Electrical characterization of piezoelectric-on-silicon contour mode resonators fully immersed in liquid

Biological sensing in the mechanical domain offers novel opportunities to measure cellular processes. Operating mechanical resonators in liquid environment for the detection of mass presents a challenge at least for electrical characterization. In this paper, we demonstrate the full electrical characterization of a micromechanical resonator that is fully immersed in DI-water. The reported device uses piezoelectric transduction through an Aluminium Nitride (AlN) film sputtered on a low damping silicon substrate to provide strong electromechanical coupling. The effect of viscous damping in DI-water is lowered by exciting the resonator to vibrate in an in-plane contour mode. We believe that incorporating a 10 µm thicker silicon substrate layer stiffens the resonant system and thereby increases the energy storage capacity in relation to a resonator structured purely by a thin AlN film. The 14 MHz length extensional (LE) mode resonator presented in this paper shows a quality factor (Q) of 200 when fully immersed in DI-water, which is twice that of previously reported resonators structured purely in AlN. The associated motional resistance of the device in DI-water is 40 kΩ. We have also measured the values of Q for several other in-plane resonant modes with higher resonant frequencies (up to 141.69 MHz) when immersed in DI water. Having found that the high dielectric constant of DI-water significant affects the characterization setup, we have also modeled the various sources of parasitics involved in the setup.