Plasma molding over surface topography: Simulation and measurement of ion fluxes, energies and angular distributions over trenches in RF high density plasmas

A two-dimensional (2-D) fluid/Monte Carlo (MC) simulation model was developed to study plasma "molding" over a trench. The radio frequency sheath potential evolution and ion density and flux profiles over the surface were predicted with a self-consistent fluid simulation. The trajectories of ions and energetic neutrals (resulting by ion neutralization on surfaces or charge exchange collisions in the gas phase) were then followed with a MC simulation. For sheath thickness L_sh comparable to the trench width D, ions were strongly deflected toward the trench sidewall, and the ion flux along the trench surface contour was highly nonuniform. Irrespective of the trench depth, the normalized spatially-average ion flux at the trench mouth showed a minimum at L_sh/D∼1.0. The normalized spatially-average ion flux at the trench bottom decreased with increasing trench depth (or aspect ratio). As the trench sidewall was approached, the energy spread ΔE of the ion energy distributions (IEDs) at the trench bottom decreased for a thin sheath, but increased for a thick sheath. At the trench bottom, the neutral flux was comparable to the ion flux over the entire range of sheath thickness studied. Simulation results were in good agreement with experimental data on ion flux, IEDs, and ion angular distributions at the trench bottom.