Proton beam writing of microstructures in silicon

The ability to directly pattern the surface of semiconductor wafers using very accurately controlled fluences of finely focused high-energy ion beams has opened up new research directions for the fabrication of a variety of high-aspect ratio, multi-level microstructures in silicon. A beam of hydrogen or helium ions, focused to 50-100 nm in a nuclear microprobe, is used to selectively damage the semiconductor lattice in the irradiated regions. A higher beam fluence at any region produces a higher damage concentration, so by pausing the focused beam for different times at different locations, any pattern of localized damage can be built up in the material. This damage acts as an electrical barrier during subsequent formation of porous silicon by electrochemical etching. This enables local modification and control of the properties of the porous silicon formed by ion irradiation, resulting in patterned porous silicon. If the etched sample is immersed in potassium hydroxide, the unirradiated regions are preferentially removed, leaving a copy of the patterned area as a micromachined three-dimensional structure. The fundamental mechanisms involved in the creation of such porous and silicon microstructures are discussed here.