Silicon Nanowire Field-effect Chemical Sensor

This thesis describes the work that has been done on the project “Design and optimization of silicon nanowire for chemical sensing‿, including Si-NW fabrication, electrical/electrochemical modeling, the application as ISFET, and the build-up of Si- NW/LOC system for automatic sample delivery. A novel top-down fabrication technique was presented for single-crystal Si-NW fabrication realized with conventional microfabrication technique. High quality triangular Si-NWs were made with high wafer-scale yield and scalable lateral dimensions down to 10 – 20 nm and lengths up to 100 ￭m. The thick microscale electrical contact regions formed a continuous layer of single crystal silicon, which provide an easy way for ohmic contact formation. The importance of impurity doping concentration control, ohmic contact formation, and interface charge/surface states reduction during fabrication was demonstrated with either electrical measurements or finite-element simulation. In order to understand the behavior of Si-NW device in solution, an electrical/electrochemical model was developed and discussed. Both 2D analytical model and 3D numerical model were developed to describe the conductance behavior of multigate Si-NW devices. The fitting to the experimental data for both models with the same dimensions and doping profiles proved the accuracy of both models. Finally, the 3D numerical model was used for the sensitivity analysis. Since we are working with Si-NW sensor for surface potential change measurements in solution, the most popular method, which is pH measurement, was used to characterize the sensor behavior. Three variations of SiO2 gate oxide and an ALD Al2O3 gate oxide had been deposited on the nanoISFET and titration experiments were used to assess the pH behavior and sensitivity. The data was analyzed with the well-established site-binding model and demonstrated the near ideal Nernstian pH response of the Si-NW nanoISFET with an Al2O3 gate oxide. Finally, an integrated LOC label-free biosensing platform was presented for automatic small volume sample transport and sensing. The entire platform consists of a Si-NW biosensor chip, integrated with a PDMS microfluidic channel, and a chip holder with all electrical read-out, which is ideal for biosensing application