Mechanochemistry etches silicon one layer at a time

Mechanical forces induced by a scanning probe tip can trigger chemical reactions that selectively remove a single atomic layer of silicon (Nat. Commun. 2018, DOI: 10.1038/s41467-018-03930-5). The finding may lead to ways of etching electronic materials with atomic precision, a level of control needed to further shrink today’s circuit components, which already sport nanoscale features. By fitting a scanning probe tip with a silica sphere and slowly moving it against a crystalline silicon surface in humid air, Xinchun Lu of Tsinghua University, Seong H. Kim of Pennsylvania State University, and coworkers methodically picked off silicon layers one at a time without inducing subsurface damage. Using microscopy, surface chemical analysis, and molecular dynamics simulations the group determined that as the surfaces start to come into contact, they react with water present at the interface, forming Si-O-H groups on silica and Si-H groups on silicon. As the probe tip slides, it bridges the modified groups on both surfaces by forming an Si-O-Si bond, which starts to break the Si-Si bond in silicon. In the final stage, the Si-Si bond breaks, transferring a silicon atom to the silica surface. Control tests indicate that this etching process is not caused by mechanical wear and does not occur in the absence of water or when the silica tip is replaced with other materials.