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.
Harley Johnson is a Professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign. His research is in the area of mechanics of electronic and optical materials, with a particular focus on understanding mechanical/electronic coupling and the effects of defects on the properties of materials and structures. He received his PhD in 1999 from Brown University, where he also earned MS degrees in Applied Math (1998) and Engineering (1996). He earned a BS degree in Engineering Science in Mechanics from Georgia Tech in 1994. He has received numerous honors and awards for his research and teaching, including the 2001 NSF CAREER Award, the 2010 ASME Thomas J. R. Hughes Young Investigator Award for Special Achievement in Applied Mechanics, and the 2011 UIUC College of Engineering Xerox Award for Faculty Research, and he has been named numerous times to the UIUC List of Excellent Teachers. In 2012 he was elected a Fellow of ASME. He currently serves as the Department Associate Head for Graduate Programs.