Rear surface spallation on single-crystal silicon in nanosecond laser micromachining

Rear surface spallation of single-crystal silicon under 5-ns laser pulse ablation at intensities of 0.6–60GW∕cm2 is studied through postablation examination of the ablated samples. The spallation threshold energy and the spallation depth’s dependences on the energy and target thickness are measured. From the linear relation between the spallation threshold energy and the target thickness, an estimation of the material spall strength around 1.4GPa is obtained, in reasonable agreement with the spall strength estimation of 0.8–1.2GPa at a strain rate of 107s−1 using Grady’s model for brittle materials. The experiment reveals the internal fracturing process over an extended zone in silicon, which is controlled by the competition between the shock pressure load and the laser ablation rate. The qualities of the laser microstructuring and micromachining results are greatly improved by using an acoustic impedance matching approach.