The edge roll off seriously restricts the further improvement of the overall surface figure accuracy in CP of large planar optics. Firstly, the half width and depth of large planar optics’ edge region is proposed to quantify the roll off degree by analyzing its radial equivalent profile. Then, a two-dimensional symmetrical model of the large planar optics and the pitch lap is established. Next, based on the finite element model, the influence of extension block’s bonding state on the large planar optics edge’s stress concentration is analyzed. Finally, the effectiveness of this proposed method is verified by CP of large planar optics with extension blocks.
Chemical mechanical polishing (CMP) is the most important process for global planarization. The micro material removal and planarization of the optical surface is a complicated process, and the surface shape of optics is effected by kinematics, pressure, and chemical conditions. Moreover, it is a remarkable fact that the distribution characterization of polishing particles also has an important effect on material removal uniformity, especially for leather pad and Tin polishing lap. Large optics were always polished to a convex shape for the low density of valid abrasives in optic center. The porosity and grooves distribution of pad plays a major role in slurry delivering. The novel model of contact and material removal is presented in which pad characterization, and polishing particles delivery and distribution effects are included. With the modified pad asperity and optimized grooves, the particles have been inclined towards well-distributed, and experiments validated that the optic figure is significantly promoted.
In this paper, the polishing properties of the zirconia slurry on the fused silica were studied, comparing with the effect of the ceria polishing. The polyurethane without dopant was applied as polishing pad. The experiment results show that it is easier to produce surface scratches when compared with ceria polishing owing to the high Mohs hardness of zirconia. The surface roughness(Rq) of the workpiece was less than 1nm after polishing with submicron size polishing slurry and suitable polishing pad. The R-on-1 test of 355 nm laser-induced damage threshold for fused silica show that zirconia polishing performance is 3.02% higher, however the exposed subsurface damages with HF etching were more than ceria polishing.The subsurface defect density is 0.10def/cm2 (@1μm)with ceria polishing and 1.19def/cm2 (@1μm)with zirconia polishing. Based on the fine polishing performance and abundant resources in nature, the zirconia slurry may be an alternative for fused silica polishing in prospect with the appropriate technological solution.
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.
In the continuous polishing process, the deformation caused by the uneven temperature distribution inside the optical element seriously affects the processing accuracy. This article combines domestic and international studies on the heat deformation of optical elements, and the formula for calculating the heat deformation of optical elements is proposed. Based on the Ansys software, the heat deformation of the element is simulated and analyzed, and the accuracy of the formula is verified. The effects of different shapes, materials, thicknesses, temperature difference on the heat deformation of the optical element are analyzed. After the optical element is processed, the temperature surface measurement experiment is performed to obtain the change of the temperature and the surface figure of the optical element in the cooling process, which has guided significance for improving the continuous polishing temperature field.
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.
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