Silicon etch using SF{sub 6}/C{sub 4}F{sub 8}/Ar gas mixtures

While plasmas using mixtures of SF{sub 6}, C{sub 4}F{sub 8}, and Ar are widely used in deep silicon etching, very few studies have linked the discharge parameters to etching results. The authors form such linkages in this report. The authors measured the optical emission intensities of lines from Ar, F, S, SF{sub x}, CF{sub 2}, C{sub 2}, C{sub 3}, and CS as a function of the percentage C{sub 4}F{sub 8} in the gas flow, the total gas flow rate, and the bias power. In addition, the ion current density and electron temperature were measured using a floating Langmuir probe. For comparison, trenches were etched of various widths and the trench profiles (etch depth, undercut) were measured. The addition of C{sub 4}F{sub 8} to an SF{sub 6}/Ar plasma acts to reduce the availability of F as well as increase the deposition of passivation film. Sulfur combines with carbon in the plasma efficiently to create a large optical emission of CS and suppress optical emissions from C{sub 2} and C{sub 3}. At low fractional flows of C{sub 4}F{sub 8}, the etch process appears to be controlled by the ion flux more so than by the F density. At large C{sub 4}F{sub 8} fractional flows, the etch process appears to be controlled more by the F density than by the ion flux or deposition rate of passivation film. CF{sub 2} and C{sub 2} do not appear to cause deposition from the plasma, but CS and other carbon containing molecules as well as ions do.