Growth of short-period epitaxial superlattices for X-ray multilayer mirrors

Soft-X-ray multilayer mirrors with amorphous structures, which are currently widely used, have almost reached their inherent maximum performance, when used with short periods for high or normal incident angle reflection. Therefore, a breakthrough that would enable further progress in the performance, such as that of reflectance, of X-ray short-period multilayer mirrors has been sought to date. To solve this issue, atomic-layer epitaxy-grown semiconductor superlattices have recently been applied to soft-X-ray short-period multilayer mirrors. Moreover, conventional molecular beam epitaxy of metallic superlattices, which was difficult to apply to X-ray mirrors in the past, has been investigated, in order to enable its application to soft-X-ray mirrors by considering the combinations of various chemical elements, taking crystal structures into account. As a result, short-period Al-based metallic superlattices have been grown successfully very recently. This paper presents a review for the current status and outlook of the development of soft-X-ray short-period multilayer mirrors, focusing on the growth behavior, structures and X-ray reflectance of these promising short-period superlattices that have potential to provide soft-X-ray mirrors with high performance.