IBM copper interconnect diffusion barrier innovation

IBM's late-1990s announcement of a copper-on-chip process with a tungsten diffusion barrier, replacing aluminum interconnects. Tom uses it to explain why 3 GHz processor clocks became achievable.

Does anybody know what we're using in the best electronic circuits now as the interconnect metal on the chip? It was a big announcement about 15 years ago by IBM, hit all the front pages of the papers around the world. They used to use aluminum, and the reason they used aluminum — aside from the fact it's easy to vapor-deposit aluminum onto a chip through the polymer mask, so you could vapor-deposit it and make little lines, connections between the little transistors — was because a little bit of aluminum contamination didn't destroy the semiconductor properties of silicon.

IBM's ~1990 announcement of copper-on-silicon with mystery barrier layer (tungsten). Aluminum had been the standard through the 1980s.

It was a big deal about 20 years ago when IBM announced that they were able to put copper on silicon. They had always been using aluminum on silicon. Copper on silicon was a very interesting concept, because ordinarily copper would diffuse into the silicon and destroy all the silicon properties. Aluminum didn't have that problem. They were using aluminized semiconductors all through the 1980s. IBM announced around 1990 that they'd come up with a mystery barrier layer that would prevent diffusion of copper into the silicon. They wouldn't tell anybody what it was, but most people could figure out it was tungsten. They'd lay down a thin layer of tungsten, then they put the copper down, and the copper wouldn't diffuse through the tungsten. Everybody knows what it is now, and most of your better semiconductors today have copper conductors.