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Title: High Power Characterizations and Infrared Sensing Applications of Nitride-based Piezoelectric MEMS Resonators
Committee:
Dr. Ansari, Advisor
Dr. Brand, Chair
Dr. Ayazi
Abstract: The objective of the proposed research is to (1) model, characterize and mitigate the thermal challenges, self-heating, power nonlinearity and quality factor (Q) degradation, in piezoelectrically actuated MEMS acoustic wave resonators, and (2) design and fabricate a large-format resonant infrared detector array (32×32 pixels) with a noise equivalent power close to the state-of-the-art kinetic inductance detectors (10-18 W/√Hz) and the capability of sensing radiation from ultraviolet to far-infrared spectrum. Piezoelectrically actuated MEMS resonators are the building blocks of modern-day bandpass filters in RF front-end (RFFE) modules. The power demands of 5G mobile protocols require today’s RFFE filters to satisfy the high power user equipment specifications (> 30dBm). In our preliminary work, we present the first experimental characterization and theoretical modeling of MBE- grown AlScN thermal conductivity values on the dependence of Sc concentration and ambient temperature. With thermal conductivity values, we compare the self-heating temperature of thin-film bulk acoustic wave resonators (FBARs) made of pure AlN and Al0.7Sc0.3N with COMSOL finite element analysis. We observe more than a tenfold decrease of thermal conductivity values by adding a small amount of Sc concentration to pure AlN. Increased self-heating temperature and decreased intrinsic Q limit are theoretically predicted for AlScN devices due to the degradation of thermal conductivity. These observations have prompted us to perform further experimental characterizations of AlScN acoustic wave resonators under high power loads. Despite the thermal challenges, one can design MEMS sensors that harness acoustic wave resonators’ thermal effects. Resonant IR detectors convert radiation energy to frequency shifts of the piezoelectric resonator through the temperature coefficient of elasticity (TCF). To push the boundary of current IR detector technology, we propose (1) using carbon nanotubes (CNT) as an IR absorber to achieve broadband sensing, (2) engineering tethers with low-thermal conductivity materials AlScN and SiO2 to enhance thermal isolation, and (3) flexural-mode resonator with stress-dependent resonant frequency to improve |TCF| beyond 100 ppm/K.
