Copper's success did not come overnight. The conventional wisdom was to stay as far away from copper as you could. Here Romankiw refers to copper's ability to diffuse through silicon dioxide insulating layers into the silicon substrate where it then changes the electrical properties of the silicon in such a way as to prevent the transistors from working. Major hurdles standing in the way of copper interconnect technology include how to deposit copper on the surface of the wafer, how to create diffusion barriers to prevent poisoning of the silicon, and how to pattern the copper layer. Deposition of copper is achieved through electrolytic plating, a process that has been rarely used in semiconductor manufacturing.
Diffusion barriers keep the high-conductivity copper from diffusing into the dielectric layers.
In conventional process flows, a metal layer is first deposited and then patterned by etching away unwanted metal, leaving the desired pattern. An insulating layer is then deposited and excess insulator material is removed by polishing. The dual-damascene, on the other hand, first forms the metal pattern by etching the insulating layer. Then the metal is deposited and the excess metal removed by polishing. This new process flow requires one fewer metal deposition and one fewer polishing steps per metal layer.