Interface decohesion is increasingly becoming a reliability concern in multilayer interconnect structures. It is therefore necessary to provide adhesion tests as a part of materials characterization procedures and to study the fundamental material properties that affect interface toughness. A sandwich structure 4-point bend test was developed for measuring the adhesion between interlayer dielectric and metal interfaces. This technique offers well defined and easily controllable fracture processes and simple analyses based rigorously on fracture mechanics. Using this technique, a number of interfaces were studied. For conventional interconnect systems consisting of Al lines and SiO2 as the interlayer dielectric, the focus was to improve the interface fracture toughness through structure design and process engineering. Potential low dielectric constant materials, such as polymers and porous SiO2, were also studied as candidates for interlayer dielectric materials in the future. Interface strengthening effects of both mechanical and chemical origin, including interface roughness and thin film plasticity, as well as alteration of chemical bonding were explored.