Free-standing diamond membranes: optical, morphological and mechanical properties

Abstract

Microwave plasma chemical vapor deposition was used to deposit diamond films on polished silicon. The substrate was back-etched to form taut free-standing membranes up to 75 mm in diameter. 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, depending on methane fraction. Surface roughness measurement by atomic force microscopy and a stylus-type profilometer shows that the film roughness scales inversely with the grain size, which is a function of the methane fraction in the feed gas. 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 blue is sensitive to the methane fraction in the gas stream. The hydrogen content in the films was measured by nuclear reaction analysis and elastic recoil detection spectroscopy. No correlation was observed between hydrogen content in the film and deposition temperature, however, a direct correlation was observed with the methane fraction. Tensile and compressive total (thermal and intrinsic) stress were observed, depending on the deposition conditions. The thermal stress is compressive and relatively constant (0.215–0.275 GPa) over the temperature range investigated. The intrinsic stress is tensile and its origin is interpreted in terms of the grain boundary relaxation model. The biaxial Young's modulus and the ultimate tensile strength of free-standing membranes were measured. Bulk values were observed at low methane fraction. Both properties decrease with increasing methane fraction due to sp 2 carbon and hydrogen incorporation.