Trilevel reactive ion etching processes for fabrication of 60 nm germanium structures with high aspect ratio

We investigated the influence of different parameters in reactive ion etching (RIE) on the anisotropy of 60 nm test structures in a 360-nm-thick layer of germanium. Structures of similar dimensions are required for zone plates in x-ray microscopy. A simple optical setup for the holographic exposure of gratings with a period of 120 nm is presented. The gratings cover an area of several mm2. This allows for splitting of the structures perpendicular to the grating lines and successive examination of the etch profiles in a scanning electron microscope. Two different trilevel systems were developed for structuring germanium in a CBrF3 plasma. Both trilevel systems use AZ 1350 photoresist on an intermediate layer of titanium. The bottom layer is either polyimide, which is structured by RIE in an oxygen plasma, or nickel, which is etched in a hydrogen plasma highly selective to titanium. The anisotropy of the germanium etch process has been found to increase with the ratio R of self-bias to gas pressure, whereas the selectivity decreases. The higher stability of the nickel mask allows us to choose higher values of R resulting in etch profiles with nearly vertical sidewalls.