Development of aluminum nitride thin films for applications in acoustic wave (AW) sensors

In this research, epitaxial growth of AlN thin films on C-plane Sapphire substrate and development of acoustic wave (AW) devices based on this material system were investigated.; AlN thin films were deposited on C-plane Sapphire by Plasma Source Molecular Beam Epitaxy (PSMBE) system. Long time etching resulted in an atomically smooth substrate surface with roughness less than 0.5 nm. AlN epi-layers grown at 650°C have considerably higher quality in term of structural and surface qualities than films grown at 400°C and 800°C. A two-stage growth method was proved to be able to improve the crystalline quality of the thicker (>1 μm) film.; Surface acoustic wave (SAW) and surface transverse wave (STW) devices were designed by Mentor Graphics. The interdigtal transducer (IDT) frequency response was simulated via MathCAD. The response was found to have the characteristic sin (X)/X behavior. AW devices with wavelength λ = 32, 16, and 8 μm were successfully fabricated via deep UV photolithography. Excimer laser micromachining techniques were utilized to fabricate the central energy-trapping microgroove gratings in one STW devices with wavelength of 32 μm. The laser micromachined grooves were 5 μm wide, 150 nm in depth and with a periodicy of 10 μm.; Acoustic wave devices were tested by network analyzer. The frequency response of device was consistent with the design. The high insertion loss and weak amplitude in the SAW devices were due to relatively thin AlN film compared to the acoustic wavelength. The oscillating wave patterns at center frequency were verified as electromagnetic feedthrough between input and output IDTs. The electromagnetic feedthrough decreases as the ratio of film thickness with wavelength increases. The frequency response of SAW resonators with λ = 8 μm showed excellent agreement with the simulation. The sign of reflection coefficient in AlN SAW resonators was experimentally determined for the first time for both open- and shorted-metal strip configurations. The phase velocities of SAW and STW propagation for AlN were about 5560 and 6568 m/s, respectively. Laser micromachined microgroove gratings were found to enhance the propagation of STW. Finally, acoustic sensors based on STW showed little attenuation in the liquid environment, while sensors based on SAW suffered excess loss when exposed to liquid.