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Ultra-precision polishing of electroless nickel molding dies for shorter wavelength applications Y. Namba a,*, T. Shimomuraa, A. Fushikia, A. Beaucampa, I. Inasaki (1)a, H. Kunieda b, Y. Ogasakab, K. Yamashitab aDepartment of Mechanical Engineering, Chubu University, 1200 Matsumotocho, Kasugai, Aichi 487 8501, Japan bDepartment of Physics, Nagoya University, Nagoya, Japan 1. Introduction Electroless nickel plating is widely used in various fi elds such as chemical, electronic, aerospace, automobile, resin, and textile industries due to its superior corrosion resistance, wear properties, hardness, solderability, magnetic properties and deposition uni- formity. In the optical industry, electroless nickel-plated molding dies have recently been used with the development of single-point diamond turning. Electroless nickel is the only material that can be single-point diamond-turned 1,2 and also polished super- smoothly. In this application, surface roughness of a few nm root-mean-square (rms) was required on the aspheric molding dies after polishing. Recent soft X-ray telescope satellites such as Chandra 3, XMM-Newton 4 and Suzaku 5 were launched in 1999 and 2005. Allthreetelescopeswereconstructedfrommultiplenestedgrazing incidence refl ecting mirrors consisting of a paraboloid and hyperboloid of revolution. Chandras aspheric X-ray mirrors were made of zero thermal expansion glassceramics, and the surfaces were well polished and coated with iridium on the inside. The other X-ray telescopes were made by replication process from gold-coated molding dies. The surface roughness specifi cation for the coating fi lm was less than 0.5 nm rms. All three telescopes are now working at observing new phenomena in space. Focusing optics in the hard X-ray region above 10 keV will open a new window on the universe. There is a new proposal called NeXT 6 to make a next-generation hard X-ray telescope from replicated platinum/carbon (Pt/C) multilayer mirrors of aspheric shapesandtolaunchitin2013.Thespecifi cationsofthemirrorsup to 500 mm in maximum diameter are as follows: the shape accuracy of the aspheric mirrors is to be less than 100 nm and surface roughness is to be less than 0.3 nm rms. More than 200 high precision thin aspheric mirrors are required. Only amorphous materials such as glass and electroless nickel canbe polishedin the aspheric shape down to less than0.3 nm rms surface roughness. It was decided to use aluminum alloy covered withelectrolessnickelplatingasamoldingdiematerial,becauseof higher machinability than glass. The electroless nickel is diamond- turned into high precision aspheric shapes and may be polished to less than 0.3 nm rms surface roughness. Then, a platinum/carbon multilayer fi lm is deposited on the electroless nickel die and a mirror substrate is glued over the die 7. The last process is the separation of the mirror substrate from the molding die. This paper deals with the ultra-precision polishing of fl at electroless nickel molding dies to less than 0.3 nm rms surface roughness, for the next-generation hard X-ray telescope. 2. Experimental procedure 2.1. Single-point diamond turning A7075 aluminum alloy was cut into 50 mm diameter and 10 mm thick plano samples and turned by a single-point diamond turning machine 8. The material was also cut into cylinders of 40 mm diameter and 35 mm length, and aspheric samples of 300 mm in nominaldiameterand 210 mmin mirror length.A layer of nickelphosphorus alloy 0.1 mm thick was deposited on the diamond-turned aluminum alloy samples by electroless nickel plating in industry. The hardness of the electroless nickel and aluminum alloy was 568 and 183 Hv, respectively. Some of the electroless nickel-plated samples are shown in Fig. 1. All samples were single-point diamond-turned again in order to obtain the specifi c shapes. CIRP Annals - Manufacturing Technology 57 (2008) 337340 A R T I C L EI N F O Keywords: Optics Polishing Molding die A B S T R A C T Electrolessnickelisoneofthebestmaterialsformakingopticalmoldingdies,becauseofitsmachinability withbothsingle-pointdiamondturningandpolishing,aswellasitssuitablehardnessanddurability.This paper deals with the ultra-precision polishing of aspheric molding dies for next-generation hard X-ray telescope mirrors, which require a super smooth surface with a roughness below 0.3 nm root-mean- square (rms). The material was machined using various methods, and asurface roughness of 0.23 nm rms was obtained on aspheric molding dies of 300 mm in diameter. A surface roughness of 0.16 nm rms was achieved on platinum/carbon multilayer mirrors, replicated from plano dies. ? 2008 CIRP. * Corresponding author. Contents lists available at ScienceDirect CIRP Annals - Manufacturing Technology journal homepage: 0007-8506/$ see front matter ? 2008 CIRP. doi:10.1016/j.cirp.2008.03.077 2.2. Cloth polishing and fl oat polishing The diamond-turned plano samples were polished with a polishing cloth or pad for semiconductor applications and CeO2 powder on the fl oat polishing machine 9 which has a hydrostatic oil bearing of high rigidity, and damping as well as high rotational accuracy.Thiscombinationofpolishingclothandpowderiswidely used in the optical industry. Then, those samples were fl oat- polished on a 460-mm diameter synthetic resin lap, having concentric grooves. The lap was made fl at with a single-point diamond tool. The polishing fl uid was a mixture of pure water and 3 wt% SiO2powder of 7 nm nominal diameter particles, and was controlled within a temperature range of 0.01 K. Plano samples were polished on a conventional small polishing machine, and cylindrical and aspheric samples were polished by special tools, with various polishing cloths for semiconductor applications and SiO2powders of 15 and 40 nm diameter particles at a concentration of 8 wt%. 2.3. Surface roughness measurement All machined plano and cylindrical surfaces were observed under a Nomarski differential interference contrast microscope. Surface micro-topography of the samples was measured with a three-dimensional optical profi ler and scanning probe microscope (SPM). Large curved surfaces were partly covered with a synthetic resin fi lm after being slightly dipped in a solvent. The fi lm was peeled off from the polished sample, and the surface roughness was measured with the optical profi ler. 3. Experimental results and discussion 3.1. Ultra-precision machining of electroless nickel plano samples Electroless nickel-plated aluminum plano samples of 50 mm diameter and 10 mm thick were single-point diamond-turned, polished with a polishing cloth and fi ne abrasives, and fl oat- polished on a resin lap with 7 nm SiO2powder. The single-point diamond-turned surface looks like a mirror with rainbow patterns, while the cloth-polished and fl oat-polished samples look just like mirrors. Fig. 2 shows Nomarski microphotographs of the various machining stages. The single-point diamond-turned surface shows fi ne directional tool marks at regular intervals. The cloth-polished surface shows fi ne microscratches all over the surface, while the fl oat-polished surface is featureless. Figs. 2 and 3 show the surface roughness of the electroless nickel samples at the various machining stages as measured with anoptical profi ler and SPM. The fl oat-polishedsurface shows0.136 and 0.110 nm rms, measured with an optical profi ler and SPM, respectively. The surface is extremely uniform, so the measured values by two different methods almost agree. This fl oat polishing method can polish CaF2single crystal surfaces to less than 0.1 nm rms surface roughness 10. 3.2. Replication of Pt/C multilayer mirrors from electroless nickel plano molding dies Replication is one other key subject to make Pt/C multilayer hard X-ray telescope mirrors, as well as polishing electroless nickel aspheric molding dies. We deposited a Pt/C multilayer fi lm on an electroless nickel plano molding die, and glued on a glass plate with an epoxy resin. After curing we took off the Pt/C multilayer mirror from the molding die. Fig. 4 shows a photograph of an electroless nickel molding die of 30 mm diameter after separation on the left, and a Pt/C multilayer X-ray mirror on a glass plate after separation on the right. Fig. 1. Electroless nickel-plated aluminum alloy samples for shorter wavelength applications. Fig. 3. Surface roughness of electroless nickel plano surfaces at various machining stages, measured with a scanning probe microscope. (a) Diamond-turned; (b) cloth- polished; (c) fl oat-polished. Fig. 2. Surface roughness of electroless nickel plano surfaces at various machining stages, measured with an optical profi ler. (a) Diamond-turned; (b) cloth-polished; (c) fl oat- polished. Y. Namba et al./CIRP Annals - Manufacturing Technology 57 (2008) 337340338 Fig. 5 shows the surface roughness of a Pt/C multilayer replicated mirror as measured with the optical profi ler. The roughness value of 0.155 nm rms almost agrees with that of the fl oat-polished sample, which is 0.136 nm rms as shown in Fig. 2(c). It becomes clear that Pt/C multilayer hard X-ray mirrors can be obtained by this replication process using polished electroless nickel molding dies. 3.3. Cloth polishing process on plano sample The electroless nickel plano samples shown in Fig. 6 were polished by incremental changes of the polishing conditions with various polishing cloths and SiO2powders. Figs. 7 and 8 show the relation between surface roughness of the cloth- polished electroless nickel samples and the polishing time. The surface roughness decreases with polishing time, and the curve gradually approaches a fi xed value which is a function of the polishing cloth as shown in Fig. 7 and of the grain size of the polishing powder as shown in Fig. 8. In this second experiment we used the polishing cloth for fi nal fi nish shown in Fig. 7. Finer powder has a smoother limit value, though the polishing rate is lower. In the case of 15 nm SiO2powder, the polished surface reached the required surface roughness for hard X-ray applica- tions, of 0.3 nm rms, whilst with 40 nm SiO2powder it did not reach that level. In order to shorten the polishing time, a two-step polishing process was used. The fi rst stage used 40 nm SiO2powder for reducing the surface roughness quickly, and then 15 nm SiO2 powderforfi nalpolishing.Thepolishingtimewasreducedbyupto two third as shown in Fig. 8. 3.4. Cylindrical polishing Before polishing large electroless nickel aspheric molding dies, small electroless nickel cylinders of 40 mm diameter were polished. Fig. 9 shows the two-step cloth polishing applied to electrolessnickelcylindersusing thepolishingclothforfi nalfi nish, and shows that 0.3 nm rms surface roughness was obtained after 13 h of polishing. 3.5. Polishing of large aspheric molding dies Fig.10showsadiamond-turnedandpolishedaspheric electroless nickel molding die of 300 mm in diameter. This die Fig. 7. Surface roughness variation of electroless nickel plano sample during cloth polishing, as a function of polishing cloth. Fig. 4. Photograph of fl oat-polished electroless nickel plano molding die on the left and Pt/C multilayer X-ray mirror on a glass plate molded from the die. Fig. 5. Surface roughness of molded Pt/C multilayer mirror. Fig. 8. Surface roughness variation of electroless nickel plano sample during cloth polishing, as a function of grain size of abrasives. Fig. 6. Nomarski microphotographs of electroless nickel plano surfaces at various machining stages. (a) Diamond-turned; (b) cloth-polished; (c) fl oat-polished. Y. Namba et al./CIRP Annals - Manufacturing Technology 57 (2008) 337340339 was designed for making Pt/C multilayer hard X-ray mirrors of 12,000 mm in focal length. The total mirror length is 210 mm, and the 10 mm upper part is hyperbolic whilst the 200 mm lower part is parabolic. This die was manually fi nished by the afore- mentioned two-step polishing process. This molding die has a perfect mirror appearance without any fl aw, although the form error was increased. Fig. 11 shows that the surface roughness of a polished paraboloid die of 300 mm in diameter, as measured with an optical profi ler, was 0.228 nm rms, better than 0.3 nm rms. So the surface roughness specifi cation on an aspheric molding die was satisfi ed. This polishing technology for electroless nickel will be used in making extremely ultraviolet (EUV), soft and hard X-ray optics applications 11,12 as well as hard X-ray molding dies. The next development will be the computer-controlled auto- mation of the polishing process for electroless nickel aspheric molding dies, implemented on a single-point diamond turning machine. 4. Summary Planosamplesandasphericmoldingdieswithelectrolessnickel plating were diamond-turned and polished to enable shorter wavelength applications, particularly dies for hard X-ray mirrors. The following conclusions may be drawn from the results of this study: 1.Surface roughness of 0.1 nm rms was obtained on electroless nickel plano samples by fl oat polishing. 2.The surface roughness of polished samples is a function of grain sizeofabrasive,type ofpolishingcloth,aswell aspolishingtime. 3.Surface roughness of less than 0.3 nm rms was obtained on cylindrical and aspheric electroless nickel molding dies by cloth polishing with SiO2powder of 15 nm in nominal diameter, and this roughness is suitable for hard X-ray optics applications. 4.Pt/C multilayer plano mirrorscan be obtained from an electroless nickel plano molding die. The surface roughness of the separated multilayer mirror matches with that of the molding die. Acknowledgements We extend our sincere thanks to S. Yoshida and M. Suzuki of Chubu University, and R. Freeman of ZEEKO Ltd. company for their help with the experiment and manuscript. This work was supported in part by a Grant-in-Aid for Scientifi c Research (B) Nos. 15360075 and 18360073 from the Japan Society for the Promotion of Science and a Grant-in-Aid for Exploratory Research (No. 16656054) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References 1 Decker DL, Grandjean DJ, Bennett JM (1979) Optical and Surface Physical Characteristics of Diamond-machined Infrared Windows, NBS Spec. Publ. 562:293304. 2 Ikawa N, Donaldson RR, Komanduri R, Ko nig W, Aachen TH, McKeown PA, Moriwaki T, Stowers IF (1991) Ultraprecision Metal CuttingThe Past, The Present and The Future. Annals of the CIRP 40(2):587594. 3 Weisskopf MC, et al, (2002) An Overview of the Performance and Scientifi c Results from the Chandra X-ray Observatory. Publications of the Astronomical Society of the Pacifi c 114:124. 4 Aschenbach B (2001) In-orbit Performance of the XMM-Newton X-ray Tele- scopes: Images and Spectra. 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