The standard stuff of such wires has long been aluminum or aluminum alloys and, in interlayer connectors, tungsten.
Copper is much more conductive than aluminum, allowing finer wires with lower resistive losses.
Copper is also significantly less vulnerable to electromigration than aluminum and less likely to fracture under stress. It readily diffuses into silicon and causes deep-level defects. Late in 1997 the first commercial copper-wired chips were announced by IBM and Motorola. To keep the copper from migrating into the dielectric and poisoning it, a diffusion barrier was used, which lined the trench walls between the copper and the substrate. IBM and Motorola produced their copper-wired chips by electroplating the copper over the diffusion barrier.