The Big Blue sez: copper interconnects are here to stay

November 28, 2017 | 00:00
World's first "copper chip" is a fully functional CMOS 7S microprocessor module on top of a copper wafer. Source: IBM
 
World's first "copper chip" is a fully functional CMOS 7S microprocessor module on top of a copper wafer. Source: IBM
 
20 years since IBM a.k.a., The Big Blue started leading the way in copper interconnects at the then sensational 180 nm node for CMOS circuits many other interconnects are being proposed to replace copper, notably graphene. Despite that, IBM, says it will stick to copper claiming that slight tweaks to the copper deposition make it the preferred option pretty much for as long as CMOS will last.

"Graphene is not readily manufacturable, and furthermore end-to-end comparisons show graphene does not flow uniformly and can't achieve the low resistances of enhanced copper interconnects," IBM Fellow Dan Edelstein said at a recent Nanotechnology Symposium talk. He explained: "copper with a thin cap of cobalt is better than graphene at carrying current and even at the smallest sizes imaginable copper interconnects are still the best solution, perhaps with cobalt, nickel, ruthenium or another platinum-group noble metals brought in to underlay it.”

Since 1997, two main objections against copper were surmounted by IBM. First, copper is said to degrade silicon when it comes into direct contact. That was solved by encasing copper in tantalum-nitride and tantalum in a diffusion barrier all around. Second, since copper had to be encased in the tantalum compound, the popular and established “substrative” method appeared impossible. This was countered by IBM coming up with an additive method with the kind of electroplating used for printed-circuit boards (PCBs).

Copper interconnect technology was initially thought to last only last one generation, but so far it has lasted 12. At the The Big Blue they firmly believe that for CMOS it will last forever, possibly excepting on the bottom layer next to the advanced node silicon transistors which may require cobalt, nickel, ruthenium or another platinum-group noble metal.

Source: EE Times
 
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