Research of selective etching in LiNbO₃ using proton-exchanged wet etching technique

Abstract

Lithium niobate material (LN) has shown great application potentials in the fabrication of integrated optical devices due to its excellent physical properties, especially with the occurrence of lithium niobate-on-insulator (LNOI) substrate. However, the greatest challenge of micro/nano optical devices based on LN material lies in the precise etching process and thus limits its applications. In this paper, we comprehensively analyze the etching results treated by the proposed proton-exchanged wet-etching method (PEWE) combining with theoretical simulations and experiments. It is found that the proton-exchanged layer in the LN material can be easily etched after using a mixture acid of HF/HNO 3 , leading to the improvement of etching rate and surface morphology. The lowest roughness of the optical waveguide is measured to be 0.81 nm, which is beneficial for the performance improvement of LN-based optical devices. Ultimately, a quasi-vertical sidewall of the upper part of optical waveguide with improved surface morphology is successfully realized by utilizing the PEWE. Moreover, this method could also be extended to improve the performance of LNOI-based optical devices and pave the way for ultra-compact photonic integrated circuits based on LNOI.

Advances in lithium niobate photonics: development status and perspectives

“…199 Therefore, the combination of PE and a mixture of HF and HNO 3 etchant is widely used for LN wet etching. 109,[199][200][201][202][203][204] Compared with dry etching, its etched sidewall is not too deep and there can be an underetching problem. Ting et al demonstrated that the etching depth and aspect ratio can be improved using a diluted PE source with a lithium compound.…”

Femtosecond-laser-assisted high-aspect-ratio nanolithography in lithium niobate

“…Previous studies have shown an improved etching rate in the inverted LN ferroelectric domains. [20][21][22][23] Based on domain engineering techniques such as electrical field poling, 20,21 ionbeam irradiation, 24,25 and laser poling, 22,26,27 the highest etching rate in x-cut LN is reported to be ∼80 nm h −1 by using 40% HF solution. 22 Currently, it is still a great challenge to selectively enhance the etching rate to several microns per hour for LN nanofabrication with a large aspect ratio and high in-plane precision.…”

Hybrid Dry and Wet Etching of LiNbO3 Domain-Wall Memory Devices with 90° Etching Angles and Excellent Electrical Properties

“…Unfortunately, the nanofabrication of the memory cells remains a significant challenge, particularly in terms of LNO etching. − Generally, acoustic and optical LNO microdevices can be fabricated through wet chemical etching processes using HF/HNO 3 mixed solution and SC-1 solution (NH 4 OH/H 2 O 2 /H 2 O). However, the wet etching processes are isotropic and hence cannot transfer the fidelity of etched patterns at the nanometer-sized scale. − Therefore, much effort has been dedicated to improve dry etching processes due to their advantages of anisotropic etching, precisely controlled etching patterns, faster processing, and compatibility with materials unsuitable for wet etching. , …”