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

• Published in: Optics and lasers in engineering 2022-01, Vol.148, p.106778, Article 106778
• Main Authors: Yi, Kiyoon, Liu, Peipei, Park, Seong-Hyun, Sohn, Hoon
• Format: Article
• Language: English
• Subjects: Coating thickness estimation; Femtosecond laser; Moving object scanning; Reflectometry; Silicon wafer; Ultrafast ultrasonic
• ISSN: 0143-8166; 1873-0302
• DOI: 10.1016/j.optlaseng.2021.106778

•Femtosecond laser enables a minimized contact time with a moving target.•Femtosecond pump-probe setup reduces signal distortion due to target movement.•Both pulse-echo and pitch-catch modes are investigated on a moving target.•Ultrathin coating thickness of a silicon wafer is successfully estimated. 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.