Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Electrical Engineering

Degree Granting Department

Electrical Engineering

Major Professor

Jing Wang, Ph.D.

Co-Major Professor

Thomas Weller, Ph.D.

Committee Member

Thomas Weller, Ph.D.

Committee Member

Sylvia Thomas, Ph.D.

Committee Member

Rasim Guldiken, Ph.D.

Committee Member

Andreas Muller, Ph.D.

Keywords

ALD, Capacitive, MEMS, Piezoelectric, Resonator

Abstract

On-chip vibrating MEMS resonators with high frequency-Q product on par with that of the off-chip quartz crystals have attracted lots of attention from both academia and industry for applications on sensing, signal processing, and wireless communication. Up to now, several approaches for monolithic integration of MEMS and transistors have been demonstrated. Vibrating micromechanical disk resonators which utilize electroplated nickel as the structural material along with either a solid-gap high-k dielectric capacitive transducer or a piezoelectric transducer have great potential to offer unprecedented performance and capability of seamless integration with integrated circuits.

Despite the frequency drift problems encountered in early attempts to use nickel as a structural material in MEMS gyroscopes, this low temperature nickel electroplating technology is amenable to post-transistor planar integration. The nickel microstructure is formed through the photoresist molding and electroplating process which enables the microstructure to have extremely high aspect ratio while retaining the overall process temperature under 60ºC. This temperature is low enough to allow the RF MEMS devices to be fabricated directly on top of foundry IC chips, thus enabling post-transistor monolithic integration with minimum parasitics. In addition, the electroplating setup for nickel deposition can be much cheaper as compared to the other deposition facilities (e.g., PVD, CVD, etc).

However, as the dimensions of the resonators are shrunk to µm range, several issues have come forth such as higher motional resistance and lower power handling ability. In order to reduce the motional resistance, high permittivity material is employed to form a solid capacitive gap instead of an air gap. As compared to the air gap, ease of the process, better stability and elimination of the particles are the additional benefits of using the solid gap. Therefore, an ultra-thin high-k dielectric layer with atomically controlled thickness down to sub-nm range can be deposited under 100ºC on the vertical sidewall of the device structure by using ALD processing technology. This enhances the efficiency of the capacitive transducer enormously, thus reducing the characteristic motional resistance of the device. This research project explored the idea of applying low temperature process of electroplated nickel and high-k solid-gap as well as partially-filled air-gap capacitive transducers. To further reduce the motional impedance, electromechanically-coupled resonator arrays have been implemented. Furthermore, the linearity of solid-gap versus partially-filled air-gap resonators has been studied through a modeling approach for RF applications.

In the meanwhile, this work also investigated electroplated nickel as a structural material for piezoelectrically-transduced resonators to demonstrate piezoelectric-on-nickel resonators with low temperature process. The thin film piezoelectric resonators can achieve high resonance frequency when increasing the piezoelectric film thickness and scaling down the device size. However, the sputtered piezoelectric films have very low deposition rate which limits the thickness to a couple of microns or less. Moreover, the yield of piezoelectric resonators is restricted after the releasing process since the stress of the thin films usually causes the structural layer to buckle or fracture. Thus, the development of piezoelectric-on-substrate resonators is an alternative solution to resolve the aforementioned issues. The previous work has been done by using single crystal silicon or nano-crystalline diamond (NCD) as resonator structural materials due to their high acoustic velocity and low loss. However, the deposition temperature for thin film silicon and diamond is too high to be allowable thermal budget of ICs. Therefore, electroplated nickel is also a reasonable substitute for silicon and diamond substrates while realizing high frequency and moderate Q. Furthermore, it is observed that a localized annealing process through Joule heating can be adopted to significantly improve the effective mechanical quality factor for the ZnO-on-nickel resonators. This work successfully demonstrated the ZnO-on-nickel piezoelectrically-actuated MEMS resonators and resonator arrays with frequencies ranging from a few megahertz to 1.5 GHz by using IC compatible low temperature process.

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