High-reflective coatings are indispensable in order to manufacture mirrors with highest possible reflectivity. The maximum reflectivity can be achieved by all-dielectric coatings; however, the spectral bandwidth of these mirrors is limited. For astronomical applications metal based coatings (Al, Au, Ag) are commonly applied, as they allow high reflectivity and at the same time a broad spectral bandwidth.
The purpose of this paper is to present concepts for an improved control of plasma ion assisted deposition (PIAD) processes which are employed for the production of optical interference coatings. While the well established PIAD technique typically comprises methods for in situ monitoring of thin film properties, there is no detailed knowledge about plasma parameters which are the foundation of magnitude and stability of plasma assistance, however. We adopt optical emission spectroscopy (OES) and active plasma resonance spectroscopy (APRS) and present schemes for controlling radiance and electron density on a batch coater equipped with an Advanced Plasma Source (APS). In a repeatability experiment of a 5-layer quarterwave stack (QWS, SiO2/TiO2), characteristics of two plasma based control schemes are compared to those of a conventional approach. For the conventional process we find systematic drifts and shifts in time traces of monitored plasma paramaters which correlate to properties of the layer stack. By using the novel concepts, stability of plasma paramaters can be improved by a factor of up to 6, while repeatability of in situ QWS transmission is strongly enhanced, exhibiting no spectral shift and minimal variation in reflectivity.
Oxide coatings have been prepared by PIAD by means of a Leybold Syrus pro 1100 deposition system. Focus was placed on tantalum pentoxide, hafnium oxide, and zirconium oxide films. Coating characterization pursued the determination of the refractive index in the VIS/UV spectral regions, as well as the geometrical film thickness. In situ spectrophotometry was used to obtain information about the shift of the coatings. Measurements of the coating stress as well as EDX characterization have further been performed to complete the picture.
Results are presented which provide information about the correlation between deposition parameters (Assistance, choice of working gas) and the optical properties of the films. Correlations between optical and non-optical (stress, EDXresults) properties are further presented and discussed in terms of classical models. Reproducibility issues are discussed, too.
In the present study the preparation and characterization of Al2O3/AlF3 material mixture coatings is described. The main focus is on the correlation between the optical properties (refractive index, extinction coefficient), elemental composition and mechanical properties, depending on deposition conditions. The thin mixture films prepared with E-Beam gun evaporation with and without plasma ion assistance show a continuous change in the refractive index between the basic materials extrema (n=1.4 - 1.75), depending on the constituents volume filling factors. At the same time, extinction coefficients vary between less than 1x10-4 and 2x10-3. In addition, it can be shown that in dependency of the applied plasma ion assistance, the residual stress inside the mixture layers is tunable. Finally, several multilayer coatings has been prepared and investigated.
Optical coatings are an integral part of superior optical components. Astronomical applications (ground- and space-based)
place especially high demands on these coatings, not only with regard to their optical performance but also to
their mechanical and environmental stability, their thermal properties, and their radiation resistance. This article presents
a short overview of several coating solutions developed in recent years at Fraunhofer IOF in order to meet the
challenging demands of astronomical applications. The focus is placed on high reflective coatings for different
wavelength regions including coatings for the VUV range below 100nm, coatings for the DUV wavelength range above
100nm and VIS/NIR/IR coatings. Further, amorphous silicon layers will be introduced which can be polished to very
low roughness values and therefore can act as polishing layer for the manufacture of ultraprecise optical components
from metal substrates.
O. Stenzel, S. Wilbrandt, N. Kaiser, C. Schmitz, M. Turowski, D. Ristau, P. Awakowicz, R. Brinkmann, T. Musch, I. Rolfes, H. Steffen, R. Foest, A. Ohl, T. Köhler, G. Dolgonos, T. Frauenheim
The PluTO project is aimed at combining thin-film and plasma technologies. Accordingly, the consortium comprises
experts in optical coating (Laser Zentrum Hannover, Fraunhofer IOF) and such in plasma technology (INP Greifswald,
Ruhr University of Bochum RUB). The process plasmas available, especially the sheath layers, will be thoroughly
characterized by means of special probes, so that the types, numbers and energies of the particles participating in the
coating formation processes can be determined comprehensively in every detail for the first time. The data thus obtained
will provide a basis for a numerical modelling of layer growth at atomic scale (Bremen Center for Computational
Materials Science BCCMS). The results are expected to deepen the understanding of the physical mechanisms
responsible for the influence of plasma action on the layer properties. In parallel, suitable tools for process monitoring
will be identified and made available. Some first results have already been achieved which prove the viability of the
approach.
The present study deals with the characterization of hafnia, alumina, and zirconia coatings as well as mixtures
thereof with respect to applications in the UV. Emphasis is placed on optical properties, particularly on the
relation between UV refractive index and absorption edge. The shift of the coatings is investigated as well as the
mechanical stress. Finally, we present the results of stress measurements performed for quarterwave stacks
deposited on different substrates in a broad range of deposition temperatures. In this study, no systematic
dependence of the result of the stress measurement on the substrate material and geometry could be identified.
We report on a hybrid monitoring strategy, which makes use of quartz crystal monitoring and broadband optical monitoring data in application to the deposition of chirped mirrors for the near infrared spectral region. We present a short description of the basic monitoring concept, the experimental setup, and the data elaboration facilities of the developed optical monitoring system OptiMon. Although being flexible enough to be implemented into different
types of deposition system, we focus here on the application of our monitoring system for coating preparation with Advanced Plasma Source (APS) assisted electron beam evaporation. Chirped mirrors have been prepared using SiO2 and Ta2O5 as low and high index materials, respectively. The layers are characterized by in-situ transmission spectroscopy, ex-situ transmission and reflection spectroscopy, and white light interferometry to determine the group delay dispersion
GDD. Basing on characterization results, we demonstrate and discuss the relative benefits of the developed monitoring strategy.
In the case of plasma ion assisted deposition (PIAD) processes either quartz crystal monitoring or optical monitoring are commonly applied to control thickness of the layers. For several oxide layer materials the final stoichiometry of the deposited film is extremely sensitive to the oxygen gas inlet during the deposition process. It is well known, that under these circumstances, variations in the
reaction gas flow or in deposition rates may cause unwanted variations of the stoichiometry of the coating.
Finally this results in film inhomogeneities and increased absorption losses, which cannot be identified early
enough and reliably by in-situ transmission spectroscopy alone. For this reason, the correlation between optical
performance of the coating and emission spectra of the APS-plasma measured by a separate analyzer has been
investigated. The synchronization in recording in-situ transmission spectra and plasma emission spectra was
achieved by developing a common trigger unit for both spectrum analyzers.
From the correlation between spectrophotometry and emission spectroscopy, we expect an earlier and more reliable assignment of absorption losses and inhomogeneities to instabilities in the process parameters of the deposition process.
Inhomogeneous coatings are promising for superior optical properties, e.g. broadband antireflection, in comparison to conventional HL-stack designs. Although a lot of excellent theoretical work on optical behaviour of rugates and gradient index films has been done during the last decades, there is no real breakthrough in industrial fabrication. The realization of such coatings leads to an extensive and time-consuming computer-aided control, because of complicated layer designs with continuously changing refractive index gradients. We describe the design and optical performance of an omnidirectional antireflection coating that essentially represents a hybrid coating composed from homogeneous layers and linear refractive index gradient layers.
Inhomogeneous layers, such as so-called gradient index layers and rugate filters represent new and prospective thin film designs. Manufacturing such systems in practice requires calculation, deposition, monitoring and characterization of optical coatings with a well-defined continuous refractive index profile along an axis that is perpendicular to the film surface. Those coatings may be manufactured in the Leybold Syrus Pro 1100 deposition system by co-evaporation of SiO2 and Nb2O5 as a sequence of several refractive index gradients. During these experiments our in-situ broadband monitoring system was used to measure the transmittance of the growing film directly at the rotating substrate. This additional information on the intermediate stages of the not yet completed film are extremely helpful in reverse engineering tasks, and clearly superior to the extent of information that may be drawn from the spectra of the completed film only. For characterization of these coatings a new model was developed, which significantly reduces the number of parameters. To generate a feasible parameter set, deposition rates for both materials recorded with quartz crystals monitor during deposition were used. This approach achieves a better accordance between in-situ measured transmittance and modelled transmittance than the intended design. During the optimisation process, a local minimization algorithm was used to vary the refractive index profile of the whole coating and film thickness of the intermediate stages. Finally, a significantly improved accuracy of the modelled transmittance was achieved.
By comparing multilayer and rugate solutions to a typical rugate synthesis problem we demonstrate that rugate synthesis methods are still of a great importance. The new accurate algorithm for the synthesis of rugate coatings with arbitrary refractive index profiles is discussed. It is shown that this algorithm can be successfully applied for solving such classical rugate synthesis problem as reducing sidelobes accompanying stopbands of rugate filters.
We present and discuss the optical properties of thin film materials, which are inhomogeneous on the nanometer scale. Particular emphasis will be given to the specific optical behavior of the cluster matter. The cluster shape, cluster size, and the inter-cluster-distance control the latter. When the dielectric functions on the constituents are given, the influence of these spatial parameters on the optical properties may be understood in terms of the Generalized Mie Theorie (GMT), which provides a quantitative description to the optical behavior of such systems, no matter whether the cluster arrangement is preferable one-, two-, or three-dimensional. We will mainly focus on the case, when two-dimensional cluster aggregates are embedded into ultrathin solid films with a thickness of only a few nanometers. In this case, the film thickness appears as a third spatial parameters that is crucial for the optical behavior of the whole system. An appropriate choice of the material combination allows us to manipulate the optical film properties by even subnanometer changes of the mentioned spatial parameters. For instance, absorption line shifts for some hundred nanometers in wavelength may be achieved this way. As examples, amorphous hydrogenated carbon as well as metal island films in various environments will be discussed.
In contrast to 'conventional' algorithms for determining the otpical and geometrical characteristics of interference coatings, we present an alternative approach using artificial intelligent systems for reverse search tasks. The goal is to develop a neural network which is able to distinguish characteristics spectral features of optical thin films, such as specific interference pattern as well as absorption lines or edges. We demonstrate the application of neural networks to determine film thickness, refractive index and surface roughness of a thin film from the interference pattern of the specular reflectance spectrum in the near IR. For simplicity, in this case both film and substrate materials were assumed to be free of absorption losses. Such tasks may be solved using considerably simple neural networks containing up to 52 neurons for 128 spectral points. Current activities include the extension of the method to absorbing thin film system. We particularly emphasize the significance of a mathematical pre-processing of the spectral data in order to keep the network dimensionality as low as possible. Basing on the first result we strongly suggest that neural networks may be successfully applied for fast estimation of thin film thickness, roughness and optical constants and may consequently supply reliable initial values for any subsequent local minimum search procedure.
Thin copperphthalocyanine layers have been deposited on quartz glass substrates and investigated by means of transmission and reflection spectroscopy. The film thickness ranged between 20 nm and the subnanometer region. The determination of the optical constants allowed the estimation of the oscillator strengths for the relevant molecular transitions. A thickness dependence of the Q-band absorption maximum position could be established for layers with a thickness below 5 nm. The contributions of several physical mechanisms to such lineshifts are discussed.
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