For the purpose of ultra-precision grinding large scale and complex off-axis aspheric optics effectively and automatically, computer-aided NC programming system was developed in this article. First the mathematical model of aspheric parallel grinding was analyzed, and the manufacture process of aspheric grinding is designed. Then the system architecture was established, which included initial grinding module, on-machine measuring module and error compensation grinding module. After inputting process and aspheric parameters, the system could calculate the grinding wheel X/Z/Y coordinates precisely and simulate the grinding pathway automatically, and then create grinding CNC program, which could control the grinding wheel to move along the aspheric surface. And the on-machine measurement CNC program was created to acquire the form error by displacement sensor. By combining the form error with the aspheric surface coordinates, the grinding wheel coordinates could be calculated and compensation machining CNC program was created. Using this system to manufacturing one large scale and off-axis aspheric optics, the PV of final form error was below 3.0μm, and the RMS was below 0.5μm.
A full aperture rapid polishing process (RPP) have been developed for batch producing high-precision large aperture optical flats required in some extreme application, such as high power laser, extreme ultraviolet lithography. Combining the theory of ultra-precision machine and chemical mechanical polishing, RPP can polish out the large aperture flat optical components in several hours or less. The material removal rate of fused silica component can be enhanced to ≥12 μm/h in RPP, which polishing efficiency is more than ten times of traditional pitch polishing process. The surface roughness also can be down to 0.3nm (RMS). Through several process improvements, the surface figure is determinately controlled, and subsurface damage can be quickly removed and suppressed. At last, high accuracy optical components can be obtained with high flatness (sub-micron), super smooth (sub-nanometer), and near-zero defects (SSD density ≤0.02 def/cm2 ).
Continuous polishing is a significant process to fabricate optical workpiece with nano figure precision. The figure of the optical workpiece is to a large extent dependent on the surface shape of the pitch lap. In this study, a novel method is proposed to determine the lap shape error by moving the measurement point in a generally radial direction while the lap rotates and correct the lap shape error by employing a small heat tool considering its viscoelastic property. It is validated that the surface shape error of the pitch lap can be corrected dramatically by the method, and the workpiece figure attempts to target the lap shape so as to reach a uniform material removal.
The pressure distribution is the most vital to material removal in continuous polishing in which removes the scratches and subsurface damage induced in grinding and lapping. Firstly, the interface response mechanism of the static contact pair is determined by uniaxial compression test. Then, the numerical model of the optics (Φ50mm×5mm ) and polishing lap(Φ80mm×7mm) contact pair is established to acquire the pressure distribution based on elastic contact mechanics. After that,the compression behavior is characterized by the pressure distribution and real contact area (RCA) measured by fujing pressure film. Attempt is made to clarify the figure of optics in continuous polishing.
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.
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.
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.
In the process of computer-controlled optical surfacing (CCOS), the key of correcting the surface error of optical components is to ensure the consistency between the simulated tool influence function and the actual tool influence function (TIF). The existing removal model usually adopts the fixed-point TIF to remove the material with the planning path and velocity, and it considers that the polishing process is linear and time invariant. However, in the actual polishing process, the TIF is a function related to the feed speed. In this paper, the relationship between the actual TIF and the feed speed (i.e. the compensation relationship between static removal and dynamic removal) is determined by experimental method. Then, the existing removal model is modified based on the compensation relationship, to improve the conformity between simulated and actual processing. Finally, the surface error modification correction test are carried out. The results show that the fitting degree of the simulated surface and the experimental surface is better than 88%, and the surface correction accuracy can be better than 1/10 λ (Λ=632.8nm).
The chemical mechanical polishing (CMP) is a key process during the machining route of plane optics. To improve the polishing efficiency and accuracy, a CMP model and machine tool were developed. Based on the Preston equation and the axial run-out error measurement results of the m circles on the tin plate, a CMP model that could simulate the material removal at any point on the workpiece was presented. An analysis of the model indicated that lower axial run-out error led to lower material removal but better polishing efficiency and accuracy. Based on this conclusion, the CMP machine was designed, and the ultraprecision gas hydrostatic guideway and rotary table as well as the Siemens 840Dsl numerical control system were incorporated in the CMP machine. To verify the design principles of machine, a series of detection and machining experiments were conducted. The LK-G5000 laser sensor was employed for detecting the straightness error of the gas hydrostatic guideway and the axial run-out error of the gas hydrostatic rotary table. A 300-mm-diameter optic was chosen for the surface profile machining experiments performed to determine the CMP efficiency and accuracy.
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