Fujitsu has developed a carbon nanotube-based package of heatsinks for semiconductor chips. The company plans to use them in high-frequency, high power amplifiers for next-generation mobile communication systems.

According to the company, the new technology represents a major step forward in developing practical applications that take advantage of the superior thermal conductivity of carbon nanotubes. It also enables the realization of high-performance amplifiers with high frequency and high power for next-generation mobile communication systems.

Fujitsu's new technology enables the simultaneous achievement of high amplification and heat dissipation, in high-frequency, high power amplifiers. The company has succeeded in the world's first application of carbon nanotubes, which are claimed to have excellent thermal conductivity, for the bumps in a flip-chip structure.

Conventionally, heat was dissipated through the use of what is known as a "face-up structure", in which a high power transistor device would be connected directly dice-bonded to the package.

At higher frequencies, amplifiers using the face-up structure suffer from reduced amplification due to an increase in inductance from the metal wire through which the electrical current flows from the chip to the package.

One solution is to flip over the chip and connect the chip and the package with short metallic bumps made from gold or other metals in a "flip-chip structure".



However, for use in high power amplifiers, conventional metallic bumps have proven inadequate in dissipating the high levels of heat generated by high-power transistors. For these reasons, it has been difficult to develop high-performance amplifiers that can satisfy both high-amplification and heat dissipation at high frequencies. Fujitsu's new nanotube-based packaging technique solves that problem.

To produce the heat sink package, Fujitsu engineers first grow carbon nanotubes to a vertical length of at least 15 micrometers on the wafer substrate. Usually, bumps for flip-chips are required to have a length of at least 10 micrometers.

Carbon nanotubes have thermal conductivity of 1400W/(m-K) -- a level much higher than that of metal, and because it is possible to connect carbon nanotube-based bumps very near to the heat-generating miniature electrodes, Fujitsu successfully achieved the high amplification of flip-chips with heat dissipation levels equivalent to face-up structures.

Compared to conventional face-up structures, ground inductance is reduced by more than half, thereby enabling an increase in amplification of at least 2 decibels at high frequencies of 5GHz or greater.

Fujitsu believes that it will be able to deploy the new technology in base stations for next-generation mobile communication systems in around three years? time.

For additional information and technical details, visit http://www.fujitsu.com.