December
5, 2005 — Fujitsu
Limited and Fujitsu Laboratories, Ltd. today announced
that they have succeeded in the world's first development
of carbon nanotube ( 1 )
-based heatsinks for semiconductor chips. The use
of carbon nanotubes as heatsinks for high-frequency
high power amplifiers successfully achieves heat
dissipation and high amplification simultaneously.
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.
Details of the technology will be presented at the
IEEE International Electron Devices Meeting (IEDM)
to be held in Washington, D.C. from December 5. This
research is part of the Advanced Nanocarbon Application
Project consigned to the Japan Fine Ceramics Center,
by Japan's New Energy and Industrial Technology Development
Organization (NEDO).
Background
The volume of information transmitted in our information
age continues to grow, and in wireless communications
there is an increasing demand for higher power and
higher frequencies in amplifiers used in mobile phone
base stations. Because high power transistors, the
output source for high-performance amplifiers, generate
high levels of heat, heat dissipation is extremely
important. Conventionally, heat was dissipated through
the use of what is known as the "face-up structure" (Figure
1), in which a high power transistor chip would be
connected directly dice-bonded to the package and
the heat would escape through the chip.
Technological Challenges
At higher frequencies, amplifiers using the face-up
structure suffer from reduced amplification. The
cause is inductance ( 2 )
from the metal wire, through which the electrical
current flows from the electrode of the transistor
chip to the electrode of the package. One solution
is to flip over the transistor chip and connect the
chip electrode and the package electrode with short
metallic bumps ( 3 ) made
from gold or other metals, in what is known as a "flip-chip
structure"(Figure 2). 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 Technology
Fujitsu's new technology enables the simultaneous
achievement of high amplification and heat dissipation,
in high-frequency, high power amplifiers. Fujitsu
has succeeded in the world's first application of
carbon nanotubes, which have excellent thermal conductivity,
for the bumps in a flip-chip structure (Figures 2
and 3).
Key features of the technology:
1. Technology to grow carbon nanotubes This is a
technology that uses an iron catalyst coating to
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.
2. Technology to connect the carbon nanotube bump
to the flip-chip Taking advantage of the process
miniaturization features of carbon nanotubes, by
forming a miniature carbon nanotube bump patterned
to match a high power transistor's miniature electrode
pattern with a width no greater than 10 micrometers,
Fujitsu succeeded in connecting the carbon nanotube
bump to the flip-chip.
Results
With the technology, Fujitsu was able to connect
carbon nanotube bumps to the miniature electrode
of a high power transistor. Carbon nanotubes have
thermal conductivity of 1400W/(m-K) - a level much
higher than that of metal ( 4 )
, 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 5 gigahertz or greater.
Future Developments
Fujitsu intends to continue to refine the site density
of carbon nanotubes in bumps to enable further improvements
in heat dissipation, paving the way for development
of high-frequency, high power flip-chip amplifiers
using carbon nanotube bumps. Fujitsu targets deployment
of this new technology in base stations for next-generation
mobile communication systems in approximately three
years.
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Figure
1: Face-up mounted high power amplifier 
