Silicon etch using SF6/C4F8/Ar gas mixtures

While plasmas using mixtures of SF6, C4F8, 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, SFx, CF2, C2, C3, and CS as a function of the percentage C4F8 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 C4F8 to an SF6/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 C2 and C3. At low fractional flows of C4F8, the etch process appears to be controlled by the ion flux more so than by the F density. At large C4F8 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. CF2 and C2 do not appear to cause deposition from the plasma, but CS and other carbon containing molecules as well as ions do.