Kinetic Monte Carlo simulations of anisotropic Si(100) etching: Modeling the chemical origins of characteristic etch morphologies.

An atomistic, chemically realistic, kinetic Monte Carlo simulator of anisotropic Si(100) etching was developed. Surface silicon atoms were classified on the basis of their local structure, and all atoms of each class were etched with the same rate. A wide variety of morphologies, including rough, striped, and hillocked, was observed. General reactivity trends were correlated with specific morphological features. The production of long rows of unstrained dihydride species, recently observed in NH(4)F (aq) etching of Si(100), could only be explained by the rapid etching of dihydrides that are adjacent to (strained) monohydrides-so-called "alpha-dihydrides." Some etch kinetics promoted the formation of {111}-microfaceted pyramidal hillocks, similar in structure to those observed experimentally during Si(100) etching. Pyramid formation was intrinsic to the etch kinetics. In contrast with previously postulated mechanisms of pyramid formation, no masking agent (e.g., impurity, gas bubble) was required. Pyramid formation was explained in terms of the slow etch rate of the {111} sides, {110} edges, and the dihydride species that terminated the apex of the pyramid. As a result, slow etching of Si(111) surfaces was a necessary, but insufficient, criterion for microfacet formation on Si(100) surfaces.