Oscillator circuit for monitoring the gas damping effect of piezoelectric microresonators

In this work, an aluminium nitride based piezoelectric resonator (05-mode) was fabricated and characterized to study how various factors, such as pressure, gas composition, the resonator geometry or the order of the vibrational mode, influence the resonant frequency and quality factor of micro resonators. In order to determine the resonant parameters of interest, an interface circuit was implemented and included within a closed-loop scheme. The effect of viscosity and density of the gases under test on the resonant parameters can be determined through a calibration process using different gases, an impedance analyser and theoretical values of density and viscosity reported in the literature. Depending on gas species different gas damping effects in the molecular, transitional and viscous flow regimes were observed. However, as the resonant mode number increases and therefore the resonant frequency, the acoustic wavelength reduces, the contribution of acoustic effects on the energy loss cannot be neglected any more in comparison with viscous effects. Our results demonstrate the performance of the resonator in different gases (Air, N2, Ar, CO2 and He) and pressures (0.1-950 mbar) by developing and applying specific experimental setup.