Formation of nanoripples in Al films during O2+ sputtering

Off-normal ion bombardment of solid targets often leads to development of periodically modulated structures on the eroded surface. For tilted incidence (theta >= 30(degrees)) of 16.7 keV O-2(+) ion beam on Al thin films, quasiperiodic ripple topography has been found to develop on the sputtered surface. As predicted by Bradley-Harper theory, below a critical angle theta(c)similar or equal to 60 degrees, the ripple wave vectors are found to be oriented parallel to ion beam projection, while for angles above theta(c) the ripple wave vectors are perpendicular to the ion beam projection. At the critical angle of ripple rotation, one observes moundlike morphology, the growth of which is similar to that of parallel ripples. It is noted that the intrinsic roughness of the film surface plays an important role in the smoothing and roughening kinetics at the early stages of sputtering.