IN-PLANE MODE RESONANT CANTILEVERS AS LIQUID-PHASE CHEMICAL SENSORS WITH PPB RANGE LIMITS OF DETECTION

To reduce fluid damping in liquids and improve the detection limit in liquid-phase chemical sensing applications, resonant cantilever sensors vibrating in their in-plane rather than out-of-plane resonant mode have been fabricated and tested, resulting in measured Q-factors up to 70 in water (up to 4,000 in air) and detection limits in the ppb range for polymer-coated cantilevers exposed to volatile organics in water.In-plane vibrations were thermally excited and detected using piezoresistors arranged in a Wheatstone bridge configuration.The special U-shaped layout of the piezoresistive Wheatstone bridge based on diffused silicon resistors ensures an output signal for the in-plane modes, but suppresses out-of-plane modes, thus enabling stable closed-loop operation of the resonant sensors at the desired first in-plane mode's frequency.