Low-stress sputtered chromium-nitride hardmasks and their etching characteristics for X-ray mask fabrication

Fabrication of precise X-ray masks is one of the most important issues in Synchrotron Radiation (SR) lithography. To achieve high-precision X-ray masks, we have developed a low-stress Chromium-Nitride (CrN) hardmask. Both the stress and its distribution (gradient) in the CrN films, which are electrically conductive, are within measurement error. Using a 75 nm-thick CrN hardmask, 0.10 mu m line-and-space patterns in a 0.4 mu m-thick Tantalum-Germanium (TaGe) alloy X-ray absorber have been demonstrated. The CrN film was etched by reactive-ion etching using chlorine gas mixed with oxygen. The etching selectivity between the TaGe alloy and the CrN was 13 when using Electron-Cyclotron-Resonance (ECR) plasma etching with sulfur hexafluoride gas. We have also investigated the durability of CrN to various acids; e.g., APM (NH3:H2O2:H2O), HPM (HCl:H2O2:H2O), and SPM (H2SO4:H2O2); widely used in silicon-wafer cleaning processes. The CrN etching rate for those acids was less than 3 nm/min. These results demonstrate that a sputtered CrN film is an excellent hardmask material for precise X-ray mask fabrication.