Enhancing silicon wafer ablation using passive Q-switch [formula omitted] lasers with third-order nonlinear effects

Abstract

We show that a passive Q-switch nanosecond Tm:Yap laser operating at 1.940μm is a promising candidate for conducting ablation process at the surface of silicon wafers. By modulating the fluence intensity of the lasing source, we can engrave the surface with a short series of pulses and obtain sharp and regular craters with a high level of repeatability. Our results show that we can get highly repeatable craters of diameters ranging from D=10.0μm up to D=40.0μm with a laser operating We easily manage the hole depth by choosing the correct fluence repetition rate between pulses, reducing the waste deposition on the borders of the holes. For fluences greater than 3.8J/cm2, third-order nonlinear effects, such as the lens converging Kerr n2 and the two-photon absorption βTPA effects, turn the silicon into an absorbing medium, improving the ablation process. At a room temperature of 300°K, n2=12.0×10−5±2.0×10−5cm2/GW and βTPA=0.56±0.02cm/GW, the resulting nonlinear factor of merit NFOM=2.1±2.0 is very large; conferring a self-focusing lensing action on the silicon. We also observe that these latter nonlinear effects modify the regular linear regression relating the ablated surface to the logarithm of the fluence, enhancing the efficiency of the ablation process at higher fluences due to these effects.