STUDY OF FLUORINE (XEF2) ADSORPTION AND OF OXYGEN FLUORINE COADSORPTION ON SILICON USING INFRARED REFLECTION ABSORPTION-SPECTROSCOPY

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 approximately 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 par-t 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.