Silicon-Based Resonant Microsensor Platform for Chemical and Biological Applications

Abstract

The main topic of this thesis is the performance improvement of microresonators as mass-sensitive biochemical sensors in a liquid environment. Resonant microstructures fabricated on silicon substrates with CMOS-compatible micromachining techniques are mainly investigated. Two particular approaches have been chosen to improve the resolution of resonant chemical/biochemical sensors. The first approach is based on designing a microresonator with high Q-factor in air and in liquid, thus, improving its frequency resolution. The second approach is based on minimizing the frequency drift of microresonators by compensating for temperature-induced frequency variations. A disk-shape resonant microstructure vibrating in a rotational in-plane mode has been designed, fabricated and extensively characterized both in air and in water. The designed resonators have typical resonance frequencies between 300 and 1,000kHz and feature on-chip electrothermal excitation elements and a piezoresistive Wheatstone-bridge for vibration detection. By shearing the surrounding fluid instead of compressing it, damping is reduced and quality factors up to 5800 in air and 94 in water have been achieved. Short-term frequency stabilities obtained from Allan-variance measurements with 1-sec gate time are as low as 1.1 10-8 in air and 2.3 10-6 in water. The performance of the designed resonator as a biological sensor in liquid environment has been demonstrated experimentally using the specific binding of anti-beta-galactosidase antibody to beta-galactosidase enzyme covalently immobilized on the resonator surface. An analytical model of the disk resonator, represented by a simple harmonic oscillator, has been derived and compared with experimental results. The resonance frequency and the Q-factor of the disk resonator are determined from analytical expressions for the rotational spring constant, rotational moment of inertia, and energy loss by viscous damping. The developed analytical models show a good agreement with FEM simulation and experimental results and facilitate the geometrical optimization of the disk-type resonators. Finally, a new strategy to compensate for temperature-induced frequency drifts of resonant microstructures has been developed based on a controlled stiffness modulation by an electronic feedback loop. The developed method is experimentally verified by compensating for temperature-induced frequency fluctuations of a microresonator. In principle, the proposed method is applicable to all resonant microstructures featuring excitation and detection elements.