Thermal reactions of fluorine and nitrogen trifluoride with silicon(110) studied by laser ionization mass spectrometry

Abstract

The techniques of laser and electron ionization mass spectrometry have been employed to study the thermal etching of Si(110) by F/sub 2/ and NF/sub 4/ at substrate temperatures between 300 and 1200 K. By two-photon resonance-enhanced ionization of SiF/sub 2/ via the B/sup 1/ tilde B/sub 2/ state, the apparent activation energy for gaseous silicon difluoride production was found to be 8.9 +/- 0.3 and 22.1 +/- 1.7 kcal/mol for F/sub 2/ and NF/sub 3/, respectively. SiF was not detected. An extensive search for SiF/sub 3/ during etching by F/sub 2/ at 1000 K, by means of resonance ionization from 320 to 325 and from 416 to 510 nm, also showed no signs of the species. Both SiF and SiF/sub 3/ are thermochemically unimportant etch products under the conditions employed. In F/sub 2/ etching, SiF/sub 4/ and total silicon fluoride ..sigma.. SiF/sub x//sup +/ signals as measured by electron ionization rose rapidly at lower temperatures and stabilized between 700 and 900 K before rising again. No such behavior was observed for SiF/sub 2/ production from F/sub 2/ or for the products formed in NF/sub 3/ etching. Apparent activation energies for total silicon fluoride and SiF/sub 4/ production are similar. Formore » F/sub 2/, they were found to be about 9 kcal/mol in the low-temperature region, and for NF/sub 3/ both were measured to be about 21 kcal/mol. A proposed reaction mechanism explaining these and related results is discussed.« less