Femtosecond laser ultrasonic inspection of a moving object and its application to estimation of silicon wafer coating thickness

In this study, an ultrasound generation and sensing system using a femtosecond laser is developed specifically for noncontact inspection of a moving object. In the developed femtosecond laser ultrasonic system, a laser pulse source is divided into pump and probe laser pulses. Using a pump laser pulse with a subpicosecond duration, ultrasounds with ultrashort wavelengths (micrometer to tens of nanometers) are generated up to THz. Then, the resulting ultrasounds are measured using a probe laser pulse based on reflectometry at a sampling frequency of up to 1.5 THz. The developed system is used to generate and measure ultrasounds from a silicon wafer while the wafer is moving in a horizontal direction. Because of the ultrashort pulse duration of the probe and pump laser pulses, the contact time of these pluses with respect to a moving object is extremely short (subpicosecond), and the distortion of ultrasounds due to object motion is minimized. Ultrasounds are measured from the silicon wafer in both pulse-echo and pitch-catch modes, and it is validated that the ultrasounds acquired from a moving condition of the silicon wafer are in good agreement with those obtained from a stationary condition. Then, the thickness of a submicrometer coating layer deposited on the silicon wafer was successfully estimated while the silicon wafer was moving up to 20 mm/s.