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.