Molecular dynamics simulations of Ar+ and Cl+ impacts onto silicon surfaces:: Distributions of reflected energies and angles

Previous profile evolution studies of plasma-assisted etch processes have shown that ions scattered from sidewalls can lead to microtrench formation on the bottom of an etched feature [see, for example, Dalton ct al., J. Electrochem. Sec. 140, 2395 (1993)]. In these studies, the ions impacting feature surfaces with incident angles above a critical value were assumed to reflect specularly from the surfaces. In the present article, we describe the energy and angle distributions of reflected atoms obtained from molecular dynamics (MD) simulations. We simulated Ar+ and Cl+ ions impacting model silicon surfaces. The ion incident energies Ei were 20, 50, and 100 eV. We varied the ion incident angles theta(i) from 0 degrees to 85 degrees from the;surface normal. The model silicon surfaces had chlorine coverages of 0 monolayers (ML) of Cl, 1 ML Cl, and 2.3 ML Cl. We determined the Ar and Cl reflection probabilities, i.e., the fraction of Ar and Cl atoms scattered from the surfaces during the 1-2 ps MD trajectories. For theta(i) greater than or equal to 75 degrees, we found that the reflection probabilities were greater than 90% in most cases. For these large incident angles, we describe the distributions of energies E-r and angles (polar theta(r) and azimuthal phi(r)) for the Ar and Cl atoms reflected from the surfaces. The results of the MD simulations are compared with the assumption of specular scattering. In addition, we compare the average energies of the reflected atoms with the predictions of two simple models based on the binary collision approximation. We discuss the effects of incident ion species, E-i, theta(i), chlorine surface coverage, and surface roughness on these results. (C) 1998 American Vacuum Society. [S0734-2101(98)04506-0].