Integrated 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 an integrated NMOS circuit detects its vibration. Vapor uptake by the polymer increases the mass-loading on the microbridge, thereby perturbing its first resonant frequency f
1
. For the prototype sensor, negative photoresist is used as the sensor polymer, and the phase between the excitation and output voltages at resonance is monitored as the output signal. Exposure to saturated xylene vapor produces a phase shift of -8° with a response time of less than seven minutes.