Properties of diamond membranes for x-ray lithography

Abstract

Microwave plasma chemical vapor deposition was used to deposit diamond films on polished silicon and then the substrate was back-etched to form taut free-standing membranes up to 75 mm diam. Raman spectroscopy and x-ray diffraction verify that the films are diamond. Transmission electron microscopy and x-ray diffraction reveal that the films are fine-grained polycrystalline diamond with an average grain size between 15 and 110 nm. The films contain microcrystalline graphite and nondiamond carbon as indicated by the broad Raman bands at 1355 and 1560 cm−1, respectively. The measured x-ray transmission of a 3.25-μm-thick membrane at the Cu Kα line is 99.2% while the optical transmission at the He-Ne laser line is 58% for a 1-μm membrane, uncorrected for reflection losses from both surfaces, absorption, and scatter. The transmission in the ir is at the theoretical limit, 71%. The surface roughness of a typical 3-μm-thick membrane is about 30 nm on the side away from the silicon substrate and 20 nm on the side adjacent to the substrate. The surface roughness scales inversely with methane fraction in the supply gas due to decreasing grain size. The typical membrane stress is 0.03–0.15 GPa. The films contain between 0.8% and 7.0% hydrogen, as determined by 15N nuclear reaction analysis. No correlation was observed between hydrogen content in the film and deposition temperature; however, a direct correlation was observed with methane fraction. The average in-plane distortion is 70 nm for a 1.5-μm-thick diamond mask with 0.2-μm-thick tungsten absorber. No measurable out-of-plane distortion was observed for evaporated gold absorber metallization while tungsten produced 0.8 μm distortion.