Transmission FTIR was used to investigate the removal of residue layers on doped oxide films and to monitor the etching mechanism of thermal oxide films in real time. Partial removal of the residue on BPSG films containing a mixture of boric and phosphoric acids was possible with a low pressure/low temperature UV-Cl_2 process. Both direct and indirect UV illumination removed a portion of the residue, but the direct process produced an opaque residue on the surface of the doped oxide that could not be rinsed off immediately afterward. In low pressure HF/vapor etching of thermal oxide films, HF hydroxylates and fluorinates the Si surface atoms during the induction time forming groups of H-bonded silanols. After induction, etching begins when HF attacks SiF_2(OH)_2 moieties, which contain weaker Si-O bonds than in siloxane, producing molecular water which activates the oxide surface and SiF_4, which is volatile.
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