Carbon buffer layers for smoothing superpolished glass surfaces as substrates for molybdenum /silicon multilayer soft-x-ray mirrors

Zerodur and BK7 glass substrates (developed by Fa. Glaswerke Schott, D-55014 Mainz, Germany) from Carl Zeiss Oberkochen polished to a standard surface roughness of varsigma = 0.8 nm rms were coated with a C layer by electron-beam evaporation in the UHV. The roughness of the C-layer surfaces is reduced to 0.6 nm rms. A normal-incidence reflectance of 50% at a wavelength of 13 nm was measured for a Mo/Si multilayer soft-x-ray mirror with 30 double layers (N = 30) deposited onto the BK7/C substrate, whereas a similar Mo/Si multilayer (N = 30) evaporated directly onto the bare BK7 surface turned out to show a reflectance of only 42%.     

Experimental Investigation of Material Removal and Surface Roughness during Optical Glass Polishing

It has been a challenge to finish optical glass surfaces due to their hard and brittle nature. Moreover, tight tolerances of surface figure and finish make polishing a more critical operation. This work reports the results of an experimental study performed for full aperture polishing of BK7 optical glass. Flat samples of borosilicate (BK7) glass are polished using an optical pitch polisher and cerium oxide (CeO₂) slurry. Taguchi's L9 orthogonal array is used for the design of experiments. Abrasive concentration, pressure and overarm speed are considered as variable process parameters. Polishing is performed for duration of 120 minutes for each combination of parameters. Material removal is measured using the precision weighing balance. Surface roughness was measured using a Form Talysurf PGI 120 profiler. Abrasive slurry concentration is observed to be one of the most significant parameters in the optical polishing process. It affects both the material removal rate (MRR) and the surface roughness. Pressure applied at the workpiece–polisher interface affects the MRR, but the variation of pressure is not found to affect the surface roughness significantly. Relative motion at the workpiece–polisher interface is also observed to be significant in defining the final polishing outputs.     

Microscopic surface structure of C/SiC composite mirrors for space cryogenic telescopes

We report on the microscopic surface structure of carbon-fiber-reinforced silicon carbide (C/SiC) composite mirrors that have been improved for the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and other cooled telescopes. The C/SiC composite consists of carbon fiber, silicon carbide, and residual silicon. Specific microscopic structures are found on the surface of the bare C/SiC mirrors after polishing. These structures are considered to be caused by the different hardness of those materials. The roughness obtained for the bare mirrors is 20 nm rms for flat surfaces and 100 nm rms for curved surfaces. It was confirmed that a SiSiC slurry coating is effective in reducing the roughness to 2 nm rms. The scattering properties of the mirrors were measured at room temperature and also at 95 K. No significant change was found in the scattering properties through cooling, which suggests that the microscopic surface structure is stable with changes in temperature down to cryogenic values. The C/SiC mirror with the SiSiC slurry coating is a promising candidate for the SPICA telescope.     

Magnetorheological nanofinishing of BK7 glass for lens manufacturing

BK7 is a high-quality crown glass which is used where additional benefits such as temperature sensitive applications of fused silica glass are not required. Due to very low inclusion content with extremely low bubbles, BK7 glass can find its application in lens manufacturing. The present work focuses on nanofinishing of the BK7 glass specimen for ratifying its utility in practical application. A programmable logic controlled 3-axis motions are fed to the magnetorheological (MR) rotating tool for finishing the glass specimen. MR polishing fluid used for nanofinishing consists of deionized water, magnetic iron particles, and cerium oxide powder. Under the influence of magnetic field, the stiffened MR polishing fluid is assisted in reducing the surface roughness of glass up to nanolevel range. Optical properties such as transmittance, absorbance, and reflectance of finished BK7 glass are analyzed and found suitable for lens manufacturing. Results of higher surface quality with excellent finishing are obtained by the present MR finishing process. After 90?min of finishing, the surface roughness values R_a and R_q are reduced to 17 and 27?nm from the initial values of 41 and 57?nm, respectively. To study the surface morphology, scanning electron microscopy is performed on BK7 glass.     

抛光液组分对硬盘盘基片超光滑表面抛光的影响

在硬盘盘基片的最终抛光中,研究SiO2溶胶颗粒、氧化剂、络合剂和润滑剂对盘基片Ni—P材料抛光性能的变化规律,获得较高的材料去除速率（MRR）和原子级光滑表面,以满足下一代硬盘盘基片制造的更高要求．结果表明,采用平均粒径25nm的SiO2溶胶颗粒,易减少微划痕等缺陷;以过氧化氢为氧化剂,可大幅提高MRR;加入有机酸络合剂后,MRR显著增大,其中含有水杨酸的抛光液MRR最大,并讨论不同络舍剂对盘基片去除的影响机理;丙三醇润滑剂的引入,使抛光中摩擦系数减小,盘基片的表面粗糙度得到明显降低．采用原子力显微镜（AFM）、光学显微镜和ChapmanMP2000＋表面形貌仪来考察盘基片抛光后的表面质量,基于抛光液各组分的影响作用,最终获得表面粗糙度尺。为0．08nm的盘基片表面,且MRR达到0．132μm／min．     

Surface finish requirements for soft x-ray mirrors

We have examined the correlations between direct surface-finish metrology techniques and normalincidence, soft x-ray reflectance measurements of highly polished x-ray multilayer mirrors. We find that, to maintain high reflectance, the rms surface roughness of these mirrors must be less than ~ 1 ? over the range of spatial frequencies extending approximately from 1 to 100 μm(-1)1 (i.e., spatial wavelengths from 1 μm to 10 nm). This range of spatial frequencies is accessible directly only through scanning-probe metrology. Because the surface-finish Fourier spectrum of such highly polished mirrors is described approximately by an inverse power law (unlike a conventional surface), bandwidth-limited rms roughness values measured with instruments that are sensitive to only lower spatial frequencies (i.e., optical or stylus profileres) are generally uncorrelated with the soft x-ray reflectance and can lead to erroneous conclusions regarding the expected performance of substrates for x-ray mirrors.     

Antireflection coating with fluoropolymer for a novel organic nonlinear optical crystal: 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (ADPA)

Prior to the frequency doubling of a laser diode through a ring resonator, reflection loss on the surface of a novel organic second-order nonlinear optical crystal, 8-(4'-acetylphenyl)-1,4-dioxa-8-azaspiro[4.5]decane (APDA), was decreased. An optically flat crystal surface with a flatness of λ/10 (λ = 633 nm) and surface roughness of 1.5 nm rms was obtained when an ultraprecision diamond turning lathe was used to machine the crystal surface. After this surface was coated antireflectively with an amorphous fluorinated polymer, Teflon AF-2400, the transmittance at 810 nm was increased to 98%. A laser-diode-pumped ring resonator was built with this crystal, and a blue second-harmonic emission at 405.5 nm wassuccessfully generated.     

Analysis and optimization of parameters in optical polishing of large diameter BK7 flat components

Optical polishing and nanofinishing of glass substrates has gained impetus owing to the continuously increasing applications of precise optical components, but still, the process is elusive and skill dependent as several unaccounted parameters induce uncertainties. This study is an effort toward deterministic surface quality of glass substrates in polishing, based upon analysis and optimization of some pronounced parameters. In the experiments, BK7 glass samples (100?mm dia.) were polished for 3 hours using Ceria slurry on a conventional eccentric one arm spindle polisher employing a polyurethane pad. The experiment was designed as per Taguchi’s L9 orthogonal array for a comprehensive analysis. Three control parameters – relative velocity, normal force (pressure), and abrasive size – were varied and the resultant average roughness value (R_a), measured by a surface profilometer (Talysurf PGI-120), constituted the response. Kinematic analysis, using existing literature, yielded velocity and MALVERN ZETASIZER measured average abrasive size using dynamic light scattering principle. Relative velocity was observed to be the most significant parameter and normal force the least significant. Optimum combination was identified by deriving a deterministic relation for the best surface finish in the given time. R_a values as low as 6.8?nm in the experiment runs and 4.4?nm in the validation experiment were obtained.     

Fabrication of High Performance Laminated Polarization Splitters Consisting of Thick a-Si:H /SiO(x):H Multilayers

We report fabrication techniques ofa-Si:H/SiO(x):H multilayers having ample thickness and flat layer boundaries for high performance laminated polarization splitters (LPS's). In the new fabrication process we used the following techniques to achieve low stress and high surface flatness: SiO(x):H film deposition by rf sputtering with a mixture of Ar/H(2), two-step deposition using rf bias sputtering, and elimination of surface roughness and defects by mechanical polishing. This process enabled deposition of a multilayer as thick as 265 mum while preserving layer boundaries as flat as 1 nm (rms). As a result, LPS's having low loss, a large aperture, and a long splitting distance were successfully obtained. The high optical performance is applicable to functional devices integrated into fibers or planar waveguides.     

精细雾化抛光氮化硅陶瓷的抛光液配制参数优化

采用超声精细雾化施液抛光对氮化硅陶瓷基体进行抛光,研究了不同的pH值、磨料浓度以及氧化剂含量对氮化硅陶瓷基体抛光的材料去除率的影响,优化了pH值、磨料浓度及氧化剂含量,并与传统的化学机械抛光进行了对比。结果表明:当二氧化硅磨粒质量分数为5wt%,氧化剂含量为1wt%,pH值为8时,材料去除率MRR为108.24nm/min且表面粗糙度Ra为3.39nm。在相同的抛光参数下,传统化学机械抛光的材料去除率MRR为125nm/min,表面粗糙度Ra为2.13nm;精细雾化抛光的材料去除率及表面粗糙度与传统抛光接近,但精细雾化抛光所用抛光液用量仅为传统抛光所用抛光液用量的1/9。     

Topographic measurements of precision-ground optical glasses

We studied the surfaces of six precision-ground optical glass samples of fused silica, BK-7, BaCD16, LaC14, NbF1, and LHG8 glass, using a mechanical profiler, a photon tunneling microscope, and a scanning force microscope. The measured roughnesses of the scanning force microscope varied from 3.5 A rms for BK-7 glass to 30 A rms for LHG8 laser glass. Good agreement was obtained among the roughness values measured with all three instruments.     

Diffuse and specular reflectance from rough surfaces

We present a reflection model for isotropic rough surfaces that have both specular and diffuse components. The surface is assumed to have a normal distribution of heights. Parameters of the model are the surface roughness given by the rms slope, the albedo, and the balance between diffuse and specular reflection. The effect of roughness on diffuse reflection is taken into account, instead of our modeling this component as a constant Lambertian term. The model includes geometrical effects such as masking and shadowing. The model is compared with experimental data obtained from goniophotometric measurements on samples of tiles and bricks. The model fits well to samples with very different reflection properties. Measurements of the sample profiles performed with a laser profilometer to determine the rms slope show that the assumed surface model is realistic. The model could therefore be used in machine vision and computer graphics to approximate reflection characteristics of surfaces. It could also be used to predict the texture of surfaces as a function of illumination and viewing angles.     

Manipulating mechanics and chemistry in precision optics finishing

Deterministic processing is critical to modern precision optics finishing. Put simply, determinism is the ability to predict an outcome before carrying out an activity. With the availability of computer numerically controlled (CNC) equipment and sophisticated software algorithms, it is now possible to grind and polish optics from a variety of materials to surface shape accuracies of ∼20 nm peak-to-valley (p-v), with surface roughness values (measured on white light interferometers over 250 μm×350 μm areas) to sub-nm root-mean-square (rms) levels. In the grinding phase the capability now exists to estimate removal rates, surface roughness, and the depth of subsurface damage (SSD) for a previously unprocessed material, knowing its Young's modulus, hardness, and fracture toughness. An understanding of how chemistry aids in the abrasive-driven removal of material from the surface during polishing is also critical. Recent polishing process research reveals the importance of chemistry, specifically slurry pH, for preventing particle agglomeration in order to achieve smooth surface finishes with conventional pad or pitch laps. New sub-aperture polishing processes like magnetorheological finishing (MRF) can smooth and shape flat, spherical, aspheric and free-form surfaces within a few process iterations. Difficult to finish optical materials like soft polymer polymethyl methacrylate, microstructured polycrystalline zinc sulfide, and water soluble single-crystal potassium dihydrogen phosphate (KDP) can be finished with MRF. The key is the systematic alteration of MR fluid chemistry and mechanics (i.e. the abrasive) to match the unique physical properties of each workpiece.     

Loose abrasive slurries for optical glass lapping

Loose abrasive lapping is widely used to prepare optical glass before its final polishing. We carried out a comparison of 20 different slurries from four different vendors. Slurry particle sizes and morphologies were measured. Fused silica samples were lapped with these different slurries on a single side polishing machine and characterized in terms of surface roughness and depth of subsurface damage (SSD). Effects of load, rotation speed, and slurry concentration during lapping on roughness, material removal rate, and SSD were investigated.     

Optical profilometry of poly(methylmethacrylate) surfaces after reshaping with a scanning photorefractive keratectomy (SPRK) system

A prototype frequency-quintupled Nd:YAG laser was used with a scanning system to create, on poly(methylmethacrylate) (PMMA) blocks, ablations corresponding to a correction of 6 diopters of myopia by photorefractive keratectomy. The topography of the ablated samples was measured with an optical profilometer to evaluate the smoothness and accuracy of the ablations. The ablation depth was larger than expected. With a 50% to 70% spot overlap, large valleylike variations with a maximum peak-to-peak amplitude of 20 μm were observed. With an 80% spot overlap, the rms surface roughness was 1.3 μm, and the central flattening was 7 diopters. This study shows that optical profilometry can be used to determine precisely the ablation per pulse and the smoothness and accuracy of surface ablations. Knowing the exact ablation per pulse is necessary to produce a smooth and accurate corneal surface by scanning photorefractive keratectomy.     

Effect of interface structure correlation on magnetoresistance of Fe/Cr multilayers

Effect of interfacial roughness on giant magnetoresistance (GMR) in Fe/Cr multilayers has been studied. A set of samples is prepared by simultaneously depositing on a set of float-glass (FG) substrates with varying rms surface roughness. This causes the correlated part of the rms roughness to vary from sample to sample. Another set of specimen is irradiated with 200 MeV Ag ions in order to induce uncorrelated roughness at the interfaces. In both the cases morphological and other microstructural features of different multilayers remained similar, thus allowing one to separate the effect of interface roughness from that of morphological changes. GMR measurements on these multilayers show that increasing interfacial roughness causes GMR to decrease nonlinearly. It is found that the effect of uncorrelated part of the roughness is much stronger than that of the correlated part.     

Measurement of group refractive index wavelength dependence using a low-coherence tandem interferometer

A technique for the measurement of the group refractive index wavelength dependence of optical materials using a low-coherence tandem interferometer and a spectrometer is proposed. Four channeled spectra resulting from interferences of light beams from different pairs of optical paths are used for the calculation of optical path differences. The group refractive index wavelength dependence is calculated from these optical path differences generated from the sample under measurement. No a priori information of the geometric thickness of a sample is required. The wavelength dependence of the group refractive index of the samples BK7 parallel plate of 5.200 and 10.025 mm from 675 to 850 nm is experimentally measured with an accuracy of the order of 10(-5) and a repeatability of the order of 10(-9).     

Analysis of a scanning pentaprism system for measurements of large flat mirrors

The optical surface of a large optical flat can be measured using an autocollimator and scanning pentaprism system. The autocollimator measures the slope difference between a point on the mirror and a reference point. Such a system was built and previously operated at the University of Arizona. We discuss refinements that were made to the hardware, the alignment procedure, and the error analysis. The improved system was demonstrated with a 1.6 m flat mirror, which was measured to be flat to 12 nm rms. The uncertainty in the measurement is only 9 nm rms.     

Effect of Surface Roughness on Laser Induced Nonlinear Optical Properties of Annealed ZnO Thin Films

Variation of nonlinear optical properties with surface roughness of ZnO thin films deposited on corning glass substrates at different annealing temperature(TA) was reported.The films were characterized by X-ray diffraction(XRD),UV-Vis-NIR transmission and single beam z-scan technique using second harmonics of Nd:YAG laser.Surface morphology of the samples was investigated by atomic force microscopy(AFM).Surface roughness was found minimum(8.4 nm) for ZnO sample annealed at 450 ℃.The nonlinear optical properties(NLO) were found to be dependent on surface roughness and the highest value of third order nonlinear susceptibility(χ～(3)=4.3×10～(-7) esu) was obtained for ZnO at TA 450 ℃.     

Epitaxial growth of relaxed germanium layers by reduced pressure chemical vapour deposition on (110) and (111) silicon substrates

The growth of Ge on (110) and (111) oriented Si substrates is of great interest to enhance the mobility of both holes and electrons in complementary metal oxide semiconductor transistors. However, the quality of thick, relaxed Ge layers grown epitaxially on these surfaces is usually much lower than similar layers grown on (100) Si, resulting in both higher defect densities (i.e. threading dislocations and stacking faults) and rougher surfaces. In this work we have investigated the growth of Ge layers on (110) and (111) Si substrates by reduced-pressure chemical vapour deposition using a two temperature process. We have found that the combination of suppressing the Ge seed layer roughness and high temperature post-growth annealing can reduce the rms surface roughness of (110) Ge layers to below 2 nm and the threading dislocation density to below 1 × 10⁷ cm^− 2. Thick (111) Ge layers were found to exhibit a very high density of stacking faults, that could not be reduced by post-growth annealing and a higher rms surface roughness of around 12 nm, which was limited by the Ge seed layer.