||In this study, the Bragg reflectors deposited by DC and RF|
magnetron sputtering are composed of alternating layers of high and low acoustic impedance materials which have a thickness of one quarter wavelength at the desired resonance frequency, while the piezoelectric layer, aluminum nitride thin film, was deposited by RF reactive magnetron sputtering.
The deposition parameters of Mo and SiO2 thin films are tuned according to the AFM measurement. With the optimal deposition parameters, the cross-sectional SEM images of multilayer Bragg reflector show smooth and clear interfaces which are important criterias for the SMR devices.
The frequency responses of SMR show distinct resonant
phenomenon near 1.4GHz and 2.5GHz with an excellent noise
restraint. It indicates that the Bragg reflector made of Mo/SiO2 is suitable for the fabrication of SMR devices. Furthermore, the two resonant frequencies show that the bulk acoustic wave transmit in two different way, the longitudinal mode and the shear mode. The cause of the shear mode vibration is the tilt of the c-axis of AlN thin
films. For c-axis inclined AlN thin films, the longitudinal and the shear mode vibration both exist, and the acoustic wave propagates with different acoustic velocities along the c-axis inclined AlN.