Papers

ICMCTF2003 Session C1: Recent Advances in Optical Thin Films and Applications

Thursday, May 1, 2003 8:30 AM in Room Sunset

Thursday Morning

Start	Invited?	Item
8:30 AM		C1-1 Improved Uniformity and Stability of DC Reactive Sputter Coating using Multiple Anodes and Fuzzy Logic (MAFL) Control C.H. Burton (SOLA International) Plastic ophthalmic lenses for vision correction have traditionally been Anti-Reflection (AR) coated using e-beam techniques - especially those involving ion-assist in the last decade or so. Such e-beam techniques have much lower throughput in general than is obtainable from drum coaters using DC reactive sputter deposition and SOLA has led the way in the application of this latter technique to high volume AR coating in the ophthalmic industry. Such sputter techniques are very cost effective and capable of producing very durable optical thin film coatings on plastic lenses. To achieve large batches it is necessary to use long magnetrons positioned alongside a rotating drum on which may reside as many as ten rows of lenses. There is an inherent problem in obtaining sufficient "row to row" coating uniformity along the length of the target in this situation. The use of multiple active anodes to successfully address this "row to row" uniformity issue will be described. DC reactive sputter is also prone to "batch to batch" variations which derive from various sources including the variable water content of plastic lenses as well as contentious "disappearing anode" or similar effects. The successful use of a fuzzy logic control scheme to reduce "batch to batch" variation will also be described.
9:10 AM		C1-3 Single and Multilayer Optical Coatings using Closed Field Magnetron Sputtering D.G. Teer, J. Hamsphire, D.G. Gibson, J.M. Walls (Teer Coatings Ltd, United Kingdom) Magnetron Sputtering has a number of advantages for multilayer thin film deposition over conventional electron beam evaporation technology. The process operates at higher energy producing dense, stable coatings with outstanding durability. Deposition rates are stable allowing the process to be computer controlled using time only. The sputtering process is also ''cold'' making it suitable for use on the widest range of substrates including polymers, glass and semiconductors. This paper will describe a strategy that allows high quality, metal-oxide thin films to be deposited at high rates using the Closed Field Unbalanced Magnetron Sputtering Ion Plating process. The Closed Field process for optical coatings uses two or more different metal targets. The metal target is held in a partially oxidised state controlled using plasma emission monitoring and the oxidation occurs in the entire volume around the substrate carrier. The substrate carrier rotates at typically 50 rpm. Silicon dioxide is used as the low refractive index material and the high index layers can be chosen from the oxides of zirconium, titanium, zirconium, hafnium or tantalum. In addition to describing features of the process, this paper will also discuss the optical properties of individual layers and its application to a range of multilayer precision optical coatings and ophthalmic coatings. The ion current density and the low bias voltage provided by Closed Field magnetron sputtering produces films at a high rate with excellent optical properties. Examples of fully oxidised SiO₂, TiO₂ and other oxide films will be shown where the refractive index is close to bulk values with no absorption. Thin film thickness control can be accomplished simply using time or optical monitoring for more demanding applications. Applications include anti-reflection coatings, edge filters, cold mirrors etc..
9:30 AM		C1-4 Nb₂O₅, Ta₂O₅, WO₃, MoO₂ and ZrO₂ Optical Coatings for LAC-Applications Obtained by DC Reactive Magnetron Sputtering of Ceramic Sputtering Targets A. Schintlmeister, P. Wilhartitz (Plansee Aktiengesellschaft, Austria) Refractory metal oxides show interesting properties for optical thin film applications like high refractive indices and low absorption coefficients (Nb₂O₅, Ta₂O₅, ZrO₂) electrochromic behaviour (WO₃) or electrical conductivity (MoO₂). As most metal compound materials, such films are usually deposited by reactive magnetron sputtering of metallic sputtering targets. In order to obtain stochiometric films, sputtering has to be carried out in the compound mode, which is normally accomplished by low deposition rates and arcing. Moreover the sensitivity of the deposition process concerning instabilities in the oxygen content of the sputtering gas makes commercial large area applications very difficult. Within this study ceramic sputtering targets of Niobium-, Tantalum-, Tungsten-, Molybdenum- and Zirconium oxide were used to obtain stochiometric films by DC reactive magnetron sputtering. All target materials were almost fully compacted (density>99%) and showed specific electrical resistivities in the range of 1,0E+02 Ωcm to 1,0E-04 Ωcm. During deposition, the process turned out to be very stable regarding oxygen variations in the sputtering gas and arcing was never observed. Comparing to reactive sputtering of metallic targets, the deposition rates were significantly higher. The thin films which were obtained show excellent optical properties. Refractive indices, coefficients of absorption and film resistivities will be presented.
9:50 AM		C1-5 Plasma Deposition of Inhomogeneous Optical Filters Using TiO₂/SiO₂ Mixtures S. Larouche, A. Amassian, J.E. Klemberg-Sapieha, L. Martinu (École Polytechique de Montréal, Canada) Graded-index optical filters offer interesting characteristics difficult to obtain with traditional multilayer coatings; this includes elimination of harmonics and sidelobes and improved mechanical performance. Plasma-enhanced chemical vapor deposition (PECVD) appears very suitable to fabricate graded-index filters since the index profile can be controlled by varying the concentration of precursors during the deposition by an appropriate adjustment of the working gas mixture. In the present work, PECVD was used to deposit graded-index films from SiO₂/TiO₂ mixtures with refractive indices between 1.50 and 2.35. In the first part of this study, we investigate the relation between the optical properties of the coatings, determined by spectrophotometry and spectroellipsometry, and their microstructure studied by infrared spectroscopy and X-ray photoelectron spectroscopy. In the second part of this study, we investigate the properties of single- and multi-band rugate filters based on a dispersion-corrected design. We evaluate the performance of different optical filters for specific instruments and devices. We finally discuss the advantages of real-time in situ monitoring using spectroscopic ellipsometry.
10:10 AM		C1-6 Highly Reflective Uranium Mirrors for Astrophysics Applications D.D. Allred (Brigham Young University); S. Lunt (Optical Sciences Center, University of Arizona, Tucson); R.S. Turley, K.R. Adamson (Dept. of Physics and Astronomy, BYU) The reported optical constants of uranium differ from that of vacuum significantly more than other elements do over the range of about 150 to 350 eV. This suggests that uranium could be used to produce high reflectance imaging mirrors for many soft x-ray applications. Elemental uranium is too chemically active to be used as a front surface mirror without protection. We computed the expected reflectance of carbon-coated uranium films and of UO₂ and UN for low-angle reflectors. Carbon is mostly transparent below its K absorption edge at about 283 eV. The reflectance at 10 degrees from grazing is computed to be greater than 50% at 277 eV (CKα). For comparison, about 5 degrees is the maximum grazing incidence angle for which conventional materials are computed to have comparable reflectance. We sputter deposited and measured the reflectance of carbon-coated uranium layers at 4.47 nm (CKα). Sample reflectance was a factor of two greater than that of nickel, the material used for low-angle mirrors. We have also reactively sputtered uranium in an O₂/Ar to prepare UO₂ films. These also show high reflectance. Coatings based on uranium should be considered for all applications where high-reflectance, broadband, low-angle soft x-ray mirrors are required.
10:30 AM		C1-7 Demonstration of the Stierwalt Effect in Dielectric Filters Caused by Scatter from Induced Coating Defects J.D. Barrie, P.D. Fuqua, N. Presser (The Aerospace Corporation) Many applications of optical technology require the use of narrow bandpass filters with high levels of out-of-band rejection. Frequently, system designs require that the filter be placed in close proximity to a detector or focal plane array. Many years ago Stierwalt discovered that some filters that met out-of-band rejection specifications in a spectrophotometer did not meet specification when integrated into a focal plane assembly. In fact, he reported that proximal to the detector, one filter passed three orders of magnitude more out-of-band light. Since then, the Stierwalt Effect has become widely referenced, but few papers have been published regarding this effect. Many investigators assume that the effect originates with scatter in the films, but very little data has been reported. In this paper, we demonstrate that optical coatings with high levels of scatter resulting from introduction of model coating defects exhibit an enhanced Stierwalt effect as compared to otherwise identical coatings which do not contain these defects. The seeding was done by depositing sparse 1 micron polystyrene spheres upon a clean substrate before sputter depositing a simple band-stop filter. Light rejection from filters prepared in this way showed a strong dependence upon the distance between the film and the detector. Filters deposited without the spheres showed a much smaller effect.
10:50 AM		C1-8 Ultrasonic Cleaning Prior to PVD-coating of Precision-Optics and Glass-Substrates M.G. Ertl (ECE, Germany) The highest requirements on the results of cleaning are made in industrial manufacturing of optics and glass substrates. Only absolutely spot-free and particle-free surfaces can guarantee the demanded high coating quality. The actual cleaning technology applied is an aqueous ultrasonic process, based on biological decomposable cleaning chemicals with regard to the various types of substrate material. The main parameters influencing the cleaning performance are: ultrasonic power and frequency, types of cleaning chemicals, sequence of cleaning and rinsing steps, the water quality and the drying methods. During industrial manufacturing two methods of cleaning are to be distinguished: "Cleaning for Inspection" and "Cleaning for Coating" The first one is applied during manufacturing in between the various process steps to inspect and to evaluate the quality of surfaces after each step. The main types of contamination we meet are cement, resins, masking lacquers and polishing compounds. A fully automatic cleaning system is provided with the advantages of a controlled process and not-damaging the surfaces. The "Cleaning-for-Coating" is a final-cleaning with more easy removable types of contaminations but also capable to remove micro-particle size contaminations. Concepts for fully automated systems, easy to service and easy for maintenance are the solution for the future to meet the high demand of quality.
11:10 AM		C1-9 Effects of the Space Environment on a Multilayer Dielectric Filter P.D. Fuqua, N. Presser, J.D. Barrie, M.J. Meshishnek, D.J. Coleman (The Aerospace Corporation) Spaceborne optical telescopes involve optical elements that are directly exposed to the space environment. In one particular design, the exposed element could be a semiconducting lens that is coated with a thick dielectric stack for the purpose of solar rejection. Test coupons were placed in a space environmental effects simulator and subject to electron and proton bombardment. Following the exposure, damage was observed that was attributed to dielectric breakdown. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. We found that the typical size of discharge pits was 50-100 microns at the surface, extending to the substrate material, where a 10-micron diameter melt region was found. Proton irradiation alone does not cause pit damage, but it does seem to make materials more susceptible to pitting during electron irradiation. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film.
11:30 AM		C1-10 Effects of Annealing on the Structure and Laser-induced Damage Threshold of Laser Reflectors Y. Zhao (Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, PR China); Y.J. Wang, Hui Gong, J. Shao, Z. Fan (Chinese Academy of Sciences, PR China) The effects of annealing on structure and laser-induced damage threshold (LIDT) of mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering, then annealed in air under the temperature from 100°C to 400°C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the reflectors was measured by surface thermal lensing technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220µs. It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature above 250°C. Atomic-force microscopy was employed in determining its micro-roughness of the surfaces before and after different temperature annealing, and in mapping laser-induced damage morphology features after irradiation.
11:50 AM		C1-11 Optical Properties of Aluminium Oxide Thin Films Prepared at Room Temperature by Off-plane Filtered Cathodic Vacuum Arc System Z.W. Zhao, B.K. Tay, G.Q. Yu (Nanyang Technological University, Singapore); L.K. Cheah (NanoFilm Technologies International, Singapore); D. Sheeja, S.P. Lau, C.Q. Sun, P. Zhang (Nanyang Technological University, Singapore) An industrial off-plane filtered cathodic vacuum arc (FCVA) system has been developed to deposit large area (up to 8 inch) optical coatings with good uniformity controlled by scanning magnetic field system. Using this industrial FCVA system, aluminium oxide thin films with high deposition rate (1.5 ~ 0.5 nm/s) were deposited at room temperature under different oxygen partial pressures (4´ 10-5 ~ 3´ 10-4 torr) in Si (100) and quartz substrates. The films were dense, surface-smooth (RMS: ~0.1 nm) and good uniformity (±2%). The grown films were all amorphous with low stress (less than 0.5 GPa). The oxygen concentration and peak positions of various elements in the films were studied by AES and XPS, respectively. The optical properties, such as film transmittance, optical constants and absorption coefficient of aluminium oxide thin films under different oxygen partial pressure were investigated. The properties exhibited by the grown films manifest the potential applications for aluminium oxide thin films in optical coatings and for FCVA technology in deposition of metal oxide optical coatings.