Design and Fabrication of Soft X ray Suppermirrors

Hanlin Wang University of Chinese Academy of Sciences Haifeng Wang Changchun Institute of Optics, Fine Mechanics and Physics Shuai Ren Changchun Institute of Optics, Fine Mechanics and Physics Peng Zhou Changchun Institute of Optics, Fine Mechanics and Physics Xin Zheng Changchun Institute of Optics, Fine Mechanics and Physics Bo Chen Changchun Institute of Optics, Fine Mechanics and Physics Zhongjun Chen Institute of High Energy Physics Shuhu Liu Institute of High Energy Physics Xiaodong Wang (  wangxiaodong@ciomp.ac.cn ) Changchun Institute of Optics, Fine Mechanics and Physics


Main Text
In X ray region, the refractive index of all materials is near to unity, extinction coe cient is near to zero, which means that all materials are transparent. In normal incidence, X ray has extremely low re ectance. Thus, re ective optical system works in glancing mode in X ray region by virtue of total re ection of materials [1]. With increasing of energy, total re ection glancing angle decreases signi cantly, and this means that optical system ought to have a long size (tens or hundreds of meters magnitude), which leads to high cost, alignment di culty, and low collecting area. Supermirror is a good solution to enhance glancing incident angle [2][3][4]. It is composed of alternating two materials, each material has different thickness, and it has a good re ection in a broad waveband at a xed angle or in a broad angle range at a xed energy by Bragg diffraction law.
There are three kinds of methods to design supermirrors: power law method [3], numerical analysis method [4], and block method [2]. In block method, a supermirror is composed of several blocks, each block is a periodic multilayer, and it has a good re ectance at speci c waveband by Bragg diffraction law. Different block has different period thickness, and this will achieve a good re ectance at broad wavelength or angle range. The supermirror designed by block method is easier to be fabricated than other two methods because it has lower requirement of thickness control accuracy.
Supermirrors are widely used in space exploration [5][6][7]. W/Si suppermirrors were used in NuStar [5], Pt/C supermirrors were used in ASTRO-H [6] and NuStar, and Ir/SiC will be used in Athena [7]. In Athena, the working energy range is from 0.1 to 10keV, and the biggest glancing angle is 1.752 degree. However, they did not give the details of multilayer design, and it seems that their theoretical and experimental results were not so good [7].
In this paper, W/Si, Ir/SiC, and Pt/C supermirrors were designed by block method and fabricated by magnetron sputtering method. The glancing incident angles are 1.0, 1.4 and 1.7 degree.
Supermirrors designed by block method are easy to be fabricated because they do not need high precision of control accuracy for deposition rate. Thus, we used block method to design supermirrors. As shown in Fig. 1, supermirrors designed by block method compose of several blocks, and one block is a periodic multilayer. Two alternating materials with xed thickness (dH, dL) constitute a period with a period thickness of d in the periodic multilayer, and two materials have high (H) and low (L) refractive index (n). One block has N periods, and this block has a high re ectance at xed wavelength based on Bragg diffraction law. Bragg diffraction law is de ned as Eq. (1), where θ is glancing incident angle, λ is wavelength, d is periodic thickness. Since with increasing of energy, absorption decreases, to reduce absorption, we put long-wavelength block near to air. That is to say, λ5>λ4>λ3>λ2>λ1, and d5>d4>d3>d2>d1, N5<N4<N3<N2<N1, and total thickness of Block5<Block4< Block3<Block2< Block1. The details about block method can be found in Ref.2 and 8. W/Si, Ir/SiC, and Pt/C supermirrors were designed by block method. The glancing incident angles are 1.0, 1.4 and 1.7 degree. The substrate is fused silica, and the roughness of the substrate and lms is assumed to be 0.45 nm. To resist space high-energy particles, we put metal layer as the nal one near to air. It was reported that Ir and Pt have good stability when exposed to space environment, especially atomic oxygen [9][10][11].      [12], Cr layer was deposited on the subsstrate prior to Ir layer deposition.
Re ectance of supermirrors were measured at Beijing Synchrotron Radiation Facility (BSFR). Re ectance at 2.0-5.5 keV was measured at 4B7A station, and re ectance at 6-10 keV was measured at 4B9A station. As shown in Fig. 3a, two axuv detectors were used, rst one was used to detect intensity of incident light, and second one was used to detect re ecting light from supermirrors after rst one moved away from optical route. First detector was electronically controlled, and measurement was automatically conducted. The step was 0.02 keV. To assure that light beam beats effective area of supermirrors, the widths of slit were manufactured by laser lithography to be 0.05mm for 1.0 degree, 0.16 mm for 1.4 degree, 0.26 mm for 1.7 degree, respectively. The base noise is less than 20 pA. As shown in Fig. 3b, only one EDEc detector was used. Measurement was manually performed. The detector detected intensity of incident light, then the sample was put onto the stage, and detector moved to 2θ position to detect intensity of re ecting light from the supermirror. The step was 0.5 keV. Figure 4 shows experimental re ectance results of W/Si supermirrors. There is large deviation between experiment and theoretical calculation. The reason for this discrepancy is that high deposition rate leads to big thickness error. Figure 5 demonstrates experimental re ectance results of Ir/SiC supermirrors. At 1.0 and 1.4 keV, there is a good agreement between experiment and theoretical calculation. The shortcoming is that re ectance is close to zero at 6 keV. At 1.7 degree, there is a good agreement between experiment and theoretical calculation at 2-5 keV, but the agreement gets worse at 5.5-10 keV. As shown in Fig. 6, there is an excellent agreement between experiment and theoretical calculation for Pt/C supermirrors. The average re ectances are 18.19%, 9.39%, 4.87% at 5-10 keV for 1.0, 1.4, 1.7 degree, respectively. In Opto-mechanic alignment, mirrors are also required to have a good angular tolerance. That is to say, when there is a little angle deviation, the re ectance ought not decrease signi cantly. Figure 7 shows angular tolerance of Pt/C supermirrors at 7.0 keV. At 1.0 degree of supermirror, there is still larger than 10% of re ectance up to 1.5 degree, at 1.4 degree of supermirror, there is still 8% of re ectance up to 1.9 degree, and at 1.7 degree of supermirror, there is still larger than 5% of re ectance up to 2.2 degree. This will bene t opto-mechanic alignment.
In summary, W/Si, Ir/SiC and Pt/C supermirrors were designed by blocked method and fabricated. Pt/C supermirrors show excellent spectral performance, and the average re ectances are 18.19%, 9.39%, 4.87% at 5-10 keV for 1.0, 1.4, 1.7 degree, respectively. They still have a good angular tolerance, and this will bene t opto-mechanic alignment.
Declarations Figure 2 Theoretical re ectance curves of Pt/C supermirrors designed by block method at 1.0, 1.4, 1.7 degrees, and for comparison, results of Ir/SiC bilayer coating are provided.  Angular tolerance of Pt/C supermirrors.