Sofie Devolder

Thin layer thickness measurements based on the acousto‐optic technique

The acousto‐optic technique presented by Wevers, Devolder, Leroy, and De Meester [Appl. Phys. Lett. 66, 1466 (1995)] was based on the information concerning the phase shift between the incident and the reflected ultrasound at critical angles. Two laser beams were used, one being diffracted by the incident ultrasound, the other being diffracted by the reflected ultrasound. Information concerning the geometrical shape of the sample as well as information concerning the thickness of a thin layer could be obtained from measurements in the near‐field of the light diffraction. In this letter it is shown that using only one laser beam, traveling through the intersection region of both ultrasounds, exact thickness information of the layer can be obtained. Theoretical calculations are derived and compared with practical measurements for a 3 μm copper layer on a stainless‐steel plate.

Acousto-optic technique: A new nondestructive technique to evaluate thin-layered structures

On laboratory scale a new nondestructive technique has been elaborated for the quality control of layered structures. This technique is called the ‘‘acousto‐optic technique’’ and it measures the modulation of ultrasound reflected by a layered material optically in amplitude and phase using laser beams crossing the ultrasonic beams (incident and reflected). The most important characteristics of layered structures which have been investigated are the thickness and the thickness variations within a layer and the presence of artificial defects. Only when the ultrasound is incident in the Rayleigh angle of the substrate material, the phase differences measured obtain important information on the quality of the layered structure. The experimental results underline the results already obtained from the theoretical models elaborated for this technique.

Surface roughness determination using the acousto-optic technique: Theory and experiment

With the acousto-optic nondestructive testing technique, phase shifts between the incident and the reflected ultrasound are measured at critical angles of the investigated material. This letter is focusing theoretically as well as experimentally on the influence of a small surface roughness (some micrometers) of a thick material on the phase information.