A Piezoelectric Square-Extensional Mode Oscillator for Mass Sensing in Liquid

We present the frequency stability measurements of a piezoelectric resonator-based oscillator operating in the liquid phase. The frequency setting element is fabricated using aluminum nitride (AlN) on silicon (Si) and is excited in square-extensional (SE) mode, which enhances the electromechanical coupling. This electromechanical coupling reduces the motional resistance of the resonator, which simplifies the overall circuit setup toward improving the performance. The resonator is embedded within a phase-locked loop (PLL), allowing for real-time frequency tracking. We achieve an Allan deviation of 70 ppb through this setup. The result demonstrates a superior stability result compared to other resonators in the liquid phase that do not apply a microfluidic cell. The strain profile of SE mode resonator is a critical aspect of this work as it enhances the performance in liquid. Additionally, we briefly discuss the liquid-air boundary effect on frequency stability in liquid sensing environments. This research highlights the potential of piezoelectric resonator-based oscillator using PLL for liquid-phase sensing applications offering significant frequency stability improvements. These findings could contribute to biomedical applications, where liquid-phase sensing is crucial.