Chemical microsystem based on integration of resonant microsensor and CMOS ASIC

The recent growth of the chemical sensor market is supported by technological advances, such as the use of microfabrication techniques, which enable low-cost, miniaturized chemical sensors. Microfabricated chemical sensors can potentially offer affordable solutions to many challenges in analytical chemistry, which cannot be solved with conventional bench-top laboratory equipment. In particular, these microsensors are useful in applications requiring continuous on-site monitoring, e.g. in threshold-level or environmental monitoring. In these applications, portability is often a key issue and, thus, reliable and low-cost hand-held chemical sensor instruments are needed. In realizing such hand-held instruments, CMOS-integrated interface electronics play a crucial role. When compared to off-chip electronics, CMOS interface circuits allow the integration of the entire system in a compact package with reduced power consumption and system cost.;The main objective of this thesis is to develop a chemical microsystem based on the integration of a silicon-based resonant microsensor and a CMOS application-specific integrated circuit (ASIC) for portable sensing applications. Two types of resonant microstructures are used as mass-sensitive sensors: cantilever and disk-shape microresonators. Based on the characteristics of the microresonators, CMOS-integrated interface and control electronics have been designed and implemented. The CMOS ASIC utilizes the self-oscillation method, which incorporates the microresonator in an amplifying feedback loop as the frequency determining element. In this manner, the ASIC includes a main feedback loop to start and sustain oscillation at or close to the fundamental resonance frequency of the microresonator. For stable oscillation, an automatic gain control loop, which regulates the oscillation amplitude by controlling the gain of the main feedback loop, has been implemented. 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. The CMOS chip has been interfaced to cantilever and disk-shape microresonators and short-term frequency stabilities as low as 3.4x10-8 in air have been obtained with a 1 sec gate time.;The performance of the implemented microsystem as a chemical sensor has been evaluated experimentally with microresonators coated with chemically-sensitive polymer films. To be able to evaluate the performance of the implemented microsystem as a chemical sensor, a gas-phase chemical measurement setup has been constructed. With this setup, gas-phase chemical measurements have been performed and different concentrations of benzene, toluene and m-xylene have been detected. The limit of detection of the implemented microsystem for benzene, toluene and m-xylene in the gas phase has been estimated as 5.3 ppm, 1.2 ppm and 0.35 ppm, respectively.;To improve the long-term stability and therefore the sensor resolution in monitoring applications with slowly changing analyte signatures, a method to compensate for frequency drift caused by environmental disturbances and aging of the microresonator has been proposed and implemented on the CMOS chip. This method uses a controlled stiffness modulation generated by a frequency drift compensation circuit to track the changes in the resonator's Q-factor in response to variations in the environmental conditions. The measured Q-factor is then used to compensate for the frequency drift using a relation between Q-factor and resonance frequency obtained through an initial calibration step. The feasibility of the proposed method has been verified experimentally by compensating for temperature-induced frequency drift during gas-phase chemical measurements.