Anisotropic Etching of Pyramidal Silica Reliefs with Metal Masks and Hydrofluoric Acid

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 16; no. 43; pp. e2002290 - n/a
• Main Authors: Kirchner, Robert, Neumann, Volker, Winkler, Felix, Strobel, Carsten, Völkel, Sandra, Hiess, André, Kazazis, Dimitrios, Künzelmann, Ulrich, Bartha, Johann Wolfgang
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.10.2020
• Subjects: anisotropic etching; Anisotropy; Atomic force microscopy; Atomic radius; Crystal structure; Etchants; Etching; Hydrofluoric acid; Light emitting diodes; Metallizing; Nanotechnology; Photovoltaic cells; plasmonics; Pyramids; self‐perfectioning effect; Silica; Silica glass; Silicon; Silicon dioxide; Solar cells; Thin films
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202002290

This work describes the fabrication of anisotropically etched, faceted pyramidal structures in amorphous layers of silicon dioxide or glass. Anisotropic and crystal‐oriented etching of silicon is well known. Anisotropic etching behavior in completely amorphous layers of silicon dioxide in combination with purely isotropic hydrofluoric acid as etchant is an unexpected phenomenon. The work presents practical exploitations of this new process for self‐perfecting pyramidal structures. It can be used for textured silica or glass surfaces. The reason for the observed anisotropy, leading to enhanced lateral etch rates, is the presence of thin metal layers. The lateral etch rate under the metal significantly exceeds the vertical etch rate of the non‐metallized area by a factor of about 6–43 for liquid and 59 for vapor‐based processes. The ratio between lateral and vertical etch rate, thus the sidewall inclination, can be controlled by etchant concentration and selected metal. The described process allows for direct fabrication of shallow angle pyramids, which for example can enhance the coupling efficiency of light emitting diodes or solar cells, can be exploited for producing dedicated silicon dioxide atomic force microscopy tips with a radius in the 50 nm range, or can potentially be used for surface plasmonics. Anisotropic, crystal‐oriented etching of silicon is well known. Anisotropic etching of amorphous silica with purely isotropic hydrofluoric acid as etchant is an unexpected phenomenon. This work demonstrates the fabrication of pyramidal structures in silicon dioxide and glass by utilizing the enhanced lateral under‐etching beneath metal masks. This self‐perfecting method is attractive for 3D surface texturing.