CNC Precision Optical Engineering in ITRC2020-03-30
In the 1970s, national education was very popular in Taiwan, but teaching equipment was still in short supply; the microscope was one of the important and expensive teaching equipment. It is against this historical background that the Instrument Technology Research Center (ITRC) was born. The Centre has been developing optical engineering technology since 1974, and its optical technology applications have evolved from teaching microscopes to optical systems for semiconductor manufacturing industries. Optical components are used in a wide range of applications, such as: microphotography lenses, optical disc readers, optical microscopes, endoscopes, optical glasses, digital/monocular cameras, high resolution lenses and precision positioning reference mirrors for semiconductor industry equipment, astronomical telescopes, reflectors for synchrotron radiation systems, and precision lenses and prisms for optical measurement systems.
The major advanced countries in the world, such as Germany, the United Kingdom, the United States and Japan, have a deep foundation in optical manufacturing technology, especially in the manufacturing of high value-added optical components, which have long been cultivated. Figures 1 and 2 show the application distribution and the optical accuracy and polishing process requirements for machining accuracy/size and optical industry, respectively. The application classification of precision optical lenses can be divided into precision machinery and precision measurement, astronomical instruments, photography and optics, microscopy/endoscopy (biomedical optics), miniature imaging module/optical storage, semiconductor manufacturing equipment, synchrotron radiation reflector, etc. according to the size and accuracy requirements. The most commonly known applications are optical instruments and products with a diameter of 100 mm or less, such as microlenses for smart devices, microscopes, optometry (glasses), cameras, projectors, etc.
Most of these products have surface roughness requirements in the range of 1-10 nm, and their manufacturing technology is relatively mature, driven by high market demand; and most of them are mass produced using mold forming technology to reduce costs. Advanced optical systems are moving towards large diameter optics and short wavelengths for better optical quality. In general, the shorter the wavelength used in an optical system, the more stringent the surface accuracy requirements. The combination of large bore and high surface accuracy poses significant technical challenges and makes it more difficult to achieve mass production with mould technology. Common large diameter optical systems include astronomical telescopes, optical systems for semiconductor manufacturing equipment, and precision machining and measurement equipment. For example, the surface roughness of an optical component for a semiconductor manufacturing device is about 1 nm and the lens diameter is between tens and hundreds of mm. Conventional optical grinding and polishing techniques uses a simple mechanical oscillation to remove the material from the lens, and are used to produce flat and spherical surfaces with the advantage of high efficiency and low cost. With advances in computer technology, optical design and analysis software and CNC machining equipment are rapidly developing, and the design, manufacturing and application of aspheric and free-form optical lenses are growing. The greatest advantage of the aspheric and free-form design is that it reduces the number of optical components in the optical system and facilitates the development of short, thin and light optical systems. Although traditional polishing techniques can achieve high surface accuracy, the new generation of optical systems can no longer be completed by simple mechanical oscillation due to the use of aspheric or even free-form design.
The history of optical engineering technology development in the instrument science center can be summarized in three stages, as shown in Figure 3. The first stage was the establishment of an optical and machining plant in the 1970s, which was based on small-scale lens manufacturing technology, mainly supporting the production of teaching microscopes and further providing services for custom optical components for scientific research. The second phase, from the late 1999s to 2010, will focus on the development of computerized production technologies complemented by conventional process technologies. In addition to computerized glass component processing and inspection facilities, the Company also introduced ultra-precision diamond turning processing equipment to expand production capacity to ductile optical materials and optical moulds for mass production, and entered the third phase in 2010s with the construction of a platform for the design and production of large diameter high-end optical lenses and lens modules as the main development direction. In this paper, we will elaborate on the highlights of the recent optical technology development of the Instrument Technology Center, including: ultra-precision machining technology and large-diameter aspherical optical polishing and inspection technology.
II. Ultra-precision machining technology
In response to the trend of advanced optical manufacturing and in line with its participation in the satellite remote sensing rewards program, the Instrument Technology Center introduced computerized optical and machining inspection equipment in the late 1990s, such as: computer-controlled grinding and forming machines and centering machines, aspheric polishing machines and computerized full-image laser interferometers, aspheric profilers, 3D measuring machines, etc. In order to diversify the optical processing and application technology, 5-axis ultra-precision processing system was also built in 2005; the material selection of optical components is based on the traditional Fig. 3. Semiconductor Manufacturing Equipment Optical Lens FV Master 1974 2000 2010 2015 FV Project FV Project Lenses / Secretaries / Reflectors Freeform Die Caps Ultra Precision Diamond Turning Endoscope Prototype Vegetation Transformation Observer Aluminum Reflector Microscope Traditional Polishing Computerized Polishing Magnetic Fluid Polishing 132 Science Instruments 200 Issue 103.9 Glass Brittle Materials, Expanded to Ductile Optical and Die Caps, e.g. Aluminum Alloy, Copper Alloy, Optical Plastic, Nickel Phosphorus Alloy, etc. The five-axis ultra-precision machining system in the Yi-Tech Center is equipped with various machining functions, including: single point diamond turning (SPDT), fast tool servo (FTS), slow tool servo (STS), fly cutting and micro milling (1-3), etc. Since its installation in 2005, it has been actively developing the aforementioned processing technologies and supporting various research projects, such as: diffractive lens applications (Microscopic Autofocus System Development Project (4), Long Working Distance Microlocation Measurement System (5), see Figure 4(c)), Cylindrical Free Surface (Laser Ink Line Project), Micro Lens Array (Micro Projection System Development Project (6-7)), Progressive Multifocal Lens Die (Progressive Multifocal Lens Development Project (8), see Figure 4(d)), Endoscopic Reflection Lens Unit (Prototype Development of Endoscope (9)).
In line with the trend of high quality and small-scale lightweight development of optical systems, the Instrument Technology Center has been focusing on the development of free-form ultra-precision machining technology. This section will describe in detail the optical free-form surface ultra-precision machining and application techniques. A free-form surface is a surface height (sag) determined by two or more dimensional polynomial parameters, such as ( , ) for cylindrical coordinates and (x, y) for right angle coordinates. The following is an example of a free-surface machining machine that can be used for high frequency microstructure and low frequency free-surface machining, as shown in Figure 4(e-f).
The CXZ 3-axis synchronous machining structure is similar to single point diamond turning; the difference is that the turning spindle with angle sensor and servo motor makes it a C-rotor with servo positioning function, as shown in Figure 4(a). By changing the surface coordinates to the machining coordinates, the turning tool can be free-formed by the CXZ 3-axis synchronous control. During machining, the rotation speed of the C-axis is limited by the speed and acceleration of the Z-feed axis, and the rotation speed is within 200 rpm; therefore, CXZ 3-axis simultaneous machining is also called slow tool servo machining. Although slow tool servo machining is similar to single point diamond turning, the front-end work is very complex and the machining planner must have complete technical and knowledge of optical surface parameter definition, interference checking of turning tools and component surfaces, machining parameter setting and verification, etc.
Generally, the optical surface parameters are determined by the optical designer, who in turn determines the machining steps and machining parameters based on the design parameters. The process planner must have a proper understanding of the optical design parameters, such as the definition of the direction of the coordinates and the definitions of the sub-nominal coefficient symbols. Optical surfaces usually have a uniform definition based on the optical path direction of the optical system; therefore, optical design surfaces and machined surfaces may have different definitions of concave and convex surfaces. In addition, for the same optical surface, the components are machined directly in the opposite direction of the surface coordinates to the die. To determine whether or not there is a need to change the positive and negative symbols when converting the optical surface parameters to the processing coordinate system, it is necessary to understand the shape and orientation of the parameter settings.
For slow cutting tool servo machining of free-form surfaces, interference checking of the turning tool and the machining element is a necessary pre-processing. Standard diamond turning tools have the following main geometrical parameters: tool radius, effective angle and clearance angle, as shown in Figure 5. The tool radius and the effective arc angle of the tool are co-faced with the cutting plane during the cutting process; the tool interference check is the same as for single point diamond turning. The curvature radius of a concave surface must be greater than the radius of the tool's arc. The radius of curvature and slope of a free-form surface are different for each angle. If the free-form surface is a parametric design, the minimum radius and maximum slope of the surface can be determined by partial differential calculations to confirm the existence of tool interference. Common tool curvature radii range from 0.1 mm-1 mm with an effective radius of 50 degrees. The slope of the circumferential tangent direction of an axially symmetrical spherical aspheric surface is zero, so there is no interference between the machining element and the front clearance angle of the tool. The standard diamond cutter has a front clearance angle of approximately 12 degrees, and special diamond cutters have a front clearance angle of up to 25 degrees. In the optical surface design optimization algorithm, the tool geometry and interference can be set to the limits of the surface parameters, and the optical surface design results can be analyzed directly.
Slow tool servo machining parameters include feed rate, feed per revolution (fr) and depth of cut (DOC). Feed rate is one of the most important aspects of slow tool servo machining, which directly affects machining accuracy and efficiency. The feed rate should be increased as much as possible under acceptable machining accuracy conditions. However, the feed rate of the machine is limited by the setting of the controller. The feed rate for slow tool servo machining is limited to the maximum speed or acceleration of the Z-axis, but may be limited to the maximum speed of the C-axis for surfaces with small depth variations. The maximum speed and maximum acceleration of each axis are determined by the inertia of each axis, the output horsepower of the driver and the response bandwidth of the controller. The feed rate fr per turn in can be determined by the tool radius Rt and the theoretical surface roughness Ra, fr=√(Ra*8Rt). In general, the cutting depth for roughing and finishing can be set to 20 and 6 um respectively.
III. Large-diameter aspherical fabrication and inspection technology
The biggest difference between optical glass aspherical fabrication and traditional spherical mirror fabrication is that aspherical mirror fabrication is achieved by polishing a small area with polishing position control technology. The current trend in aspheric polishing equipment is to use computer numerical control of the position of the polishing head relative to the machined component. Since the late 1990s, the Instrument Center has been developing large diameter aspherical optical processing and inspection technology, and in line with the independent development of remote sensing satellite imaging systems, the large diameter polishing and inspection system and aspherical splicing interferometer have been established since 2009, and the large diameter aspherical polishing and inspection technology team has been established. At present, the technical team has established a reliable and efficient aspherical mirror production process by combining the existing traditional polishing and measurement systems of the Instrument Technology Center. This section will outline its basic steps including the following; (1) approximate spherical polishing, (2) spherical curvature radius measurement, (3) aspheric polishing, (4) aspheric profile monitoring, (5) aspheric shape error measurement, and (6) aspheric corrective polishing, as shown in Figure 6 (11-13); in addition, aspheric corrective polishing and inspection techniques are explored in more detail.
Best-fit-sphere (BFS) polishing is performed with a spherical grinding bowl using traditional mechanical oscillating polishing techniques. The spherical grinding and polishing process has a large contact area between the spherical grinding bowl and the lens, and the lens surface material removal efficiency is high, and the sub-surface damage (SSD) caused by grinding and forming the lens surface can be quickly removed. The first embryo of a glass lens is usually formed into a design surface by a grinding process, followed by a surface refinement by polishing. The purpose of grinding is to remove the material quickly, but also to produce a subsurface destruction layer of about a few dozen microns. In addition to trimming the lens to a curvature radius close to the design value, the subsurface damage layer is also removed. A sphereometer can be used to quickly check whether the radius of curvature of a large diameter lens is close to the design value. In order to accurately monitor the curvature radius of the approximate spherical surface of the large aperture, it is proposed to use a cubic measuring bed for measurement.
Once the approximate spherical polishing results have been confirmed by curvature radius measurements, the aspherization polishing procedure can then be continued. Aspheric polishing is mainly used to remove the difference between the approximate spherical and aspheric contours, and then shape correction is performed by removing the material. Aspheric polishing requires computer numerical control for material removal in specific areas and monitoring of the aspheric shape with a surface profiler to ensure a consistent material removal rate during the aspheric polishing process. The approximate spherical and aspherical contours of large diameter aspherical mirrors may differ by tens or even hundreds of microns; therefore, aspherical polishing has to be performed in stages. The maximum removal depth of material per aspheric polish is recommended to be within 10 microns.
If the aspheric shape can be recessed to within 1-2 microns, the laser interferometer can be used with computerized full-image or stitching interferometer to check the accuracy of the aspheric three-dimensional shape. The aspheric correction polishing can be carried out after obtaining the three-dimensional shape error of the aspheric surface. The aspheric correction polish requires about 3 iterations of the aspheric shape error measurement to correct the aspheric lens shape error to within 0.1um.
The most critical step in the manufacturing process of these large diameter lenses is the cycle of aspherical shape accuracy detection and aspherical correction polishing. As mentioned above, aspheric polishing is done by polishing small areas and then shape correction is done by controlling the position of the polishing head relative to the lens. The basic principle of small area polishing is shown in Figure 7 (14). The polishing head is a sphere made of a flexible material with a polishing skin applied to the surface of the sphere. During the polishing process, the polishing head rotates at high speed to bring the polishing fluid into the polishing zone. The abrasive in the polishing fluid moves tangentially with the polishing element and removes the surface material of the polishing element by a shear mechanism.
Polished workpiece material removal efficiency is related to the following factors: polished element material type, polishing abrasive (particle size and material type), polishing fluid concentration, polishing head rotation speed, polishing head to polished element surface contact (tool offset), polishing head internal pressure, polishing head curvature radius, etc. In order for the polishing process to have a consistent material removal rate for good surface shape error correction, several key polishing parameters are usually controlled so that they are proportional to the material removal efficiency. Material is removed by a shear mechanism with a material removal rate (material removal depth per unit time) that is proportional to the following parameters: abrasive and component surface normal pressure, abrasive relative speed to component surface, amount of abrasive, and polishing time. The normal pressure and relative speed can be controlled by the amount of contact of the polishing head relative to the surface and the speed of the polishing head, respectively. Surface contact is less sensitive to the amount of material to be removed and the change in polishing head speed may cause system vibration; therefore, it is recommended to maintain a fixed surface contact and polishing head speed during the polishing process. In addition, the polishing solution is proportional to water and abrasive and is stirred to maintain uniformity during the polishing process; therefore, the polishing is usually carried out at a fixed concentration of polishing solution. The slower the polishing head moves, the more material is removed in the area; conversely, the less material is removed. The purpose of polishing is to correct the shape error if it is possible to obtain the shape error of the surface of the mirror by distributing the movement speed of the polishing head on the surface of the mirror.
The measurement of large diameter aspherical mirrors can be performed by using an interferometer with a computer generated holography (CGH) or an apheric stitching interferometer (ASI). The lens shape error measurement is based on the calculation of the polishing head movement speed during polishing. The following two factors should be taken into account in the measurement of morphological error of large diameter lenses Firstly, a stable optical measurement system. Large diameter lenses have a longer measurement range and are more susceptible to environmental factors such as airflow disturbance and temperature and humidity variation. If the ambient temperature and humidity can be effectively controlled, the air disturbance can be overcome by repeated measurements of the average value. In addition, the weight of the large diameter lens itself can cause the surface of the lens to deform due to gravity; therefore, special attention must be paid to the way the lens is set up during measurement. The large-diameter lens measuring frame uses elastic support to evenly distribute the weight of the lens to avoid excessive gravity deformation due to uneven force.
In recent years, the Centre has focused on ultra-precision machining technology and large-diameter aspherical lens fabrication and testing technology, and has completed a number of key components required for major projects, such as the prototype of endoscope, Foswell V primary and secondary lenses and semiconductor manufacturing equipment for optical lenses. In the future, the company will continue to integrate with local industries in Taiwan to develop system integration equipment, including: exposure machine optical systems, precision positioning platforms and automated optical inspection equipment. In addition, the Centre will also combine its years of optical engineering experience with that of domestic automation equipment manufacturers to jointly develop optical processing systems. It is hoped that the optical technology of the Instrument Technology Center can be effectively utilized and the competitiveness of the precision optical and mechanical industry in China can be enhanced.