Advanced resonant ultrasound spectroscopy for measuring anisotropic elastic constants of thin films

This paper presents an advanced resonant ultrasound spectroscopy (RUS) method to determine the elastic constants C_ij of thin films. Polycrystalline thin films often exhibit elastic anisotropy between the film growth direction and the in‐plane direction, and they macroscopically show five independent elastic constants. Because all of the C_ij of a deposited thin film affect the mechanical resonance frequencies of the film/substrate layer specimen, measuring resonance frequencies enables one to determine the C_ij of the film with known density, dimensions and the C_ij of the substrate. Resonance frequencies have to be measured accurately because of low sensitivity of the C_ij of films to them. We achieved this by a piezoelectric tripod. Mode identification has to be made unambiguously. We made this measuring displacement–amplitude distributions on the resonated specimen surface by laser Doppler interferometry. We applied our technique to copper thin film and diamond thin film. They show elastic anisotropy and the C_ij smaller than bulk values of C_ij. Micromechanics calculations indicate the presence of incohesive bonded regions.