Coating thickness estimation in silicon wafer using ultrafast ultrasonic measurement

Abstract

Considering the diminishing dimensions of modern silicon wafers (50-100 mu m thick and their coating layers are in an nm scale) and potential defect sizes, spatial resolutions for the corresponding non-destructive evaluation (NDE) solutions should be in the order of sub-mu m or nm. However, the spatial resolutions of current NDE techniques are often in the range of mu m or mm. In this study, an ultrafast ultrasonic measurement system is developed using a femtosecond pulse laser. The proposed ultrafast ultrasonic measurement system can generate ultrasonic waves up to several THz (10(12) Hz), and measure the corresponding responses with a sampling rate up to PHz (10(15) Hz). In this system, the femtosecond pulse laser beam is split into pump and probe pulses for a pulse-echo ultrasonic measurement. The pump pulse produces ultrafast ultrasound onto the target silicon wafer, and the ultrasound travels through the thickness direction. Then, the waves reflected from the coating layer are measured using an optically delayed probe pulse. Using the proposed system, the coating thicknesses of silicon wafers in the range of 50 nm to 200 nm were successfully estimated.