Resonant-microbridge vapor sensor

A novel integrated vapor sensor is described that incorporates a polycrystalline silicon microbridge coated with a thin polymer film. The microbridge is resonated electrostatically and its vibration is detected capacitively using an integrated NMOS circuit. Vapor uptake by the polymer increases the mass-loading on the microbridge, thereby perturbing the first resonant frequency of the microbridge. In the prototype device, a 150-nm-thick layer of negative photoresist coats a 153-µm-long 1.35-µm-thick polycrystalline silicon microbridge. The phase between the excitation and output voltages at resonance is monitored as the sensor output signal. Exposure to saturated xylene vapor produces a phase shift of -8° with a response time of less than 7 min.