Composition of trench sidewalls and bottoms for SiO2-masked Si(100) etched in Cl2 plasmas

We have investigated the possible role of redeposition of silicon–chloride etching products on profile evolution by studying the influence of etching product partial pressure on the surface layer formed during chlorine plasma etching of SiO2-masked p-type Si(100). Samples were etched with high and low etching product (SiCly)-to-etchant (Cl, Cl2) concentration ratios by changing the Cl2 flow rate (1.4 or 10.0 sccm, respectively) at a constant pressure of 4 mTorr. Compositional analysis was performed using angle-resolved x-ray photoelectron spectroscopy (XPS). Electron shadowing and differential charging of the insulating SiO2 regions were exploited to spatially resolve the composition of the trench sidewalls and bottoms (2.0, 1.0, 0.5, 0.3, and 0.22 μm wide). Chlorine content and stoichiometry of the etched surfaces were determined by quantifying the XPS intensities of both the Cl(2p) peak and the silicon chloride containing tail of the Si(2p) peak. Comparisons of chlorine content and stoichiometry were also made to unmasked Si areas etched on the same samples. For trenches etched with 10 sccm Cl2, the chlorine coverage (2.6×1015 Cl/cm2, equivalent to ∼3 monolayers) and the silicon chloride stoichiometry (SiCl:SiCl2:SiCl3=1:0.45:0.33) were identical for the unmasked Si areas and the bottoms of the trenches. The trench sidewalls, however, contained roughly 50% less Cl than the unmasked areas, all in the form of SiCl. Virtually identical results were obtained for trenches etched with 1.4 sccm Cl2, indicating that increased SiCly etching product concentrations do not result in the formation of a thick, passivating sidewall layer on trench sidewalls during Cl2 plasma etching of Si masked with SiO2.