Boffins boost circuits with copper connections

Paul Kohl, Thomas L. Gossage chair and Regents Professor at Georgia Tech's School of Chemical and Biomolecular Engineering, explained that the vertical connections between chips and boards are currently formed by melting tin solder between the two pieces and adding glue to hold everything together.

The research shows that replacing the solder ball connections with copper pillars creates stronger connections and the ability to create more connections.

"Circuitry and computer chips are made with copper lines on them, so we thought we should make the connection between the two with copper also," said Professor Kohl.

Solder and copper can both tolerate misalignment between two pieces being connected, but copper is more conductive and creates a stronger bond.

With funding from the Semiconductor Research Corporation, Kohl and graduate student Tyler Osborn developed a novel fabrication method to create all-copper connections between computer chips and external circuitry.

The researchers first electroplated a bump of copper onto the surface of both pieces, a process that uses electrical current to coat an electrically conductive object with metal.

A solid copper connection between the two bumps is then formed by electroless plating, which involves several simultaneous reactions in an aqueous solution without the use of external electrical current.

Since the pillar, which is the same thickness as a $1 bill, is fragile at room temperature, the researchers heated it in an oven for an hour to remove defects and generate a strong solid copper piece.

Osborn found that strong bonds were formed at an annealing temperature of 180 degrees Celsius. He has also been investigating how misalignments between the two copper bumps affect pillar strength.

"I have also studied the optimal shape for the connections so that they are flexible and mechanically reliable, yet still have good electrical properties so that we can transmit these high frequency signals without noise," he said.

The researchers have been working with Texas Instruments, Intel and Applied Materials to perfect and test the technology.

Jim Meindl, director of Georgia Tech's Microelectronics Research Center, and professor in the School of Electrical and Computer Engineering, and Sue Ann Allen, professor in the School of Chemical and Biomolecular Engineering, have also collaborated on the work.

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