Aluminum nitride thin films on titanium: Piezoelectric transduction on a metal substrate.

摘要

With many microsystems recently fabricated on titanium substrates---mechanical arrays, sensing elements, and fluidic devices---titanium has emerged as a platform for MEMS fabrication. The success of these devices relies upon the advantageous material properties of titanium: high fracture toughness, oxide biocompatibility, three-dimensional stacking ability, and the integration of micro- and macromachining techniques. However, due to the relative immaturity of this technology, the number of established processes are still limited; thus far, titanium MEMS rely on electrostatic actuation and incorporate few, if any, functional materials.; This dissertation introduces aluminum nitride thin film deposition and piezoelectric transduction to the titanium MEMS design toolkit. A process has been developed for depositing AlN films onto polished titanium wafers by middle-frequency sputter deposition. By utilizing the hysteretic behavior of reactive sputtering, thin films with strong c-axis orientation have been deposited; XRD rocking curves for these films have FHWM as low as 1.8°. Further characterization has shown that these films have piezoelectric, optical, and electrical properties that approach single crystal AlN.; Piezoelectric unimorph cantilever beams have been fabricated from the deposited films and used for the first demonstration of piezoelectric transduction on titanium. At large displacements, these beams exhibit nonlinear spring softening. An analytical model has been developed to accurately describe the frequency response of these beams and to calculate the material properties of the film. Test results from these beams show that d 31 and k231 for the films are within 10% of the reported values.; Surface acoustic wave (SAW) filters have also been fabricated from AlN films on titanium. Although the conductivity of titanium results in severe electromagnetic feedthrough between the input and output ports, the transmission response of the filters has been measured using a time-gating method. The electromechanical coupling and power attenuation have been extracted from s12 measurements; though k2 is high, the attenuation is roughly an order of magnitude higher than SAW filters fabricated from AlN films on silicon.