Corrective finishing of a micro-aspheric mold made of tungsten carbide to 50 nm accuracy.

The increasing demand on the optical performance of micro-aspheric glass lenses used for consumer, medical, as well as industrial applications places a high requirement on the surface quality of the molds used for replicating these lenses. However, it is difficult, almost impossible to generate a supersmooth surface with extremely high form accuracy by using the current cutting or grinding techniques. Therefore, this paper presents a corrective polishing process to finish micro-aspheric molds made of tungsten carbide by applying the vibration-assisted polishing method, aiming to obtain tens-of-nanometer form accuracy and subnanometer surface roughness. A polishing system which provides precise position, angle, and force control in 5 degrees of freedom (DOF)is employed for the experiments, and a tilting angle control method is introduced for the ease of the precisely controlling polishing force so as to keep the material removal rate stable. Then a dwell time algorithm is proposed by considering the scanning path of the polishing tool and the tilting angle of the workpiece. The experimental results show that after corrective polishing, the form accuracy of a micro-aspheric mold with high numerical aperture (NA) is successfully improved from 230 nm peak-to-valley (PV) to under 50 nm PV, while the surface roughness is reduced from 7.2 nm root-mean-square (rms) to 0.5 nm rms.