Photochemical etching of silicon

Silicon that was immersed in hydrofluoric acid can be etched photochemicall y by laser, and it was found to produce long and regular columnar structure , if the laser power density is greater than 10 mW/mm(2). Another criterion is that the laser wavelength should be at the blue end of visible spectrum . Fine wires with diameter 300-200 nm were also observed at the top of thes e columns. The dimension of these fine wires is near to quantum confinement dimension, thus can be taken as supporting evidence for quantum confinemen t. The photoluminescence spectra full width half maximum was narrower than that from porous silicon fabricated from conventional anodisation method. T he narrower full width was attributed to the uniformity of the porous silic on structure. A physical model is proposed to explain the observed strong d irectional etching. The model showed that once the etch sites have randomly initiated, the etching rate becomes directional under the influence of las er. The intensity of laser controls the etching direction such that silicon columns are formed if the intensity of the laser is strong enough.