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Silicon Nanowires for Chemical Sensing, pH Measurement and Ion Species Identificaiton in Solution



Silicon Nanowires for Chemical Sensing, pH Measurement and

Ion Species Identification in Solution


Maria E. Pace

Doctor of Philosophy in Applied Science and Technology

University of California, Berkeley

Professor Tarek I. Zohdi, Co-Chair

Dean Albert P. Pisano, Co-Chair

In situ measurement of true pH would be useful in many applications.

True pH is the negative log of the Hydrogen ion concentration, however, measurement of this is elusive in many practical applications due to the presence of interfering ions, such as sodium and potassium. Monitoring true pH in surgical procedures, for instance, would be very useful, however, interfering ions necessitate preprocessing of the blood and laboratory analysis making it not in situ, and not in real time. This work presents a measurement system capable of measuring true pH in the presence of interfering ions using nanowire sensors and electrospectroscopy. This system is also able to operate as a chemical sensor by discriminating between different ionic species in solution and can separately measure concentrations of other ionic species for leak detection and chemical identification. Nanowire sensors offer many advantages such as small size, low power,

and inexpensive fabrication. These advantages allow real time, in situ monitoring in many applications.

In addition, silicon nanowires are integrated as a semiconductor pH sensor and species identification chip. Using electrospectroscopy, ions drift in the fluid at different times allowing the nanowire to make measurements of different species present in the fluid. To accomplish this, various modes of operation including “the time of flight” have been developed to maximize ion identification and species concentration measurement. The advantages of these sensors include high sensitivity at low concentrations, 80% sensitivity at 1e-6 M with ion species identification and measurement of true pH.

Depending on the species of interest, a particular mode of operation can be employed to achieve desirable results. Advantages of these modes of operation are isolation of hydrogen ions from other species including sodium to measure true pH in real time and a method for deconvolving the species both in temporal and spatial maps.

Additionally, a method for electrically cleaning the nanowires sensors and a method to re-zero the nanowires has been explored allowing more accurate measurement of the species and true pH.

A novel top down fabrication process has been developed which reduces the line edge roughness of the nanowire for more reproducible sensors, reduces dielectric pin hole density for minimal sensor drift over time and reduces parasitic resistance for higher ion sensitivity. This novel fabrication process is truly CMOS compatible allowing more compatibility with other electronics. The SiNW is covered by thin film which protects SiNW from liquid penetration and can also work as ion sensitive film or functionalized surface. As a fabrication simplicity, the entire structure above can be built on a standard SOI (Silicon on Insulator) wafer. Experimental results have shown a linear relation between resistance change in the nanowire and pH in the fluid.

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