afterglow of pulsed CF₄ plasmas, by laser-induced fluorescence (LIF). Spatially and temporally resolved measurements show that there is a major source of these radicals at, or very close to, the etched substrates, and that desorption of SiF₂ as one of the primary products is the most likely explanation. As the absolute concentration was not determined, it is not currently possible to say whether SiF₂ is a mayor etch product, although the observed signals were large. With an Si substrate, SiF₂ is produced both under steady-state RIE conditions (i.e. in the presence of ion bombardment) and in the afterglow of a pulsed discharge (i.e. by pure chemical etching by F atoms). In contrast, with an SiO₂ substrate, SiF₂ is only produced in the steady-state plasma. The net surface reaction probability of SiF₂ was found to be close to unity on the reactor walls. Some possible gas-phase reactions of SiF₂ are also discussed. The fluorescence lifetime of the excited A state of SiF₂ was measured for the first time, giving a value of 6.2 ± 1 ns, in good agreement with theoretical estimates.