Gas-mediated electron beam induced etching
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Gas-mediated electron beam induced etching is a direct-write nanolithography technique. In this thesis, through experimental observation and numerical simulation, descriptions of reaction kinetics of electron beam induced etching were refined to include effects of residual contaminants, substrate material properties, and temperature dependence. Reaction kinetics of electron beam induced etching are of interest because they affect resolution, throughput, proximity effects, and topography of nanostructures and nanostructured devices fabricated by electron beam induced etching. A number of mechanisms proposed in the literature for electron beam induced removal of carbon were shown to be insignificant. These include atomic displacements caused by knock-on by low energy electrons, electron beam heating, sputtering by ionised gas molecules, and chemical etching driven by a number of gases that include N₂. The behaviour ascribed to these mechanisms was instead explained by chemical etching caused by electron beam induced dissociation of residual contaminants such as H₂O present in the vacuum systems that are typically used for EBIE. Reaction mechanisms in single crystal and ultra nano-crystalline diamond were shown to be dependent on substrate material properties. Single crystal diamond etch morphology is attributed to anisotropic etching along crystal planes, which varies with precursor composition. In contrast to single crystal diamond, etching of ultra nano-crystalline diamond was shown to proceed via an electron activated pathway. A refined electron beam induced etching model incorporating the role of electron induced damage in ultra nano-crystalline diamond yields higher order reaction kinetics, predicting a new reaction regime limited by the concentration of chemically active surface sites. A temperature dependent, cryogenic electron beam induced etching technique was implemented to increase the residence time of adsorbates on the surface. This technique efficiently increases the rate of electron beam induced etching, demonstrated using nitrogen trifluoride as the etch precursor for silicon. Cryogenic cooling broadens the range of precursors that can be used for electron beam induced etching, and enables high-resolution, deterministic etching of materials that are volatilised spontaneously by conventional etch precursors. Determining the reaction kinetics of electron beam induced etching enables new applications in nanoscale material modification. Methods for the fabrication of optically active, functional diamond structures from single crystal diamond and rapid Stardust particle extraction were demonstrated. Electron beam induced etching is ideal for these applications, where high-resolution, damage-free etching is required.
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