Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Jing Wang, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Jiangfeng Zhou, Ph.D.

Keywords

Microelectromechanical Systems, Motional Resistance, Quality Factor, Vibrating Micromechanical Resonators

Abstract

Over the past few decades, the usage of mobile communication devices such as smartphones, laptop computers, tablets has grown significantly. Clearly, smartphones and other mobile gadgets have been leading the current and emerging market thanks to the fast growing of wireless communications. Therefore, the wireless communications market is constantly looking for new ways to miniaturize the RF front-ends, while reducing the size, cost, and power consumption. Radio frequency microelectromechanical systems (RF MEMS) is widely viewed as a potential enabling technology for the multi-standard monolithic transceivers on a single chip with high reliability, high performance, and very low DC power consumption. Among a wide variety of RF MEMS device components, resonators offer unique benefits because of their very high-quality factors, which enable the implementation of advanced functions such as low insertion loss filters, mixer-filters, ultra-low phase noise oscillators, and even potential future RF front-end channel selection instead of band selection for realizing true software defined radio all in GHz frequencies. The film bulk acoustic resonators (FBAR’s) and surface acoustic wave (SAW) resonators are piezoelectric resonators that have already been successfully implemented into commercial products in mass consumer electronics for more than a decade. Apparently, vibrating micromechanical resonators such as FBAR’s can perform really well while enabling significant power reduction as compared to conventional RF systems. Nonetheless, both FBAR and SAW devices fall short of satisfying the performance requirements, such as quality factor at a frequency below 2 GHz by future single-chip transceiver with multi-frequency functionality. Moreover, the quality factor drops notably as the resonance frequency approaches 3 GHz and beyond. Therefore, contour-mode resonators have been developed to replace FBAR at VHF and UHF frequencies, since their resonance frequencies are defined by lateral dimensions while being able to deliver moderate quality factor and low insertion loss to enable multi-frequency functionality on a single chip. Nevertheless, the motional impedance of these devices is still considerably higher than the value needed to match to 50 Ω RF front-end electronics. On the other hand, capacitively-transduced micromechanical resonators can achieve very high quality factor and can operate at even higher frequency, which is challenging for piezoelectric counterparts to achieve. However, relatively high motional impedance and high insertion loss is still a big issue for these devices to interface with standard 50 Ω systems. Shrinking the capacitive transducer air gap between the electrodes and the resonator body to sub-10 nm might partially solve this problem, which demands very complex fabrication processes as compared to piezoelectric resonators. Furthermore, the employment of nanogap capacitive transducers gives rise to higher fabrication cost, lower yield, poorer reliability, and reproducibility of those devices. Another solution to lower the motional impedance by improving the transduction efficiency is to use piezoelectric transducers that are capable of achieving fairly low motional impedances. For instance, piezoelectrically transduced TPoS resonators enabled by the SOI technology are capable of operating at frequencies up to a few GHz while retaining its performance reliability. Even after being equipped with all the benefits of both piezoelectric and electrostatic (capacitive) resonators, neither of these two resonator technologies can undoubtedly fulfill all the needed performance specifications for single-chip multi-frequency RF front-end applications. Meanwhile, it is obvious that these two transduction mechanisms can efficiently complement each other. Hence, a novel hybrid resonator is proposed herein by combining electrostatic micromechanical resonator with piezoelectric thin-film transducer to exhibit low acoustic losses along with high electromechanical coupling coefficients, which could resolve the aforementioned issues hindering MEMS resonators.

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