Elastic constants of chemical-vapor-deposition diamond thin films: resonance ultrasound spectroscopy with laser-Doppler interferometry

Abstract

Polycrystalline thin films show transverse isotropy (or hexagonal symmetry) and show five independent elastic constants because of columnar structure, texture, residual stress, and local incohesive bonds (or microcracks). In this paper, we developed an advanced method for measuring the anisotropic elastic constants of thin films using resonance ultrasound spectroscopy and determined the elastic constants of chemical-vapor-deposition diamond thin films. Mechanical resonance frequencies of a film/substrate layer specimen were measured, from which elastic constants of the thin film were determined by an inverse calculation. Mode identification for observed resonance frequencies is the key for their successful determination. We achieved this by measuring the displacement distribution on the vibrating-specimen surface. Determined elastic constants are smaller than those of bulk diamond, and they are elastically anisotropic between in-plane and out-of-plane directions. We attribute these compliant and anisotropic thin films to the incohesive bond at grain boundaries. A micromechanics model consistently explains the observation.