Integrated silicon-based chemical microsystem for portable sensing applications

This paper presents a chemical microsystem based on the integration of a silicon-based resonant microsensor and a CMOS ASIC for portable sensing applications in air. Two types of resonant microstructures are used as mass-sensitive microsensors: cantilever-based and disk-shape microresonators. Based on the characteristics of the microsensors, CMOS-integrated interface and control electronics have been designed and implemented. The CMOS ASIC utilizes the self-oscillation method in which a main feedback loop starts and sustains oscillation at or close to the desired resonance frequency of the microresonators. For stable oscillation, an automatic gain control loop regulates the oscillation amplitude by controlling the gain of the main feedback loop. In addition, an automatic phase control loop has been included to adjust the phase of the main feedback loop to ensure an operating point as close as possible to the resonance frequency, which results in improved frequency stability. To improve the long-term stability, a method to compensate for frequency drift has been proposed and implemented on the CMOS chip. The performance of the implemented microsystem as a chemical sensor has been evaluated experimentally by detecting different concentrations of volatile organic compounds such as benzene, toluene and m-xylene with microresonators coated with chemically sensitive polymer films.