Feedthrough Engineering to Enable Resonant Sensors Working in Conductive Medium for Bio Applications

Operating micro-nanoscale sensors in conductive liquids faces challenges due to liquid damping and high feedthrough floor, leading to low signal-to-feedthrough ratio. This work presents an innovative feedthrough engineering technique for resonant sensors immersed in ionic liquids, eliminating the need of isolation layers or additional processing for the sensing device. By leveraging the feedthrough path through substrate, the proposed technique counteracts the feedthrough induced by the ionic liquid, and successfully resumes the desired motional signal of the sensor. A thin-film piezoelectric-on-silicon (TPoS) resonator and oscillator operated in ionic environment are introduced to demonstrate that this technique not only enables the measurement of resonant signals in conductive liquids but also offers suitable options regarding the mode shape and resonant frequency of the sensor in different ion concentration environments. Additionally, the cancellation phenomenon shows potential as a concentration detector for ionic liquids. The fundamental (5MHz) and higher (15MHz) frequency modes of the PZT-based resonator are thoroughly investigated. Measurements show that regardless of the frequency where it operates, the resonator features decent stopband rejection (SBR) of around 18
$\sim$
20dB using the cancellation approach, which is even better than operating in deionized water. When employed as an oscillator, the results indicate a remarkable frequency resolution of approximately 1.8 Hz for both fundamental and higher mode frequencies. These measurements highlight the improved resonant behavior and real-time sensing capability offered by the proposed technique in conductive liquids. Such MEMS resonant transducers using this engineered feedthrough cancellation mechanism would serve as crucial building blocks for chemical and biosensing applications. 2023-0194