Optical remote sensing of the earth from air and space typically utilizes several channels in the visible and near infrared spectrum. Thin-film optical interference filters, mostly of narrow bandpass type, are applied to select these channels. The filters are arranged in filter wheels, arrays of discrete stripe filters mounted in frames, or patterned arrays on a monolithic substrate. Such multi-channel filter assemblies can be mounted close to the detector, which allows a compact and lightweight camera design. Recent progress in image resolution and sensor sensitivity requires improvements of the optical filter performance. Higher demands placed on blocking in the UV and NIR and in between the spectral channels, in-band transmission and filter edge steepness as well as scattering lead to more complex filter coatings with thicknesses in the range of 10 - 25μm. Technological limits of the conventionally used ion-assisted evaporation process (IAD) can be overcome only by more precise and higher-energetic coating technologies like plasma-assisted reactive magnetron sputtering (PARMS) in combination with optical broadband monitoring. Optics Balzers has developed a photolithographic patterning process for coating thicknesses up to 15μm that is fully compatible with the advanced PARMS coating technology. This provides the possibility of depositing multiple complex high-performance filters on a monolithic substrate. We present an overview of the performance of recently developed filters with improved spectral performance designed for both monolithic filter-arrays and stripe filters mounted in frames. The pros and cons as well as the resulting limits of the filter designs for both configurations are discussed.
Optical remote sensing of the earth from air and space typically utilizes several channels from visible (VIS), near infrared (NIR) up to the short wave infrared (SWIR) spectral region. Thin-film optical filters are applied to select these channels. Filter wheels and arrays of discrete stripe filters are standard configurations. To achieve compact and light weight camera designs multi-channel filter plates or assemblies can be mounted close to the electronic detectors. Optics Balzers has implemented a micro-structuring process based on a sequence of multiple coatings and photolithography on the same substrate. High-performance band pass filters are applied by plasma assisted evaporation (plasma IAD) with advance plasma source (APS) technology and optical broad-band monitoring (BBM). This technology has already proven for various multi spectral imager (MSI) configurations on fused silica, sapphire and other substrates for remote sensing application. The optical filter design and performance is limited by the maximum coating thickness micro-structurable by photolithographic lift-off processes and by thermal and radiation load on the photoresist mask during the process Recent progress in image resolution and sensor selectivity requires improvements of optical filter performance. Blocking in the UV and NIR and in between the spectral cannels, in-band transmission and filter edge steepness are subject of current development. Technological limits of the IAD coating accuracy can be overcome by more precise coating technologies like plasma assisted reactive magnetron sputtering (PARMS) and combination with optical broadband monitoring (BBM). We present an overview about concepts and technologies for band-pass filter arrays for multi-spectral imaging at Optics Balzers. Recent performance improvements of filter arrays made by micro-structuring will be presented.
KEYWORDS: Coating, Ultraviolet radiation, Thin film coatings, Humidity, Optical storage, Silicon, Manufacturing, Standards development, Process control, Digital video discs
A spin coating technique for the 100 micron cover layer for Blu-Ray Disc production has been developed. The process includes a controlled manipulation of the viscosity of the lacquer with heat and UV light during spin coating. Stability and cost efficiency, two vital criteria for mass production, have been considered.
Masahiko Tsukuda, Eiichi Ito, Morio Tomiyama, Shinya Abe, Eiji Ohno, Stephane David, Martin Dubs, Chris Bayliss, Colin Dix, Nick Ogilvie, Simon Proffitt, David Scott-Maxwell, Bernard Fantin, Thomas Eisenhammer
A full-length 50GB dual layered BD-ROM disc was producted by improvement of EBR. We got sufficient track pitch deviation and jitters through whole radius.
Multilayer systems of thin silver films and dielectric layers are used for example in low-e windows. The optical properties of the silver films differ markedly from the optical properties of bulk silver and are influenced by the deposition of the top dielectric layer. Recently, we presented a dual-ion-beam sputtering apparatus with in-situ monitoring of reflectance and dc- resistance. The dielectic, Si3N4, was produced by sputtering silicon and assisting with an energetic nitrogen beam (200 eV). A sharp reduction in reflectance and a corresponding increase in dc-resistance of the silver film was observed during the first stage of the deposition. It was speculated that a thickness reduction of the silver film due to sputtering by the energetic nitrogen ions might cause some of the changes. In the present paper, we report measurements of film thicknesses by grazing incidence x-ray reflectometry (GIXR), which allow to exclude thickness reduction. We show that the optical absorptance of the sandwich system increases during deposition of the top dielectric layer. Different elements (Al, Si, Ta, and Ti) were sputtered onto the silver film without assisting, causing similar changes as dual- ion-beam sputtered Si3N4. Furthermore, Si3N4 was sputtered directly from a dielectric target. In contrast to the other materials, we observed only small changes in reflectance and dc-resistance.
Heat mirrors based on multilayers of silver and anti-reflective dielectrics can be used in solar process heat applications at elevated temperatures (approximately equals 250 degree(s)C). The optical properties of silver films depend on preparation conditions and differ from bulk material properties; especially in multilayer systems they are at present not well understood. An apparatus for simultaneous measurement of reflectance at one wavelength in the range 600 - 2000 nm and DC-resistivity during ion-beam sputtering of thin films is presented. The device is sensitive for changes in reflectance smaller than 0.001. Si3N4 / Ag / Si3N4 heat mirrors were produced on float glass. During growth of the silver film, reflectance initially decreases and then increases sharply. An interpretation with effective medium theories is presented. During deposition of the second dielectric layer, a sharp wavelength independent initial decrease in reflectance of approximately equals 2.5% is observed. After an increase of approximately equals 0.5% the reflectance decreases as predicted by Fresnel's equations. DC-resistivity closely follows this behavior. Different mechanisms for the observed effects are discussed.
Heat mirrors based on thin silver and dielectric films for thermal solar energy applications at elevated temperatures (200 - 300 degree(s)C) are studied numerically. The well known three layer design dielectric/silver/dielectric can be improved by additional anti-reflective films. Thicknesses and selection of dielectrics of these more complex designs are optimized with a genetic algorithm. At absorber temperatures above 200 degree(s)C and for bulk optical constants the optimal sequence with five layers contains two silver films: dielectric/silver/dielectric/silver/dielectric. This design is compared to optimal designs with four and five layers, but only one silver layer. The five layer design with two silver layers allows a reduction in hemispherical emittance in comparison to the design with only one silver layer. An improvement of 14% in the figure of merit at a solar radiant flux of 500 W/m2 and an absorber temperature of 300 degree(s)C is possible. However, optical constants of thin silver films differ substantially from bulk optical constants. Thickness dependent optical constants modeling experimental data were evaluated within a Drude model. The figure of merit is generally smaller than for bulk optical constants. For the design with two silver layers, the figure of merit is smaller than for designs with one silver layer, except at the highest investigated absorber temperature of 300 degree(s)C.
Smooth TiNxOy-Cu absorbers show low hemispherical emittancies (epsilon) < 0.05 at 250 degree(s)C and poor absorptances of < 0.08, when choosing optimal thickness and composition. Thin TiNxOy films were deposited on rough copper substrates in order to increase the absorptance. Specular and diffuse reflectance measurements of these coatings are compared with measurements on smooth films. An increase of 0.05 ... 0.1 in absorptance can be achieved. With moderate roughnesses (delta) 01 approximately equals 50 ... 60 nm, the absorptance can be increased to values above 0.85 without increasing the emittance significantly. Thin films of approximately equals 30 nm and moderate roughness (delta) 01 equals 55 nm should be favored against thick films because the emittance is lower at high temperature. A theoretical model is presented, explaining the experiments for not too rough interfaces. This model is suitable to be used for optimizations, provided the temperature dependence of optical constants is taken into account. Coatings on very rough substrates (delta) > 150 nm can achieve absorptances of more than 0.9 but accompanied with emittancies greater than 0.1. The interference peak at short wavelengths, typical for TiNxOy-Cu, cannot be suppressed completely; (alpha) is limited by this interference peak. For high temperature applications roughnesses greater than 60 to 80 nm are of no use.
TiNxOy films were deposited by activated reactive evaporation (ARE) on copper to produce a tandem solar absorber with a low emittance of 0.042 at 153 degree(s)C. The influence of oxygen and nitrogen partial pressures and the plasma current in the ARE-process were investigated. The optical selectivity of the absorber is determined mainly by the thickness and oxygen content of the coating. Increasing the oxygen partial pressure leads to higher oxygen content, lower emittance, and dielectric-like properties, i.e., small imaginary parts of the refractive index and small dispersion. Other preparation parameters are shown to have minor importance on the selective properties. The effective cut-off wavelength of the transition from high short wavelength absorptance to low long wavelength emittance can be shifted between 0.5 and 2 micrometers by changing either the thickness from 17 to 110 nm or the nitrogen to oxygen pressure ratio in the reactive gas from 1 to 200. The slope of the transition is affected by the pressure ratio.
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