Laser micromachining of free-standing structures in SiO2-covered silicon

Tunnels and free-standing microstructures were fabricated in SiO2-covered silicon, using a laser-based micromachining technique. We employed a focused CW Ar+ laser beam, at a wavelength of 514 nm, to melt the silicon and provoke its etching in a chlorine ambient; the oxide layer is transparent to the laser beam, and is not affected by the process. Tunnels were etched for different incident beam powers and scanning speeds, at a chlorine pressure of 500 Torr. For an incident power of 1.6 W and a scanning speed of 25 μm/s, tunnels 7.5 μm deep and 3.6 μm wide were obtained. For a given scanning speed, defects were observed at the tops of the tunnels after a critical length, around 400 μm for a scanning speed of 5 μm/s, ripples and tiny holes were generated in the oxide film. The defect-free length of the tunnels, Lc, was investigated as a function of the reciprocal of the scanning speed, (Vscan)−1; for an incident laser power of 1.2 W, (LcVscan)=2400 μm2/s. Furthermore, various free-standing microstructures, cantilevers and square membranes, were successfully machined.