High power density laser etching of silicon

Silicon that was immersed in hydrofluoric acid can be etched photochemically by laser, and it is 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 the visible spectrum. Fine wires with diameter 300-200 nm were also observed at the top of these columns. The dimension of these fine wires is near to quantum confinement dimension and thus can be taken as supporting evidence for quantum confinement. The photoluminescence spectra full width half maximum was narrower than that from porous silicon fabricated from the conventional anodization method. The narrower full width was attributed to the uniformity of the porous silicon structure. A physical model is proposed to explain the observed strong directional etching. The model showed that once the etch sites have been randomly initiated, the etching rate becomes directional under the influence of the laser. The intensity of the laser controls the etching direction such that silicon columns are formed if the intensity of the laser is strong enough. (C) 1998 American Institute of Physics. [S0021-8979(98)02003-9].