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)crys-tal 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 opto-electronic devices.Any imperfections,such as scratches,cracks,or embedded abrasives,can lead to scattering,absorp-tion,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 re-quirements for high-performance optoelectronic devices.The stability and reliability of optoelectronic devices are signi-ficantly influenced by the generation and evolution of surface and subsurface damages.The hardness,fracture tough-ness,Young's modulus,and other material properties of LiNbO3 along different crystallographic orientations are invest-igated using methods such as nanoindentation and scratch tests.The surface damage patterns of various planes are ana-lyzed.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 pre-cise 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 re-move material from the LiNbO3 crystal.A crystal substrate with a thickness of 80 μm is prepared effectively by optimiz-ing 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 effect-ively 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 dam-age from previous processes,achieving a surface roughness(Ra)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-preci-sion processing,achieving ultra-smooth and low/no-damage results.High-shear and low-pressure grinding and magnet-orheological 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 ad-vanced 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.Re-search 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 machin-ing.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-preci-sion machining of LiNbO3 crystals.Considering the complex interplay between material properties,processing paramet-ers,and underlying mechanisms,the ongoing exploration of new ultra-precision machining techniques and process op-timizations for LiNbO3 crystals is critical.Such advancements are essential for enhancing machining efficiency,improv-ing 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 con-trol,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 investig-ation and development in this field.
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