This paper presents the optimization for obtaining ultra-high aspect ratio nanostructures by GLancing Angle Deposition (GLAD). GLAD is a bottom-up, physical deposition process for creating nanometer-level features by shadows cast by seeds on the substrate at high incident angles. Based on the seeds used, GLAD can be categorized into two types: GLAD with natural seeds and pre-defined seeds (pre-seeds). When natural seeds are used, the seeds are randomly distributed with sub-100 nm feature sizes, and the percent coverage of the substrate is determined simply by the incident angle of the vapor. When the pre-seeds are used, the features can be redistributed and regrouped to generate new periodic nanostructures. This paper discusses how to obtain ultra-high aspect ratio nanopillars from natural seeds and nanoribbons from pre-defined line seeds by GLAD. In the discussion on GLAD with natural seeds, a study on the dependence of the aspect ratio on the incident angle is provided; resolvable nanopillars are obtained with aspect ratio over 1:20, and the growth mechanism is proposed for pillars with high deposition angles. Next, line seeds used in the GLAD process for creating high aspect ratio nanoribbons are discussed. Proper design and process parameters are discussed for controlling the morphologies of the nanoribbons. The ultra-high aspect ratio nanostructures are potentially used for applications including sensing, surface property alteration, and the creation of nanoporous structures.

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