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The mechanism for material removal in ductile mode cutting of CaF2 brittle crystals

Single-crystal calcium fluoride (CaF2) is an indispensable optical material for the deep ultraviolet radiation lithography system. The requirement for extremely high performance using in lithography system can be not satisfied by using conventional polishing because the polishing powder are easy to embed to surface due to its relative softness. In the process of rough finishing of CaF2, ductile cutting of CaF2 is considered as a suitable technology instead of tradition polishing to efficiently achieve super-smooth surface without impurity. In this paper, a theoretical and experimental study on the ductile cutting of CaF2 is presented. An energy model for ductile mode cutting of CaF2 is developed, in which the critical undeformed chip thickness can be predicted from the workpiece material characteristics and cutting parameters. The model is verified with experimental results from the micro-grooving of CaF2 on an ultra-precision lathe using a circle diamond crystals tool. The pattern of micro-grooving is inspected using optical microscope, the critical cutting depth of ductile-to-brittle is measured using white-light interferometer. It is shown that the predicted results for the critical undeformed chip thickness corresponding to ductile cutting agree well with the experimental results. Finally, ductile cutting of CaF2 is carried out under the cutting conditions as the maximum undeformed thickness less than the critical undeformed thickness of 233nm obtained above, ductile mode cutting of CaF2 has been achieved a super smooth surface with roughness of 3.50nm (Ra) measured by AFM is obtained.