Towards resonant sensing in liquids by using CMOS MEMS capacitive oscillators

Liquid properties can be studied by monitoring the in-liquid resonant behavior of micromechanical devices. Most of these devices are electromagnetically or piezoelectrically driven to oscillation and rely on externally sensed signals to sustain oscillation. This work presents the first attempt to implement capacitive micromechanical oscillators on a CMOS (complementary metal-oxide-semiconductor) chip with the driving and sensing circuitry for miniaturization and enhancement of the signal-to-noise ratio. The oscillator comprises of a wide-range CMOS phase-locked-loop (PLL) circuit to maintain oscillation in accordance with the large frequency shift for in-liquid operation. Measurements show that the capacitive micro-resonator is driven to oscillation at 51.2 kHz in air, and the frequency increases to 78.1 and 203 kHz, respectively, in ethanol and deionized water due to the squeeze-film damping effect and the increased mass density and viscosity.