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 ratioRof the ion sheath transit time to the rf period, which determines the spread in the ion energy distribution at the wafer. The effect is most pronounced whenR⩽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 thatR⩽0.3 for all ions, the effect disappears. ©1997 American Institute of Physics.