Morphological stability and failure mechanisms for thin metal and barrier films on porous low-kappa dielectrics

This work examines the morphological stability of contiguous metal and barrier thin films deposited on porous dielectrics used in the microelectronics industry. Two different thin film instabilities, agglomeration and buckling, are studied and the underlying mechanisms governing each of these instabilities are identified. The relationship between the underlying dielectric internal properties, namely porosity and pore size distribution, and the stability behavior of the contiguous films is explored. The overall objective of this work is to gain insights into the initiation and propagation of thin film instabilities and their relationship to the substrate properties.; To achieve this goal, Nanoporous Silica (Xerogel), is fabricated as a "model" porous low-kappa dielectric. Metal (Copper) films deposited on these model substrates fail by agglomeration, which initiates by void formation due to grain boundary grooving. A maximum in film stability against void formation is observed for a particular substrate porosity range. Thermodynamic modeling of the grain grooving process is able to predict this maximum in film stability and tie the behavior to porosity induced micro-roughness in the substrate. The thermodynamic analysis is extended to study the kinetics of copper film agglomeration on solid substrates. The effects of copper film thickness and processing temperature on dewetting pathways are examined. Films thinner than 20 nm dewet in two kinetic limiting sequential steps: void nucleation by grain-boundary diffusion, followed by void growth and agglomeration by surface diffusion. However, for thicker films, the results indicate that dewetting is limited by one-step process of grain boundary diffusion.; Barrier films (Tantalum and Tantalum Nitride) fail by buckling due to the high intrinsic stresses in these films. A telephone-cord pattern of the buckles is observed and characterization of this pattern allows calculation of critical fracture energy of the barrier-dielectric interface. The fracture energy is also calculated using standard adhesion tests like four-point bending. The results of fracture energy imply that the key property governing fracture and adhesion of the barrier layers to porous substrates is the substrate pore size and not its porosity. A general property relationship is derived for the same.