Feng Jiangtao, Huang Qiushi, Wang Hongchang, Yang Xiaowei, Giglia Angelo, Xie Chun, Wang Zhanshan
MOE Key Laboratory of Advanced Micro-Structured Materials, Institute of Precision Optical Engineering, School of Physics Science and Engineering, Tongji University, Shanghai 200092, People's Republic of China.
Diamond Light Source Ltd, Harwell Science and Inovation Campus, Didcot OX11 0DE, UK.
J Synchrotron Radiat. 2019 May 1;26(Pt 3):720-728. doi: 10.1107/S1600577519001668. Epub 2019 Apr 1.
Cr/C multilayer optics are a suitable choice for the tender X-ray range (1-4 keV) that covers the K absorption edges of P, S, Cl and 3d transition metals as well as the L absorption edges of 4d transition metals. In particular, these optics are studied in order to optimize the optical properties of collimated plane-grating monochromators. In this paper, the structure, stress and optical properties of Cr/C multilayers (fabricated using direct-current magnetron sputtering) with bi-layer number of 20 and the same period (about 11.64 nm) but different Cr thickness ratio (0.20-0.80) are investigated. Firstly, the grazing-incidence X-ray reflectivity at 8.04 keV was measured. These measurements were fitted assuming a multilayer structure with a four-layer and non-periodic model. Results and fitting show that interface widths increase with the Cr thickness ratio. The results obtained from X-ray diffraction at 8.04 keV were consistent with high-resolution transmission electron microscopy which showed an increase in grain size of the Cr layers. In addition, the stresses of the Cr/C multilayers have been measured and the results show that the stress value approaches zero when the Cr thickness ratio is about 0.45. The reflectivity of a Cr/C multilayer with Cr thickness ratio of 0.37 was measured and reaches 26.6% at 1.04 keV. The measured reflectivity matches very well with the predicted value using the four-layer and non-periodic model, which confirmed the viability of the prediction. Thus, the reflectivity at 1.04 keV of a Cr/C multilayer with different Cr thickness ratio was predicted and was found to drastically decrease when the Cr thickness ratio is larger than 0.37. It has been determined that a Cr thickness ratio value of 0.37 is the best choice for a Cr/C multilayer in view of high reflectivity and low stress.
Cr/C多层光学器件是适用于软X射线波段（1 - 4 keV）的选择，该波段涵盖了P、S、Cl和3d过渡金属的K吸收边以及4d过渡金属的L吸收边。特别地，对这些光学器件进行研究以优化准直平面光栅单色仪的光学性能。本文研究了双层数为20且周期相同（约11.64 nm）但Cr厚度比不同（0.20 - 0.80）的Cr/C多层膜（采用直流磁控溅射制备）的结构、应力和光学性能。首先，测量了8.04 keV下的掠入射X射线反射率。这些测量采用四层非周期模型的多层结构进行拟合。结果和拟合表明，界面宽度随Cr厚度比增加。8.04 keV下X射线衍射得到的结果与高分辨率透射电子显微镜一致，后者显示Cr层的晶粒尺寸增加。此外，还测量了Cr/C多层膜的应力，结果表明当Cr厚度比约为0.45时应力值接近零。测量了Cr厚度比为0.37的Cr/C多层膜的反射率，在1.04 keV时达到26.6%。测量的反射率与使用四层非周期模型预测的值非常匹配，这证实了预测的可行性。因此，预测了不同Cr厚度比的Cr/C多层膜在1.04 keV下的反射率，发现当Cr厚度比大于0.37时反射率急剧下降。鉴于高反射率和低应力，已确定Cr厚度比为0.37是Cr/C多层膜的最佳选择。
