用于太空望远镜的大口径薄膜菲涅尔衍射元件

为了满足空间衍射成像系统对大口径、轻量化衍射元件的需求,设计制作了直径为400mm的聚酰亚胺(PI)薄膜菲涅尔衍射元件。通过紫外光刻、离子束刻蚀等微细加工方法在石英基底上制作衍射图形,然后将衍射图形复制到PI薄膜上得到菲涅尔衍射型薄膜元件。结合有限元法探究了薄膜复制过程中热应力的变化规律及降低热应力的方法,分析了影响薄膜衍射效率的因素及薄膜制作误差、温度变化对薄膜成像的影响,最终实现了大面积薄膜与基底的分离,并通过局部氧气等离子体轰击提高了薄膜衍射效率的均匀性。经测试,薄膜菲涅尔衍射元件的厚度约为20μm,在波长633nm处的实际衍射效率平均值为33.14%,达到了理论效率的81.83%,衍射效率的均方根值RMS=0.01。实验结果表明,通过紫外光刻、离子束刻蚀和薄膜复制的方法可以得到大口径、高衍射效率的薄膜菲涅尔衍射元件。

Large-diameter membrane Fresnel diffraction elements for space telescope

In order to meet requirements of large diameter and light-weighting diffraction elements for space diffraction imaging system, the polyimide (PI) membrane Fresnel diffraction elements with the diameter of 400 mm were designed and manufactured. With ultraviolet lithography, ion beam etching and other microfabrication methods, diffraction patterns on quartz substrate were made and then reproduced onto PI membrane, thereby obtaining the Fresnel diffraction type membrane elements. The change rule of thermal stress during the membrane reproduction process and the methods for reducing thermal stress were researched by Finite Element Method (FEM). The influence factors of the membrane diffraction efficiency, and impact of membrane production error and temperature change on membrane imaging were analysed. Finally, the separation of large-diameter membrane and substrate was realized, and the uniformity of membrane diffraction efficiency was improved by local oxygen plasma bombardment. By testing, the thickness of membrane Fresnel diffraction element is 20 μm, and the average value of actual diffraction efficiency is 33.14% when wavelength is 633 nm, reaching 81.83% of the theoretical efficiency, and Root Mean Square (RMS) of diffraction efficiency is 0.01. The experimental results show that large diameter and high diffraction efficiency membrane Fresnel diffraction elements can be manufactured by ultraviolet lithography, ion beam etching and membrane reproduction.