The mechanism of anisotropic, electrochemical silicon etching in alkaline solutions

Some experimental data on the anisotropy, selectivity, and voltage dependence of anisotropic silicon etchants are given. An attempt is undertaken to provide a general unifying model describing the reaction mechanism and the key features of all alkaline anisotropic etchants of silicon. It is shown that the reaction is electrochemical, comprising the transfer of four electrons between the electrolyte and the solid for the dissolution of one silicon atom. The crystallographic anisotropy can be attributed to small differences of the binding energy of surface atoms depending on their respective surface orientations. High boron concentrations induce the shrinking of a space charge layer on the silicon surface, which in turn leads to the fast recombination of electrons injected into the conduction band making them no longer available for the reduction of water. The electrochemical etch stop at positive potentials is due to the anodic oxidation of silicon. The finite etch rate observed at etch stop potentials was found to correspond well to the etch rate of SiO/sub 2/.<>