Electrical Characterization of Piezoelectric MEMS Resonators in Liquid Phase for Sensing Applications


Student thesis: Doctoral Thesis

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Award date6 Apr 2018


Devices based on resonant micro- and nano-electromechanical systems (MEMS/NEMS) have been of interest for a wide range of sensing applications. These devices have great potential for implementing extremely high throughput resonant sensors suited to operate in various environments such as air, vacuum, and fluids. This thesis investigates some of the notable challenges associated with operating resonators in a liquid environment for sensing applications and proposes some solutions to advance the state of art. Given the challenges for working resonators in liquid, we focus on Thin-film Piezoelectric-on-Silicon (TPoS) resonators due to the strong electromechanical coupling afforded by piezoelectric transduction relative to capacitive transduction.

Using a 14MHz TPoS length-extensional (LE) mode resonator, we illustrate some of the challenges involved in electrically characterizing MEMS resonators in a liquid environment. We demonstrate a Q factor of 200 in deionized (DI) water, which is twice that of AlN-body resonators reported in the literature. The same levels of Q factors have been found in several other in-plane mode resonators with higher resonant frequencies (up to 141.69 MHz) immersed in DI water. Having found that the high dielectric constant of DI water significantly affects the characterization setup, we have also modeled the various sources of parasitics involved in the setup.

We then proceed to describe a method to cancel feedthrough by targeting parasitics on the package level rather than parasitics intrinsic to the device. The proposed technique is tested on three different resonator designs to investigate its scalability. We show that the proposed method is most beneficial when the resonators are smaller and thus highly relevant to resonant mass sensing applications. Reductions in feedthrough by as much as a factor 90 are demonstrated experimentally using the proposed method.

Apart from looking at feedthrough, this thesis also proposes novel modes of vibration that deliver higher Q factors in water. This includes what we call the square wine glass (SWG) mode based on a square plate resonator. The non-zero anti-symmetric strain profile of the SWG mode allows for fully-differential piezoelectric transduction to reduce feedthrough. It has been found from the experimental results that the SWG mode resonator delivers higher Q factor compared to commonly-reported contour modes in water. These enhancements in transduction result in a resonant peak-to-feedthrough ratio of 29.8 dB in DI water with no anti-resonance observed and a Q factor of 377 with the smallest square plate resonator tested.

Apart from the SWG mode, another novel mode has been proposed that we called the Button-like (BL) mode. The BL mode has a characteristic lateral strain profile (based on the sum of orthogonal strain components in the plane of fabrication) that resembles a shirt button, hence the name for this mode. The BL mode not only offers higher Q factor than the DWG mode (as well as the commonly-reported elliptical mode), but also higher coupling factor. An average Q factor of 410 has been demonstrated in water, which is the highest value among piezoelectric resonators reported to date.