Study On Dislocation Defect Characteristics And Heat Treatment Of Calcium Fluoride Crystal

CaF₂ crystal is an important optical material,which has excellent optical and physical properties,and can be used as a window material from vacuum ultraviolet to infrared band.Especially in the field of UV photolithography,CaF₂ crystal has become the preferred lens material in the objective lens system of lithography machine because of its high transmittance and high laser damage threshold.However,with the development of UV lithography technology,the stress birefringence and transmittance of CaF₂ crystals are required to be less than 1 nm/cm,and the transmittance is more than 99.8%/cm@193 nm.However,a large number of dislocations in crystals will lead to local stress concentration and stress birefringence,which will seriously affect the practical application of crystals.Therefore,the formation and evolution mechanism,accurate characterization and suppression of crystal dislocations are the key basic problems that must be solved in order to realize the development of CaF₂ crystal in lithography.In this paper,the dislocation characteristics of CaF₂ crystal are studied,especially the morphology and spatial extension of corrosion pits on{111}crystal face,the structure of dislocations and the formation mechanism of dislocations.Based on the accurate characterization and formation principle of dislocation defects,the change of dislocation during heat treatment was studied,and the heat treatment conditions that can effectively reduce the dislocation defects were found.The specific research contents are as follows:（1）The origin of etch pits on the{111}crystal surface of CaF₂ crystal was studied.In order to solve the problem of incomplete measurement of dislocation density by etch pit density,the dynamic evolution behavior and kinematics characteristics of etch pit on{111}crystal face of CaF₂ were studied.Based on the kinematic characteristics of corrosion pits,the kinematic model of etch pits on{111}crystal face of CaF₂ crystal was established,which reveals the dynamic evolution process of etch pits.By conducting chemical corrosion experiments on crystal surfaces,it was found that the triangular conical pits and conical flat bottomed pits in the crystal surface have spatial extension characteristics,and the etch pits originate from dislocations in the crystal;However,the flat bottom pit has not been observed to have a spatial extension characteristic,which is the residual body left after the defect is completely corroded.It is suggested that the etching pits on the crystal face do not originate completely from the dislocations in the crystal,and that the accurate characterization of the dislocation density in the crystal should be realized,in the calculation of pit density,it is necessary to exclude the dislocation density evaluation technique,which is used to calculate pit density.（2）The dislocation characteristics and formation mechanism of CaF₂ crystal were studied.In response to the current problems of unclear understanding of the dislocation structure of CaF₂ crystals and insufficient research on the dislocation formation mechanism,transmission electron microscope（TEM）was used to systematically analyze the crystal dislocation at micro scale.The results show that there are a large number of dislocation lines in the crystal,which have two forms:annular and reticular.The size of annular dislocation lines（dislocation rings）ranges from a few nanometers to several hundred nanometers,and there are two forms of formation:discrete irregular arrangement and linear regular arrangement.The dislocation rings were characterized by transmission electron microscope light and dark field images and electron diffraction,and it was found that the direction of the Bernoulli vector was<110>,and the linearly arranged dislocation rings were<110>along the crystal slip direction.Based on the morphology,specific alignment direction,and crystal growth theory of dislocation ring formation,it is speculated that the principle of dislocation ring formation is the formation of vacancy diffusion and aggregation during crystal growth,followed by collapse.The occurrence of linear alignment is due to the sliding of dislocation rings along the<110>direction caused by excessive thermal stress.（3）Dislocation suppression techniques and processing methods for CaF₂ crystals were studied.Based on the formation mechanism of dislocation rings in CaF₂ crystals,the effects of different annealing temperatures on dislocations in CaF₂ crystals,as well as the half peak width and stress birefringence of X-ray double crystals,were systematically studied.It is found that when the annealing temperature increases from1000℃to 1120℃,the dislocation density of the crystal decreases significantly,from10⁵dis/cm² to 10⁴dis/cm²;The stress birefringence in the same crystal can be reduced to less than 1 nm/cm,the stress birefringence reduction index reaches 53.33%,and the half peak width of the crystal is reduced from 79.6"to 44",which can significantly improve the optical and crystalline properties of the crystal.