Determination of the multiple local properties of thin layer with high lateral resolution by scanning acoustic microscopy.

Abstract

Simultaneous determination of the multiple local acoustic and geometrical properties of the thin layer with a high lateral resolution is of great interest in ultrasonic non-destructive evaluation. In this paper, we propose a technique based on the V(z, t) data to simultaneously determine the four local properties of the thin layer, namely, the thickness, the sound velocity, the acoustic impedance, and the density. First, the V(z, t) data are collected from both the thin layer and the reference material. Then the sound velocity and the thickness are calculated by focusing the point-focusing transducer on the front and back surfaces of the thin layer, with the confocal positions determined by averaging the peak positions in the V(z) curves at different frequencies. Second, the acoustic impedance of the thin layer is obtained based on the experimental and theoretical two-dimensional reflection spectrum using the echo from the front surface of the layer. Finally, the density can be obtained by dividing the acoustic impedance by the sound velocity. The four local properties of an aluminum layer are accurately obtained using our method. The largest relative error of determining the four properties is around 1%. This technique opens a new way of simultaneously measuring the multiple local acoustic and geometrical properties of thin layers.