"We discovered that copper's 'magic' was that in the process of preparing it, trace impurities vastly improved its reliability," Edelstein told EE Times. "Our electroplated copper had minimal electro-migration [the bane of interconnections in microelectronics] because of these traces of carbon, nitrogen, sulfur, chlorine and phosphorus, all of which were present in as little as 10 parts per million."
Cyprian Uzoh, the chemist on the team (whose name in his native Nigerian language means “copper”) came up with the electroplated copper "recipe" and said at the time of the impurities that "a little salt and pepper never hurt anybody."
"I firmly believe that the discovery of the superior, cheaper and easier interconnection of CMOS transistors with copper instead of aluminum resulted from IBM Research's multi-disciplinary expertise across chemistry, electrical engineering and physics," Edelstein told EE Times. "Plus, we built our own PCBs, chips and their packaging, which together gave us the expertise to discover how electroplating copper could replace aluminum. All our competitors sub-contracted many of these steps, putting IBM in the unique position to solve the puzzle."
A SEM image of a portion of the CMOS 7S microprocessor, with the insulator removed for clarity.
The dual damascene process, for instance, essentially added silicon dioxide as insulation between layers while simultaneously permitting the tantalum-coated copper wires to be electroplated into the chips trenches. These techniques depended on multidisciplinary expertise, enabling IBM to produce the first prototypes in 1997 and the first production PowerPC chips in 1998. When compared to the previous generation 300-MHz PowerPCs, the 1998 versions experienced a 33 percent boost in speed attributable to their unique copper interconnect. And putting the rest of the industry on the trail to figure out how IBM was doing it.
"At first our competitors said that it would only last one generation, but so far it has lasted 12. And we believe that for CMOS it will last forever, except perhaps on the bottom layer next to the advanced node silicon transistors which may require cobalt, nickel, ruthenium or another platinum-group noble metals," Edelstein told EE Times.
Besides Edelstein and Uzoh other researchers at IBM contributing to the project included Chao-Kun Hu, Frank Kaufman, and Barbara Luther. The group presented their preliminary research in Planar Copper Polyimide Back End Of The Line Interconnections at the 1993 International VLSI Multilevel Interconnection Conference and later in more detail in Damascene Copper electroplating for chip interconnections in the IBM Journal of Research and Development.
— R. Colin Johnson, Advanced Technology Editor, EE Times