Nanopatterning of silicon surfaces by low-energy ion-beam sputtering: dependence on the angle of ion incidence

Abstract

We report on the production of nanoscale patterning onSi substrates by low-energy ion-beam sputtering. Thesurface morphology and structure of the irradiated surface werestudied by atomic force microscopy (AFM) and high-resolutiontransmission electron microscopy (HRTEM). Under ion irradiationat off-normal incidence angle (~50°), AFM images showthe formation of both nanoripple and sawtooth-like structuresfor sputtering times longer than 20 min. The latter featurecoarsens appreciably after 60 min of sputtering, inducing alarge increase in the surface roughness. This behaviour isattributed to the preferential direction determined on the substrate by the ion beam for this incidence angle, leading toshadowing effects among surface features in the sputteringprocess. Under irradiation at normal incidence, the formationof an hexagonal array of nanodots is induced for irradiationtimes longer than 2 min. The shape and crystallinity of thenanodots were determined by HRTEM. At this incidence angle, the surface roughness is very low and remains largely unchanged evenafter 16 h of sputtering. For the two angle conditionsstudied, the formation of the corresponding surface structurescan be understood as the interplay between an instability dueto the sputtering yield dependence on the local surfacecurvature and surface smoothing processes such as surfacediffusion.