Fabrication of silicon nanowires with controlled nano-scale shapes using wet anisotropic etching
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Silicon nanowires can enable important applications in energy and healthcare such as biochemical sensors, thermoelectric devices, and ultra-capacitors. In the energy sector, for example, as the need for more efficient energy storage continues to grow for enabling applications such as electric vehicles, high energy storage density capacitors are being explored as a potential replacement to traditional batteries that lack fast charge/discharge rates as well as have shorter life cycles. Silicon nanowire based ultra-capacitors offer increased energy storage density by increasing the surface area per unit projected area of the electrode, thereby allowing more surface “charge” to reside. The motivation behind this dissertation is the study of low-cost techniques for fabrication of high aspect ratio silicon nanowires with controlled geometry with an exemplar application in ultra-capacitors. Controlled transfer of high aspect ratio, nano-scale features into functional device layers requires anisotropic etch techniques. Dry reactive ion etch techniques are commonly used since most solution-based wet etch processes lack anisotropic pattern transfer capability. However, in silicon, anisotropic wet etch processes are available for the fabrication of nano-scale features, but have some constraints in the range of geometry of patterns that they can address. While this lack of geometric and material versatility precludes the use of these processes in applications like integrated circuits, they can be potentially realized for fabricating nanoscale pillars. This dissertation explores the geometric limitations of such inexpensive wet anisotropic etching processes and develops additional methods and geometries for fabrication of controlled nano-scale, high aspect ratio features. Jet and Flash Imprint Lithography (J-FIL™) has been used as the preferred pre-etch patterning process as it enables patterning of sub-50 nm high density features with versatile geometries over large areas. Exemplary anisotropic wet etch processes studied include Crystalline Orientation Dependent Etch (CODE) using potassium hydroxide (KOH) etching of silicon and Metal Assisted Chemical Etching (MACE) using gold as a catalyst to etch silicon. Experiments with CODE indicate that the geometric limitations of the etch process prevent the fabrication of high aspect ratio nanowires without adding a prohibitive number of steps to protect the pillar geometry. On the other hand, MACE offers a relatively simple process for fabricating high aspect ratio pillars with unique cross sections, and has thus been pursued to fabricate fully functional electrostatic capacitors featuring both circular and diamond-shaped nano-pillar electrodes. The capacitance of the diamond-shaped nano-pillar capacitor has been shown to be ~77.9% larger than that of the circular cross section due to the increase in surface area per unit projected area. This increase in capacitance approximately matches the increase calculated using analytical models. Thus, this dissertation provides a framework for the ability to create unique sharp cornered nanowires that can be explored further for a wider variety of cross sections.