Interfacial instability of copper-silver multilayer thin films at elevated temperatures

The interfacial instability of copper-silver (Cu-Ag) multilayer thin films during aging at elevated temperatures was characterized. Microlaminates with Cu:Ag layer thicknesses of 2:2, 1:4, 4:1 and 4:0.1 microns were fabricated by electron-beam deposition onto Nb substrates held at 530 K. The specimens were aged for 10 minutes to 192 hours, at temperatures ranging from 700 to 900 K.;The as-deposited and annealed structures were analyzed in cross-section and in plane using atomic force, scanning and transmission electron microscopy. The as-deposited microlaminates had a fine-grained columnar microstructure, with an average in-plane grain diameter of approximately 0.4 microns, and had well-defined interfaces. Upon annealing, the morphology evolved over three time regimes. In the first regime, the grains grew quickly from columnar to equiaxed, and the grain growth followed parabolic behavior. Activation energies for grain growth in this regime were approximately 84 +/- 34 and 83 +/- 33 kJ/mol, for Cu and Ag, respectively. During this time, the interface roughness remained approximately constant. However, the grain boundary migration slowed and eventually stopped; the terminal grain diameter depended on the layer thickness and the ratio of the interfacial (gammai) to grain boundary energies (gammagb). In the second time regime, there was little to no additional grain growth. However, grooves formed at the intersections of grain boundaries and layer interfaces, and grew with a t0.25 dependence, where t is the aging time. Groove growth appeared to be independent of layer thickness and was approximately equal in Cu and Ag. The microlaminates started to break down in the third time regime as grooves intersected along grain boundaries.;Models for grain growth and grooving in thin films were modified for multilayer microlaminates, and shown to fit the experimental data reasonably well. This suggests the terminal grain size is reached when groove drag overcomes the capillary forces driving grain growth, and that grooving kinetics are dominated by interfacial diffusion. The time to layer breakdown scales as (agammai/gammagb)4/gammai Di' where a is the layer thickness.