Two-dimensional crystalline hard masks for high-aspect-ratio nanofabrication

Hard masks with high etch selectivity are essential for fabricating high-aspect-ratio nanostructures via deep and anisotropic plasma etching. While most two-dimensional materials are susceptible to plasma damage, we report that van der Waals metal oxyhalides, specifically CrOCl and FeOCl, exhibit extraordinary resistance to aggressive SF₆/O₂ plasma, far surpassing conventional hard mask materials. CrOCl achieves etch rates as low as ~2.4 nm min^-1 and an etch selectivity >200:1 relative to silicon, representing improvements of ~30× over Si₃N₄, ~2.3× over Al₂O₃ and ~20× over TiN under identical conditions. CrOCl maintains subnanometre surface roughness after etching, even exhibiting plasma-induced surface smoothening. Beyond its inherent etch resistance, CrOCl can be chemically patterned using Cl₂ plasma and mechanically transferred onto a broad range of substrates, including perovskite oxides, polymers, glasses and monolayer two-dimensional semiconductors, enabling patterning on materials that are typically incompatible with conventional hard masks. Using CrOCl masks, we demonstrate deep silicon etching with aspect ratios exceeding 39:1 and minimal feature distortion. These findings establish van der Waals metal oxyhalides as a versatile and scalable platform for next-generation nanofabrication, combining extreme plasma robustness, high-resolution patternability and broad substrate compatibility in one material system.