Study of fluorine (XeF2) adsorption and of oxygen/fluorine coadsorption on silicon using infrared reflection absorption spectroscopy

Abstract

Infrared reflection absorption spectroscopy has been used to study the chemisorption of fluorine (by exposure to XeF2) on polycrystalline Si surfaces in ultrahigh vacuum and under low-pressure steady-state conditions. Adsorption at 300 K leads to the sequential formation of mono-, di-, and trifluoride SiFx groups. Annealing at successively higher temperature forms more ≡SiF by decomposition of =SiF2 and –SiF3. After a 770 K anneal, only the 890 cm−1 ≡SiF band remains and is removed by F desorption at ∼820 K. Similar results are obtained in a continuous XeF2 flux with, in addition, the appearance near 300 K of a weak feature possibly due to adsorbed SiF4. Based in part on these results, an estimate can be made of the rate-limiting step in the reaction. In the high-temperature limit (T≥650 K) it appears to be the conversion of ≡SiF to =SiF2; whereas, near 300 K, it is the formation of SiF4 from –SiF3. The effects on fluorination of pre- and postexposure to O2 have also been observed. Preadsorbed O stabilizes F, shifting the F desorption temperature higher by at least 100 K. In the presence of coadsorbed F, the Si–O–Si antisymmetric stretch appears at 1150 cm−1, significantly higher than on a similar, F-free surface. Preadsorbed F impedes subsequent chemisorption of O2, and the O that is adsorbed has no apparent effect on SiFx bonding as reflected in the Si–F stretching vibrations.