Proximal probe-like nano structuring in metal-assisted etching of silicon

We use silicon having multiple crystalline orientation domains and high metal doping in metal assisted chemical etching (MACEtch) in HF/H₂O₂. In device-quality silicon, MACEtch produces high-aspect ratio anisotropic (1-D) structures (wires, columns, pores or holes) and to a lesser degree non-high-aspect ratio luminescent (0-D) nano structures. While the 1-D structure symmetry is understood in terms of crystallography axis-dependent etching, predominantly along the <100> direction, the isotropic 0-D spherical symmetry etching is not understood. We observe in silicon having multiple crystalline orientation domains formation of metal tips (needles or whiskers) of diameters as small as 2-3 nm that bridge the metal to silicon and cause AFM/STM-like nanofabrication, producing 0-D mounds, indentations, or clusters. The formation of sharp needles can be understood in terms of charge injection/electric breakdown between metal clusters and silicon due to charge build-up. Silicon with high degree of impurities as well as with multiple crystalline orientation domains allow imaging these effects using electron spectroscopy without cross sectional cuts.