Structural and sputtering effects of medium energy ion bombardment of silicon

Molecular dynamics simulation is used to study argon ion bombardment of an initially perfect silicon crystal up to its damaged state at a total fluence of 4· 10 impacts/cm. Lower and higher energy processes are considered: one process with ions at 500 eV and another process with ions at 700 eV, which are like those used in a particular microelectromechanical systems (MEMS) fabrication technique. These energies are intermediate relative to most previous silicon ion bombardment studies, higher than those typically used in ion-assisted deposition and lower than in typical ion implantation. In all, up to 118 impacts are simulated in a 5.43 by 5.43 nm periodically replicated cell of a target (0 0 1) surface of silicon. After an impact, the material is cooled slowly to 77 K by a process that models thermal conduction in to the bulk target material. It is assumed that defects are immobile at this temperature and that no further structural relaxation occurs before the next impact. Multiple simulations of more than 100 ion impacts are conducted for both ion energies and averaged to converge statistical descriptions of structural evolution and sputtering. Surface roughening is observed with increase in ion fluence. Damage throughout the sample is studied using a planar radial distribution function. Using a crystallinity measure based on this function, it is found that the damage region extends 2.2 nm into the material for 500 eV ions and 3.0 nm for 700 eV ions after 80 impacts. The damaged region is separated from deeper, undamaged crystal by a nearly flat interface. Sputter yields are in reasonable agreement with experimental data, reaching nearly steady rates of 0.5 and 0.7 sputtered atoms per incident argon atom for the 500 and 700 eV cases, respectively. For the number of ions considered, implanted argons do not significantly affect Si sputtering. 2004 Elsevier B.V. All rights reserved. PACS: 79.20.R; 61.43.B; 61.43.D; 61.20.J; 81.05.G