The effects of ion sheath collisions on trench etch profiles

The effects of ion collisions in the plasma sheath on etch profiles of a long trench is modeled. The pattern transfer step is for trilayer lithography where the photoresist etches down to a silicon dioxide mask. and the trench is etched in the substrate. The etch rate in the ion flux limited regime varies as the ion energy flux which is calculated from the ion velocity distribution function. The form of the ion velocity distribution function consists of a sum of drifted Maxwellians. This sum fits experimental data for ion energy distribution functions with angular dependence for an argon plasma by a simulated annealing optimization procedure. Etch-rate expressions for a drifted Maxwellian in a collisionless sheath are extended to the collisional sheath. The etch profiles are computed numerically by integration of characteristic equations for the profile points and by numerical computation of the profile at a given time. (C) 1997 American Institute of Physics.