ION MASS EFFECT IN PLASMA-INDUCED CHARGING

Simulations of charging and profile evolution during etching of high a spect ratio polysilicon gates in typical high-density plasmas containi ng heavy ions (e.g., BCl3+, Cl-2(+)), predict a reduction in charging and notching when lighter ions (e.g., He+) are added. The reduction oc curs because of the influence of the ion mass on the radio R of the io n sheath transit time to the rf period, which determines the spread in the ion energy distribution at the wafer. The effect is most pronounc ed when R less than or equal to 0.1-0.2 for light ions and, simultaneo usly, R greater than or equal to 0.6 for heavy ions; then, more light ions arrive at the patterned structure with low energies, where they h elp decrease localized charging. When the rf bias frequency is reduced so that R less than or equal to 0.3 for all ions, the effect disappea rs. (C) 1997 American Institute of Physics.