Graphene nanotubes promise better stacked chips

A Swedish team of researchers has linked two chips using carbon nanotubes, which they say promise to be more reliable than copper interconnects for commercial production of 3D chip stacks

Swedish researchers have used carbon nanotubes to connect two stacked-up chips, which they say could pave the way for commercial production of 3D circuits.

Graphene nanotube connections

Researchers in Sweden have used carbon nanotubes to connect two stacked chips. Image credit: Teng Wang, Kjell Jeppson, Lilei Ye, Johan Liu/Chalmers University

The experiment involved nanotubes made of graphene being run through silicon to create vertical connections between two processors, scientists at the Chalmers University of Technology in Gothenburg, Sweden, said on Monday.

Other work is underway on using copper interconnections to stack chips, and in September, IBM said it was working with 3M to develop an adhesive that allows shorter copper connections. However, the Swedish researchers believe the thermal limitations of copper mean it would lead to less reliable 3D electronics than carbon.

"Potentially, carbon nanotubes have much better properties than copper, both in terms of thermal and electrical conductivity," Kjell Jeppsson, a member of the research team, said in a statement.

"Carbon nanotubes are also better suited for use with silicon from a purely mechanical point of view. They expand about the same amount as the surrounding silicon while copper expands more, which results in mechanical tension that can cause the components to break," he added.

Manufacturing potential

Chip-stacking using carbon nanotubes to conduct electrical impulses could allow electronic components to be packed densely and let manufacturers "build tiny, well-functioning units", the Chalmers team said.

The researchers linked two chips using thousands of carbon nanotubes per interconnection. The chips were bonded using an adhesive so the interconnects were touching, allowing a current to be conducted through the chips.

One difficulty with production of the nanotubes was the length of the tubes needed to go through the chip, according to researcher Teng Wang.

"We have produced tubes that are 200 micrometres long, which can be compared to the diameter, which is only 10 nanometres," said Wang. "Their properties, however, are not yet perfect."

Moreover, work is needed on bringing down the temperatures used in production of the carbon nanotubes before the process can be used in commercial manufacturing of 3D electronics, Wang said. Production of nanotubes is at temperatures in excess of 700°C, and needs to be brought down to around 450°, he noted.

Graphene is carbon one atom thick that is being put to use in a number of chip technologies. IBM outlined some uses of graphene in chips at an IEEE meeting last week, including a silicon wafer with graphene components.

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