氟化钙(CaF2)晶体具有紫外透过率高(＞99%@193 nm)、抗激光损伤阈值高等特点, 在高功率紫外激光器、深紫外光刻机等领域具有广泛的应用。应力双折射是氟化钙晶体在实际应用中的关键性能指标, 会导致通过晶体的光束发生形变, 严重影响成像质量。采用坩埚下降法制备了尺寸为210 mm的氟化钙晶体, 系统研究了CaF2晶体的位错和小角度晶界以及结晶质量对晶体应力双折射的影响, 实验结果表明, 位错密度的增高、小角度晶界的聚集、结晶质量的变差, 会引起局部残余应力的集中, 加剧应力双折射现象。

Calcium fluoride (CaF2) crystals have high UV transmittance (>99%@193 nm) and high resistance to laser damage threshold, which have been widely used in the fields of high-power UV lasers and ArF laser lithography stepper. Stress birefringence is the key performance index of calcium fluoride in practical application, stress birefringence can cause deformation of the optical wavefront, which seriously affects image quality. Calcium fluoride crystals 210 mm in diameter were prepared by Bridgman-Stodebarger(B-S) method. The effects of crystal dislocation and small angle grain boundaries and crystal quality on stress birefringence were systematically studied. The experimental results show that the increase of dislocation density, the aggregation of small angle grain boundaries and the deterioration of crystal quality can cause the concentration of local residual stress and exacerbate the phenomenon of stress birefringence.