Simultaneous measurement of local longitudinal and transverse wave velocities, attenuation, density, and thickness of films by using point-focus ultrasonic spectroscopy

Abstract

This paper presents an ultrasonic technique for simultaneous determination of the complete set of acoustical and geometrical properties of a film or a thin layer in a stratified material embedded between two known materials using point-focus ultrasonic spectroscopy, which provides a high lateral resolution. The theoretical model of the two-dimensional spectrum Rt(θ,ω) of the stratified material is calculated as a function of six parameters of the unknown layer: longitudinal and transverse velocities cl, ct, attenuation αl, αt, density ρ, and thickness h, which fully determined the properties of the film. The experimental spectrum Re(θ,ω) can be measured by V(z,t) technique. A two-step algorithm is presented to decompose the searching process of parameters from one six-dimensional to two three-dimensional spaces. The sensitivity of the two-dimensional spectrum to individual properties and its stability against experimental noise are studied. The full set properties of a 250 μm thick stainless steel film and a 930 μm thick SiO2 thin layer of a three layered stratified material immersed in water are determined. The proposed technique used a point-focus transducer, which makes the setup simple and reliable. It allows measurement of the local properties of the film and enables precision material characterization.