Nano-fabrication and its Application in Plasmonic Chemical and Bio-Sensors
This thesis is focused on the nanofabrication and its application in plasmonic chemical and bio- sensors. The contribution thus is the development of novel nanofabrication techniques and nano- structures for the sensors based on surface plasmon (SP).
The first part is about novel nanofabrication techniques, especially the nanoimprint lithography (NIL) and electron beam lithography (EBL). For NIL, the four major aspects of NIL were discussed, including resist, mold, imprint process and equipment for NIL. Combined with NIL and soft lithography, the hybrid nanoimprint-soft lithography were investigated. To overcome the difficulty of mold fabrication, a more robust solution of mold fabrication through a sacrificial poly(dimethyl glutarimide) (PMGI) master mold was designed in this work. Based on this method, the mold was fabricated without structure distortion, and pattern replication with sub-10 nm resolution was demonstrated. For EBL, several aspects were discussed to improve the performance of EBL, including the resist, development, and exposure condition. The charging effect to the pattern distortion was studied systemically for the e-beam exposure in large area with high current (>nA). Tilted periodic nanostructure was achieved by e-beam scanning on tilted sample with dynamic mode. EBL on irregular surface was realized by the exposure on evaporated polystyrene.
For the application in chemical and bio-sensors based on surface plasmon, the first type of sensors is surface enhanced Raman scattering (SERS) sensors based on localized SP. Bowtie-shape nano-antenna structure of sub-10 nm gap was fabricated with the breakthrough of EBL resolution to 3 nm by exposing resist on Si3N4 membrane. By controlling the gap size during lithography, the surface plasmon enhancement was tuned accurately. High sensitivity of bowties antenna with sub-10 nm gap was achieved at low concentration of the target molecule (10-7 mM 1,2-di(4-pyridyl)ethylene in ethanol solution) and high enhancement of 107 resulting from the corresponding bowtie structure.
The second type of sensors is extraordinary optical transmission (EOT) sensors based on propagating SP. The process of double liftoff was developed for nanohole arrays fabrication on a thick layer (100 nm) of gold utilizing EBL. This technique is versatile for the fabrication of many kinds of high-aspect-ratio noble metal structures. Additionally, annealing method was employed in this work to improve the smoothness of Au film. It was found that the RMS roughness of the deposited film was reduced by 72% and the sensitivity was increased from 7.8 nm/RIU to 42 nm/RIU as a result of annealing. It was also found that the optical transmission intensity of the annealed NHA of similar hole diameter was increased more than twice which is due to smaller absorption/scattering of the incident light and surface waves from the Au film surface. Besides the double liftoff process, several techniques were developed for EOT structures, including electroplating, imprint method, deposition on membrane and EBL on optical fiber.