Thermal reactions of F₂ and NF₃ with silicon(110) studied by laser ionization mass spectrometry

The techniques of laser and electron ionization mass spectrometry have been employed to study the thermal etching of Si(110) by F₂ and NF₃ at substrate temperatures between 300 and 1200 K. By two-photon resonance-enhanced ionization of SiF₂ via the B̃¹B₂ 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 F₂ and NF₃, respectively. SiF was not detected. An extensive search for SiF₃ during etching by F₂ 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 SiF₃ are thermochemically unimportant etch products under the conditions employed. In F₂ etching, SiF₄ and total silicon fluoride ΣSiF_x ⁺ 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 SiF₂ production from F₂ or for the products formed in NF₃ etching. Apparent activation energies for total silicon fluoride and SiF₄ production are similar. For F₂, they were found to be about 9 kcal/mol in the low-temperature region, and for NF₃ both were measured to be about 21 kcal/mol. A proposed reaction mechanism explaining these and related results is discussed.