Fabrication and characterization of silicon and polysilicon nanogaps electrodes using size reduction technique for chemical and biomolecules detection
Thakra., S. Dhahi
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Nanogap electrodes may be defined as a pair of electrodes separated by nano-scale spacing. Two important studies enabled the non-scientists to envision the development of nanogap-based electrical and electronic sensors and devices for the ultrasensitive detection of DNA. In the first study DNA can transport charges and can bridge the electrodes separated by a nanoscale gap whereas for second study, charge transport is interrupted when the molecule undergo denaturation from double-stranded to single-stranded conformation. The aim of this research work is to design, fabricate, characterize, and test nanogap-electrode based device for biochemical detection and DNA immobilization and hybridization detection. However, the focus of this research is to investigate electrically and chemically the effect of different materials and gap sizes on the nanogap electrodes design. Fabrication and characterization of less than 10 nm gap for both silicon and polysilicon nanogap electrodes structures being the main target in this research. Two masks were designed for the fabrication of nanogap and electrodes pad on silicon (Si) and silicon-on-insulator (SOI) wafers as a starting substrate to fabricate polysilicon and silicon nanogap devices respectively. Nanogap electrodes devices were fabricated using a size reduction technique which involves sequential and repeated thermal oxidation and wet etching processes. A thick layer of silicon and polysilicon materials were used to provide device stability throughout the fabrication process. The surface morphology of the fabricated nanogap structure was characterized using SEM and FESEM. The observed results showed the gap size of a 6-nm and 5-nm for silicon and polysilicon electrodes structure respectively. Gold pad electrodes were then fabricated on the silicon and polysilicon nanogap structures to increase the electrical conductivity and permittivity of the devices especially during bio-molecules detection. Capacitance, permittivity and conductivity are measured electrically to characterize the fabricated nanogap structures using a dielectric analyzer. However, sourcemeter equipment was first used to measure the current and characterize the resistivity of the nanogap structures as a function of applied voltage. It was found that the resistivity decreases with the reduction in gap sizes to aid the passage of current flow between the electrodes. Furthermore, the devices were chemically tested for the measurement of pH and yeast concentrations. It was found that the capacitance, permittivity and conductivity increased with pH and decreased with yeast concentrations. Finally, the devices were used as a DNA sensor for nucleic acid hybridization detection which is a key step in molecular diagnostics, gene profiling and environmental monitoring. Amine functionalized group from APTES were used to modify the silicon and polysilicon electrodes surface. Amine- groups (NH2) were labeled with gold nanoparticles to tag a thiol-modified DNA probe onto the nanogap surfaces. The developed biosensors clearly differentiated complementary, noncomplementary and single mismatched DNA targets through the measurements of capacitance, conductance and permittivity. The detection limit of the sensors was 5 nmol/L of target DNA. As a conclusion, this research successfully demonstrated the process to design, fabricate, characterize and test nanogap based biosensor using size reduction technique for DNA hybridization detection.