Metal-less x-ray phase-shift masks for nanolithography

In this article the theoretical and experimental results of metal-less x-ray phase-shift masks are reported. Features of 40 nm have been printed using gaps on the order of 5–10 μm, and lines of 0.1 μm have been printed with gaps of 30 μm with synchrotron radiation. A first prototype has been constructed of 2.75 μm of PMMA patterned on 0.8 μm Si3N4. They can be created out of a variety of low-Z materials. The model predicts a mask contrast between 5–6. Among its advantages is its ability to form nanometer lines without actually having to write them directly, because, with this technique, only the edges of the features are patterned on the mask where the two phases destructively interfere. In addition, the images do not degrade with increasing gap as quickly as a standard metal masks, resulting in a larger process window.