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

## TL;DR

A safer method for fabricating LiTaO3 metasurfaces replaces toxic aqua regia with a thin Ag or Au layer for mask removal.

## Contribution

A novel, non-toxic lift-off strategy using Ag or Au sacrificial layers for Cr-Pt mask removal in LiTaO3 metasurface fabrication.

## Key findings

- Ag or Au sacrificial layers enable complete mask removal in 60 seconds with low-toxicity etchants.
- Surface roughness and optical performance are comparable to conventional aqua regia processing.
- SEM imaging confirms intact sidewall angles of 10–11° after mask removal.

## Abstract

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.