ION-ASSISTED ETCHING AND PROFILE DEVELOPMENT OF SILICON IN MOLECULAR CHLORINE

An ion beam etching study designed to characterize the kinetic and tra nsport processes important in the ion-assisted etching of silicon in m olecular chlorine was performed. Monoenergetic argon ion beams were di rected normal to a silicon wafer that was simultaneously exposed to a background of molecular chlorine, thereby simulating a low temperature rf plasma etching process. The ion-induced etching yield scaled with the square root of the ion energy for a surface saturated with adsorbe d chlorine. Moreover, the yield was found to depend strongly on the io n to neutral flux ratio which was varied over two orders of magnitude. A kinetic model in which the dissociative sticking probability and th e yield scale linearly with surface coverage was developed and was fou nd to be consistent with the yield data in the ion energy range of 90- 300 eV. Its applicability to etching topography was tested with additi onal experiments where patterned silicon wafers (with oxide masks and with a range of features and linewidths) were etched at saturation in the apparatus. Minimal lag effects and some microtrenching and sidewal l sloping were encountered. The etching selectivity of silicon over ox ide at 100 eV was greater than 30. Computer simulations of the etching process and profile development were performed using the kinetic mode l and a line-of-sight re-emission model for the chlorine transport. Kn udsen diffusion was found to be important in the transport of neutrals to the base of the feature, with the aspect ratio dependence of the e tch rate becoming significant only at high values of the ion to neutra l flux ratio. The agreement between simulation and experiment was good , with the initial sticking probability on a clean surface and the ave rage product stoichiometry as parameters comparing favorably to publis hed results. However, the observed microtrenching and sidewall sloping were not predicted by these simulations. (C) 1997 American Vacuum Soc iety.