Plastic deformation and interfacial sliding in Al and Cu thin film: Si substrate systems due to thermal cycling

As a result of the large difference in thermal expansion coefficients between metal and Si, high stresses can develop in thin metallic films attached to Si substrates in microelectronic devices during thermal excursions experienced in processing steps or during usage. These stresses may induce plastic deformation of the thin films accompanied by creep and interfacial sliding, and have a pronounced effect on the reliability of microelectronic devices and components. Even though various methods have been proposed to study thermal stress, methodologies for studying plastic deformation of thin films are not well established. Here, we report the results of a study of plastic deformation and interfacial sliding of thin Al and Cu films on Si substrates during thermal cycling. Cross-sectional profiles of pattern-grown Al and Cu films of nominally 250 nm thickness were measured before and after thermal cycling by employing an atomic force microscope. Through statistical analysis, the size changes of the thin films induced by thermal cycling were determined. Finite element (FE) analyses were conducted to compute the stress and strain states within the thin film and at the interface, and the results were utilized to interpret the atomic force microscopy (AFM) observations. Experiments revealed that, following thermal cycling, Al films expanded relative to the Si substrate, whereas Cu films shrank, resulting in an alteration of the film-footprint on the substrate in both cases. Based on the FE calculations, this was attributed to net inelastic deformation of the thin films via creep and yielding, with the deformation being accommodated at the interface by diffusion-controlled interfacial sliding.