X-ray gratings and projection lithography by means of laterally structured multilayers

Starting from multilayer systems as X-ray mirrors with a high normal incidence reflectivity (for example measured to be 60% at 14 nm), laterally structured multilayers serve as X-ray optical components such as gratings and reflection type masks for X-ray lithography. Mo/Si (30 bilayers) and Mo-Si/Si multilayer systems (33 bilayers) are fabricated by electron beam evaporation in UHV. Analysis of the quality of the stack is made by using an in situ monitoring system measuring the reflection of the C-K line (4.47 nm) and ex situ grazing incidence X-ray reflection of the Cu-K$_\alpha$ line (0. 154 nm). A smoothing of the boundaries is obtained by thermal treatment of the multilayer system during growth and by ion polishing. The microstructure of the multilayer systems is investigated by means of Rutherford Backscattering, Sputter/AES techniques, electron-microscopy, scanning tunnel microscopy and atomic force microscopy. Baking the final stack after deposition up to 900 °C is applied to study its thermal stability. This paper reports the figure of merits of laterally structured multilayers such as X-ray gratings when laterally structured silicon wafers are coated with multilayers, or masks for X-ray projection lithography when multilayer mirrors are structured by reactive ion etching.