Quantitative Studies of Long-term Stable, Top-down Fabricated Silicon Nanowire pH Sensor
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Quantitative Studies of Long-term Stable, Top-down Fabricated Silicon Nanowire pH Sensor Sun Choi1, Albert P. Pisano1 1
Berkeley Sensor and Actuator Center (BSAC)
ABSTRACT We report simple and effective methods to develop long-term, stable silicon nanowirebased pH sensors and systematic studies of the performance of the developed sensors. In this work, we fabricate silicon nanowire pH sensors based on top-down fabrication processes such as E-beam lithography and conventional photolithography. In order to improve the stability of the sensor performance, the sensors are coated with a passivation layer (silicon nitride) for effective electrical insulation and ion-blocking. The stability, the pH sensitivity, and the repeatability of the sensor response are critically analyzed with regard to the physics of sensing interface between sample liquid and the sensing surface. The studies verify that the sensor with a passivation layer over critical thickness show long-term, stable sensor response without longterm drift. The studies also show the detection of pH level with silicon nanowire sensors is repeatable only after proper rinsing of sensor surfaces and there exists trade-off between the stability and the pH sensitivity of sensor response. INTRODUCTION Silicon nanowires are good sensing materials for detecting Bio-chemicals such as pH level, the concentration of DNA, and the concentration of protein because highly sensitive detection of analytes can be achieved thanks to its high surface to volume ratio.[1-8] Recently, top-down fabricated silicon nanowire sensors have been developed by the combination of Ebeam lithography and conventional photolithography[4-6,9] and these sensors are highly favored in modern Integrated Circuit (IC) technology because the dimensions and the electrical properties of wires can be tuned accurately and the entire fabrication processes are CMOS (Complementary Metal-Oxide-Semiconductor) process-compatible. So far, previous works on top-down fabricated silicon nanowire sensors are primarily focused on the studies on the sensitivity, response time of sensors,[3,4,9] or computational modeling of surface charge effect on the sensors. In order for silicon nanowire sensors to be used in industrial applications or in bio-monitoring in the long run, it is very crucial to implement reliable and systematic ways to silicon nanowire sensors to improve the stability of the sensors. Herein, we introduce a straightforward way to develop long-term and stable silicon nanowire pH sensors. We fabricate silicon nanowire pH sensors by the combination of E-beam lithography and photolithography, major techniques in top-down fabrication. A passivation layer (silicon nitride) is coated on the surface of silicon nanowire to provide superb electrical insulation and ion-blocking properties of the sensor. The stability, the pH sensitivity, and the repeatability of the sensor response are characterized and the sensing response is interpreted.
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THEORY The detection of pH of fluid is achieved based on the principle of
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