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A resonant electrostatic charge sensor with high sensitivity based on micro electromechanical systems (MEMS) technology is proposed to measure electric charge. Input charge produces lateral electrostatic force to change effective stiffness of double-ended tuning forks resonator, and leads to a resonant frequency shift. The sensitivity of the charge sensor is 4.4 × 10 −4 Hz fC −2 . The proposed sensing scheme of effective stiffness perturbation has higher sensitivity than the traditional axial strain sensing methods. Experimental results show that the frequency modulation has better resolution and stability than the amplitude modulation. The proposed sensing scheme also creates additional energy transmission paths inside the device to improve quality factor and stabilize frequency fluctuation. The instability of resonant frequency induced by mechanical nonlinearity are investigated.

Abstract:In this paper, we report attenuation characteristics of aluminum nitride (AIN) film-based surface acoustic waves (SAWs) in liquids and their potential as liquid ethanol sensors. An AIN film-based SAW resonator was fabricated for liquid sensing application. The fabricated SAW device had a Rayleigh wave mode at a resonant frequency of 147.1 MHz and a low temperature coefficient of frequency (TCF) of −21.7 ppm/K. The signal attenuation in the transmission line of the SAW device was presented when ethanol (ETH) droplets and deionized water (DIW) with different concentrations and volume (0.2-1 µL) were dropped on the sensing area respectively. The attenuation of SAW as a function of time and liquid position was investigated. Residues left on the wave propagation path resulted in a frequency shift of the SAW device after liquid evaporation. For ETH, there was a 49 kHz frequency shift caused by a large amount of residues, while the frequency shift of DIW was not distinct, on account of a clean surface. The linear relationship between evaporation rate and ethanol concentration was demonstrated. The evaporation rate of ethanol droplets showed good consistency, and the evaporation time variation was less than 5% at each concentration level. Therefore, the proposed SAW device had great potentials to determine ethanol concentrations based on evaporation rate.

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