Silicon nanowire : fabrication, characterisation and application

Abstract

This thesis focuses on the fabrication considerations and the characterisation of silicon nanowires and their integration into chemical sensors. One aim is to optimize a top-down fabrication process for silicon nanowires, in particular the methods that use optical lithography, wet etching and thermal oxidation. The main concerns here are to achieve a reproducible and high yield fabrication process and to obtain a controllable structure. Extensive work was carried out to study the parameters that affect the repeatability of the process. The properties of silicon nitride films, the oxidation method and the characteristics of the anisotropic etchant were found to be key parameters affecting the reproducibility of the process. Several silicon nitride films were deposited under various conditions and their optical properties were tested before and after thermal oxidation. It was found that the oxynitride thickness depends on the refractive index of the nitride film: the lower the refractive index, the thinner the oxynitride. Then an etching process was developed to selectively etch the oxidised silicon nitride over silicon dioxide. The etching process included two steps: firstly ion milling to remove the oxynitride film and secondly using boiling phosphoric acid to strip the silicon nitride film. Nitride-rich silicon nitride films exhibited higher etching selectivity over silicon dioxide compared with silicon-rich silicon nitride. Based on the etch selectivity, oxynitride thickness, and silicon dioxide thickness the maximum thickness of silicon nitride film that can be used to act as a mask during the fabrication of silicon nanowires was determined. The impact of oxidation method on the reliability of the process was studied, and SOI and bulk silicon samples were oxidised at the same temperature and time using lamp-based RTP radiation and also a furnace with resistive heating. The results showed that the SOI sample is colder than the bare silicon sample when both were heated using the lamp-based RTP. This effect was considered during the fabrication of silicon nanowires to obtain a reliable process. Comprehensive experimental measurements were carried out to compare the characteristics of Tetra-Methyl Ammonium Hydroxide (TMAH) and Potassium Hydroxide (KOH) etching to optimise the fabrication process. The use of TMAH was found to lead to a more reliable process. ii Another aim of the project was to characterise the fabricated devices, and for this the contact properties and the electrical properties of the silicon nanowires needed to be evaluated. Extensive electrical measurements were carried out to study the thermal stability and ohmic contact formation for the silicon nanowire. Three metallization schemes were studied: Al/Ti, Al/W/Ti and Al/Ti/AlOx. All these exhibited ohmic contact to the nanowires. However, Al/Ti/Si and Al/W/Ti/Si were found to be unstable after 425 °C RTP annealing. Al/Ti/AlOx/Si withstood this level of temperature but the contact resistance was about ten times higher than that of Al/W/Ti. The electrical resistivity of the silicon nanowires was then studied; it was found that the measured electrical resistivity decreases with the nanowire thickness. Several models were then developed to explain the apparent increase in resistivity. It was suggested it can be largely attributed to the reduction of the conductive area of the nanowire due to interface traps. Finally, a silicon nanowire sensor was designed and fabricated, and this sensor was used to detect the changes in pH. The preliminary results showed that the sensor detected the change of pH in the buffer solution. However, reliability and yield were low, which was assumed to be due to the large parasitic current between the source/drain and the buffer solution.