Development of multilayer coatings (Ni/C-Pt/C) for hard x-ray telescopes by e-beam evaporation with ion assistance

Abstract

A number of X-ray astronomical missions of near future (XEUS, Constellation-X, SIMBOL-X, HEXIT-SAT, NEXT) will make use of hard X-ray (10-100 keV) optics with broad-band multilayer coatings. To this aim we are developing a multilayer deposition technique for large substrates based on the e-beam deposition technique, improved by the implementation of an ion beam assistance device, in order to reduce the interfacial roughness and improve the reflectivity. The e-beam deposition with ion assistance keeps the film smoothness at a good level and takes the advantage of a reduction of the interlayer stresses 11 . This approach is well suited for the manufacturing of highreflectance multilayer mirrors for hard X-rays space telescopes where, in addition to a high quality of the deposited films, a volume production is also requested. Moreover, we are also up-grading the replication technique by nickel electroforming, already successfully used for the gold coated soft X-ray mirrors of Beppo-SAX 1 , XMM 2 , JET-X/SWIFT 3 missions, to the case of multilayer coated mirrors. In this paper we will present the technique under development and the implemented deposition facility. Some preliminary, very encouraging, results achieved with the Xray (8.05 and 17.4 keV) and topographic characterization on flat samples will be discussed. SCOPE OF THE WORKThe extension to the hard X-ray band (10 -100 keV) foreseen in next focusing X-ray telescopes will mostly implement multilayer-coated mirrors. A suitable technique for the production of multilayer mirrors is an extension of the Nickel electroforming replication from shaped mandrels 4,5 . This approach is very attractive since, in addition to get very good optical performances, in the case of multimodular telescopes it implies a reduction of cost and integration time since the shells are monolithic and many elements can be replicated from the same master.In order to extend the Ni replication technique to the case to the case of multilayer coated mirrors, in the framework of a development project funded by ASI (the Italian Space Agency) which involves INAF/OAB, Media Lario S.r.l., LABEN, IASF, Politecnico di Milano, we are developing a multilayer deposition technique on mandrels based on the e-beam deposition. The choice of this kind of deposition is determined by the requirement of uniform coating of large surfaces, a feature which is achievable with difficulty with techniques like the ion-beam sputtering. We can, moreover, reuse the same coating facilities installed at Media Lario, which were used to produce the mirrors of XMM by Nickel electroforming replication.

Analysis of microroughness evolution in X-ray astronomical multilayer mirrors by surface topography with the MPES program and by X-ray scattering

“…Another advantage of e-beam evaporation is the uniform coating over large surfaces, like mandrels used for mirror shell replication. The main drawback of this method is the evaporation rate instability; very good results were, indeed, obtained in terms of peak reflectivity 12 . Sputtering processes are suitable to deposit compact and uniform films with good optical and mechanical properties: multilayers deposited by sputtering (like sample C) exhibit a good smoothness and durability in time, an important requirement for space telescopes with long lifetime.…”

Microstructure evolution with varied layer thickness in magnetron-sputtered Ni/C multilayer films

“…They are also suited as monochromators for Cu Kα radiation in virtue of low divergence of the reflected beam and the superior suppression of Cu Kβ radiation678. Moreover, depth-graded Ni/C multilayers are used in astronomical telescopes in hard X-ray region910.…”

Comparative study of single-layer, bilayer, and trilayer mirrors with enhanced x-ray reflectance in 0.5- to 8-keV energy region

“…To address these problems, many authors have suggested covering heavy materials with a thin layer of a low-Z material, such as C or B 4 C [14][15][16][17] or even an organic compound (polyethylene 18 ). As shown in Fig.…”