Surface roughening of silicon under ultra-low-energy cesium bombardment

Ripple formation on silicon due to oblique (ultra-)low-energy cesium bombardment was studied by measuring the degradation in depth resolution during sputter profiling of an antimony doped delta layer sample (six Sb deltas with an interlayer spacing of 5 nm). Additional insight into the evolution of surface roughness was obtained by recording depth dependent changes in matrix negative-ion signals. The surface topography of the sputtered crater bottoms was investigated ex situ by scanning electron microscopy. At beam energies between 0.25 and 1 keV, ripple formation was found to take place at “critical” angles exceeding 50–60°. Depending on the impact angle θ, the onset of ripple growth could be observed at depths as small as about 2 nm, i.e. immediately after saturating the sample with cesium. The rate of ripple growth increased with increasing θ. Almost complete loss of resolution of the Sb deltas occurred at angles about 10° above the critical angle, at depths as low as 15–20 nm.