Orientation-resolved chemical kinetics: Using microfabrication to unravel the complicated chemistry of KOH/Si etching

Microfabricated test patterns are used to measure the orientation-dependent rates of KOH/silicon etching of 180 surfaces in the Si[110] zone. The concentration and temperature dependence of the reaction is quantified, and a pronounced kinetic isotope effect is observed for all orientations. Although the kinetics of the KOH etching of silicon are complicated, the magnitude of the kinetic isotope effect, the morphology of the macrosteps on vicinal Si(111) surfaces, the pronounced hydrophobicity and H-termination of the etched surfaces are all consistent with a chemical mechanism that is rate-limited by cleavage of a Si-H bond by OH-. There is no evidence of a gross change in chemical mechanism with surface orientation. Silicon surfaces in the [110] zone can be divided into four regions of similar reactivity: vicinal Si(100), vicinal Si(110), and two types of vicinal Si(111) surfaces. Within each region, all surfaces display remarkably similar chemical kinetics. These regions are separated by morphological transitions of unknown origin. The orientations of the morphological transitions are temperature dependent, which implies that they are not associated with surface structural transitions, such as reconstructions. The etch rate of vicinal Si(111) surfaces is well fit by a simple step flow model; however, etching-induced step bunching is also observed. The observed kinetics are inconsistent with existing theoretical models of step bunching. Low miscut vicinal Si(110) surfaces have very isotropic etch rates, which are attributed to etching induced faceting. The macroscopic etch rate displays markedly non-Arrhenius behavior (the etch anisotropy actually increases with temperature!), and the concentration dependence cannot be fit by a simple empirical rate law. These phenomena are attributed to the multisite nature of the etching reaction.