Graduation Year

2021

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

Arash Takshi, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Jiangfeng Zhou, Ph.D.

Abstract

Microelectromechanical system (MEMS) resonators emerged a few decades ago, and now they have been well developed and commercially utilized as a core part in many radio frequency (RF) communication hardware as well as micro-sensing and detections for many different latest applications. Since MEMS-based devices are made small thanks to the microfabrication technology's improvement, the cost can be drastically reduced when mass-produced on wafer batches, and the devices still maintain a decent performance with negligible power consumption. This perfectly fits into the requirements of the newest internet-of-thing (IoT) and headset applications. Our previous research mainly focuses on designing and fabricating MEMS resonators based on piezoelectric thin film while lacking systemic study and understanding the acoustic behavior in design variants. This research addresses the mentioned concern and further improves many resonators' performance, such as higher qualify factors, lower static capacitance, and higher power handling. In the meantime, diamond technology has been reviewed and adopted in this research, delivering high-performance MEMS acoustic resonators fabricated on Diamond-on-Silicon (DOS) wafers. The measured unloaded quality factor is as high as 7,370 near 200 MHz, which can be an excellent candidate for sensing and detection purposes. It is the first time thicker macrocrystalline diamond films (10-20 μm) are implemented into MEMS acoustic resonators design and fabrications. Meanwhile, tether (also known as anchor) designs have been thoroughly studied, including various dimensions and quantities. Different-shaped phononic crystals have been exploited to improve the resonators’ quality factors and suppress spurious mode at lower frequencies.

An innovative post-process technique has been introduced, aiming to create notched air gaps in between interdigital transducer (IDT) electrodes of a MEMS resonator. By substituting lossy Zinc Oxide (ZnO) by air, the static capacitance can be reduced, leading to a lower feedthrough noise level by 10 dB or more while having less impact on insertion loss. By using this strategy, the signal-to-noise ratio (SNR) at the resonance can be enhanced, which is favorable for sensing and detection capabilities. The capacitance ratio is defined as the ratio of static capacitance over the motional capacitance. Due to a smaller static capacitance, the capacitance ratio is lower, resulting in a higher electromechanical coupling coefficient. In the meantime, it is observed several spurious modes can be effectively suppressed or fully eliminated due to the modified distribution of electric field and displacement/strain field in the piezoelectric transducer layer adjacent to IDT electrodes thanks to the newly introduced air notches. The mentioned post-fabrication technique adds another degree of freedom in vertical coordinate in MEMS resonators design variants. Moreover, the SNRs can be further improved by cascading multiple contour-mode resonators so that the static capacitance can be canceled. Last but not least, a droplet test has been conducted to sense different levels of mass. To make the testing unit portable, a 3-D printed evaluation board has been carried out to avoid the need for probing.

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