Non contact measurement of thin layer thickness by zero group velocity Lamb modes

A homogeneous elastic plate can support non-propagating Lamb modes having a zero group velocity (ZGV) for nonzero values of their wave number, giving rise to sharp resonances. For plates covered with a thin layer, it was shown that the frequency of the S 1 S 2 ZGV resonance was shifted proportionally to the mass loading through a factor which depends on the mechanical properties of the layer and of the substrate. As they are non-contact and broadband, Laser Based Ultrasonics techniques are very efficient to investigate mechanical vibrations. In our experiments, Lamb modes were generated by a Nd:YAG pulsed laser. An optical interferometer was used to measure the displacement normal to the plate surface at the same point on the non-coated face. Gold layers of a few hundred nanometers were detected through a 1.5-mm thick Duralumin plate. The shift of the 1.9-MHz resonance frequency of the fundamental ZGV mode is proportional to the layer thickness: typically 10 kHz per μm for a gold layer deposited on a Duralumin plate. Taking into account the influence of the temperature, a 240-nm gold layer was measured with a \pm 4% uncertainty. This thickness has been verified by scanning the coated face with an optical profiling system.