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This paper reports an investigation into the various dissipation mechanisms that can affect polycrystalline diamond micromechanical resonators. Double-ended tuning fork and cantilever resonators were fabricated from 1–5- \mu \textm thick microcrystalline diamond films. It is shown that the quality factor of the low frequency (<500 kHz) resonators is limited by surface loss, whereas the thermoelastic damping limits the quality factor of the higher frequency resonators. In resonators where surface loss is the dominant effect, the dependence of quality factor on resonator thickness is demonstrated. The addition of a lossy surface layer of Al2O3 deposited via atomic layer deposition is shown to degrade quality factor, and an experiment that further demonstrates the effect of surface dissipation and results in a reduction in quality factor that scales with the thickness of the Al2O3 layer. Heat treatment of cantilever resonators in N2 for various times up to 660 min is used to modify the resonator surface and is shown to result in a threefold increase in quality factor up to 365 000 at 26.6 kHz. [2015-0094] [ABSTRACT FROM AUTHOR]

This paper presents a 1.2-mm diameter high fill-factor array of 1261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz in fluid for intravascular ultrasound imaging. At 1061 transducers/ \mathrmmm^2 , the PMUT array has a 10–20 times higher density than previous PMUT arrays realized to date. Aluminum nitride (AlN)-based PMUTs described in this paper are fabricated using a process compatible with the fabrication of inertial sensors, radio frequency (RF) resonators, and CMOS integrated circuits. The PMUTs are released using a front-side sacrificial etch through etching holes that are subsequently sealed by a thin layer of parylene. Finite element method and analytical results, including resonant frequency, pressure sensitivity, output acoustic pressure, and directivity are given to guide the PMUT design effectively, and are shown to match well with measurement results. Due to the PMUTs thin membrane (750-nm AlN/800-nm SiO2) and small diameter, a single 25- $\mu $ m PMUT has approximately omnidirectional directivity and no near-field zone with irregular pressure pattern. PMUTs are characterized in both the frequency and time domains. Measurement results show a large displacement response of 2.5 nm/V at resonance and good frequency matching in air, high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz, when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused high-output pressure acoustic beam. [2014-0114] [ABSTRACT FROM PUBLISHER]