Cheng Xianchao, Zhang Lin
LCLS, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.
J Synchrotron Radiat. 2019 Jan 1;26(Pt 1):234-237. doi: 10.1107/S1600577518015047.
Hard X-rays with energies higher than several kiloelectronvolts can be focused to spot sizes below 10 nm with the present synchrotron beamlines, offering unique advantages for the chemical, elemental and structure analysis of matter. Nevertheless, a surface precision on the nanometre scale for the focusing optics is required and remains the main hurdle limiting X-ray analytical techniques with single-nanometre spatial resolution. On the other hand, to preserve the wavefront properties of coherent X-ray beams, precise control of the reflective mirror surface quality at the nanometre scale is demanded for X-ray free-electron laser applications. In this work, the surface shape of a multilayer-coated X-ray mirror is controlled by layer stresses. The desired surface profile of the mirror is differentiated to its second order to obtain its corresponding curvature profile. With a step size of 1 mm along the mirror length, different coating thicknesses are applied to create different layer thermal stresses from uniform temperature change. The mirror surface profile can be obtained by integrating the curvature profile to its second order and further corrected by moving constant values for the slope and height. The technical process is simulated by finite element analysis (FEA). A case study showed that the residual slope error and the residual height error between the desired shape and the FEA result are 0.22 µrad (r.m.s.) and 1.42 nm (r.m.s.), respectively.
能量高于几千电子伏特的硬X射线，利用目前的同步辐射光束线可聚焦到尺寸小于10纳米的光斑，这为物质的化学、元素和结构分析提供了独特优势。然而，聚焦光学器件需要纳米级的表面精度，这仍然是限制具有单纳米空间分辨率的X射线分析技术的主要障碍。另一方面，为了保持相干X射线束的波前特性，X射线自由电子激光应用要求对反射镜表面质量在纳米尺度上进行精确控制。在这项工作中，多层镀膜X射线镜的表面形状由层应力控制。将镜子所需的表面轮廓对其进行二阶微分以获得相应的曲率轮廓。沿着镜子长度以1毫米的步长，施加不同的涂层厚度，以通过均匀的温度变化产生不同的层热应力。镜子表面轮廓可通过将曲率轮廓积分到二阶并通过移动斜率和高度的常数进行进一步校正来获得。通过有限元分析（FEA）对该技术过程进行模拟。一个案例研究表明，所需形状与FEA结果之间的残余斜率误差和残余高度误差分别为0.22微弧度（均方根）和1.42纳米（均方根）。
