Microelectromechanical (MEMS) resonators have been of interest for mass sensing applications, leveraging on several advantages such as small form factor, increased sensitivity through miniaturization, compatibility with complementary metal-oxide-semiconductor (CMOS) electronics for compact and integrated solutions, and versatility to sense various entities (gas, chemical, biological analytes). Potential applications of such devices span a range of domains that include biomedical engineering, medical diagnosis, food quality monitoring, microplastic contamination monitoring in seawater. Enhancing liquid phase quality factor (Q) and lowering motional resistance (R_m) is critical to improving the performance of a close-loop oscillator that embeds the MEMS resonant device for real-time frequency tracking in sensing applications. This thesis addresses some notable challenges and proposes designs for sensing applications in a liquid environment to advance the state-of-the-art. Among the available choices, device designs focused on Thin-film-Piezoelectric-on Silicon (TPoS) resonators due to their superior electromechanical coupling efficiency than capacitive resonators and avoidance of direct current power dissipation inherent in thermal-piezoresistive resonators.

In this thesis, a novel TPoS resonator design was proposed, shaped as an elliptical plate. Thus, it is referred to the resonator topology as an elliptical plate resonator (EPR). The proposed EPR resonant mode can be preserved as scale down the device’s size, as desired for mass sensing towards enhancing mass sensitivity by scaling down the proof mass of the resonator. The EPR delivers lower R_m relative to other tether-supported disk-based resonator modes and has a reasonable Q in water. The low R_m arises from enhanced transduction efficiency associated with the modal lateral strain profile. Experimental results in water are demonstrated for a 500 µm by 400 µm EPR, which delivers an R_m of only 2.68 kΩ in water. Scaling the device down to 300 µm by 200 µm, an R_m demonstrated just 5.5 kΩ and Q of 245 in water. For piezoelectric contour mode resonators, R_m is generally inversely proportional to the area of the device. R_m could be reduced by increasing device area but at the direct cost of lower mass sensitivity. As such, the figure-of-merit (FOM) of R_m×A captures the tradeoff or balance between lowering R_m for enhanced electrical performance versus reducing resonator area A for enhancing mass sensitivity. Compared to other resonator designs, the EPR delivers the lowest R_m value normalized for A, thus offering the lowest R_m×A. The EPRs were fabricated on silicon (100) wafers and oriented against the <100> direction, where the piezoelectric layer was deposited on the device layer of a silicon-on-insulator (SOI) wafer.

Having the merits of the EPR designs, EPRs were further considered exploring the effect of device orientation on the liquid-phase performance of EPRs in terms of R_m and Q in liquid. We then compared the performance of EPRs oriented along the <110> direction against the same designs oriented along the <100> within the (100) plane. We showed that orienting the device along the <110> direction delivers significant improvements in both liquid-phase Q and R_m compared to the <100> orientation. We have found that the change in orientation significantly affects the trends on how the choice of geometric ratios used to design the EPR impacts Q and R_m. We show that for the same EPR design 500 µm by 250 µm EPR, the change in orientation reduces 4 times in R_m and 2 times rise in Q. The difference in orientation results in a factor of 4 difference in the R_m×A.

Apart from designing low R_m tether supported in-plane mode resonators, a class of fully clamped membrane resonators proposed that completely seal the domain above the resonator from the domain below. Such isolation between the top and bottom of a resonator is preferable when encasing the device in a microfluidic cell for liquid-phase sensing measurements. This setup can be beneficial for sensing applications. Most fully clamped membrane resonators are hampered by low liquid-phase Q and large R_m. We described the design of higher-order transverse (2, 2) mode square membrane and (2, 0) mode circular membrane resonators that show higher liquid-phase Q and R_m compared to other lower-order membrane designs.

The low motional resistance R_m, a reasonable Q factor, and the scalability feature obtained from the resonant mass sensors presented in this thesis significantly impact designing a stable close-loop oscillator to allow a lower detection limit in liquid. Therefore, it highlights the potential as a resonant mass sensor for future biosensing applications.