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

Abstract

ABSTRACTThis paper presents an advanced resonant ultrasound spectroscopy (RUS) method to determine the elastic constants Cij 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 Cij of a deposited thin film affect the mechanical resonance frequencies of the film/substrate layer specimen, measuring resonance frequencies enables one to determine the Cij of the film with known density, dimensions and the Cij of the substrate. Resonance frequencies have to be measured accurately because of low sensitivity of the Cij 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 Cij smaller than bulk values of Cij. Micromechanics calculations indicate the presence of incohesive bonded regions.