Aluminum nitride thin film based acoustic wave sensors for biosensing applications

In recent years, SAW devices have drawn enormous interest from the analytical assay and sensing business, especially in the biosensing area where highly sensitive, cost efficient and miniaturized sensors are in urgent needs. This dissertation focuses on the development of AIN thin film based SAW devices suitable for biosensing applications.; AIN thin films have been synthesized on different orientations of sapphire substrates by a plasma source molecular beam epitaxy system. Surface and structural characterization techniques have been applied to investigate the film quality and the results show that high quality c-plane AIN was epitaxially grown on both c-plane and a-plane sapphire substrates.; Complete process flows have been developed for the fabrication of SAW delay line and resonator devices. Important electrical parameters such as the insertion loss, bandwidth, and impedance have been measured to assist the design optimization and derivation the phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency.; On both c-plane and a-plane sapphire substrates, the SAW phase velocities (∼5700 m/s) and electromechanical coupling coefficients (∼0.3%) have been thoroughly mapped out with respect to the propagation direction and film thickness to wavelength ratio. The data are of practical importance for designing AIN-based SAW devices. A higher velocity (>6000 m/s) shear horizontal SAW mode has been discovered only at isolated propagating directions. This mode is especially suitable for aqueous biosensing due to its weak energy coupling to liquid. Much stronger response of the SH-SAW mode has been detected on the c-plane AIN on a-plane sapphire structure than on the c-plane AIN on c-plane sapphire structure, which could be attributed to large anisotropy in a-plane sapphire substrate. Linear frequency-temperature relationship has also been observed for both modes.; We further quantify the mass sensitivity of the SAW and SH-SAW by monitoring the frequency and phase changes in response to the coating of aluminum thin films onto the device surface. The derived mass sensitivity indicates that both modes could potentially reach an extremely low detection limit at the level of picograms.