Three-dimensional nanostructures by direct laser etching of Si

Nanostructures have been machined into Si by a high-resolution laser direct write system. The Si substrate is locally heated above its melting point by a continuous-wave laser and rapidly etched by dry Cl2 gas. If the solid-to-liquid transition is adjusted to occur only at the peak of the temperature profile, the melt size is significantly smaller than the diffraction-limited spot size. This can translate to extremely small etched features because of the high selectivity of the etching process for liquid Si compared to crystalline Si. By using objectives with a high numerical aperture, 488 nm as well as 351 nm light from an Ar ion laser, and X/Y/Z translation stages for moving the substrate instead of steering the beam, we have achieved line widths below 200 nm combined with very high scanning accuracy and speed. The resolution limit for Si machining is determined by the selectivity of the chemical reaction rather than the laser spot size. Interfacing to computer-aided design (CAD) software allows us to remove layer by layer of a truly three-dimensional nanostructure.