Fujitsu turns to carbon nanotubes as heatsinks

Fujitsu Laboratories has reported success in using carbon nanotubes as a heatsink for semiconductor chips.

Fujitsu Laboratories has reported success in using carbon nanotubes as a heatsink for semiconductor chips. The company turned to the tiny tubes as a potential replacement for metallic heatsinks for amplifiers that will be used in next generation cellular telephone base stations and said the same basic technology could one day be employed with more common system LSI (large scale integrated circuit) or processor chips.

Carbon nanotubes are hollow cylinders made from carbon atoms and are just a few nanometers in diameter. A nanometer is one-millionth of a millimeter. Since being discovered in 1991 they've been the subject of much research because of properties that are attractive for many applications: they can carry a high current, have a low electrical resistance comparable to copper, and a low thermal resistance comparable to diamond.

For the heatsink application the latter two properties are key, said Yuji Awano, a research fellow at Fujitsu Laboratories' nanotechnology research center, in an interview.

Current high-power amplifiers in cell phone base stations have transistors mounted on top of metallic blocks connected to a heatsink to get rid of the heat generated in use. In this arrangement the transistors are above the circuit board so wire connections are needed between to board and transistor but these increase inductance and that brings down the gain of the amplifier. New cell phone systems that operate at higher frequencies will need greater gain than today's amplifiers so the wires become more of a problem, said Fujitsu.

One potential solution is to flip the chip over and connect the transistors to small metallic bumps on the circuit board. This does away with the wires but such an arrangement isn't good at dissipating heat, said Awano.

To get over both the induction and heat problems Fujitsu turned to carbon nanotubes because they are good at both carrying electricity and heat, said Awano.

Carbon nanotubes can be accurately grown in patterns so Fujitsu designed a pattern that matches the layout of the transistors. The pattern consists of groups of nanotubes, called 'bumps', which measure as little as 10 microns in width and about 15 microns high.

The result is a gain of up to 2dB over a traditional amplifier using wires at frequencies over 3GHz and this gain could be greater with more optimization, said Awano.

Fujitsu will continue research of the system to improve its performance and other characteristics and is hoping to have it ready for use in practical applications, such as amplifiers for cellular base stations, in around three years from now, said Awano.

The technology could also be used as a base for heat dissipation in other applications including powerful chips, such as LSIs (large scale integrated circuit) or computer processors. Such chips often require large heat sinks so carbon nanotubes represent a potential way to dissipate that heat although more research is required before such applications become possible, said Awano.

Details of the carbon nanotube heatsinks were also presented at the IEEE International Electron Devices Meeting that took place in Washington, D.C., last week. The work is the result of a joint project between Fujitsu Laboratories and Fujitsu that received funding from Japan's New Energy and Industrial Technology Development Organization (NEDO).

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Martyn Williams

IDG News Service
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