Planar Defects in Metallic Thin Film Heterostructures

Twin and phase interfaces are two important types of planar defects in nanocrystalline (NC) thin film heterostructures. Formation of twins in NC metals plays a critical role in their physical and mechanical properties. The introduction of twins in NC metal films has been reported to result in a combination of ultra-high strength and good ductility, good electrical conductivity and excellent resistance to current-induced diffusion, much better thermal stability (stable at 800 C for 1 hr) and negligible grain growth (little grain size change at room temperature for 1 yr). Meanwhile, the stress relief by formation of twins in thin film fabrication helps to avoid damage in a microelectronic device and prolong its service life. Despite the reported advances in properties, the twinning mechanisms in NC thin film heterostructures have remained poorly understood. In this project, the following aspects of twinning mechanisms in NC thin film heterostructures were studied: 1) the formation of twins and grain size dependence of twin density in as-fabricated NC Cu films were investigated for the first time; 2) twin intersection mechanisms in NC fcc metals were understood by observing twin transmission across the boundary of another twin. A partial dislocation reaction model was proposed for twin transmission phenomenon and evidenced by experimental observation. The observations and mechanisms proposed in this project can be applied in other twin intersection phenomena in all fcc materials; 3) the long-existing question about the dominant mechanism for the formation of deformation twins in NC fcc metals with zero macrostrain was answered, by a novel design of experimental approach in this project.;The interface between different phases is another type of important planar defect. It will play an increasingly important role in device performance when device size shrinks to several monolayers thick. In this project, the following aspects of interfacial phase formation in NC thin film heterostructures were studied: 1) the world's first bcc/fcc Cu and MgO structure was fabricated on Si using TiN as the buffer layer and the detailed atomic structure at Cu/MgO interface was studied; 2) epitaxial Ni/MgO stack was successfully integrated on Si substrate and the atomic structure at Ni/MgO interface was studied. The distinctive magnetic properties as a function of Ni thin film morphology were investigated for the first time; 3) the world's first Cu3Ge/sapphire thin film structure was fabricated and the first study of twinning structure inside the Cu3Ge thin films was performed.