The concept of treating interconnections as a device and designing them while keeping both materials and structures in mind is presented. An example using molybdenum and copper is demonstrated. Copper introduces new problems such as diffusion in addition to the traditional problems for interconnections such as adhesion. A new structure called a sidewall barrier is used as part of a copper interconnection. This structure can be combined with a multilayer thin film resulting in a completely encapsulated interconnection. The technique is versatile enough that almost any material including dielectrics can be used as the encapsulation material and the sidewall barrier can be either on the outside of a feature or the inside of a space. Several potential metals (Mo, TiN, W) for encapsulating copper are examined and molybdenum is chosen and used. Both sputtering and switched-bias sputtering are used to deposit molybdenum sidewall barriers followed by anisotropic etching for patterning. Electromigration measurements of bilayered copper films reveal that there are problems with TiN and tungsten barriers. Copper oxidation, stress, electromigration, hillock growth, resistivity, diffusion and adhesion are all studied.