A Novel Feedthrough Cancellation Technique for Piezoelectric MEMS Resonant Sensors in Ionic Liquid Medium

This work presents a thin-film piezoelectric-on-silicon (TPoS) resonator and oscillator operating in an ionic (i.e., conductive) liquid environment without suffering from existing ionic background floor through a novel feedthrough cancellation technique. Such MEMS resonant transducers would serve as an important building block for mass sensing used in physical/chemical/bio applications demanding sensing in ionic liquid medium, such as saline solution. Resonators implementing both PZT and AlN piezoelectric materials are thoroughly explored in this work. With feedthrough cancellation, the PZT TPoS resonator features decent stopband rejection (SBR) of around 21 dB and also enables oscillation in an ionic solution, with an oscillating frequency of 5.75 MHz and Allan deviation (ADEV) of 2.05 ppm, which implies a mass resolution of only 3.7 pg. The PZT TPoS oscillator phase noise (PN) performance in the conductive liquid is -83.15 dBc/Hz and -127.04 dBc/Hz, respectively, at offset frequencies of 1 kHz and 100 kHz. To the best knowledge of the authors, this is the first real-time measurement of TPoS resonators and oscillators working in ionic circumstance without isolation between signal pads and liquids. Furthermore, the feedthrough cancellation makes the SBR of the resonator in ionic water (21 dB) even greater than that in DI (deionized) water (11 dB). This new technique has great potential in chemical and biosensing applications in future.