Time dependence study of the anisotropic etching of silicon by electrochemical impedance spectroscopy and atomic force microscopy

A study on the etching of monocrystalline silicon p(100) was carried out in 2 M KOH solution at 50°C in the dark using electrochemical and surface analysis techniques such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). Two different surface pre-treatments were used. The study was carried out at three different potentials: negative to the open circuit potential (ocp): -1.8 V saturated calomel reference electrode (SCE); near-ocp: -1.42 V (SCE); positive to the ocp: -1.18 V (SCE). From electrochemical experiments it was clear that a fundamental feature in the study of silicon etching was a time dependence. The surface of the silicon working-electrode, which is smooth at the beginning of the etching becomes rough due to the formation of pyramidal hillocks. After about 20 min the CVs remain constant, indicating that a steady state has been reached on the surface. The Nyquist plots change in dimension and shape with time and as well show a steady state after about 20 min of etching. The EIS experimental data have been fitted using an equivalent circuit containing a constant phase element and a Warburg element (W). A diffusion coefficient for electro-active species was found to be of the order of 10^-12 cm² s^-1. This low value is indicative of hindrance to the diffusion process due to the possible presence of polymeric species such as silicate reaction products on the silicon surface.