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A Study on Fabrication and Noise Characteristics of Silicon Nanowire BioFET

A Study on Fabrication and Noise Characteristics of Silicon Nanowire BioFET
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Advances of the information technology, biotechnology and nanotechnology enable the concept of the ubiquitous-health care (U-health) service. Due to its superior gate-to-channel controllability and large surface-to-volume ratio, the Silicon-nanowire biologically-active FETs (Si-NW BioFETs) are the most promising devices for a U-health biosensor platform. In this study, the fabrication and bio/chemical detection are presented. The ability to prepare a large number of sensors on a wafer, standard silicon top-down micro-fabrication techniques resulting in cost savings and potential sensitivity are significant advantages in favor of nanoscale BioFETs for future biosensor requirements. The Si-NW BioFETs are produced using a combination of oxide-grown Si-NWs and integrated Ag/AgCl reference electrodes. The Si-NW BioFETs are fabricated on a standard silicon-on-insulator wafer using electron-beam lithography and conventional semiconductor processing techniques. To form an Ag/AgCl reference electrode, a 250-nm thick Ag layer are deposited and later chlorinated in 100 mM KCl solution. SEM analysis reveals Si-NWs with a width of ~40 nm and a length of 10 m. The DC characteristics are measured by placing a BioFET in a 0.1x PBS buffer solution. The current-voltage characteristics show an n-type FET behavior with a relatively high on/off current ratio, reasonable sub-threshold swing value, and low gate-leakage current. The pH responses of the BioFETs with different pH solutions are characterized at room temperature. A lateral shift of the ID-VG curve is clearly observed by changing the pH value of the solution. In the early stage of the research, the pH sensitivity was less than 35 mV/pH. It is caused by relatively thick (~20 nm) gate oxide thickness on the nanowires. The thickness of the gate oxide is decreased as thin as 5 nm and the pH sensitivity increases up to 40 mV/pH. Since Bergveld introduced the BioFET structure and its operation principles, the pH sensitivity of the BioFETs have been increased because of improvement of micro fabrication process. Due to strict gate oxide quality control, the pH sensitivity of the BioFETs has faced its theoretical limit (40 mV/pH). The noise spectral density on drain current (SID) is measured and analyzed. The method to extract pH sensing resolution of the Si-NW BioFET is proposed and the pH resolution is 0.016 pH. The pH resolution could be an evaluation method for FET type sensor devices. For the U-Health application, the sensor device should be miniaturized which can attached on client’s body and operated by battery. Therefore the Si-NW BioFETs should operate in the region that consumes as low power as it can unless decreasing its sensitivity. The SID is measured and analyzed. From the measured noise characteristics, a pH sensing resolution and signal-to-noise ratio (SNR) of the Si-NW BioFET is extracted by simple calculation. The SNR is independent of the gate bias whereas the power consumption increases with the gate bias. This clearly suggests that the Si-NW BioFET should be operated near or below the threshold voltage for low power consumption and high sensitivity. Finally, the fabricated Si-NW BioFETs are applied in a demonstration of carcinoembryoic antigen (CEA) detection using the monoclonal antibody (mAb) of CEA. The immobilization process was confirmed by atomic force microscopy. The Si-NW BioFET with immobilized mAb on SiO2 was used to successfully detect the CEA across a wide range of concentrations. By diluting with 0.01× PBS, these concentrations ranged from a few aM levels to hundreds of pM levels. The total threshold voltage shift was 22 mV. Methods to extract the detection limit and signal-to-noise ratio (SNR) are proposed based on the measured noise characteristics of the Si-NW BioFET. The Si-NW BioFET showed a sensing limit of 83 aM and a SNR of 11.8 dB. These parameters are useful tools to evaluate FET-type sensor devices, especially for use in a ubiquitous health care system.
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