Double focus technique for quantitative measurements in time-resolved acoustic microscopy

Abstract

Simultaneous measurement of sound velocity and thickness of thin specimens is a well-known technique in time-resolved acoustic microscopy. However, the applicability is restricted, because a substrate in contact with the back of the sample is required. Furthermore, the substrate must have an area without coating, because a reference signal from the substrate’s surface is needed. In order to overcome these performance limitations, a double focus technique has been developed. By focusing the lens on the front, and afterwards on the back of the sample, only signals from the sample itself are required. Based on a simple geometrical model, the relevant acoustical equations for sound velocity and thickness were deduced analytically. Afterwards the method was applied to PVDF film in order to experimentally examine the feasibility of the approach. The measured results were compared with reference values and showed reasonable agreement. Based on these experiments, the influence of factors limiting the precision of the model, for example the spherical aberration, will be discussed, yielding an approach for an improved model. Eventually, the significance of the results for applying the method to manufacturing processes of thin films or foils, especially for online process control, will be described.