Application of the pressure wave propagation method for adhesion defects detection and quantification in bilayer structures

The pressure wave propagation method, usually used to study electrical behavior of dielectric materials, is applied here to nondestructive detection and quantification of adhesion defects in a bilayer structure. This method relies on a very simple idea: as an electric field is created in a bilayer dielectric sample placed between short-circuited electrodes, the propagation of a pressure pulse induces an electric signal. If the field distribution is known, the signal leads to the pressure profile all along its propagation through the sample and therefore gives information on the interface. First, we extend the signal expression already established for a monolayer structure to a multilayer structure and consider particularly bilayer structures. After explaining the signal analysis in perfectly bonded and totally disbonded structures, a model is proposed to describe and analyze the signal in a partially disbonded structure and relate it to the percentage of totally disbonded area in the zone being tested by the pressure pulse. To assess this analysis, measurements were carried out on kapton (130 mu m)-adhesive (98 mu m) transparent samples. Some samples are perfectly bonded, some are totally disbonded and the others are partially disbonded. The pressure pulse is created by the impact of a laser pulse on an absorbing target coupled to the sample. The excellent agreement between the measurements carried out on perfectly bonded and totally disbonded samples and simulations assesses the correctness of the signal expression for a multilayer structure. In the case of partially disbonded structures, the value of the percentage of totally disbonded area, determined by measurement and simulation, is very close to that deduced from a photograph of the sample, only possible for transparent materials. The spatial resolution of the method is related to the spectrum of the pressure pulse, which extends up to 200 MHz. This method presents an excellent spatial resolution. If the transit time of the pressure pulse in each medium is superior to 11 ns, measurements and simulations show that owing to this method, it is possible to detect and localize in the structure submicron gaps and a percentage of totally disbonded area as low as 5% in the tested zone by analyzing the signals in the time domain in a very simple manner.(C) 1999 American Institute of Physics. [S0021-8979(99)02214-8].