Fabrication of nanostructures on sapphire surfaces can enable unique applications in nanophotonics, optoelectronics, and functional transparent ceramics. However, the high chemical stability and mechanical hardness of sapphire make the fabrication of high density, high aspect ratio structures in sapphire challenging. In this study, we propose the use of optical emission spectroscopy (OES) to investigate the sapphire etching mechanism and for endpoint detection. The proposed process employs nanopillars composed of polymer and polysilicon as an etch mask, which allows the fabrication of large-area sapphire nanostructures. The results show that one can identify the emission wavelengths of key elements Al, O, Br, Cl, and H using squared loadings of the primary principal component obtained from principal component analysis of OES readings without the need of domain knowledge or user experience. By further examining the OES signal of Al and O at 395.6 nm, an empirical first-order model can be used to find a predicted endpoint at around 170 s, indicating the moment when the mask is completely removed, and the sapphire substrate is fully exposed. The fabrication results show that the highest aspect ratio of sapphire nanostructures that can be achieved is 2.07, with a width of 242 nm and a height of 500 nm. The demonstrated fabrication approach can create high sapphire nanostructures without using a metal mask to enhance the sapphire etch selectivity.

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