High-aspect-ratio germanium zone plates fabricated by reactive ion etching in chlorine

This article describes the fabrication of soft x-ray germanium zone plates with a process based on reactive ion etching (RIE) in Cl2. A high degree of anisotropy is achieved by sidewall passivation through cyclic exposure to air. This enables structuring of higher aspect ratios than with earlier reported fabrication processes for germanium zone plates. The results include a zone plate with a 30 nm outermost zone width and a germanium thickness of 310 nm having a first-order diffraction efficiency of 70% of the theoretical value. 25 nm half-pitch gratings were also etched into 310 nm of germanium. Compared to the electroplating process for the commonly used nickel zone plates, the RIE process with Cl2 for germanium is a major improvement in terms of process reproducibility.