Routing (interconnecting standard cells ) is done over standard cells. As technology is shrinking standard cell size is decreasing and they are packed in very small area but interconnect scaling proportion to the standard cell size. So interconnect density is increasing. It is not possible to route VLSI chips using single layer. We need multiple layer of metals to route. Traditionally aluminium was used for interconnect material but aluminium has serious problem of electromigration. Now as copper is used as interconnect material as it don't have electromigration problem and resistance is also less compared to aluminium. So copper interconnects has less RC delay, hence faster.