  Wet etching is an essential and complex step in semiconductor device
processing. Metal-Assisted Chemical Etching (MacEtch) is fundamentally a wet
but anisotropic etching method. In the MacEtch technique, there are still a
number of unresolved challenges preventing the optimal fabrication of
high-aspect-ratio semiconductor micro- and nanostructures, such as undesired
etching, uncontrolled catalyst movement, non-uniformity and micro-porosity in
the metal-free areas. Here, an optimized MacEtch process using with a
nanostructured Au catalyst is proposed for fabrication of Si high aspect ratio
microstructures. The addition of isopropanol as surfactant in the HF-H2O2 water
solution improves the uniformity and the control of the H2 gas release. An
additional KOH etching removes eventually the unwanted nanowires left by the
MacEtch through the nanoporous catalyst film. We demonstrate the benefits of
the isopropanol addition for reducing the etching rate and the nanoporosity of
etched structures with a monothonical decrease as a function of the isopropanol
concentration.
