Piezoelectric MEMS for acoustic sensing, contactless RF switching, and resonant mass sensing

This thesis presents microelectromechanical systems (MEMS) for acoustic sensing, contactless RF MEMS switching, and resonant mass sensing based on piezoelectric transduction using zinc oxide (ZnO) thin films.;For acoustic sensing, a sensitive, broad-bandwidth piezoelectric MEMS transducer based on frequency interleaving of resonant transducers is designed and fabricated. High compliance cantilever and spiral-beam-supported diaphragms are designed and built on the edge-released MEMS structure to relieve initial residual stress and to avoid in-plane tension when bent. For a given pressure level and diaphragm size, the maximum strain on the spiral-beam-supported diaphragm is about an order of magnitude larger than that of a rectangular cantilever diaphragm. Also, the acoustic transducer built on the spiral-beam-supported diaphragm has a much higher sensitivity (but with less tolerance on the fabrication process variation and at the cost of lower usable bandwidth) than the one built on a rectangular cantilever diaphragm. By connecting many transducers in parallel, both the sensitivity and acoustic output were improved by about 30 times. The interleaving of the transducers increased not only the sensitivity, but also broadened the useable bandwidth.;For RF MEMS switch, the novel contactless piezoelectric switch employing ZnO actuation has been developed. The cantilever is built on a 0.3 um thick nitride with a 0.1 mum/0.35 mum/0.1 mum Al/ZnO/Al sandwich structure. The piezoelectric deflection is demonstrated to be linear and bi-directional with fast frequency response. The contactless RF MEMS switch is composed of two surface-micromachined tunable capacitors and two bonded-wire inductors. By applying a dc bias voltage between the two piezoelectrically-actuated cantilevers, the free-standing bridge is pulled down to the cantilevers for simply-supported boundary conditions. The ON-state and OFF-state of the switch are achieved by deflecting a pair of piezoelectrically actuated cantilevers to move a simply-supported bridge up and down to vary the capacitance by a factor of 4 without any physical contact between the bridge and substrate.;For resonant mass sensing, film bulk acoustic resonators (FBAR) can be made into label-free mass sensors for identifying biological and chemical species. They have higher sensitivity than a quartz crystal microbalance (QCM) due to their higher resonant frequency. Unlike current probe methods labeled with fluorescence, chemiluminescence, electrochemiluminescence, or radioactive tags, the resonant frequency of FBAR changes in response to mass added to its surface, and can be used for label-free quantitative measurement. A novel FBAR array is fabricated by connecting four resonators in parallel. Each device in the array has a distinct resonant frequency and sensing/reaction chamber, allowing for simultaneous detection of four different bio-chemical agents. Ligand--protein interactions are chosen as a demonstration vehicle of the arrayed FBAR due to their well-known high affinity binding.