title: Thin layer thickness measurements by zero group velocity Lamb mode resonances.
abstract: Local and non-contact measurements of the thickness of thin layers deposited on a thick plate have been performed by using zero group velocity (ZGV) Lamb modes. It was shown that the shift of the resonance frequency is proportional to the mass loading through a factor which depends on the mechanical properties of the layer and of the substrate. In the experiments, ZGV Lamb modes were generated by a Nd:YAG pulsed laser and the displacement normal to the plate surface was measured by an optical interferometer. Measurements performed at the same point that the generation on the non-coated face of the plate demonstrated that thin gold layers of a few hundred nanometers were detected through a 1.5-mm thick Duralumin plate. The shift of the resonance frequency (1.9 MHz) of the fundamental ZGV mode is proportional to the layer thickness: typically 10 kHz per μm. Taking into account the influence of the temperature, a 240-nm gold layer was measured with a ±4% uncertainty. This thickness has been verified on the coated face with an optical profiling system.
authors: Institut Langevin - Ondes et Images, Univ Paris Diderot, UMR CNRS 7587, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France.
authors: Maximin
authors: M
authors: Cès
authors: Dominique
authors: D
authors: Clorennec
authors: Daniel
authors: D
authors: Royer
authors: Claire
authors: C
authors: Prada