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Reducing geometry size of lithium niobate (LN)-based photonic elements is imperative for realizing ultra-compact LN photonic systems with novel functionalities. However, precision fabrication of LN structures of tens-of-nanometers and below is a challenging task. Focused ion beam (FIB) is promising for ultra-smooth LN nano-patterning due to its flexibility and high resolution. However, tremendous surface roughness was left on the LN surface during FIB milling because of the inhomogenous etching of the metallic film, coated on LN surface to avoid charge accumulation. To achieve ultra-smooth patterns, in this paper, an organic conductive resist was used as an alternative to metallic films. The results showed that surface roughness was kept lower than 0.6 nm with etching depth from hundreds of nanometers to several nanometers. Ultra-shallow steps were fabricated on LN surface with controllable 10 nm step height. This work paves the way to the ultra-smooth and ultra-shallow FIB patterning of LN nano-structures. [ABSTRACT FROM AUTHOR]

Focused ion beam (FIB) milling has been used for fast prototyping of lithium niobate (LiNbO 3 , 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. • Investigation of etching depth and surface roughness evolution on focused ion beam milled lithium niobate (LN) surface. • Revealing the mechanism of non-negligible LN surface roughness especially in the ultra-shallow etching range. • The inhomogeneous etching of the pre-metallized coating had a detrimental effect on LN surface roughness control. • Several-nanometer ultra-shallow steps with surface roughness below 1 nm was presented on LN with a 7-nm-thick Au coating. [ABSTRACT FROM AUTHOR]