| Summary: | 碩士 === 國防大學中正理工學院 === 應用物理研究所 === 94 === In this study, the mechanical properties of nano-films were investigated with picosecond ultrasonic waves, which were produced by an ultrafast laser. The topic of this study was divided into three parts: First, a single film of TaN was reactively sputtered with various nitrogen partial pressures, and the effect of the nitrogen partial pressure on acoustic velocity will be investigated using picosecond ultrasonics.
Next, an investigation on the difference of acoustic impedances between TaN and Mo will be carried out using the impedance mismatch of TaN/Mo by picosecond ultrasonic waves. TaN has many advantages, such as high impedance, controllable resistance, and wide application in semiconductor industry. Our study suggests that TaN can replace Mo as a high-impedance layer in Bragg reflectors to solve the problem of parasitic capacitance.
The last part discusses the echo time measurement in quarter-wavelength SiO2/Ni films in order to accurately calibrate the thickness of the high-and low-impedance layers in Bragg reflectors. This result offers designers for Bragg reflectors useful information.
The velocities of TaN films grown under three different N2 partial pressures were measured. In addition, we found that TaN films have higher acoustic impedance than Mo, and thus can replace metals as the high-impedance layers in the Bragg reflectors to reduce the problem of parasitic capacitance. Finally, we successfully demonstrate a new method to verify whether the film thickness is quarter wavelength. This method can facilitate the design of the quarter-wavelength layers in Bragg reflectors.
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