Homogenous and ultra-shallow lithium niobate etching by focused ion beam

Focused ion beam (FIB) milling has been used for fast prototyping of lithium niobate (LiNbO3, LN) devices with feature size from sub-to hundreds of micrometers. However, a promising and challenging depth range of tens-of-nanometers or below is rarely attended. Moreover, the surface roughness, related closely with device performances, is particularly non-negligible for such an ultra-shallow etching. Here, the surface roughness evolution was studied on ultra-shallow FIB etched LN structures. It was found that the inhomogeneous etching of the metallic film, coated on LN surface to avoid charge accumulation, had a detrimental effect on the LN surface roughness control. By thinning the gold thickness to 7 nm, sub-nanometer surface roughness was reported for etching depth of several nanometers. This work paves the way towards a homogenous and ultra-shallow FIB milling of LN nano-structures.