Mechanical characterization of thin film structures using a laser spallation technique.

摘要

The laser spallation technique has been developed to measure the interface strength between different materials, especially thin film structures. In this work, it is refined and applied to various material systems. With these advances, the laser spallation technique is now fully mature for applications not only to measurement of material interface strength, but also to the study of laser-material interaction, dynamic fracture mechanics, as well as to the measurement of material bulk properties.; In the first part of this work, the laser spallation technique was examined quantitatively for signal processing and stress wavefield recovery. It is shown that the short time Fourier transformation is another appropriate means for recovering the free surface displacement from the acquired optical signal. Two methods have been chosen to recover the stress field inside the sample. When the displacement of the coating's free surface is recorded directly, it is convenient to use a special finite difference strategy. When the free surface displacement is recorded on the bare substrate surface, it is more convenient to use the finite element method to calculate the interface strength.; The application work includes several topics. The first one was the evaluation of the effect of substrate orientation and deposition mode on the interface strength of Nb-sapphire interfaces. The interface strength is higher for the sapphire substrate with prismatic orientation, and RF deposition mode yields higher interface strength than the DC mode. The second application estimated the effect of substrate roughness on the interface strength of Nb-alumina system. The effect of chemical composition of thin films on the interface strength was also investigated.; The final application investigated the dynamic fracture mechanics of thin film structures. The purpose of this chapter is to clarify the controversial topic regarding the limit speed of bimaterial interface crack propagation. We were successful in using the microlithography technique to generate microcracks with controlled geometries between a thin film and its underlying substrate. The sample was mechanically loaded by using a laser generated stress pulse as discussed above. The crack propagation was monitored via a fast response circuit involving micro-wires on top of the cracked sample. The interface crack was observed to propagate at a speed close to the higher Rayleigh wave speed.; Thus, all the necessary quantification techniques have been developed for the laser spallation technique. The application to the measurement of interface strength and dynamic fracture in thin film structures showed that this is a reliable, powerful, and unique technique that has opened up new fields for experimental and theoretical research.