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 major 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) over tilde state of SiF2 was measured for the first time, giving a value of 6.2 +/- 1 ns, in good agreement with theoretical estimates.