A Kinetic Model for Plasma Etching Silicon in a SF6/O2 RF Discharge

Time-dependent Boltzmann electron distribution calculations have been made at constant power and pressure in a SF6/O2 plasma with a varying oxygen mole fraction. The results show that as the oxygen fraction increases in a SF6/O2 plasma, the number of high-energy electrons in the tail of the electron distribution and the mean electron energy both increase significantly while the plasma is kept at the same reduced electric field E/N. Rate coefficients have been computed for the electron kinetic processes of these plasmas and merged within a kinetic equilibrium model for the plasma etch process, including neutral gas-phase chemistry, ion chemistry, and surface reactions. Model simulations show good agreement with experimental results for SF6/O2 etching of polysilicon and demonstrate that the anisotropic character of dilute SF6 plasma etching is related to the shift in the electron distribution with increasing oxygen fraction. Competition between F and O species for adsorption to silicon etching sites is also shown to be a factor in determining etch rates, but this competition is not significant until very large (> 80 percent) oxygen concentrations are present. Ionization rates and ion transport to the surface are shown to be much more important. The model simulations provide a rationale for explaining the very high etch rates observed at low-SF6 partial pressures and the increasing anisotropic etch character with greater oxygen dilution of SF6.