We have systematically investigated the thermal and particle stability of several state-of-the-art EUV multilayer coatings suitable for use in high-performance solar instrumentation. Our research has been motivated principally by the performance requirements for extreme solar missions such as Solar Orbiter, an approved ESA mission with an expected launch date of 2013. The goal of this particular mission is to explore the solar atmosphere with both in situ and remote sensing instrumentation at a close encounter. At perihelion the mission will reach 0.2 A.U. providing a unique viewpoint where the instruments can both 'see' and 'feel' the dynamic atmosphere. But the orbit is technically challenging- no remote sensing instrument has been put in such close proximity to the Sun before. Furthermore, the thermal and particle environment will not only be severe but will suffer huge fluctuations as the elliptical orbit changes from 0.2 A.U. to 1.1 A.U. Several of the remote sensing packages on the strawman payload of the mission contain multilayer coatings, thus the stability of these coatings to the expected thermal and particle environment must be established. In this paper, we investigate the impact on the integrity of several candidate EUV multilayer coatings after long-term thermal annealing, and upon exposure to energetic protons and neutrons. In summary, we find no significant degradation in any of the multilayer samples tested. These results suggest that the multilayers we have studied can be safely used for Solar Orbiter or other extreme solar missions.
Scandium containing multilayers have been produced with very high reflectivity in the soft x-ray spectrum. Accurate optical constants are required in order to model the multilayer reflectivity. Since there are relatively few measurements of the optical constants of Scandium in the soft x-ray region we have performed measurements over the energy range of 50-1,300 eV. Thin films of Scandium were deposited by ion-assisted magnetron sputtering at Linkoping University and DC Magnetron sputtering at CXRO. Transmission measurements were performed at the Advanced Light Source beamline 6.3.2. The absorption coefficient was deduced from the measurements and the dispersive part of the index of refraction was obtained using the Kramers-Kronig relation. The measured optical constants are used to model the near-normal incidence reflectivity of Cr/Sc multilayers near the Sc L2,3 edge.
Cr/Sc multilayers have been grown on Si substrates using DC magnetron sputtering. The multilayers are intended as condenser mirrors in a soft x-ray microscope operating at the wavelength 3.374 nm. They were designed for normal reflection of the first and second order with multilayer periods of 1.692 nm and 3.381 nm, and layer thickness ratios of 0.471 and 0.237, respectively. At-wavelength soft x-ray reflectivity measurements were carried out using a reflectometer with a compact soft x-ray laser-plasma source. The multilayers were irradiated during growth with Ar ions, varying both in energy (9-113 eV) and flux, in order to stimulate the ad-atom mobility and improve the interface flatness. It was found that to obtain a maximum soft x-ray reflectivity with a low flux (Cr=0.76, Sc=2.5) of Ar ions a rather high energy of 53 eV was required. Such energy also caused intermixing of the layers. By the use of a solenoid surrounding the substrate, the arriving ion-to-metal flux ratio could be increased 10 times and the ion energy could be decreased. A high flux (Cr=7.1, Sc=23.1) of low energy (9 eV) Ar ions founded the most favorable growth condition in order to limit the intermixing with a subsistent surface flatness.
This paper describes a new method for improved determination of multilayer period using a soft x-ray reflectometer based on a line-emitting high-brightness water-window liquid-jet laser- plasma source. The use of line emission with well-known wavelengths allows accurate measurements of multilayer period without source monochromatization and calibration. By using a new multi-line data analysis procedure the multilayer period of W/B4C mirrors can be determined with an accuracy of 0.001 nm.