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

Abstract

The nano-mechanical properties of as-deposited thin Au/Cr films deposited on Si(100) substratesare investigated using a nanoindentation technique. Nanoindentation is performed to a maximumdepth of 1000 nm, and selected specimens are then annealed at temperatures of 250, 350 or450 °C for 2 min. The nanoindentation results show that the loading–unloading curve is continuousand smooth in both the loading and the unloading steps, which suggests that no debondingor cracking occurs. Furthermore, very little elastic displacement is observed in theunloading curve, which indicates that the deformation is primarily plastic in nature. Thehardness and Young’s modulus of the Au/Cr/Si thin films are found to vary with thenanoindentation depth, and have values of 1.7 GPa and 88 GPa, respectively, at themaximum indentation depth of 1000 nm. The microstructures of the as-deposited andannealed nanoindented specimens are examined using scanning electron microscopy (SEM)and transmission electron microscopy (TEM) techniques. The microstructural observationsreveal that nanoindentation induces an atomic reorganization, and results inthe formation of high-stress plastic deformation regions beneath the indenter. Inthe as-deposited specimens, the plastic deformation results in a pile-up of Auaround the entrance of the indentation. However, the diffusion of the Au atoms isenhanced at higher temperatures, and hence the annealing process prompts ahomogenization of the high-stress areas and leads to a full recovery of the pile-upeffect. The high temperature induced in the annealed thin film specimens alsoprompts a silicidation of the Cr layer, which results in a direct contact between theAu film and the Si substrate. As a result, annealing has a beneficial effect onthe interfacial bond strength. Following annealing at the highest temperature of450 °C, an Au–Si eutectic phase is formed, which further enhances the strength of the interfacialbond.