Development of an ultrasonic piezoelectric mems-based radiator for nonlinear acoustic applications

The development of a piezoelectric micromachined ultrasonic transmitter is presented. The transducer is evaluated as an ultrasonic source for parametric arrays. A parametric array is an acoustic technology that leverages the nonlinear demodulation of ultrasound to create a highly directional beam of audible sound analogous to a flashlight.;The transmitter was formed by a circular composite diaphragm that was radially non-uniform. The diaphragm consisted of molybdenum annular electrodes and an aluminum nitride diaphragm. The composite diaphragm was released using a combination of deep reactive ion and oxide etching.;Transduction of the diaphragm occurred when an electric field was induced across the annular piezoelectric layer of aluminum nitride. The electric field caused mechanical strain within the piezoelectric layer through the piezoelectric effect. The strain coupled into force and moment resultants that generated diaphragm deflection. By supplying an ac voltage between the electrodes, an oscillating electric field caused diaphragm vibration. The vibrating diaphragm in turn generated acoustic waves.;The overall device model was formed using composite plate mechanics, lumped element modeling (LEM), and acoustic theory. Through LEM an equivalent circuit of the device was formed that incorporated electrical, mechanical, and acoustic components. Nonlinear acoustic theory was used to predict ultrasonic demodulation. The device performance model was used in a geometric constrained optimization scheme.;The optimal design based on the LEM was used to form the primary design. A series of secondary designs were formed by constraining the device layer thicknesses and performing optimization using radial geometry as design variables and considering deviations in stress. The device was fabricated at Avago Technologies Limited's foundry. A unique package with back cavity depth control was designed and fabricated. This was followed by experimental characterization. Electrical characterization consisted of measurement of device impedance. Mechanical characterization included mode shape and resonant frequency measurements. Acoustic characterization encompassed farfield acoustic measurements of a single device.;The characterization results showed significant mismatch between devices as well as the equivalent circuit model. Four out of the six devices tested had resonant frequencies near 60 kHz. The remaining two devices had resonant frequencies of 31 kHz and 44 kHz. The equivalent circuit predicted a resonance of 39 kHz. The variation between the results was attributed to stress variations across the wafer that occurred during fabrication. A variable back cavity was used to tune the devices and maximize sensitivity at resonance. A 220% improvement in the resonant deflection sensitivity of the 31 kHz resonant device was found by tuning the back cavity. A nonlinear acoustic calculation was used to project the performance of a 150 mm diameter device array as an acoustic source for a parametric array. The sound pressure level of a 5 kHz audible tone was 42 dB at 1 m. The low projected audible output does not show good promise of the application of this design to a conventional audible parametric array. Recommendations for future work focus on a more robust device design and packaging improvements for other ultrasonic applications.