氟化钙单晶超精密抛光技术

Research on Ultra-Precise Figuring for CaF2 Single Crystal

针对紫外光学系统对氟化钙(Calcium fluoride,CaF2)晶体高精度、超光滑表面的加工要求,提出通过化学机械抛光(Chemically mechanical polishing,CMP)获得超光滑的表面,并利用离子束抛光(Ion beam figuring,IBF)进一步提升其面形精度的超精密加工工艺新方法。对CaF2晶体表面粗糙度和表面形貌随着CMP去除深度的演化规律进行试验研究,发现CMP可有效去除传统机械研磨抛光过程产生的划痕,并获得表面粗糙度为0.268 nm RMS(0.94 mm 0.7 mm)的超光滑的表面;对CaF2晶体表面粗糙度随IBF中离子入射角度和去除深度的演化规律进行试验研究,发现IBF后CaF2晶体表面变得粗糙,且在入射角度为40°左右时变粗糙的程度最小。因此,利用离子束倾斜40°对CMP后的CaF2晶体平面进行表面面形误差修正,得到面形精度13.14 nm PV,1.06 nm RMS和表面粗糙度为0.281 nm RMS的高精度、超光滑CaF2晶体表面。

In order to machine ultra-precise and extremely smooth calcium fluoride（CaF2） surface for optical system in ultra violet or deep ultra violet applications,a novel polishing method combining advantages of chemically mechanical polishing（CMP） and ion beam figuring（IBF） is put forward.CMP is aimed at extremely smooth surfaces,while ion beam figuring is aimed at ultra-precise figures without destroying the extremely smooth surface.The evolution of surface roughness and surface shape with material removal depth in CMP process are researched.Scratches produced in mechanically polishing process are removed by CMP and CaF2 plane with surface roughness of 0.268 nm RMS（0.94 mm 0.7 mm） is obtained.Evolutions of surface roughness in dependence of sputtering depth and ion incidence angle are studied.The surface tends to roughen after ion sputtering,but roughen slightly with optimum parameters such as incidence angle of 40°.So,Polished by combined method of CMP and IBF with incidence angle of 40°,CaF2 single crystal plane with 13.14 nm PV,1.06 nm RMS in surface contour errors and 0.281 nm RMS in surface roughness is obtained.