The effect of annealing temperature on the microstructure of nanoindented Au/Cr/Si thin films

The nano-mechanical properties of as-deposited thin Au/Cr films deposited on Si(100) substrates are investigated using a nanoindentation technique. Nanoindentation is performed to a maximum depth of 1000 nm, and selected specimens are then annealed at temperatures of 250, 350 or 450°C for 2 min. The nanoindentation results show that the loading-unloading curve is continuous and smooth in both the loading and the unloading steps, which suggests that no debonding or cracking occurs. Furthermore, very little elastic displacement is observed in the unloading curve, which indicates that the deformation is primarily plastic in nature. The hardness and Young's modulus of the Au/Cr/Si thin films are found to vary with the nanoindentation depth, and have values of 1.7 GPa and 88 GPa, respectively, at the maximum indentation depth of 1000 nm. The microstructures of the as-deposited and annealed nanoindented specimens are examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The microstructural observations reveal that nanoindentation induces an atomic reorganization, and results in the formation of high-stress plastic deformation regions beneath the indenter. In the as-deposited specimens, the plastic deformation results in a pile-up of Au around the entrance of the indentation. However, the diffusion of the Au atoms is enhanced at higher temperatures, and hence the annealing process prompts a homogenization of the high-stress areas and leads to a full recovery of the pile-up effect. The high temperature induced in the annealed thin film specimens also prompts a silicidation of the Cr layer, which results in a direct contact between the Au film and the Si substrate. As a result, annealing has a beneficial effect on the interfacial bond strength. Following annealing at the highest temperature of 450°C, an Au-Si eutectic phase is formed, which further enhances the strength of the interfacial bond.