This step is achieved by a technique such as sputtering, chemical vapor deposition, or an electroless process. At the end of the deposition step, the metal fills the cavities and forms a layer above them. This layer, as deposited, is not planar due to the uneven topography of the underlying pattern. It is necessary to remove the metal above the surface and leave it only in the cavities. When aluminum alloys or tungsten were used as the conducting metals, they were removed by techniques such as reactive ion etching.
Copper has a lower electrical resistivity than that of aluminum.
Hence the use of copper instead of aluminum as the interconnect metal leads to a corresponding, and substantial reduction in both the power dissipated and the RC delay of the circuits. Also, copper offers better resistance to electromigration which is a phenomenon that causes reliability problems with aluminum interconnects. Since copper cannot be patterned easily by reactive ion etching it is processed using the so-called damascene technique. In this method, a planarization process is used to remove small layers of copper of thickness of the order of micrometers leaving a flat surface behind. The importance of planarity arises from the need for the next series of lithographic steps.