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

摘要

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 macro-scopically 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.