Recent advances in ultra-precision machining of lithium niobate crystals

Significance: The fabrication of high-performance optoelectronic devices requires substrate materials with exceptional optoelectronic properties, robust mechanical stability, and broad application versatility. These substrates are crucial for advancing the information technology sector and driving economic growth. Lithium niobate (LiNbO₃) crystal has the advantages in piezoelectric, electro-optic, nonlinear optical, and photorefractive effects, and exhibits superior thermal stability, chemical resilience, and tenability, making it an ideal substrate for the development of optoelectronic components such as optical modulators, frequency doublers, and optical filters. The LiNbO₃ crystal is quite promising for application in cutting-edge technologies, such as 5G communication systems, micro/nano-integrated photonics, and artificial intelligence. Achieving an ultra-smooth, low/no-damage crystal surface is paramount for LiNbO₃-based optoelectronic devices. Any imperfections, such as scratches, cracks, or embedded abrasives, can lead to scattering, absorption, or diffraction of optical signals, adversely affecting device performance. However, the challenges posed by LiNbO₃’s intrinsic properties—namely, its relatively low hardness, high brittleness, and significant anisotropy—complicate the precise surface processing. High-efficiency, high-quality, and low/no-damage ultra-precision machining technology for large-sized high-quality crystals is a critical bottleneck in enabling the widespread application of LiNbO₃ crystal devices. Progress: Thinning, lapping, and polishing are essential for LiNbO₃ crystals to meet industrial application requirements for high-performance optoelectronic devices. The stability and reliability of optoelectronic devices are significantly influenced by the generation and evolution of surface and subsurface damages. The hardness, fracture toughness, Young's modulus, and other material properties of LiNbO₃ along different crystallographic orientations are investigated using methods such as nanoindentation and scratch tests. The surface damage patterns of various planes are analyzed. The material removal behaviors under different parameters are revealed. Ion slicing and grinding are two critical processes for thinning LiNbO₃ crystals. Ion slicing, which relies on ion implantation and wafer bonding, enables the precise thinning of crystals. Currently, it is possible to produce high-quality LiNbO₃ films with thicknesses varying from several hundred nanometers to a few micrometers. Grinding utilizes the mechanical behavior of abrasives to rapidly remove material from the LiNbO₃ crystal. A crystal substrate with a thickness of 80 μm is prepared effectively by optimizing the grinding parameters. Free-abrasive lapping has a wide range of applicability. However, lapping for LiNbO₃can easily lead to surface damage and abrasive embedding. During fixed abrasive lapping, abrasive embedding is effectively prevented, and surface and subsurface damage are reduced. It also exhibits notable advantages for continuous batch grinding. Chemical mechanical polishing is a widely adopted final polishing method that effectively reduces damage from previous processes, achieving a surface roughness (R_a) of less than 1 nm. With advancements in grinding and chemical mechanical polishing, techniques such as photolithography, etching, and femtosecond laser ablation have been employed to fabricate LiNbO₃ crystal metasurfaces, facilitating the development and application of multifunctional and ultra-compact integrated optoelectronic devices. Additionally, innovative methods, such as optimizing polishing slurry compositions with nanomaterial additives and adaptive shearing-gradient thickening polishing, have enabled ultra-precision processing, achieving ultra-smooth and low/no-damage results. High-shear and low-pressure grinding and magnetorheological shear thickening polishing under the coupling of magnetic, stress, and flow fields hold significant promise for the ultra-precision polishing of LiNbO₃ crystals. Conclusions and Prospects: As critical functional materials in advanced applications, such as 5G wireless communication, integrated/micro-nanophotonics, and big-data processing, LiNbO₃ crystals have garnered significant attention for their potential in ultra-precision machining technologies. Research shows that the development and evolution of surface/subsurface damage have been examined using methods such as nano-indentation and scratch testing. Ion slicing and grinding are effective techniques for thinning lithium niobate crystals. Lapping and chemical mechanical polishing are commonly used techniques to achieve ultra-precision machining. Furthermore, high-quality LiNbO₃ metasurfaces can be generated using micro-nano manufacturing methods such as femtosecond laser ablation, etching, and photolithography. New technologies, such as high-shear and low-pressure grinding and magnetorheological shear thickening polishing, are the most promising methods for achieving ultra-precision machining of LiNbO₃ crystals. Considering the complex interplay between material properties, processing parameters, and underlying mechanisms, the ongoing exploration of new ultra-precision machining techniques and process optimizations for LiNbO₃ crystals is critical. Such advancements are essential for enhancing machining efficiency, improving surface quality, and minimizing damage. However, future work, including the material removal mechanism of LiNbO₃ crystals, the critical machining conditions of elastic-plastic-brittle transition, and surface/subsurface quality control, needs to be systematically studied to provide theoretical and technical guidance for the ultra-precision machining of LiNbO₃ crystals. Given the fundamental challenges and technological implications, the ultra-precision machining of LiNbO₃ crystals is expected to remain a focal point of research for the foreseeable future, warranting continued investigation and development in this field.