Etching of crystalline Si in $Cl2$ atmosphere by means of an optical fiber tip

We report experiments on laser-induced chemical etching of silicon in chlorine atmosphere using a near-field optical configuration. Crystalline (100) Si surfaces were locally illuminated in $300 mbar Cl2$ through a tapered fiber tip. In most of the experiments, we used UV argon ion laser lines around 350 nm for illumination. The etched samples were analyzed by means of atomic force microscopy. Patterns with a width of 140 nm at full width half maximum and a vertical etch rate of the order of 15 nm/s have been achieved. Comparison of etching with a visible wavelength at intensities that cannot cause surface melting shows that the etching is primarily a photophysical process.