Air-coupled aluminum nitride piezoelectric micromachined ultrasonic transducers

This work focuses on air coupled piezoelectric ultrasonic transducers (PMUTs) for range finding and gesture recognition applications. Such applications require an array of identical PMUTs operating at center frequencies from 40-900 kHz, a fractional bandwidth greater than 5%, and center frequency variation between PMUTs within the same array that does not exceed the fractional bandwidth.;The first contribution of this work aims to reduce the in-wafer and in-die variation of the resonance frequency without degrading the PMUT performance. By designing a variable thickness diaphragm, a more robust diaphragm was designed while widening its bandwidth compared to flat solid plate with the same resonant frequency. The membranes are partially etched to remove mass, in a radial ribbed pattern that maintains the stiffness of the structure. This design achieves a 10 fold lower variation in the resonance frequency, while maintaining low quality factor for PMUTs at about 200 kHz.;The second contribution of this work concerns the design of PMUTs for higher frequencies. Traditionally, air couple ultrasonic transducers operate at 40-200 kHz, in order to minimize loss in air. However, for some applications, the transmission range can be traded off in order to achieve better resolution. PMUTs at resonance frequencies from 200-900 kHz were fabricated and characterized, and their loss in air was confirmed. The devices were fabricated using wafer-level bonding of a MEMS wafer to a CMOS wafer; therefore the acoustic effects due to the presence of the CMOS wafer were investigated. A back side cavity was required in order to prevent squeeze film damping between the membrane and the CMOS.;Finally, a novel method to recycle the back-side acoustic pressure by redirecting it to the front-side through concentric venting rings was demonstrated. The ring diameter determines the phase-shift between the sound emerging from the front-side port and the ring, and can be adjusted to either amplify the far-field sound pressure level (SPL) or change the directivity of the output beam. Nine deferent ring designs were fabricated and characterized, and a 4.5dB increase in on-axis SPL was measured.;The methods used include analytic and numeric modeling of the piezo-acoustic systems as well as fabrication and characterization of devices. Multi-physics finite element models (FEM) were conducted using COMSOL and included the piezoelectric devices and the acoustic domains. Also a general dynamic model for a PMUT system that can evaluate performance in transmission and sensing was developed. It can also be adjusted for different boundary conditions and different diaphragm shapes. The model is not limited to linear coefficient, and therefore can also be used to study the non-linearity of the system. Then, devices with an aluminum nitride (AlN) piezo layer were fabricated both at the Marvell Nanofabrication Laboratory and in an industrial foundry. Frequency responses were studied in-air, using a laser Doppler vibrometer (LDV) and acoustic measurements were conducted using a B&K high frequency microphone.