Carbon nanotube heatsink

Fujitsu has developed a carbon nanotube-based package for semiconductor devices that it plans to use to enhance the performance of high frequency amplifiers.

In wireless communication systems, there is an increasing demand for higher power and higher frequencies in amplifiers used in mobile phone base stations. And because the high power transistors used in the amplifiers generate high levels of heat, heat dissipation is extremely important.

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.

Then, by forming miniature carbon nanotube bumps patterned to match a high power transistor’s miniature electrode pattern with a width no greater than 10 micrometers, they connect the carbon nanotube bumps to the flip-chip.

 Face-up mounted high power amplifier

 Flip-chip high power amplifier using carbon nanotube bumps

 Carbon nanotube bumps

Compared to conventional face-up structures, the inductance of the new package 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.