Effect of surface roughening on secondary ion yields and erosion rates of silicon subject to oblique oxygen bombardment

The phenomenon of ripple formation on silicon surfaces subject to oblique bombardment with O+2 ions was studied by means of secondary ion mass spectrometry (SIMS). The measurements were performed with a versatile ion microprobe which allows the angle of beam incidence and the impact energy to be varied independently. Bombardment with 10 keV O+2 ions caused facetting of the whole bombarded area provided the impact angle θ ranged between about 32° and 58° to the surface normal. A smooth crater bottom with a few etch pits was produced at 30°. Facet formation gave rise to significant secondary ion yield changes. Rather large yield enhancement (up to a factor of 6) was observed for species like O+, SiO+, and Si2+ in which case the yields are very sensitive to the oxygen concentration at the surface. The sputtered depth at which the yield changes become noticeable decreases with increasing impact angle from about 4 μm for θ=35° to 0.4 μm for θ=55°. For as many as eight different ion species the stationary intensities relevant to secondary ion emission from smooth surfaces were measured as a function of θ (0°<θ<75°). On the basis of these ‘‘calibration curves,’’ the observed yield changes induced by facetting can be attributed in an essentially quantitative manner to the variation of the microscopic impact angles with depth of erosion. The interpretation is supported by measurements of yield changes observed upon varying the impact angle from oblique to normal, subsequent to short term or long term oblique bombardment. As a result of facetting the erosion rate of silicon was found to change appreciably.