Exploiting nonlinearity of micromachined antisymmetric weakly coupled resonators toward multifunctional sensors

In the past few years, the topic of multiple sensing has received more attention due to the increasing demand for multiple physical, chemical, and environmental parameters sensing applications. The traditional solution for multiple physical parameters’ sensing normally integrates different kinds of sensors into a single chip with the corresponding circuit board. However, the integration of multiple sensors will inevitably lead to higher power consumption and a more complicated system. In this thesis, a novel multifunctional MEMS sensor, which comprises an antisymmetric weakly coupled resonator including cantilever and bridge resonators mechanically coupled by coupling beams, is developed to simultaneously monitor multiple sensing targets on a single device. Particular research has been directed to: (i) theoretical research of binary gas mixture sensors, (ii) theoretical research of triple sensing approach, and (iii) experimental development and improvement of multi-gas sensors.

Distinguishing multiple gas mixtures is a challenging topic. Aiming to decrease cost and simplify the detection process of binary gas mixture detection. Different sensing techniques will be operated on cantilever and bridge resonators of the coupled system, respectively. The cantilever resonator would utilise the micro-gravimetric gas sensing technique for ammonia (NH3) absorbing through surface functionalisation, while the electrothermal heated bridge uses the thermal conductivity gas sensing aiming at Helium. Such two sensing mechanisms have independent effects on the first two global modes' resonance frequencies of the coupled resonators. The continuous simulation based on a numerical model of the coupled system with geometric and electrostatic nonlinear terms is obtained, revealing the full nonlinear dynamics of the coupled system and the effect of different parameters. The concept is theoretically validated, proving the promising performance of multi-gas sensing.

Based on a similar design philosophy of the multi-gas sensor, further exploration of triple sensing MEMS structure aiming at monitoring three kinds of physical parameters is conducted. The proposed sensing scheme relies on mass (i.e., due to mass absorption on coating), stiffness (i.e., due to convective cooling/heating or temperature variation), and the external acceleration at the horizontal direction alteration of the inertial mass, respectively. The sensing target of triple sensor could be customised by modifying the sensing theory, for instance, changing coating material for micro-gravimetric sensing and linking the stiffness variation to ambient gas flow velocity.

Finally, the practical MEMS multi-gas sensor development is presented. Due to the facility limitation, the fabricated sensor utilises piezoelectric actuation theory instead of electrostatic actuation however maintains the multi-sensing dynamics approach (maintaining similar geometric nonlinearities). A novel theory to improve the sensor's power consumption during resonance frequency tuning by adopting a specialised Aluminium Nitride (AlN) layer is modelled and tested. The system dynamics analysis at different veering and buckling zones under different AC actuation are experimentally tested and compared with simulated results, showing a good agreement. The experimental Helium sensing results yield high linearity and sensitivity results. Nonlinear fold bifurcation jump was exploited to enhance the sensor sensitivity as an alarming gas sensor with adjustable thresholds. The Helium concentration variation (i.e. stiffness variation on bridge resonator) only influences the 2nd global mode resonance frequency dominated by bridge resonator, proving the multi-sensing concept and the potential of being used in future for multi-sensing applications.