Imaging and characterization of film coating using scanning acoustic microscopy

Abstract

The qualities of thin and thick film coatings are closely related to their mechanical parameters such as mass density, acoustic velocity and film thicness, and the defects at the interface. Simultaneous measurements of these three parameters was carried out using a wide-band acoustic microscope operating in reflection mode at the frequency range of 50–250 MHz. This simultaneous measurement was achieved by first identifying the resonant frequency at which the film thickness was equal to one-quarter of the acoustic wavelength and the subsequent determination of the amplitude and phase of the signal reflected from the specimen. Several film coatings have been characterized and the results agree well with both the published data and the data obtained by other techniques. The adhesion of the film coating is greatly affected by the voids and defects at the film-substrate interface. The same acoustic microscope operating in transmission mode was used to detect these voids and defects. Specifically, it was employed to image an AuSn eutectic layer which was melted on a gold-coated alumina substrate, and a similar eutectic layer that was used to bond a GaAs chip and an alumina substrate. Voids and defects were clearly observed and identified. These experiments have demonstrated that the scanning acoustic microscope is a useful instrument for non-destructive characterization of film materials and detection of voids and defects that occur at the film-substrate interfaces.