Stress Evolution During Growth of Metal Thin Films

As mechanical stress strongly influences the reliability and performance of nano- and microtechnological devices, control of stresses in deposited films is of significant interest. To obtain this control an understanding of the involved physical processes and their contributions to total stress must be reached. Work focuses on the stresses that form during the e-beam evaporative growth of high mobility metals on different amorphous substrates. A typical stress curve obtained measuring substrate bending with a capacitive displacement sensor during deposition. During early stages, islands nucleate and grow on a surface, causing a compressive stress thought to be related to the surface state of the growing islands. As deposition continues, islands begin to coalesce and form a continuous layer, during which the stress becomes increasingly tensile as surface energy of the islands is transferred into grain boundary and elastic energy. After the film becomes continuous the stress reverses and approaches a constant compressive value, again thought to be related to the state of the film surface during deposition.

One possible method of controlling stress currently being explored is island size modification using growth interrupts before island coalescence. Islands coarsen on time scales according to the mobility of an atom on the substrate. The resulting larger islands coalesce at higher thicknesses and therefore lead to an analogous shift of the tensile peak to higher thicknesses. Flash depositions of Ta on the surface of deposited films allows for atomic force microscopy imaging of “frozen” films, providing a method for correlating the island size after a given length of interrupt and corresponding changes in measured stress curves.

The ability to lock in a surface state at any stage of deposition or interrupt also provides the means for exploration of several other fundamental film stress generators, including the relaxation of stress due to surface roughness and due to island and grain coarsening.