Absolute etch
rates in alkaline etching of silicon (1 1 1)
I.A. Shah, W.J.P. van Enckevort, and E. Vlieg
Abstract
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.