A Study on Thin Film Microstructure and Its Effects on Acoustic Film Velocity Through Picosecond Ultrasonics Technique
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A Study on Thin Film Microstructure and Its Effects on Acoustic Film Velocity Through Picosecond Ultrasonics Technique Ta-Ching Li, Nen-Wen Pu, Ben-Je Lwo, and Chin-Hsing Kao, Chung-Cheng Institute of Technology, National Defense University, Dahshi , Taoyuan 335,Taiwan, ROC. Long-Jang Hu, Chung-Shan Institute of Science and Technology, Longtung, Taoyuan 325,Taiwan, ROC. ABSTRACT In acoustic devices such as film bulk acoustic resonators (FBAR), it is most essential to accurately determine the thin-film sound velocities in situ. In this work, we analyzed the microstructure properties of the zirconia thin films deposited by RF magnetron reactive sputtering with various oxygen partial pressures, and measured the longitudinal film velocity with picosecond ultrasonic technique. The picosecond ultrasonic waves were produced by irradiating the testing samples with an ultrafast laser pulse generated by a self-made mode-locked Ti: Sapphire laser, and detected by a delayed probe laser pulse. The acoustic velocities of the thin films were next determined from the echo times of the ultrasonic waves. To derive more accurate and reliable velocity, three different reflective layers were employed so that the echo shapes and intensities of ultrasonic wave can be compared. It was found in this work that the thin film velocities we measured were less than the bulk value, which can be calculated from Young’s modulus and the density. Meanwhile, with the measurement results, it is also found that the measured acoustic velocity and the microstructure of films have strong dependence on the growth conditions. Consequently, accurate thin film velocity will be obtained for an SMR designer through better controlling on deposition conditions during manufacturing process. INTRODUCTION The increasing employment of thin films in acoustic devices, such as the thin film structure in a solidly mounted resonator (SMR), demands accurate mechanical characterization of the films. This is because the qualities of the Bragg mirror and the piezoelectric layer of SMR, and the thickness of the electrode play the most important role that influence the Q factor and the insertion loss [1] [2] [3] of a resonator. In this work, the zirconia was chosen for studying the microstructure effects on the film acoustic velocity due to different processing conditions. For acoustic velocity measurement on zirconia films, the picosecond ultrasonics technique,
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which has recently proven useful for studying acoustic velocity of thin films [4][5], was employed. This technique measures the optical transient reflection change (ΔR) through an pump-probe detection technique, with which the acoustic waves are excited and detected by ultrafast laser pulses. The acoustic velocities are next obtained by measuring the time period of the elastic strain waves (i.e. acoustic waves) reflected from the interface of the film and the reflective substrate [6][7]. Based on these measurements, accurate thickness design will be highly promising for the SMR-type FBAR. EXPERIMENTAL
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