Ion mass effect in plasma-induced charging

Simulations of charging and profile evolution during etching of high aspect ratio polysilicon gates in typical high-density plasmas containing heavy ions (e.g., BCl₃⁺, Cl₂⁺), predict a reduction in charging and notching when lighter ions (e.g., He⁺) are added. The reduction occurs because of the influence of the ion mass on the ratio R of the ion sheath transit time to the if period, which determines the spread in the ion energy distribution at the wafer. The effect is most pronounced when R ≤0.1-0.2 for light ions and, simultaneously, R≥0.6 for heavy ions; then, more light ions arrive at the patterned structure with low energies, where they help decrease localized charging. When the rf bias frequency is reduced so that R≤0.3 for all ions, the effect disappears.