Mechanistic aspects of anisotropic dissolution of materials Etching of single-crystal silicon in alkaline solutions

Abstract

The origin of chemical anisotropy in the dissolution of single-crystal silicon in alkaline solutions is discussed in terms of the atomic conÐguration of silicon in the pentacoordinated transition state for (100) and ( 111) surfaces. It is proposed that tetravalent silicon, which is bonded in a tetrahedral geometry, is attacked in the etch process by the hydroxide ion, forming a pentacoordinated transition state. Owing to the number of bond angles that are Ðxed by the atomic arrangement at the surface, the energetically favoured trigonal bipyramidal geometry for a pentacoordinated complex is only slightly distorted for the former plane but signiÐcantly distorted for the latter, resulting in a higher activation energy for the dissolution of ( 111) surfaces. The di †erence in the activation energies for the dissolution of Si( 100) and ( 111) surfaces, arising from steric hindrance in the transition state, can be estimated from the activation energy for a pseudo-rotation of a similar system.

Investigation of the Chemical Purity of Silicon Surfaces Reacted with Liquid Methanol

“…Baum and Schiffrin have proposed that the number of bonds to a crystal lattice restricts the ability of that silicon atom to engage in hypervalent transition states. 102 The coupled Si-O-Si bonds, which are assumed to form a more intergral part of the physical SiO 2 matrix, are expected to be less reactive toward methanol based on the difficulty of that Si atom to reach a pentavalent transistion state necessary for reaction (Figure 11). In contrast, the uncoupled Si-O-Si bonds have a much higher reactivity with methanol.…”

Anisotropy in the Anodic Oxidation of Silicon in KOH Solution.

“…This result might be due to a relatively higher rate constant for reaction 2e at (111) electrodes, possibly due to steric effects. 27 Dehydration of Si-OH bonds (reaction 2f) to give oxide is more likely to occur at (111) surfaces. 31,32,35 With two intact back-bonds, the surface silicon atom is more firmly anchored in the lattice, thus favoring dehydration and oxide formation (reaction 2f); in the (100) case the surface atom with just one back-bond and an Si(OH) 3 configuration is much more likely to dissolve (reaction 2g).…”

“…Intermediate I results from nucleophilic attack by an OH -ion on the Si-H surface bond (step 1a, Figure 9); the "departing hydride" reacts with water to give hydrogen gas and an OHion (step 1b). 27 Intermediate II very likely results from the interaction of an electron lone pair on the oxygen of a water molecule with the surface silicon atom positively polarized by the OH group (step 2a, Figure 10). …”

“…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.…”

Anisotropy in the wet-etching of semiconductors

“…Such materials will be important for applications in photonics. The activated state of this reaction is a pentacoordinated Si atom [13]. The departing 'hydride' reacts with H 2 O to give hydrogen gas and the surface atom is hydroxylated.…”