Profile Accuracy and Surface Roughness of CaF2 Cylindrical Microlens Arrays Machined by Form Tool Cutting
Fabrication of high-precision cylindrical microlens arrays (CMLA) on single-crystal calcium fluoride (CaF2) substrates is critical for advanced lithographic illumination systems. However, the intrinsic soft-brittle characteristics and weak elastic anisotropy of CaF2 often induce brittle fracture and surface damage during ultra-precision machining, limiting achievable form accuracy and surface quality. This study systematically investigates the coupling relationship between profile accuracy and surface integrity in diamond machining of CaF2 CMLA, emphasizing material properties, auxiliary cutting strategies, tool geometric accuracy, and tool wear evolution. First, fundamental mechanical properties of CaF2 are analyzed from elastic constants. Calculations show a Young's modulus of 110.04 GPa, Poisson's ratio below 1/3, and hardness of 5.71 GPa, confirming typical soft-brittle nature. The elastic anisotropy factor ranges between 0 and 1, indicating relatively weak anisotropy compared with many crystalline optical materials. Two auxiliary ultra-precision machining techniques, ultrasonic elliptical vibration cutting (EVC) and fly-cutting, are comparatively investigated. Experimental results show that EVC locally improves surface finish via intermittent cutting and reduced cutting forces, but periodic reversal of tool motion in the elliptical trajectory inevitably generates scratches and defects at the groove bottom of microlens structures. In contrast, fly-cutting consistently produces superior surface quality across the entire CMLA surface, attributed to significantly reduced tool-workpiece contact time and suppression of instantaneous cutting forces, effectively mitigating brittle fracture. A systematic parameter study reveals that tool geometric errors are directly replicated onto the workpiece surface, dominating profile error. After correcting the non-circular-arc tool profile, workpiece profile error (RMS) decreased from 200 nm to 16.8 nm, validating the decisive role of tool contour accuracy. Wear analysis indicates significant tool tip wear due to long cutting strokes, while middle sections experience lighter wear, primarily from repeated edge cutting and localized thermo-mechanical friction. These findings provide critical process guidance for high-precision, low-damage machining of CaF2 CMLA and other brittle optical crystals.