Absolute etch rates in alkaline etching of silicon (1 1 1)

The absolute etch rate of silicon (1 1 1) during wet chemical etching in aqueous KOH solution has been investigated with optical interferometry, using masked samples. The etch rate is constant at 0.62 ± 0.07 μm/h and independent of alkaline concentration for 1–5 M KOH solutions at 60 °C. Only at lower alkaline concentrations, the etch rate decreases. Adding isopropanol slightly lowers the absolute etch rate. The activation energy of the etching reaction is 0.61 ± 0.03 eV in standard KOH solutions and 0.62 ± 0.03 eV with 1 M isopropanol added to the solution. This indicates that the reaction is determined by reaction kinetics and not by transport limitations. In all cases the surfaces are covered by shallow etch pits, not related to defects in the crystal. This implies that the actual factor that determines the etch rate is the 2D nucleation of new vacancy islands at the bottom of these pits. This process is likely catalyzed by a local accumulation of reaction products, which preferentially occurs near the mask edges.