Controlled Fabrication of High-Aspect-Ratio Microstructures in Silicon at Etching Rates Beyond State-of-the-Art Microstructuring Technologies

The controlled electrochemical etching at room temperature of deep (up to 200 mu m) silicon microstructures with aspect ratio ranging from 5 to 100 and etching rates from 10 to 3 mu m/min, respectively, is here reported and discussed. This allows silicon microfabrication technology entering a region in the parameter space etching-rate vs. aspect-ratio that was so far unreachable for silicon microstructuring technologies both at commercial and research levels. Addition of an inert oxidant molecule, namely H2O2, to a standard aqueous hydrofluoric (HF) acid electrolyte is used to drop the valence (number of electrons required to dissolve a single silicon atom) of the dissolution process down to 1 (from the standard value of about 3) under anodic biasing. This makes the electrochemical etching of silicon more effective without losing control accuracy in microfabrication, on the one hand, and allows to increase the etching rate by opening a more efficient silicon dissolution path with respect to the well-known Gerischer mechanism, both at lower and higher aspect ratios, on the other hand.