Maria Pace - University of California, Berkeley
Co-Author(s): Albert P. Pisano and Tarek I. Zohdi
Sensing and quantifying ions in liquid is important in many research and commercial applications. For instance, pH is often a key component in manufacture. In biology, sensing ions in solution, such as lab-on-a-chip applications, can be pivotal to effective applications. In many of these technology areas, miniaturization and low cost production of ion sensors is critical. To address these challenges, MEMS silicon nanowires (SiNWs) have been developed. Silicon nanowires offer various advantages, for example, they are small, scalable and CMOS compatible so they can easily be integrated with other electronics.The sensor presented in this work, employs a MEMS device where ions are sensed with nanowires using electrospectroscopy. The MEMS nanowire ion sensor can serve as a chemical sensor. For example, the nanowire ion sensor can be employed as a pH sensor or pH monitor for environment, infrastructure or plant facility. In biologic applications, the nanowire ion sensor can serve as a biosensor for biomolecule detection, DNA sequences, blood tester and an ion species identifier, among others.
A p-type Silicon Nanowire is submerged in a fluid. Positive electrical charges (eg. hydrogen ions (H+)) in solution cause positive majority carriers (holes) in the nanowire to be electrostatically repelled around the periphery of the nanowire. As a result a depletion region in the nanowire causes an increase in electrical resistance of the nanowire which can be measured. This change in resistance of the sensor corresponds to a particular pH. To implement these techniques, silicon nanowire sensors have been developed using a novel fabrication process that improves three problems in the state of the art fabrication process: line edge roughness, pin hole density and sensor drift over time. 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. 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 specie concentration measurement. The advantages of these sensors include high sensitivity at low concentrations, 80% sensitivity at 1e-6 M with ion specie identification and measurement of true pH.
In conclusion, a novel sensor has been developed using a new fabrication process which improves the state of the art fabrication process by reducing pin-hole density, line edge roughness and sensor drift overtime. In addition, the development of new methods for driving the nanowire sensor using electrospectroscopy allows selective ion screening benefits which include ion specie identification, measurement of true pH, high sensor sensitivity by optimizing new driving mechanism and functionalization of the nanowire sensor for bio-detection. Noise reduction is still a challenge when making measurements which needs to be addressed in future work.Not Submitted
Funder Acknowledgement(s): NSF, Hitachi
Faculty Advisor: Albert P. Pisano, email@example.com