Twelve nanometer half-pitch W–Cr–HSQ trilayer process for soft x-ray tungsten zone plates

The authors describe a new W–Cr–HSQ trilayer nanofabrication process for high-resolution and high-diffraction-efficiency soft x-ray W zone-plate lenses. High-resolution HSQ gratings were first fabricated by electron-beam lithography and high-contrast development in a NaCl/NaOH solution. The HSQ pattern was then transferred to the Cr layer by RIE with Cl2/O2, and subsequently to the W layer by cryogenic RIE with SF6/O2. The anisotropy of the W etch as a function of substrate temperature was investigated, and the best etch profile was achieved at −50 °C. Using this optimized process, W gratings with half-pitches down to 12 nm and a height of 90 nm were fabricated. For a zone plate with corresponding parameters, this would result in a theoretical diffraction efficiency of 9.6% (at λ = 2.48 nm), twice as high as has been reported previously.