Aqua Regia-Free Removal of Cr-Pt Hard Masks Using Thin Ag or Au Sacrificial Layers for High-Fidelity LiTaO3 Metasurfaces

For the method of focused ion beam (FIB) milling to fabricate lithium tantalate (LiTaO₃) metasurfaces, the use of a Cr-Pt mask can enhance imaging contrast and enable superior drift correction. However, removing the Pt component necessitates the volatile and toxic etchant aqua regia, presenting considerable safety risks. This work introduces a novel lift-off strategy that incorporates thin Ag or Au sacrificial layers (≤30 nm) between the LiTaO₃ substrate and Cr-Pt mask. Systematic evaluation identifies Ag or Au as optimal candidates due to their high sputtering yield for efficient FIB patterning and compatibility with a low-toxicity KI + I₂ etchant. Experiments showed complete mask removal within 60 s while preserving structural fidelity: atomic force microscopy (AFM) results reveal a surface roughness comparable to conventional aqua regia processing, and scanning microscope (SEM) imaging confirms intact sidewall angles (10-11°). The second-harmonic generation (SHG) measurements reveal comparable optical performance upon the introduction of Ag or Au sacrificial layers. This approach eliminates hazardous etchant and maintains process precision, offering a scalable and safer fabrication route for LiTaO₃-based photonic devices.