Characterization of internal material properties based on laser picosecond ultrasonics

Materials such as tungsten (W), copper (Cu) for interconnects, and amorphous carbon (A-Carbon) for hard masks are critical in semiconductor manufacturing. Their internal properties, including Young's modulus and acoustic attenuation, are vital for predicting process parameters like etching depth and rate. However, techniques for non-destructively characterizing these properties are limited. This study leverages laser picosecond ultrasonic technology to develop a methodology for evaluating both Young's modulus and the relative ultrasonic attenuation coefficient to assess variations in internal material properties. For materials with sufficient optical penetration depth, this technique detects Brillouin scattering and high-frequency (GHz) ultrasonic echo signals, from which longitudinal wave velocity and acoustic impedance are derived to calculate Young's modulus. Furthermore, by comparing the picosecond ultrasonic echoes from a target material with those from a reference, the relative ultrasonic attenuation coefficient is determined, providing insights into microstructural changes. Using a custom-built laser picosecond ultrasonic system, we excited Brillouin scattering and ultrasonic echoes in A-Carbon films. The results exhibit strong linear correlations (R2 > 0.98) between material preparation temperature (varied by 20°C) and the measured properties. Specifically, a 20°C temperature increase corresponds to a sound velocity decrease of ~5 Å/ps, a Young's modulus decrease of ~16 GPa, and a ~16% increase in the relative acoustic attenuation coefficient.