Surface Figure Accuracy Research Articles

Mask-Shifting-Based Projection Lithography for Microlens Array Fabrication

Microlens arrays play a critical role in enhancing imaging systems due to their outstanding optical performance, compact size, and lightweight nature. However, traditional fabrication methods for microlens arrays suffer from low precision, inefficiency, high costs, and a lack of adequate surface figure control. In this paper, we present a novel approach for microlens array fabrication, using a projection lithography process with mask-shifting. The method employs a 0.2× projection objective lens to enhance linewidth resolution. By employing a projection-based mask-shift filtering technique, we achieve superior surface figure accuracy while reducing the complexity of mask preparation. The experimental results for four microlenses with different aperture sizes demonstrate surface figure accuracy in the submicron range and surface roughness at the nanometer level. In addition, 3D profilometer scanning equipment was employed to measure the surface roughness of these microlens arrays, and the measurement results of these microlens arrays processed using the proposed method for their surface roughness are 18.4 nm, 29.6 nm, 34.4 nm, and 56.1 nm. Our findings indicate that this method holds great potential in microlens array fabrication, offering the ability to achieve lower linewidths and higher surface figure accuracy compared to conventional methods.

A Review of Lightweight Design for Space Mirror Core Structure: Tradition and Future

With the continuous improvement of the imaging quality requirement of the space optical system, the large-aperture mirror becomes the research focus. However, the increase of the aperture will increase the whole weight which results in high launch cost and degrades the mirror surface figure accuracy. Therefore, the lightweight design method of the mirror structure is of great importance. In recent years, many space telescope system schemes have demonstrated the progress of the structural lightweight design of mirrors, such as Spitzer, SOFIA, JWST, etc. This article reviews the main content and innovations of the research on the structural designs of mirrors including conventional machining designs and topology optimization structures. Meanwhile, some emerging designs (e.g., lattices and Voronoi structures) considering additive manufacturing (AM) are also introduced. Several key elements of different structural design approaches for lightweight mirrors are discussed and compared, such as material, lightweight ratio, design methods, surface figure, etc. Finally, future challenges, trends, and prospects of lightweight design for mirrors are discussed. This article provides a reference for further related research and engineering applications.

Process Optimization Based on Analysis of Dynamic and Static Performance Requirements of Ion Beam Figuring Machine Tools for Sub-Nanometer Figuring

Extreme ultraviolet lithography objective lenses require surface figure accuracy of approximately sub-nanometer root mean square (RMS). As the key equipment for sub-nanometer accuracy figuring, the dynamic and static performance of ion beam figuring (IBF) machine tools are critical. However, the related research is not sufficient and comprehensive. To this end, a general model of dynamic and static performance requirements on three-axis IBF machine tools was established. The requirements on dynamic and static performance under different figuring process for different surface shape were comprehensively analyzed. Analysis results revealed that the three-axis IBF machine tools require typical motion accuracy better than 100 μm and certain dynamic performance for achieving sub-nanometer accuracy. According to the theoretical and simulation results, a process optimization based on analysis of dynamic and static performance requirements of IBF machine tools for sub-nanometer figuring is proposed. To verify the proposed method, a Φ90 mm mirror with 2.594 nm RMS was figured to 0.251 nm RMS by optimizing the processing parameters to ensure that the IBF machine tool with measured performance (positioning error of 52.74 μm, 53.04 μm, 37.71 μm, and maximum acceleration of 1.0 m/s2, 1.3 m/s2, and 1.5 m/s2 for axes x, y, and z, respectively) meets the performance requirements. The proposed method can promote the application of three-axis IBF machine tools in sub-nanometer accuracy figuring.

Self-calibration interferometric stitching test method for cylindrical surfaces

The surface figure accuracy requirement of cylindrical surfaces widely used in rotors of gyroscope, spindles of ultra-precision machine tools and high-energy laser systems is nearly 0.1 µm. Cylindricity measuring instrument that obtains 1-D profile result cannot be utilized for deterministic figuring methods. Interferometric stitching test for cylindrical surfaces utilizes a CGH of which the system error will accumulated to unacceptable extent for large aperture/angular aperture that require many subapertures. To this end, a self-calibration interferometric stitching method for cylindrical surfaces is proposed. The mathematical model of cylindrical surface figure and the completeness condition of self-calibration stitching test of cylindrical surfaces were analyzed theoretically. The effects of shear/stitching motion error and the subapertures lattice on the self-calibration test results were analyzed. Further, a self-calibration interferometric stitching algorithm that can theoretically recover all the necessary components of the system error for testing cylindrical surfaces was proposed. Simulations and experiments on a shaft were conducted to validate the feasibility.

Adaptive Deployable Thin Spherical Shell Reflectors

This paper begins with a quick survey of potential space applications and a brief review of previous experiments on the shape control of a spherical shell reflector with a thin film of PVDF-TrFE. Next, the problem of thermal sensitivity is addressed numerically; it is found that, because of the large thermal expansion of the active material, the surface figure error generated by a linear thermal gradient on a unimorph reflector is considerable and its correction requires large control voltages. The surface figure accuracy can be greatly improved by a balanced design (i.e., adding a passive layer symmetrical to the PVDF-TrFE layer) and using a low CTE substrate. Finally, the paper considers a petal reflector; the unimorph design is even more sensitive than the full reflector to the thermal gradient, but the balanced design turns out to be better than the full reflector, both from the point of view of the surface figure error and the control voltages.

Highly Accurate Digital Processing of Large Stroke Guideway with an Optical Material-Corning Code 7972

Currently, meter-long guideways rarely achieve an accuracy of dozens of nanometers due to processing difficulties such as the material and the edge effect. In this paper, we focus on this problem and propose a set of optimization processing methods to cope with it. In the grinding stage, a grinding tool is designed to improve the reciprocating processing and address the problem of warping; in the polishing stage, three different processes are compared, and the combination of magnetorheological finishing technology and the polyurethane disc technology process is purposed to reduce the polishing cycle and improve the surface figure accuracy. Moreover, through the combined process of magnetorheological finishing and smoothing, the edge effect and medium- and high-frequency error are essentially suppressed. The meter-long guideway is achieved with an accuracy of dozens of nanometers. Although the sizes of surface A/C and B/D are 1000 mm × 240 mm and 1000 mm × 160 mm, the surface figures are 20.33 nm, 22.78 nm, 39.23 nm and 26.58 nm RMS (Root Mean Square), respectively. The nanometer accuracy guideway is critical to an ultra-precision machine tool. Finally, the X-axis straightness of the profile measurement system formed by the guideway reaches 200 nm/600 mm.

Structural optimization design of lightweight rectangular reflective mirror

In order to solve the problem that the mass and the surface figure accuracy of the space reflective mirror are often contradictory in the lightweight design process, a structural optimization design of a lightweight rectangular reflective mirror of an off-axis three-reflection optical system is performed. In this study, a lightweight structure based on the center support of SiC materials is proposed. At the same time, a multi-objective optimization method is introduced. With the RMS value and Mass as the optimization targets at the same time, a mirror optimal structure model is obtained with a mass of 2.32 kg. Compared with the solid mirror, the lightweight ratio is 73.8%. Then the mirror subassembly is designed and the integrated performance of it is simulated. It shows that the RMS value of the mirror reaches respectively 2.5 nm, 2.2 nm and 7.3 nm when gravity load is applied in the directions of X, Y and Z axes. Furthermore, the RMS value is 3.2 nm when the mirror subassembly is under the load condition of uniform temperature rise of 4 ℃, which is far less than the requirement of RMS≤λ/50(λ=632.8 nm). Therefore the data meets the design requirements.

Characterization of thermal sprayed Si on sintered SiC for space optical applications

ABSTRACT Silicon carbide (SiC) ceramic exhibits several outstanding properties, such as superior chemical and thermal stabilities, high radiation resistance and low thermal expansion, that qualify it as a prime candidate for space applications. Optical quality with surface figure accuracy of the order of 60 nm peak-to-valley (PV) and micro-roughness (root-mean-square, RMS) less than 2 nm is necessary for space optical applications. Sintered SiC (α-HCP) is inherently porous and challenging to make high-grade optics. Hence, it calls for a surface modification of the sintered SiC by means of a clad layer that has superior adhesion to the substrate and facilitates optical polishing. This paper describes a typical method of synthesis and characterization of a thick silicon coating on sintered SiC substrate, directly machinable to aspheric for optical telescope applications. Present approach describes the quantitative opto-mechanical characterization of silicon film over SiC by thermal spray coatings, eventually leading to its space flight-worthiness.

Flexible support structure based on spring principle for a high precision reflecting mirror

A new type of three-leaf flexible structure based on the principle of spring is designed for high precision support structure of reflecting mirror which is under multi-working conditions (0° and 90°) in the optical inspection system of the on-orbit assembly space telescope validation prototype. Firstly, the external load and its action mechanism which can affect the surface figure accuracy and rigid body displacement are demonstrated; Secondly, the detailed structural form of the new flexible structure is given, and on the basis of this new structural form, the stress release principle and the theoretical analysis of the support stiffness are carried out; Finally, the finite element analysis method is used to optimize the size parameters of the flexible structure. The design results show that under the gravity load and the thermal load of ±2℃, surface figure error caused by supporting structure is 4.5nm, the maximum translational displacement is 0.0073mm, and the maximum angular displacement is 1.22″.The test results show that the surface figure accuracy is better than λ / 60 (λ = 632.8nm), and the displacement of the rigid body meets the requirements of 0.01mm and 1.5″.

Support structure and optical alignment technology of large-aperture secondary mirror measured by back transmission method

The diameter of secondary mirror measured by back transmission method in the on-orbit assembly space telescope validation prototype with a 1-meter aperture is Φ322 mm, and the distance between secondary mirror and mounting surface is 2.5 m. The requirements for supporting structure are as follows: surface figure error caused by supporting structure should be better than 3.88 nm, the tilt stability of secondary mirror is better than 2 ", and the displacement error in the direction of optical axis is 0.04 mm.According to the above requirements, the supporting structure and alignment method of the secondary mirror are illustrated in detail, and the physical testing is completed. Firstly, the size parameters are optimized by the method of Orthogonal Optimization and Finite Element Analysis (FEA),based on the optimized result, the supporting structure is designed, the bonding mode of the mirror body, the flexible structure to release stress and the implementation method of 4-DOF precise adjustment are emphasized, the results simulated by FEA show that the optical axis displacement of the secondary mirror is 5.2um and the error of the surface figure accuracy is 3 nm under the temperature range of 4℃ and gravity load. Finally, the alignment process and steps without the primary mirror as the benchmark are described in detail. the stability of the supporting structure is tested, and the test period is 14 days. The results and practice indicate that surface figure accuracy is 0.01 λ(λ = 632.8 nm), structure remains stable to excellent levels that the tilt is 1″and the displacement is 0.02 mm under gravity and thermal loading, all of them meet the requirements of the optical system for the supporting structure, the design is reasonable, the effect is good.

Design and analysis for the flexible support structure of high precision lens assembly

In order to achieve support and positioning of lens assembly which have large aperture and high precision in the on-orbit assembly space telescope validation prototype, A novel lens support structure with multi-point flexible units and four point length-adjustable radial holding is proposed. Then, the structure optimization is finished based on gravity deformation and thermal deformation. Finally, the influence of gravity and thermal load on the lens surface figure under this support structure is given by finite element analysis. The results show that: the largest first lens in the lens assembly is taken as an example, lens surface figure (RMS) of the upper surface causing by gravity and +2°C thermal load is 3.09 nm, while the lower surface is 3.03 nm, and lens surface figure RMS of the upper surface causing by gravity and -2° C thermal load is 3.43 nm, while the lower surface is 3.39 nm. The results and practice indicate that the multi-point flexible support structure can effectively reduce the influence on lens surface figure by gravity and thermal load, and can be satisfied with the lens surface figure accuracy (RMS < 4 nm).

Review of several precision finishing processes for optics manufacturing

The ultrasmooth optical components with atomic-order surface roughness and nanometre-level shape accuracy are in immense demand with the rapid advancement of modern optical technology. In recent years, aspherical and free-form surfaces are gaining more interest for its favorable properties. Moreover, the new optical materials with immensely enhanced mechanical properties are being developed to meet the stringent requirements of modern optics. Fabrication of complex-shaped ultrasmooth optical components becomes a significant challenge as conventional finishing techniques are unable to machine aspherical or free-form surfaces precisely. This situation demands some highly deterministic finishing processes. Mostly, the optical components are fabricated by shaping or pre-finishing methods followed by final finishing processes. In the shaping or pre-finishing methods, the rigid abrasive tools are used to remove the material at an enhanced rate and near net shape of the elements can be attained. Surface finish and shape accuracy can also be improved to some extent. Owing to the presence of residual finishing marks generated by shaping methods, the application of the components is limited to the infrared (IR) optics. Final finishing processes include more deterministic and flexible polishing techniques that can achieve desired surface finish, figure accuracy and surface integrity to make it suitable for shorter wavelength applications. In recent years, single point diamond turning, precision grinding, plasma chemical vaporization machining and magnetorheological fluid-based finishing are widely used for fabricating ultrasmooth optics. In this article, principle, mechanism of material removal and applicability of the aforementioned precision finishing processes to different materials are discussed.

Research on Surface Processing of Quartz Wafer Based on Magnetorheological Finishing and Ion Beam Figuring

The common methods of manufacturing the quartz wafer are grinding and finishing. These processes not only cause micron-scale flaws, but also limit the accuracy. In this paper, firstly, magnetorheological finishing (MRF) technology is applied as the follow-up treatment of traditional process to correct the defects on the surface of the wafer. Then ion beam figuring (IBF) technology is used to improve the surface figure accuracy to nanoscale. After MRF, micron-scale scratches and pit defects on the surface are effectively eliminated, and the Rq value of the surface roughness descends from 2.46 nm to less than 1 nm. Meanwhile, the RMS value of the surface accuracy descend from 35.60 nm to 5.06 nm through IBF. The experimental results show that MRF and IBF technology can improve the surface quality of the quartz wafer.

超大口径光学制造均力支撑布局优化

When fabricating ultra-large aperture mirrors, it's always very crucial to reduce the control difficulty and to lower the cost of the support system while providing the support accuracy required for on-line fabrication and testing. For this purpose, layout optimization method of supporting points for the equal-force support system is proposed. Firstly through surface fitting, surface figure accuracy of supported mirror is analyzed, and the objectives for layout optimization are auto-updated. Afterward support layout optimization, whose model has the capability of adaptive finite element analysis, of arbitrary-shaped thin flat mirrors is carried out. Sequentially load transfer structure for lightweight mirrors, which are widely used, is designed and the support location within each transfer is optimized. Finally the optimization method is applied to 30 m telescope (TMT) tertiary mirror and a 2-m aperture mirror. Results show that the proposed method has the required accuracy for the optical fabrication of ultra-large mirrors.

Ultra-smooth polishing of high-precision optical surface

A new method of ultra-smooth uniform polishing was presented, which can avoid high-precision surface figure getting worse after ultra-smooth polishing. At first, the fundamental and process were introduced. Then the process was simulated with “Gauss” and “V” type removal function. It shows that there will be no significant influence on optical surface figure after ultra-smooth uniform polishing with any type removal function. To demonstrate the process, a high-precision Ø100mm fused silica flat optical element was polished, which was prior figured by IBF. Its surface figure accuracy root-mean-square (rms) value is improved from initial 3.624nm to final 3.393nm, the mid-spatial frequency surface roughness rms value is improved from initial 0.477nm to final 0.309nm, and the high-spatial frequency surface roughness rms value is improved from initial 0.167nm to final 0.0802nm. At last, the surface quality of the lens was analyzed by power spectral density (PSD). The result indicates that the surface roughness of high-precision optical element could be improved by ultra-smooth uniform polishing method without the surface figure destroyed.

Structural Design of Primary Mirror Subassembly for Space Telescope

In order to design a large aperture primary mirror with the diameter of 1150 mm, the parameters impacting on gravity deformation of the mirror were analyzed and discussed in detail, including material selection, diameter-thickness ratio, the number of the support points, location selection and lightweight structure, etc. A novel space mirror structure system was put forward, and its back was opened and supported by six points on back. The dynamic and static and thermal characteristics were analyzed based on the finite element method. Analysis results showed the surface figure accuracy reached to RMS 14.5nm under the action of gravity load along the optical axis direction and the first-order natural frequency was 214Hz which met to the designed indexes requirements.

Research on compensation of suction deformation error of potassium dihydrogen phosphate crystal

Potassium dihydrogen phosphate (KDP) crystals used in high power laser systems require high figure accuracy. Deformation caused by vacuum suction is one of the most significant factors affecting the final surface figure accuracy. It is difficult to compensate the error caused by the suction deformation when using a single point diamond flycutting method. This paper presents a spiral turning method to machine KDP crystals on a common ultraprecision lathe, and the transfer rule and compensation theory of the suction deformation error are studied. The in situ measurement of deformation error is accomplished with a displacement test apparatus, which can acquire submicron precision, and then compensates for it by utilizing the three-axis servo control technology. Meanwhile, cutting parameters are strictly controlled to achieve full-aperture ductile material removal. The compensation experiments for both reflected and transmitted wavefront error are carried out on a Φ270 mm KDP crystal and the transmission wavefront error of 0.591λ (peak-to-valley, λ = 632.8 nm) and 25 nm/cm (gradient root-mean-square) are obtained after one compensation with full-aperture surface roughness of about 2 nm RMS.

Research on Process Parameter Effects on Surface Roughness and Subsurface Damage in CMP of Single Crystalline Silicon

Reported in this paper is an investigation of the process parameter effects on surface roughness and subsurface damage (SSD) in CMP of single crystalline Silicon. For the given experiments, the appropriate method to examine the SSD can be obtained. The surface roughness and figure accuracy were measured with an atomic force microscope (AFM) and Taylor-Hobson profilometer. The experiments results indicate proper process parameter for the best surface roughness, which can be divided into two stages. It should use longer time in the finish polishing stage, while shorter time and reduce the ratio of polishing pads and head in the ultra-finish polishing stage. Generally speaking, the isotropic etching of single crystalline Silicon, anisotropic etching of single crystalline Silicon and hand burnishing are mostly used to find the SSD and it is found that the last method is the best one to see the SSD by SEM.

长条形空间反射镜及其支撑结构设计

A flexible support structure was proposed to keep the higher surface figure accuracy of space mirrors under gravity and uniform temperature change load cases.According to the optical design requirements,the structural form of a mirror was determined,and then a flexible support structure of the primary mirror was designed.By adopting finite element analysis software,the mirror component was analyzed and its structure was optimized.Analysis results show that the first order natural frequency of the mirror component is 179 Hz,and the surface figure accuracy RMS of the mirror reaches 5.06,4.43 and 7.59 nm when gravity load is applied in the directions of X,Y,and Z axes,respectively.Furthermore,the integrated surface figure accuracy RMS of the mirror reaches 6.08,6.32 and 8.08 nm respectively under the load cases of gravity in three directions coupled with uniform temperature rise of 4 ℃.Laboratory test results indicate that the mirror can offer a good image quality under the condition of 4 ℃ uniform temperature change,and the mechanical test results are consistent with that of the theoretical analysis.The change of the surface figure accuracy is not obvious after dynamic and thermal vacuum tests.Analysis and experimental results demonstrate that designs of the mirror and its support structure are effective,which can meet the requirements of space applications.

Modeling and Experimental Study of Magnetorheological Flexible Gasbag Polishing

Magnetorheological flexible gasbag polishing based on the special application of magnetorheological fluid (MRF) in robotic gasbag polishing technique is a novel efficient approach in the field of mould finishing. It can control the polishing pressure by changing the magnetic force generated by MRF inside of the gasbag with the effect of variable magnetic field of electromagnetic coil. Its mathematical model is established to study the main factors influencing the material removal. The orthogonal tests are applied to analyze these important parameters. From the experimental results, it can be seen that this new approach is desirable in realizing the control of surface figure accuracy and improvement of surface quality under certain condition.