Abstract. Ion bombardment is used to modify semiconductor surfaces for a variety of purposes, including etching, milling, smoothing, patterning, and doping.  The formation of defects due to individual ion impact events in irradiated material has been widely studied, but several interesting mechanics questions remain about stress generation, surface instability, and material removal in low-energy ion bombardment and focused ion beam (FIB) processing.  Using atomistic and continuum computational methods, we explore these questions.  First, we note that surfaces exposed to low energy ion bombardment are unstable with respect to formation of submicron-scale ripples and dots.   We show that smoothing balanced by impact angle dependent mass redistribution explains the atomistic origin of ripple formation and orientation, particularly angle dependent transitions between different orientations.  We develop a multiscale atomistic-to-continuum computational model that generates surface morphology results consistent with experimental observations for a range of ions, ion energies, and target materials.  Second, we consider FIB machining processes, which require higher energy bombardment, and we find that material removal is even less dependent on ballistic or sputtering effects.  We show that the behavior is a cooperative effect of mass redistribution and thermal effects associated with phase transformations in the target material.  Our results are consistent with a broad range of experimental observations.