Fabrication of Silicon Monolithic Tubes with High-Aspect-Ratio Cavities Using Langmuir-Blodgett Colloidal Lithography and Bayesian-Optimized Continuous Plasma Etching.

The fabrication of semiconductor devices using submicron- and nanometer-scale silicon structures is based on lithography (patterning on a substrate) and etching (transferring a pattern onto the substrate) technologies. These processes typically require complex and expensive equipment, as well as extensive experimental optimization of etching parameters, especially for structures with an aspect ratio greater than 10. This work demonstrates a productive and relatively simple approach for fabricating an ordered array of vertically oriented monolithic silicon tubes with high-aspect-ratio internal cavities. The creation of these structures is based on Langmuir-Blodgett colloidal lithography, vacuum magnetron sputtering, and a continuous plasma etching process performed at "room" temperature of the substrate holder. To develop the fabrication process for high-aspect-ratio (>10) structures, we employed Bayesian optimization (a machine learning method), which proved highly efficient in reducing the number of experiments compared to a full factorial analysis. The resulting silicon tubes exhibited an average total reflectance of 3.7% over the 200-1600 nm wavelength range.