Effect Of Crystal Anisotropy On The Mechanical Removal Of Calcium Fluoride

Due to its excellent optical properties,such as visible-transparent and high laser-induced damage threshold,single crystal calcium fluoride?CaF₂?has been as the irreplaceable material of object lens in ultraviolet lithography.With the development of 13.5 nm extreme ultraviolet lithography?EUV?,it puts forward higher requirements on the surface accuracy and quality of CaF₂ object lens.For example,the surface accuracy and RMS roughness must be in angstrom level and the substrate crystal structure should be in nondestructive state.However,due to the anisotropic mechanical properties and material removal of crystal CaF₂,the accurate establishment of anisotropic removal function and the achievement of consistent and controllable material removal during polishing process have become the key factors to restrict the manufacture of super-precision aspheric surface of CaF₂ lens.Therefore,with the aid of nano-scratch tester and atomic force microscope,this paper firstly investigated the effect of crystallographic orientation on mechanical removal of CaF₂,and further explored the mechanism of different sliding speed on Ca F₂ material in the region where produced obvious anisotropic wear.Finally,through the comparison of the influence of atmosphere and water environment on crystal CaF₂ wear,the behavior mechanism of anisotropic material wear and influence mechanism of water molecules on crystal wear was studied.The main research contents and innovations are presented as follows:?1?Sliding test results based on different crystal surfaces displays that,with the change of sliding direction,the critical value of elastic-plastic-brittle wear showed overt differences.In the meantime,the wear depth also showed an obvious anisotropy with the change of crystal direction.CaF₂ material plastic removal was mainly affected by the correspondence between the direction of mechanical stress and the slip system,cracks were preferentially occurred at the cleavage plane.In addition,based on the experimental results,The contact stress range of10.8 GPa to 11.4 GPa is defined as the security region in which the material removal of CaF₂can be controlled in the ductile wear under all crystal planes and orientation conditions.?2?By studying the effect of sliding speed on the material removal of CaF₂ wear at different wear conditions,it was found that the wear depth,sliding friction and recovery of elasticity varied with sliding velocity.The mechanism of sliding speed affected the removal of the material was attributed to the change of the shear strain rate that leaded to the change of the shear force at the contact region.The distinction of wear behavior at different crystal faces was mainly caused by the different elastic recovery ability of crystal faces at varied speeds.And this difference was related to the mechanical stress direction and the axis direction of the crystal and the movement of the slip planes.Moreover,because the ratio of loading rate to sliding rate was constant,the sliding speed had little effect on the elastic-plastic-brittle transition of the crystal.?3?By contrasting the distinction of the wear behavior of crystal CaF₂ indentation in atmospheric environment and water,it was found that crystal orientation had a profound impact on CaF₂ material wear.At the same time,water played a certain role in promoting crack propagation on the crystal surface.The anisotropy of the CaF₂ material indentation damage resulted from the relative position between the direction of the stress action,the slip plane and the cleavage plane.The formation mechanism of the crack was also mainly related to the compressive stress and the location of the cleavage plane.And the mechanism of water molecules promoting crack propagation was attributed to stress-induced dissolution and exfoliation at the tip of crack.In conclusion,mechanical removal of crystal CaF₂ is not only related to anisotropic characteristics of the material itself,but also closely relates with the processing parameters and the processing environment.In addition,water molecules contribute significantly to the expansion of the crack.The results of this study will be in favor of optimizing the process parameters for calcium fluoride materials in actual manufacturing process,and provide theoretical support for further realizing complex shape controllable processing of CaF₂ material.