INVESTIGATIONS OF THE SURFACE-CHEMISTRY OF SILICON SUBSTRATES ETCHED IN A RF-BIASED INDUCTIVELY-COUPLED FLUOROCARBON PLASMA USING FOURIER-TRANSFORM INFRARED ELLIPSOMETRY

In situ Fourier-transform infrared (FTIR) ellipsometry has been perfor med on silicon substrates processed in a rf-biased transformer coupled plasma reactor. Plasmas in CHF3, CF4, C2F6, and C4F8 have been used. The reaction layer, which is present on the surface of the silicon waf er during the plasma process, has been analyzed in detail, addressing both chemical composition and thickness. The absolute reliability (exp ressed in terms of thickness) of the results is of the order of 0.01 n m, which corresponds to 3% of a monolayer. The instabilities of a sili con surface, which have been observed under specific conditions, can b e of the order of tens of percents of a monolayer, which clearly illus trates the advantage of using a real in situ technique like FTIR ellip sometry over quasi in situ techniques like x-ray photoemission spectro scopy and Auger electron spectroscopy. For CHF3 plasmas it has been fo und that, if the bias increases to moderate levels (30-50 V), the fluo rocarbon film deposition rate decreases and the silicon etching reacti on rate increases. The reaction layer changes from a thick, predominan tly CFx polymerlike film to a thin, carbon dominated layer of plasma a nd etching products showing vibrational absorptions of SIFx, C-C, and CF2. Increasing the bias voltage in a CHF3 plasma has a similar effect as increasing the F/C ratio of the feed gas. (C) 1998 American Vacuum Society.