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

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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.