Graduation Year

2007

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Shekhar Bhansali, Ph.D.

Keywords

Mems, Pressure sensor, Electrochemical, Nitrate sensor, Nanowires

Abstract

In this research, novel high resolution reinforced diaphragm MEMS piezoresistive pressure sensors were designed, fabricated and tested to measure physical phenomena (such as depth/pressure variations) in the ocean. To complement the physical sensing elements, a microfluidic electrochemical nitrate sensor, was also developed to detect chemical fluxes. The electrochemical sensor was designed and packaged to conform to a flow through system. The multisensor approach will enable better measurement quality compared to the current ocean sensors. This, in turn, will potentially improve the current understanding of physical and biogeochemical processes from coastal to deep-sea environment. The pressure sensor element utilized a reinforced bulk micromachined diaphragm to achieve both higher sensitivity (27% higher, model data) and wider linear pressure operating range (> 400 psi, from combination of inner and outer bridge) compared to the conventional single diaphragm design.

A temperature compensation bridge was incorporated on the sensor die to account for temperature drifts. A two-level packaging (wafer and system-level) scheme with protective coatings were developed to test the sensor in "simulated" ocean conditions. Finally, the reinforced diaphragm edge and the bossed structures were designed and fabricated using the masked-maskless etching process and their sensor performance were evaluated against the single diaphragm design. A nanowire-based Electrochemical detection-on-Chip (EoC) system was also developed to detect chemical/biological markers, especially nitrate fluxes. Different sensing modalities,involving a variety of nanosensor electrodes and different assembly techniques were investigated for suitability as electrochemical nitrate sensor. These architectures were also evaluated for robustness as a sensing platform.

Enzyme-modified Au nanowires based electrochemical sensor showed excellent sensitivity (µM level) to biomarkers (cholesterol) in biological fluids (blood).These sensors, however, exhibited poor detection limits towards nitrate ions. Doped polypyrrole nanowire electrodes proved to be effective as nitrate sensors. A detection limit of 4.5±1 µM,sensitivity of 1.65 nAµM and stability of <15% variation from interfering ions were achieved on testing in a flow through environment. The nitrate sensor performance was at par with the current state of the art. Additionally, these sensors are batch fabricated (as arrays) reducing cost, require smaller sample volume, lesser space, power and are less prone to contamination problems.

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