Space optic instrumentation for the sun observation in the Extreme Ultraviolet (EUV) spectral region is often based on multilayer coating technology. Such coatings have not negligible bandwidth, and therefore, often the detected signal is due to the contribution of different very close spectral lines. In this work we present a study of innovative capping layer structures covering the multilayer coating able to improve the rejection of the unwanted lines and at the same time preserving the reflectivity peak at the working wavelength.
The Multi Element Telescope for Imaging and Spectroscopy (METIS) is a coronagraph onboard of Solar Orbiter. It will perform simultaneous observations at HeII Lyman-α line, HI Lyman-α line and in visible. To achieve such capability, instrument mirrors need to be coated by multilayer (ML) structures with high efficiency at all three spectral ranges. Coatings with higher performances with respect standard Mo/Si are desirable. An instrument prototype of METIS has just flown onboard of a NASA sounding rocket: in this case, optics were coated with Mg/SiC MLs. Better performances have been obtained in terms of reflectivity, but long term stability of this coating is an open problem. Moreover the harsh conditions of the environment met during the Solar Orbiter mission given by plasma particles and high temperature could affect the lifetime of the optical components on the long term. We present the design and reflectivity tests of multilayer structures in which performances improvement is obtained by the use of novel capping layers. All multilayers are tuned at 30.4nm line but the design also maximize the performances at 121.6nm and 500 – 650 nm visible range. Analysis of Solar Orbiter environment have been carried on in order to point out the main damaging sources for the nanostructures. Computer simulations with a devoted software have been performed to preliminary evaluation of the possible instabilities in multilayers. Experimental tests for investigating the effects of the thermal heating and particles bombardments in the reflectivity performances have been planned.
Pulsed DC reactive sputtering is a very interesting technique for coating applications. Reactive sputtering can give very dense layers, low stress of the deposited multilayer film, high reproducibility, very high hardness (up to 1200 Vickers hardness) with unbeatable high rates ideal for industrial applications. SP-100 is Satisloh reactive sputtering systems with only one target material but can deposit various film materials simply by using different gases such as argon, as well as the reactive gases nitrogen and oxygen. Silicon-oxides, silicon-nitrides and all kinds of silicon-oxy-nitrides (SiOx-SixOyNz-SixNy) with a refractive index range of 1.44-2.05 in the visible range can be obtained. In the reactive sputtering the material it is usually deposited in the so called “transition mode” where it must be found the correct equilibrium point between the target voltage and the reactive gas flow. The transition mode assures a dense film with a stable rate. Condition to find such equilibrium point is given by the so called “material hysteresis” in which the target voltage is depicted in function of the reactive gas voltage. The hysteresis and the consequent equilibrium point are strongly depended by the power supplied to the target and the inert gas (argon) flow which could affect the optical characteristics and the deposition rate. We checked the refractive indexes of the SiOx and SixNy of very thin (1 QW Optical thickness at 520 nm) and thicker (3, 5 and 9 QW @520 nm) reporting how the different conditions can affect the refractive index and the deposition rate of the different materials.
SP-100 from Satisloh is the perfect coating machine for application in precision optics. Thanks to its innovative concept SP-100 can coat materials in a range of refraction indexes from 1.47 up to 2.05 in the visible (with all the intermediate indexes in between) and up to n=3.5 in the infrared by using only one target material. SP-100 is well suitable for application in the field of microscopy, laser optics, watches, optical filters, endoscopy, semiconductors and more. By replacing the target material the application range of the machine can be further extended. SP-100 is based on the reliable reactive bipolar Direct Current (DC) pulsed magnetron sputtering technology which guarantees high density of the deposited species, low stress of the deposited multilayer film, high reproducibility, very high hardness (up to 1200 Vickers hardness) with unbeatable high rates ideal for industrial applications. DC-pulsed sputtering assure less arc events and a lower heat load than Radio Frequency (RF) sputtering making SP-100 suitable for different substrates material and for cemented lenses. The small chamber of the SP-100 ensures very fast processes and a broadband AR can be coated in less than 15 minutes process time door to door. Thanks to its flexible substrate holder SP-100 can hold lenses of different sizes and shapes: from small size optics up to 100 mm diameter lenses, rod lenses up to 50 mm length or even glass fibers.
The next SOLO (SOLar Orbiter) mission will carry onboard the METIS (Multi Element Telescope for Imaging and
Spectroscopy) instrument which will perform broad-band and polarized imaging of the visible K-corona and
narrow-band imaging of the UV (HI Ly α, 121.6 nm) and EUV (He II Ly α, 30.4 nm) corona as well as in the visible
spectral range. Several multilayer optics with high reflectivity in the all ranges of interest have been studied. Since
SOLO will fly at the short distance from the Sun of 0.23 AU at its perihelion, a careful determination of the heat load
and the solar wind effect on the multilayers must be carried in order to check if degradation occurs.
To test thermal stability, a thermal analysis experiment has been conceived: the proposed multilayer structures, which are
based on different pairs of materials and different capping layers design, must be subjected both to heating and cooling,
reproducing the temperatures experienced in orbit. Reflectance in the EUV range of interest has been measured before
and after each treatment to verify possible degradation.
Silicon carbide (SiC) is an attractive material for EUV and soft X-ray optics. CVD-deposited silicon carbide (deposited
at 1400° C on Si substrate) is the best reflective material in the whole EUV interval (with about the 48% of reflectance at
121.6 nm). Despite of this, SiC thin films deposited with PVD techniques, such as magnetron sputtering, on silicon
substrate, do not have the same performances and they undergo to a degradation with time, probably because of some
stoichiometry reason (carbon rich). Depositing stable SiC with PVD techniques is crucial in building ML's, like Si/SiC
and SiC/Mg for soft X-ray applications (such space telescope and photolithography).
We deposited some preliminary samples using the Pulsed Laser Deposition (PLD) and the Pulsed Electron Deposition
(PED) techniques achieving a good reflectance in the whole EUV range (27% at near normal incidence at 121.6 nn) on a
silicon substrate. The higher energy involved in these deposition processes could lead to a film with a stoichiometry
much closer to the target one. The reflectivity of the deposited films has been measured at the BEAR beamline of the
ELETTRA synchrotron in Trieste (Italy) and the optical constants retrieved at six wavelength from 121.6 nm down to 5
nm.
A numerical method to design multilayer coating (ML) is presented. The mathematical tool is based on an "evolutive
strategy" algorithm which provides aperiodic solutions by maximizing input merit functions. It allows the optimization
of any kind of structures, comprising interlayers and capping layers, and modelling also inter-diffusion and interface
roughness. It has been applied to the design of MLs for different applications, as photolithography, space instrumentation
and short pulse preservation/compression. The optimization allows the control of the standing wave distribution inside
the ML. When the EUV radiation interacts with the structure, the superposition of the incident and reflected
electromagnetic wave generates a standing wave field distribution in the ML. An aperiodic design allows the regulation
of the distribution of this field, attributing specific properties to the ML. An experimental technique to recover standing
wave intensity on top of the ML is also cited. The technique is based on electron photoemission measurements, which
allow to determine both reflectivity as well as phase on top of ML. Thanks to this technique, both tests of the ML
performances compliance with expected theoretical ones and of degradation through time can be carried on.
B4C optical coating represents, together with Ir, Pt, SiC, one of best choice for high reflectance in the extreme ultraviolet
region (EUV 160-30 nm). This material is also used in multilayer for soft X-ray applications, such as Si/B4C or W/B4C,
or as interlayer in Mo/Si multilayer to avoid interdiffusion also because of its high thermal stability. Our work concerns
on B4C thin films deposited on Si [100] substrates by means of three different deposition techniques: RF plasma
magnetron sputtering, e-beam evaporation, pulsed laser ablation (PLD). We performed reflectance vs incidence angle
measurements from 5 nm to 152 nm on different samples deriving the optical constants with a least-square fitting
method. Complete films characterization have been carried out with compositional (XPS), structural (XRD) and
morphological (AFM, SEM) analyses.
In the extreme ultra-violet region, multilayer coatings are the only technique to obtain high reflectivity in normal incidence optical configurations. The interference process which regulates periodic multilayers behavior offers narrow-band spectral filtering without the use of additional filters, fact that makes these coatings particularly suitable for lines emission observations. Despite the large amount of possible materials combinations, Mo/Si multilayers are the standard choice for space research on plasma physics in the 13 - 30 nm spectral region. In this work Si/B4C is presented as an alternative material couple for the 30.4 nm selection. Attractive features are the better spectral purity and the second order reflectivity reduction. A possible application to the Sounding CORonagraph Experiment is described as an example. B4C thin films have been used to characterize this material in terms of optical constants in the 40 nm - 150 nm spectral region where, currently, only few data are available.
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