Uniform high-aspect-ratio 3D micro-and nanomanufacturing on silicon by (electro)-metal-assisted chemical etching: fundamentals and applications

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Li, Liyi
Wong, C. P.
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This dissertation is focused on a novel wet etching method, named metal-assisted chemical etching (MaCE), for fabrication of uniform high-aspect-ratio (HAR) structures on silicon (Si) in micro- and nanometer scale. In MaCE, a layer of noble metal thin film is deposited on the surface of Si and serves as the catalyst. The metal-loaded Si is immersed in hydrofluoric acid (HF)-hydrogen peroxide (H2O2) solution. A redox reaction occurs at the metal-Si interface where the Si under the metal film is preferentially etched. The metal catalyst can continue etching into Si to form HAR structures. In this dissertation, the challenge of obtaining uniform HAR structures by MaCE is firstly addressed where random movements of the metal catalyst during MaCE are observed. Then suitable experimental conditions are presented, under which uniform HAR holes and trenches on Si are successfully fabricated. The uniform MaCE phenomena are explained by the microscopic transport processes of HF and electronic holes (h+). Further, the influence of h+ transport on the 3D etching profiles is discussed. By applying external electric bias, the 3D etching profiles is effectively controlled. Further, the transport of h+ is also found to be influenced by the dopants type and the doping level of the Si substrates. Based the above findings, HAR trenches and holes with vertical sidewalls are successfully fabricated and devices built on these structures are demonstrated to work properly. The established method further shows compatibility with a novel low-cost lithography method, constituting an economic overall approach for HAR structures fabrication. Finally, uniformity of MaCE is achieved across multiple wafers that are etched simultaneously, paving the way for its application in high-volume manufacturing.
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