Photodisplacement techniques for defect detection

Abstract

The absorption of a focused, modulated laser beam generates a thermal wave in a solid, the propagation of which is sensitive to the presence of surface or subsurface discontinuities. In the photodisplacement technique, one measures the periodic displacement of the object surface due to thermal expansion, with an interferometrical laser probe. Some instability problems of the probe are analysed and a modified version of it is described. A spatial resolution approaching that of conventional optical microscopy can be attained. Two basic effects play a role in the photodisplacement detection of defects: thermal-wave interaction and elastic deformation. As a consequence, the technique is particularly appropriate for the detection of ‘ physical ’ defects such as cracks, subsurface voids, and structural damage in a lattic such as caused by ion implantation. Images of such defects are presented. A novel photodisplacement configuration, where thermal-wave generation and detection are effected with a single laser, is described.