Proceedings Volume 5870

Advances in Thin-Film Coatings for Optical Applications II

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Proceedings Volume 5870

Advances in Thin-Film Coatings for Optical Applications II

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Volume Details

Date Published: 2 September 2005
Contents: 6 Sessions, 21 Papers, 0 Presentations
Conference: Optics and Photonics 2005 2005
Volume Number: 5870

Table of Contents

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Table of Contents

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  • Advances in Material Processing
  • Advances in Substrate Cleaning and Deposition Techniques
  • Advances in Optical and Process Monitoring
  • Advances in Ultraviolet Films
  • Advanced Thin Film Applications and Measurement
  • Poster Session
Advances in Material Processing
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Inorganic polarizing materials grown by physical vapor deposition
Ian Hodgkinson, Lakshman De Silva, Matthew Arnold
Currently there is a need for retarders that do not degrade under elevated temperatures and intense illumination for use in displays and devices such as rear projection televisions. A stack of inorganic planar layers with alternating high and low refractive indices behaves as a form birefringent uniaxial material, but the sign of the birefringence is always negative. In the presentation we outline the use of serial bideposition with 90 deg incremental substrate rotations to generate positive uniaxial materials with a typical difference of 0.12 between the ordinary and extraordinary refractive indices. Optical methods for displaying the axial symmetry of the coatings and for measuring the birefringence are described and contrasted with previous and current work on inorganic biaxial materials.
Double-handed circular Bragg reflection bands in chiral thin films
Andy C. van Popta, Jeremy C. Sit, Michael J. Brett
Titanium dioxide was evaporated onto rotating substrates at highly oblique deposition angles to create thin films exhibiting a nanostructure which resembles a polygonal helix. Abrupt, periodic rotations of the substrate were used to create triangle, square, pentagon, and star-shaped film morphologies. Experimental optical measurements show that polygonal-helix thin-films exhibit double-handed circular Bragg phenomena. Unlike a standard chiral filter, a polygonal-helix thin-film reflects left-handed circularly polarized light at one frequency band and right-handed circularly polarized light at a second frequency band. The relative wavelength-dependence of the reflection bands is controlled by the angular rotation between arms of a polygonal helix. Spectral-hole polarization filters, produced by adding twist and layer defects to a polygonal helix, are also reported. Twist-defects tend to produce a narrow passband within both circular Bragg reflection bands of a polygonal helix, while a spacing layer defect can be used to produce a passband within only one of the reflection bands.
Optical and thermal characterizations of AgSbTe chalcogenide-based thin films
Y. D. Sharma, P. Bhatnagar, T. Suhara
A systematic investigation of the Ag x -Sb 2 (1-x) -Te 3 (1-x) (x=0.16, 0.18 and 0.20) is reported. The alloy of Ag-Sb-Te glass system, obtained by rapid quenching technique has been characterized by calorimetric measurements and differential thermal analysis (DTA) for different heating rates. The optical properties of the material were studied using UV spectrophotometer. The optical band gap (E o) was estimated for these samples by measuring absorption coefficient as a function of wavelength of light in the range 300 -900 nm. The films of the different compositions of Ag were studied for both the cases of before and after annealing. The present study is an attempt to understand the effect of alloying Ag into amorphous Te chalcogenide glasses.
Advances in Substrate Cleaning and Deposition Techniques
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Ion-air gun cleaning of substrates prior to thin film deposition
It is a common practice to give cleaned mirror substrates, and other optical surfaces, a final "dusting" with high pressure Nz, or air, just prior to deposition of thin reflective coatings. The object of this procedure is to remove any particles that may have settled on the substrate since chemical cleaning and washing. Some types of "guns" used for blowing dust off a surface work well. However, other types, such as those containing a radioactive polonium source, can charge the substrate so that it attracts even more dust. In this report, the connection between electrostatic charge on a substrate and its dust "gettering" (dust collecting) ability is established. Also, test results are presented for the charging and discharging capabilities of various types, and manufacturers, of dusting guns.
Silvering substrates after CO2 snow cleaning
There have been some questions in the astronomical community concerning the quality of silver coatings deposited on substrates that have been cleaned with carbon dioxide snow. These questions center around the possible existence of carbonate ions left behind on the substrate by CO2. Such carbonate ions could react with deposited silver to produce insoluble silver carbonate, thereby reducing film adhesion and reflectivity. Carbonate ions could be produced from CO2 via the following mechanism. First, during CO2 snow cleaning, a small amount of moisture can condense on a surface. This is especially true if the jet of CO2 is allowed to dwell on one spot. CO2 gas can dissolve in this moisture, producing carbonic acid, which can undergo two acid dissociations to form carbonate ions. In reality, it is highly unlikely that charged carbonate ions will remain stable on a substrate for very long. As condensed water evaporates, Le Chatelier's principle will shift the equilibrium of the chain of reactions that produced carbonate back to CO2 gas. Furthermore, the hydration of CO2 reaction of CO2 with H20) is an extremely slow process, and the total dehydrogenation of carbonic acid is not favored. Living tissues that must carry out the equilibration of carbonic acid and CO2 use the enzyme carbonic anhydrase to speed up the reaction by a factor of one million. But no such enzymatic action is present on a clean mirror substrate. In short, the worst case analysis presented below shows that the ratio of silver atoms to carbonate radicals must be at least 500 million to one. The results of chemical tests presented here support this view. Furthermore, film lift-off tests, also presented in this report, show that silver film adhesion to fused silica substrates is actually enhanced by CO2 snow cleaning.
Gold coatings for cube-corner retro-reflectors
The Space Interferometry Mission (SIM) PlanetQuest is managed by the Jet Propulsion Laboratory for the National Aeronautics and Space Administration. SIM requires, among other things, high precision double cube-corner retroreflectors. A test device has recently been fabricated for this project with demanding specifications on the optical surfaces and gold reflective coatings. Several gold deposition techniques were examined to meet the stringent specifications on uniformity, optical properties, micro-roughness and surface quality. We report on a comparative study of optical performance of gold films deposited by resistive and e-beam pvaporation, including measurements of the scattering from the coated surfaces. The effects of oxygen bombardment and titanium under-layer on optical properties and adhesion were evaluated. The influence of surface preparation on the optical properties was examined also.
Improved AR coating reflection color uniformity and stability using multiple anodes and fuzzy logic control in DC reactive sputter coating
DC reactive magnetron sputter techniques have been developed for the application of antireflection (AR) coatings to plastic ophthalmic lenses. The method described here uses the Deposition Sciences Inc. (DSI) MicrodynTM process in which the lenses are mounted on a spinning drum adjacent to two 40" long planar magnetron cathodes. Improvement in deposition uniformity along the length of the cathodes was accomplished by the use of multiple active anodes. Improvement in run to run stability was accomplished by use of fuzzy logic control techniques. Details of this Multiple-Anode Fuzzy-Logic (MAFL) process will be discussed in relation to the significant improvement in control of reflectance color of anti-reflection coated eyeglass lenses. For a specific anti-reflection coating, control of color coordinate a* was improved by a factor of 2.5 and b* by a factor of 5.0
Atomic layer deposition of TiO2 / Al2O3 films for optical applications
Atomic layer deposition (ALD) is an important technology for depositing functional coatings on accessible, reactive surfaces with precise control of thickness and nanostructure. Unlike conventional chemical vapour deposition, where growth rate is dependent on reactant flux, ALD employs sequential surface chemical reactions to saturate a surface with a (sub-) monolayer of reactive compounds such as metal alkoxides or covalent halides, followed by reaction with a second compound such as water to deposit coatings layer-by-layer. A judicious choice of reactants and processing conditions ensures that the reactions are self-limiting, resulting in controlled film growth with excellent conformality to the substrate. This paper investigates the deposition and characterisation of multi-layer TiO2 /Al2O3 films on a range of substrates, including silicon <100>, soda glass and polycarbonate, using titanium tetrachloride/water and trimethylaluminium/water as precursor couples. Structure-property correlations were established using a suite of analytical tools, including transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), X-ray reflectometry (XRR) and spectroscopic ellipsometry (SE). The evolution of nanostructure and composition of multi-layer high/low refractive index stacks are discussed as a function of deposition parameters.
Advances in Optical and Process Monitoring
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Ellipsometric monitoring of multilayer coatings
The spectral performance requirements of optical thin film coatings continue to be increasingly demanding. Optical thin film design programs are now so sophisticated that they can meet the majority of these specifications. However, producing these designs is often very difficult. Many of the resulting designs have many layers (>50) of unequal optical thicknesses (non-quarter-waves) including both very thin layers (<5 nm) and thick layers. The tolerances on layer thicknesses and refractive indices are usually very tight (<1 nm per layer). Techniques for the monitoring of optical coatings range from crystal oscillator thickness and rate monitoring, time monitoring for sputtered films, single- wavelength turning point monitoring (particularly narrow-band interference filters such as DWDMs), and multiwavelength monitoring for multilayers having complex designs with possible real time re-optimization. This presentation will describe the use of ellipsometry for optical monitoring. Advantages of the technique are that both refractive index and thickness are measured during deposition, meaning that accurate design re-optimization is possible after each layer is terminated. Very thin layers can be measured accurately, and unlike photometric monitoring the technique has high sensitivity for all layer thicknesses. Examples will be given that illustrate the advantages of ellipsometric monitoring, including a laser notch plus band-blocker filter and a very broadband antireflection coating.
Usage of total reflection for optical quality control of anisotropic thin films
Alisa Konstantinova, Konstantin Konstantinov
The problem of non-destructing quality control of anisotropic thin films is presented. Usage of total reflection from anisotropic thin films as an instrument of optical quality control is considered. It is shown, that several effects, namely multiple reflection, total reflection and anisotropy of the film influence on the reflection. Examples of total reflection from uniaxial thin films are presented. It is shown that small changes in optical properties of the films may substantially influence on the reflection. It was suggested that effects of media anisotropy coupled with total reflection and multiple reflection may result in sharp change of reflectivity for small changes of optical properties and /or film thickness and / or angles of orientation of media axes for thin films. It is suggested that effect of total reflection from thin films on a substrate may be used for determining optical properties of thin films and for controlling orientation of the optical axes within such films and /or thicknesses of such films.
Double optical monitoring of time-dependent film formation
Alexandre F. Michels, Thiago Menegotto, Hans-Peter H. Grieneisen, et al.
A brief overview of optical monitoring for vacuum and wet bench film deposition processes is presented. Interferometric and polarimetric measurements are combined with regard to simultaneous real-time monitoring of refractive index and physical thickness. Monitor stability and accuracy are verified with transparent oil standards. This double optical technique is applied to dip coating with a multi-component Zirconyl Chloride aqueous solution, whose time varying refractive index and physical thickness curves indicate significant sensitivity to changes of film flow properties during the process.
Virtual deposition plant
A general structure of the software for computational manufacturing experiments is discussed. It is shown that computational experiments can be useful for checking feasibility properties of theoretical designs and for finding the most practical theoretical design for a given production environment.
Advances in Ultraviolet Films
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Correlation between mechanical stress and optical properties of SiO2/Ta2O5 multilayer UV narrow-bandpass filters deposited by plasma ion-assisted deposition
Jue Wang, Robert L. Maier
Multilayer SiO2/Ta2O5 UV narrow-bandpass filters were deposited by a plasma ion-assisted process. The optimized PIAD process leads to densified multilayer coatings with a stabilized build-in compressive stress and un-shifted center wavelength. The correlation between stress reduction and center wavelength upward shift was established via post-deposition annealing at temperature ranging from 120 oC to 500 oC. Following 300 oC annealing, increased porosity and physical thickness of single layers of Ta2O5 and SiO2 were observed, via EMA modeling of ellipsometric data acquired around the quasi-Brewster angle. This is consistent with AFM measurement. The CWL upward shift was attributed to the micro-structural changes originating from intrinsic stress relaxation. Good agreement between the calculated CWL shift based on single layer test, and measured total CWL shift of the UV NBF suggests that the multilayer interfacial coupling effects might be ignorable for CWL shift and stress calculations.
Effects of ion assist and substrate temperature on the optical properties and microstructure of MgF2 films produced by e-beam evaporation
Cheng-Chung Jaing, Ming-Hua Shiao, Cheng-Chung Lee, et al.
Magnesium fluoride thin films were prepared by electron-beam evaporation and ion-assisted deposition. The effects of ion assist and substrate temperature during the deposition on the optical properties and microstructure have been studied. The grain size, the crystallinity and the surface roughness of MgF2 films prepared without IAD all decreased with the decrease of substrate temperature. The MgF2 films deposited with IAD had small grain size, rough surfaces, fluorine deficiency and large optical loss over the 200-500nm wavelength region due to argon-ion bombardment.
Comparison of silicon oxide films deposited by RF ion beam sputtering and e-beam gun evaporation in visible to UV ranges
Silicon oxide thin films were deposited by RF ion beam sputtering and E-beam gun evaporation from two different starting materials, silicon and silica. The properties were studied by using the UV-Visible spectrometer, Fourier transform infrared spectrometer (FTIR) and atomic force microscopy (AFM). The refractive indices, extinction coefficients, surface roughness and molecular bonding structure of these films were discussed for visible to UV range application.
Advanced Thin Film Applications and Measurement
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Low mechanical loss coatings for LIGO optics: progress report
Roger P. Netterfield, Mark Gross, Fred N. Baynes, et al.
A significant limiting factor on the sensitivity of interferometric gravitational wave detectors has been identified as thermal noise generated by mechanical loss in the high reflectivity dielectric mirror coatings on the test masses. The development of coatings which maintain high optical performance and minimize mechanical loss is therefore vital if the current designs of interferometers are to achieve adequate sensitivity. While the origins of the mechanical loss are yet to be fully elucidated, some progress has been made toward minimizing it, although there is still some way to go before specifications can be met. The work reported here is progress made toward achieving low mechanical loss coatings on behalf of the LIGO consortium. The current directions include attempts to reduce the loss in the coating materials by control of the coating stoichiometry and intrinsic stress. This includes such methods as ion bombardment of the growing films and optimization of post-deposition thermal treatments.
Low-loss gratings for next-generation gravitational wave detectors
By combining electron beam lithography with coating processes, very shallow gratings with diffraction efficiencies between 0.02% and 7% have been realized. Advantages and disadvantages of different layout concepts for such gratings, their fabrication regime and measurements of their scattering will be discussed. The application of such gratings as coupling components to a reflective Fabry-Perot cavity resulted in a finesse of more than 400.
Angular and spectroscopic ellipsometry of ion bombarded surface layer
L. V. Poperenko, P. B. Kosel, M. V. Vinnichenko
The results were analyzed and systematized and approaches were elaborated to nondestructive monitoring of the surface ion modification. The damage profiles were simulated for metals (Cu, Al, Mo, stainless steel) being irradiated by different ions (H+/D+, Ar+, metal ions) within wide energy range (E=1 keV - 1 MeV). Optical properties of mono-, polycrystalline and amorphous materials were studied by means of angular (wavelength λ=632.8 nm) and spectral ellipsometry (probing photons energy range 0.05-5 eV). The surface of metals were also probed by Auger electron spectroscopy, scanning electron microscopy and atomic force microscopy. As a result of IR ellipsometric studies it has been revealed that treatment of the polycrystalline Ti surface by the mixed Ti+ and C+ ion beam caused much stronger changes of its subsurface optical properties that treatment of polycrystalline Mo by the same ion beam, because the bombarding ions (Ti+) have an affinity with Ti surface atoms. The treatment resulted in formation on Ti surface disordered layer with non-Drude-like behavior of the optical properties. Irradiation of Mo surface by the same ion beam induced the changes of free electron scattering mechanism while optical conductivity remains Drude-like.
Poster Session
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Dielectric coatings for customised tunable lithium niobate filters
Narrow bandpass Fabry-Perot etalons are widely used in solar astronomy for spectroscopic imaging. Solid electro-optically tunable filters made of thin, single-crystal lithium niobate are presented in this article. The pass-band is typically ~0.02nm at 550nm. We describe customized corrective and high-reflectivity optical coatings designed and manufactured to tailor the filter for the specific application. Spectral reflectance is calculated to satisfy wavelength requirements and to achieve optimal optical performance. The measured optical thickness of the lithium niobate wafer is an important factor in determining the optimal design of the etalon mirrors. Out-of-band rejection and bandwidth requirements are also taken into account, as well as the influence of the spectral properties of a high-order filter which blocks adjacent etalon orders. Design customization is particularly important in the case of tandem and double-pass etalons.
Hafnium oxide for optical applications deposited by different CMOS compatible methods
Matthias Albert, Thorsten Roessler, Barbara Adolphi, et al.
Hafnium oxide is a promising candidate for electronic applications. It also offers interesting properties for a wide variety of optical applications as antireflective coatings, dielectric mirrors or protective coatings. Besides favorable optical properties, the mechanical stability and chemical inertness of hafnium oxide offers further advantages. Microelectronics require ultra thin layers in the 3-5nm range for gate dielectrics. For optical applications a wider range is necessary. As optical coatings should be capable to be integrated in a CMOS or MEMS technology, only compatible deposition processes can be used. In this presentation we report on atomic layer deposition (ALD) for thicknesses in the 2-30nm range and r.f. sputtering from 30 to above 150nm. Thus almost every wavelength from EUV to NIR can be covered for λ/4 applications, keeping in mind, that the refractive index is 2.1 at 586nm. Deposition took place on 2x2 cm2 silicon substrates for ALD and on 150mm silicon wafers for sputtering each either HF etched of thermally oxidized. The layers have been analyzed by AFM, XPS, XRD, TEM to gather information about morphology, composition, bonding and structural properties. Optical properties have been evaluated by ellipsometry. The different deposition methods are compared as well as the effects of thermal annealing after deposition. All layers are very smooth and reveal optical properties close to bulk HfO2. As deposited the layers are predominately amorphous, thermal annealing leads to crystallization.
Design of transmission linear partial polarizers using a negative film-substrate system
It is possible to design a transmission linear partial polarizer (LPP) in the negative thin-film system (N1<√(0N2)) as a function of the film thickness and its refractive index, where N0, N1, and N2 are the refractive indices of the ambient, film, and substrate, respectively. This LPP can be designed to operate at more than one angle of incidence as well as over a range of angles, allowing it to be tunable. The design procedure is discussed, the relevant design equations are presented, and an example of an LPP design is given and its performance is analyzed.