Micromachining of silicon by short-pulse laser ablation in air and under water

Micromachining of silicon was conducted using a short-pulse (FWHM=25 ns) KrF (λ=248 nm) excimer laser that generates laser energy in the range of 100–480 mJ. Laser ablation drilling tests were conducted on a silicon workpiece both in air and under water. The drilled surfaces were characterized using conventional optical and scanning electron microscopes, as well as a laser interference microscope. Once the laser fluence exceeds a certain threshold value (1.7 J/cm2), the ablation depth was found to increase rapidly with respect to the laser fluence with the variation being nonlinear. The ablation depth was also found to depend on the number of pulses, increasing with increasing number of pulses. The differences between laser ablation under water and in air are enumerated.