For ground- and spaced based applications, Ag coated reflectors are indispensable because of their high reflectivity. The transport, assembling and storage of these reflectors takes a long time, before they are finally commissioned for the actual applications. To endure this period without a decrease of reflectivity, protective coatings with a final layer, which offers a high resistance to aqueous solutions and a low mechanical stress should be used. These criteria were taken into account for the selection of a final layer for a protected Ag-coating, which was applied for reflectors utilized in the CRIRES+- instrument (an IR spectrograph used at the VLT). Reactively sputtered Al2O3, SiO2 and Si3N4 layers were investigated with regard to these criteria. In aqueous (basic) solutions, the investigated Si3N4 layers are more stable than the SiO2 layers, and the SiO2 layers are more stable than the Al2O3 layers. This shows the influence of the intrinsic material properties. The mechanical stress of the sputtered layers depends on the deposition conditions and thus on the selected parameters. A Si3N4 layer with a high resistance to aqueous solutions also offers a low and stable mechanical stress. Therefore, the deposition-parameters which have been used for this layer were applied for sputtering the final layer of the protected Ag-coating for the reflectors.
Time durability and environmental stability of silver-coated glass mirrors improve if silver layer is protected by a transparent thin film coating. The choice of the protecting layer material and of the methods for mirror manufacturing influences the mirror optical and mechanical properties. This work reports on a systematic study of silver mirrors overcoated by silicon oxide, nitride and oxy-nitrides. Variable angle spectroscopic ellipsometry was implemented to get an insight on metal-dielectric interface of the coatings. The results have been analyzed considering the coating deposition conditions and physical-chemical properties of the dielectric materials used as protective layers.
Order sorting filters had to be coated for the CRyogenic InfaRed Echelle Spectrograph upgrade (CRIRES+)-instrument, a high-resolution IR spectrograph to be set up at ESO’s Very Large Telescope in Chile. Therefore SiO2 was chosen as material with low refractive index. Si and Ge have been investigated as materials with high refractive index, whereby Si has been chosen for the application of the coating. Three types of high-pass filters were deposited with transmission bands starting at 0.96μm, 1.47μm and 2.9μm. These filters need to block effectively all wavelengths between 0.5 μm and the respective band. Therefore, in the blocking range, an optical density above four, or above three for the filter starting at 2.9 μm respectively, had to be achieved. The filter-coatings also needed to survive thermal cycling down to 65K while only introducing a small wave front error. The lower total thickness, compared to coatings consisting of other materials, and the low film-stress are favorable properties for coatings deposited onto prisms and other more complex optical components.
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 optical system of the hyperspectral imager of the Environmental Mapping and Analysis Program (EnMAP) consists of a three-mirror anastigmat (TMA) and two independent spectrometers working in the VNIR and SWIR spectral range, respectively. The VNIR spectrometer includes a spherical NiP coated Al6061 mirror that has been ultra-precisely diamond turned and finally coated with protected silver as well as four curved fused silica (FS) and flint glass (SF6) prisms, respectively, each with broadband antireflection (AR) coating, while the backs of the two outer prisms are coated with a high-reflective coating. For AR coating, plasma ion assisted deposition (PIAD) has been used; the high-reflective enhanced Ag-coating on the backside has been deposited by magnetron sputtering. The SWIR spectrometer contains four plane and spherical gold-coated mirrors, respectively, and two curved FS prisms with a broadband antireflection coating. Details about the ultra-precise manufacturing of metal mirrors and prisms as well as their coating are presented in this work.
In order to manufacture mirrors metal based coatings (Al, Au and Ag) are applied, as they enable a high reflectivity and at the same time a broad spectral bandwidth. Of all metals, Ag provides the highest reflectivity from VIS to IR.
Silver is a noble metal. However, corrosion activators (e.g. S and Cl) can lead to corrosion. Thus, a protective layer is required to prevent the corrosion and sustain the high reflectivity of the mirror. However, damage of the Ag-coating can occur, even in the case of protected-Ag. Inhomogeneous film growth of the protective layer can lead to a permeation of corrosion activators and thus to a damage of the Ag. But also the deposition of impervious protective layers is not sufficient for long-term environmental stability. Hygroscopic air borne particles can weaken the protection and therefore subsequently lead to a permeation of corrosion activators and thus to a damage of the Ag.
These damage mechanisms lead to criteria for a durable and efficient protection. AlOxNy and nanolaminates have been tested with respect to these criteria. In particular the protection based on nanolaminates shows a great potential for the protection of Ag. In addition, also the optical performance can be improved by UV-enhancement based on different nanolaminates.
Metal mirrors are an attractive solution for scan mirrors working with ultra-short pulse lasers. Small mechanical inertia and a small mirror mass are required. Therefore, the mirrors have to be very stiff and a high quality optical surface has to be provided. This can be achieved with lightweight AlSi based mirrors with diamond-turned NiP polishable plating.
Different coating options were evaluated in order to provide the necessary high reflectivity and a satisfactory laser damage threshold for ultrashort laser pulses in the few ps to fs regime at λ = 1030 nm. High-reflective metal layers enhanced by dielectric HfO2/SiO2 stacks were found to be the most advantageous coating option due to their comparatively small thickness and measured damage thresholds above 1 J/cm2@8ps.
In the context of the conceptual design study for the European Solar Telescope (EST) we have investigated
different metallic mirror coatings in terms of reflectivity, polarization properties and durability. Samples of the
following coating types have been studied: bare aluminum, silver with different dielectric layers for protection
and UV enhancement, and an aluminum-silver combination. From 2009 to 2011 we have carried out a long-term
durability test under realistic observing conditions at the VTT solar telescope of the Observatorio del
Teide (Tenerife, Spain), accompanied by repeated reflectivity measurements in the EST spectral working range
(0.3 - 20 μm), and by polarization measurements in the visible range. The test results allow us to find the
optimum coatings for the different mirrors in the EST beampath and to eventually assess aging effects and
re-coating cycles. The results of the polarization measurements are a valuable input for an EST telescope
polarization model, helping to meet the stringent requirements on polarimetric accuracy.
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.
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.
As EUV lithography is on its way into production stage, studies of optics contamination and cleaning under realistic
conditions become more and more important. Due to this fact an Exposure Test Stand (ETS) has been constructed at
XTREME technologies GmbH in collaboration with Fraunhofer IOF and with financial support of Intel Corporation.
This test stand is equipped with a pulsed DPP source and allows for the simultaneous exposure of several samples. In the
standard set-up four samples with an exposed area larger than 35 mm2 per sample can be exposed at a homogeneous
intensity of 0.25 mW/mm2. A recent update of the ETS allows for simultaneous exposures of two samples with
intensities up to 1.0 mW/mm2. The first application of this alternative set-up was a comparative study of carbon
contamination rates induced by EUV radiation from the pulsed source with contamination rates induced by quasicontinuous
synchrotron radiation. A modified gas-inlet system allows for the introduction of a second gas to the
exposure chamber. This possibility was applied to investigate the efficiency of EUV-induced cleaning with different gas
mixtures. In particular the enhancement of EUV-induced cleaning by addition of a second gas to the cleaning gas was
studied.
We report on optics contamination rates induced by exposure to broad-bandwidth, high-intensity EUV radiation peaked
near 8 nm in a new beamline at the NIST synchrotron. The peak intensity of 50 mW/mm2 allows extension of previous
investigations of contamination by in-band 13.5 nm radiation at intensities an order of magnitude lower. We report nonlinear
pressure and intensity scaling of the contamination rates which is consistent with the earlier lower-intensity
studies. The magnitude of the contamination rate per unit EUV dose, however, was found to be significantly lower for
the lower wavelength exposures. We also report an apparent dose-dependent correlation between the thicknesses as
measured by spectroscopic ellipsometry and XPS for the carbon deposits created using the higher doses available on the
new beamline. It is proposed that this is due to different sensitivities of the metrologies to variations in the density of the
deposited C induced by prolonged EUV irradiation.
Comparative lifetime studies of Mo/Si multilayer mirrors have been conducted at the Exposure Test Stand (ETS) using a pulsed Xe-discharge EUV source at XTREME Technologies GmbH (Göttingen, Germany). Due to the large, homogeneous exposed sample area a multi-technique study of EUV induced carbon contamination and cleaning can be conducted using standard surface science techniques. EUV-reflectometry, X-ray photoelectron spectroscopy (XPS), small-angle X-ray reflectometry (SAXR), and Out-of-band (OOB) reflectometry (200 - 1000 nm) were applied to investigate exposed samples and study EUV-induced changes of the surface composition. With this approach the
influence of EUV-dose, cleaning-gas pressure and composition, and capping-layer material of the Mo/Si multilayer samples on the degradation and cleaning mechanism can be studied.
A high refractive index dielectric layer of thickness λ/4 on the backside of a metal layer can reduce the absorption losses of metallic beamsplitters considerably. This was published by H. Pohlack in 1953. The practical importance of this knowledge may even be greater nowadays, as the availability of sputtering techniques makes the combination of metallic and dielectric layers within an optical coating much easier. In this contribution it will be shown, that the inclusion of dielectric layers into a metallic beamsplitter can possibly have further benefit. For some applications, where achromatic transmission and/or reflectance are needed, an additional dielectric layer can enhance the performance of a metallic beamsplitter. As an example the coating of a beamsplitter cube will be shown. The projected application of this cube demands a high reflection and only a low transmission over a spectral range from 400 nm to 800 nm. Using a metallic beamsplitter coating enhanced by a high refractive index dielectric layer it is possible to fulfil this demand with a comparably simple coating and at the same time to obtain a constant transmission of about 5% over a large spectral range.
The source output power and lifetime, including the collector optics lifetime, are among the key issues for EUV lithography systems. In order to meet the requirements for the EUV collector mirror, both the reflectivity and the long-term thermal stability of its multilayer coating have been enhanced considerably during recent development efforts. Sub-aperture ellipsoidal mirrors of different substrate materials with outer diameters of about 320 mm were coated with
laterally graded high-temperature multilayers. The interface-engineered Mo/Si multilayer mirror (MLM) coatings were optimized in terms of high peak reflectivity at 13.5 nm and working temperatures above 400°C. Thin barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon and to provide long-term optical stability of the coating at elevated temperatures. A normal-incidence reflectance of R ~ 60 %
at 13.5 nm was measured on Si wafer samples after heating up to 600°C. No degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests and multiple annealing cycles. On highly polished collector substrates with improved surface roughness a reflectance for s-polarized light exceeding peak values of R = 57 % was obtained. With optimized layer gradient the degree of wavelength matching was improved, as well,
resulting in peak reflectivity values above 56 % throughout the clear aperture for a series of measurement points across the mirror. The corresponding area-weighted 2% in-band average reflectance for this collector mirror coating exceeds 52 % for unpolarized light.
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