Investigation on near-plane varied-line-spacing gratings made by electron beam lithography and near field holography

变线距光栅(VLSG)以其高分辨率、像差校正和扫描机制简单等优点，不断被开发用于同步辐射、等离子体诊断和其他应用。VLSG的制备方法有机械刻划和全息离子束刻蚀两种。机械统治存在鬼线、背景散射和表面粗糙等问题。相比之下，全息刻蚀VLSG克服了上述缺点。然而，全息曝光的光学配置对于VLSG来说是复杂的，并且被认为在产生周期分布方面不如EBL方法灵活。目前，大口径VLSGs的精确、快速微纳加工技术仍然具有挑战性，特别是对于高密度、小密度增量的软X射线VLSGs。因此，我们提出了一种高端电子束光刻和光学近场全息术(EBL-NFH)的创新组合，用于在长衬底上开发具有前所未有的约0.1 nm步长周期增量的VLSG。从本质上讲，首先采用电子束外延技术制作了VLSG的初始可重复使用的熔融二氧化硅掩模。然后，通过NFH将熔融二氧化硅掩模的图案复制并转移到光致抗蚀剂层，从而获得可复制到衬底中的光致抗蚀剂掩模。最后，在NFH工艺中，通过光刻胶掩模，经过一个或几个反馈周期，就可以制作出优化的VLSG。据我们所知，这种方法至少还没有应用于极紫外光和软X射线系统，因为它在复制过程中需要高精度和空间分辨率。该项目中最重要和最耗时的过程大概是EBL的制造。因此，用EBL实现可复用的初始掩码是至关重要的。EBL和NFH的结合正好实现了这一目标。与其他图形转移技术相比，NFH的优点是不仅可以转移掩模的二进制密度分布，而且可以根据其设计要求控制光刻胶的轮廓。我们的目标是开发一种灵活、方便、高效的高精度VLSG微纳制作方法，改善其光学性能，扩大EBL-NFH技术的应用范围。关键词：微纳制造；VLSG；电子束光刻；近场全息术；软X射线。

Varied-line-spacing gratings (VLSGs) are continuously being developed for synchrotron radiation, plasma diagnosis, and other applications with their advantages of high resolution, aberration correction, and simple scanning mechanisms. VLSGs have been fabricated by either mechanical ruling or holographic ion beam etching. Mechanical ruling suffers from ghost lines, background scattering and rough facets. In contrast, holographically etched VLSGs overcome the above drawbacks. Nevertheless, the optical configuration for holographic exposure is complex for VLSGs and is considered as less flexible in the generation of the period distribution than the EBL method. At present, a precise and fast micro- and nano-fabrication technique for large aperture VLSGs is still challenging, especially for soft x-ray VLSGs with a high density and small density increment. Therefore, we propose an innovative combination of high-end electron beam lithography and optical near field holography (EBL-NFH) for the development of VLSGs with an unprecedented step period increment of ~ 0.1 nm on a long substrate. In its essence, first an initial and reusable fused silica mask of a VLSG is fabricated by EBL. Afterwards, the pattern of the fused silica mask is replicated and transferred to a photo resist layer by NFH, such that a photo resist mask is obtained that can be copied into the substrate. Finally, an optimized VLSG will be fabricated after one or a few feedback cycles via the photo resist mask in NFH process. To our knowledge - this method has not yet been applied at least to the EUV and soft x-ray regime, with its needs for high accuracy and spatial resolution in the copy process. The most important and time-consuming process in the project is presumably the EBL fabrication. Hence it is crucial to realize a reusable initial mask by EBL. The combination of EBL with NFH achieves this aim exactly. Compared to other pattern transfer techniques, the advantage of NFH is that it not only transfers the binary density distribution of the mask, but may also control the photo resist profile according to its design requirement. Our goal is to develop a flexible, convenient and high-efficient micro- and nano-production method for high-precision VLSGs, to improve their optical properties and to extend the application range of the EBL-NFH technique. Keywords: micro- & nano-production; VLSG; electron beam lithography; near field holography; soft x-rays.