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Excitation and detection of MEMS electrostatic resonance in water
The resonance of electrostatically actuated amorphous silicon microbridges immersed in de-ionized water is characterized. It is shown that under correct actuation conditions resonance can be measured in water without electrolysis or electrode screening. The resonance frequency of these non-passivated structures is also studied in air and under vacuum for microbridges with different lengths. When the operating medium is changed from vacuum to air, the resonance frequency decreases by 5% and the quality factor decreases from approximately 1000 to 100. Operation in deionized water produces a 60% shift in resonance frequency to lower values and the quality factor decreases to 10.
The detection of the resonance frequency of a microbridge operating in aqueous solutions with high electrical conductivities, up to 8 mS/cm, and high viscosities, up to 0.15 Pa.s is demonstrated. The range of actuation voltages needed to excite resonance frequencies in the MHz in these microstructures under aqueous solutions allows electrostatic actuation without electrode screening and electrolysis. Values of the quality factor between 1 and 10 in aqueous solutions will allow the detection of ~1 pg of added mass to the resonators.
Top left: SEM micrograph of a 30 mm-long bridge with the gate underneath. Bottom left: schematic illustration of the resonance frequency measurements of a microbridge in DI water. Right: vibration amplitude as a function of the actuation frequency of an L=40 mm electrostatically actuated microbridge measured in vacuum, air and DI water. The peak indicates the resonance frequency, fres.