Real-time detection of surface cracks on silicon wafers during laser beam irradiation

In this paper, a real-time in situ method to detect surface crack initiation on silicon wafers during laser beam irradiation is proposed. This method collects scattered light from the silicon wafer surface subjected to the laser irradiation. When the crack is initiated, the laser beam is strongly scattered by the crack so that the proposed method can monitor the time of crack initiation based on the increases of the level of the scattering signal. In order to demonstrate the performance of this method, a silicon wafer specimen was illuminated by a continuous wave (CW) fiber laser beam (wavelength of 1,070 nm) and the scattered light was detected at three different laser powers. The scattering signal showed a very high level at the time of crack initiation. The detected crack initiation times were 11.6 s, 5.5 s, and 2.5 s at irradiances of 130 W/cm₂, 149 W/cm₂, and 168 W/cm₂, respectively. These results agree well with the theoretical predictions. Based on these results, we demonstrated that the proposed method is very effective for the real-time in situ detection of surface cracking induced by laser beam irradiation on silicon wafers.