Silicon wafer crack detection using nonlinear ultrasonic modulation induced by high repetition rate pulse laser

In this study, a high repetition rate laser ultrasonic system is developed for noncontact detection of silicon wafer crack. A high repetition rate pulse laser is used to generate two narrowband ultrasonic waves on a target silicon wafer, whereas a high-speed laser Doppler vibrometer is used to measure the corresponding ultrasonic responses. Nonlinear interaction occurs between the two generated ultrasonic waves due to crack formation, producing nonlinear ultrasonic modulation at the sum and difference of the two excitation frequencies. Outlier analysis is performed on the amplitudes of the nonlinear modulation components, and a fatigue crack is identified when the amplitudes of the nonlinear modulation components exceed a user specified threshold. The uniqueness of this study is as follows. (1) A noncontact, high-speed, and scanning laser ultrasonic system is developed using a high repetition rate laser for ultrasonic generation and a laser Doppler vibrometer for sensing as well as scanning the two laser beams simultaneously in a pitch-catch mode. (2) Two narrowband ultrasonic waves up to 1 MHz are simultaneously generated from a single laser pulse. (3) Micro cracks with widths of less than 10 mu m are successfully detected by extracting the crack induced nonlinear ultrasonic modulation components. (4) The probability of detection of the proposed crack detection technique is evaluated. The performance of the proposed technique is experimentally validated using eight silicon wafer specimens with varying crack lengths. Results show that the proposed technique yields a 90% probability of detection in silicon wafers when the crack length is over 3.77 mm.