Resilience of van der Waals metal oxyhalide hard masks for substrate-agnostic nanofabrication

Abstract High-aspect-ratio plasma etching requires hard masks that maintain high etch selectivity, pattern fidelity, and substrate compatibility under aggressive plasma conditions. Conventional hard-mask materials often suffer from erosion, roughening, and process incompatibility with fragile or low-thermal-budget substrates. This Research Highlight discusses the recent demonstration of van der Waals metal oxyhalides, particularly CrOCl, as two-dimensional crystalline hard masks for substrate-agnostic nanofabrication. CrOCl exhibits exceptional resistance to SF6/O2 plasma, high etch selectivity relative to silicon, subnanometer surface roughness, and the ability to define deep silicon nanostructures with aspect ratios exceeding 39:1. Its mechanical transferability further decouples hard-mask preparation from target-substrate processing, enabling patterning on heterogeneous, flexible, and atomically thin materials. Finally, we discuss key challenges for wafer-scale implementation, including scalable thin-film growth, transfer reliability, and standardized etch-performance metrics.