The significance of redeposition and backscattering in nanostructure formation by focused ion beams

Focused ion beam (FIB) milling allows for the direct patterning of three-dimensional shapes at very fine length scales. In this work the roles of redeposition and second-order sputtering are investigated using a segment-based model in two dimensions. Angular distributions and yields for sputtering and backscattering are extracted from Monte–Carlo simulations. Deep trenches exhibiting microtrenching are simulated. It is found that redeposition leads to inclined sidewalls and a bending up of the bottom towards the sidewalls, while secondary sputtering causes the formation of a pair of microtrenches that increase in size until they meet in the center of the trench. Redeposition leads to a reduction of the effective milling rate, while secondary sputtering may increase it, particularly once the two microtrenches have met. A scaling rule is shown to apply to our simulations, which reduces the number of parameters the observed profile shape depends on. In addition, results are presented of flux calculations on experimentally obtained ripple profiles, suggesting that redeposition and backscattering may play a role in late stage ripple development.