Fabrication of ordered arrays of silicon cones by optical diffraction in ultrafast laser etching with SF₆

We report the fabrication of ordered rows of conical spikes on a Si(111) substrate by etching with a femtosecond pulsed laser in the presence of SF6 gas. This is achieved by the creation of an optical near-field diffraction pattern when a copper wire (diameter similar to140 mum) is placed on the surface. Measurements of the height, base width and average separation of the silicon cones at two irradiation wavelengths (780 nm and 390 nm) confirm the important role of the optical parameters in the photochemical etching process. Moreover, the dependence of average cone separation on the laser repetition rate indicates that long-timescale processes such as post-pulse chemical interactions are important in cone formation.