Orientation-Resolved Chemical Kinetics:  Using Microfabrication to Unravel the Complicated Chemistry of KOH/Si Etching

Abstract

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.

Quantification and Reduction of the Residual Chemical Reactivity of Passivated Biodegradable Porous Silicon for Drug Delivery Applications

“…However, even if silicon-hydride is completely thermally removed, the interaction of residual elemental silicon in the porous (control) no image after 100 hours exposure structure with aqueous environments can lead to its reintroduction. Convincing evidence for silicon-hydride creation from silicon surfaces during alkali etching [38], tribochemical etching in water [39,40] and storage in weakly alkali solutions like body fluids [41] is available in the literature. Gaseous silane emission is a second potential source of reactivity [7], especially for biological molecules stored for long periods within the mesoporous structure.…”

Anisotropy in the Anodic Oxidation of Silicon in KOH Solution.

“…When the surface silicon atom dissolves, 25 after its remaining backbonds are broken, an Si-H species is exposed to solution that is subsequently replaced by Si-OH via an OH --catalyzed reaction (eq 1). 13,21,[26][27][28][29] In contrast to the "covalent" Si-H bond, the Si-OH is quite polar; 30,31 the OH group polarizes the Si-Si back-bond, making it susceptible to attack by water in reaction 2. 31,32 Whether the silicon surface is mainly OH-or H-terminated during anisotropic etching will depend on the relative rates of reactions 1 and 2.…”

Study, Modelling and Implementation of the Level Set Method Used in Micromachining Processes

“…A thorough characterization of KOH and TMAH etchants was performed by Sato et al by etching silicon hemispheres [188,189]. Nevertheless, another method that consists in etching a silicon structure similar to a wagon wheel was used to characterize KOH [190] and it has recently been proved to obtain accurate characterizations for KOH and TMAH [191]. Furthermore, the usage of additional elements that modify the original anisotropy of the etchant have been studied, including the addition of Isopropyl alcohol (IPA) to KOH solution [192][193][194] and the addition of surfactant Triton X-100 to TMAH solutions [195].…”

“…These etch rates values are usually obtained by etching and measuring structures that resemble a wagon wheel [190,191]. These type of experiments produces many spokes, each of them has been etched a specific distance (see Fig.…”