O Copper On - Chip Interconnections A Breakthrough in Electrodeposition to Make Better Chips

The Electrochemical Society Interface • Spring 1999 FIG. 1. Cross section of a 6-level copper wiring structure fabricated by IBM showing wiring hierarchy. n-chip interconnections comprise a multilevel structure of fine wiring located on the top of the transistor circuitry of logic or memory chips, whose role is to connect circuits together as shown in Fig. 1. To avoid significant degradation of circuit speed, on-chip interconnections should permit rapid signal transmission among the various parts of the circuitry. Ever since the development of the integrated circuit about 40 years ago, the most pervasively used materials for the fabrication of the wiring structure have been aluminum as the conductor (or more recently an aluminum-copper alloy for better reliability)1 and silicon dioxide as the insulator. The transition to copper as the conductor and to a better insulator2 began with IBM’s announcement in September 19973 and product shipment since June 1998. This signals one of the most important changes in materials that the semiconductor industry has experienced since its creation. Copper metallization was implemented first since significant gains can be obtained by copper alone. It is often stated that the three major challenges in the implementation of copper have been: (1) the method of depositing; (2) patterning the metal, and (3) finding a suitable barrier material that prevents the copper metal from diffusing into the insulator material. Although this generalization neglects the enormous integration and manufacturing challenges, it serves to highlight the fact that a combination, equivalent to blanket sputter deposition and reactive ion etching (RIE) technology used to deposit and pattern aluminum, was not available for copper. It is precisely this technological challenge that was met by electrodeposition used in a process-integration approach called Damascene4 because of its resemblance to an ancient method of metal inlaying. Thus, electrodeposition, a technology frequently considered to be more of an art than a science, has been O Copper On-Chip Interconnections