Chemical mechanical polishing (CMP) is the key application process in the fabrication of large optical flats to achieve global planarity and smooth surface. The large-diameter pad is a significant part and plays a vital role in CMP. To improve the polishing quality and efficiency, high-profile accuracy of the pad, conditioned by a diamond conditioner, is necessary. However, the conventional conditioning method (CCM) has been unable to satisfy the machining requirements for optical flats, and it is a challenge to improve the conditioning accuracy of the large-diameter polyurethane pad. In this study, we propose an advanced conditioning method (ACM) using the idea of subaperture conditioning, which reduces the size of the conditioner and adds the traverse movement to control the removal of different regions. Based on the conditioning density distribution model, the effect of conditioner diameter and process parameters on the pad planarity is investigated. The conditioning accuracy of the pad and polishing quality of the optical flats obtained with ACM are compared with those of a CCM. Experimental results showed that ACM can create the surface shape of the pad more uniformly than CCM. Furthermore, the polishing accuracy of the large optical flats of ACM exceeds that of CCM.
In view of the ultra-precision machining for large aperture optics developing rapidly but lacking effective rapid polishing methods, the research problems worth further studying are pointed out. The current situation, removing mechanism and processing difficulties of large aperture ultra-precision machine are introduced. The quantity and diameter of plane optics are increasing year by year, but currently the rapid removing of ultra-precision machine based on deterministic control of surface shape of the meter size optical components is still a blank. The development of related field is seriously affected huge losses caused by the low precision and low processing efficiency. The research situation of the high accuracy and rapid polishing methods are introduced for the main factors in the polishing processing system, such as movement patterns, polishing pad, polishing slurry and process parameters. According to the characteristics of the polishing machining process that bad deterministic control of the surface shape and the low removal efficiency, the research emphasis should be the methods that deterministic control and rapid removing of the machine, a new generation of high precision and high efficiency rapid polishing equipment and application process suitable for the large aperture and flat optical components.
Continuous polishing technology is an important means to realize batch processing of large aperture and high precision planar optical components. However, traditional continuous polishing process largely depends on the operator's experience, with poor controllability of component surface figure and unstable processing efficiency. In order to solve this problem, the in-situ shape measurement technologies including measurement of pitch lap surface figure and workpiece surface figure have been proposed in this paper. The real-time states of the pitch lap flatness and the workpiece surface figure in polishing process are obtained by in-situ measurement technologies, which provide the quantitative informations for adjusting process parameters. In the experiment, a large aperture mirror (material as K9; size as 800mm×400mm×100mm) was polished. The results show that the surface figure of the component was improved from λ/2 (1λ=632.8nm) to λ/6 by using the in-situ measurement technologies during the continous polishing process.
A method of photomask substrate fabrication is demonstrated ,that the surface figure and roughness of fused silica will converge to target precision rapidly with the full aperture polishing. Surface figure of optical flats in full aperture polishing processes is primarily dependent on the surface profile of polishing pad, therefor, a improved function of polishing mechanism was put forward based on two axis lapping machine and technology experience, and the pad testing based on displacement sensor and the active conditioning method of the pad is applied in this research. Moreover , the clamping deformation of the thin glass is solved by the new pitch dispensing method. The experimental results show that the surface figure of the 152mm×152mm×6.35mm optical glass is 0.25λ(λ=633nm) and the roughness is 0.32nm ,which has meet the requirements of mask substrate for 90~45nm nodes.