Highly selective anisotropic dry etching of smooth SiO2 nanostructures using SF6 plasma and Cr hard mask: Toward sustainable plasma etching

Smooth, high-aspect-ratio directional structures in oxides such as silica (SiO2) are essential for efficient diffraction in optical and nanophotonic devices. Achieving these features requires high pattern-transfer fidelity, anisotropic profiles, smooth surfaces, and uniform processing suitable for scalable, high-throughput production. We present a simplified model describing SiO2 etching dynamics in pure sulfur hexafluoride (SF6) plasmas, supported by experimental evidence from fabricated sub-300 nm structures via reactive ion etching using a chromium (Cr) hard mask. The etch rate exhibits a pronounced etch rate maximum of around 5 mTorr, which seems to be correlated with Paschen behavior. The influence of the reactor wall, electrode, and mask materials is examined, with an emphasis on mechanisms for the self-formation and suppression of nanoroughness via a cyclic etch process that allows for unprecedented selectivity. Furthermore, we will discuss the appearance of abnormalities such as faceting, footing, and microtrenching and how to suppress them. The developed process enables repeatable and scalable transfer of nanoscale resist patterns into dielectric materials. It results in anisotropic and highly selective etching with respect to Cr, without preconditioning or cleaning the etch tool. The method provides a gas-efficient alternative to conventional fluorocarbon-based etching processes.