Fujitsu Technology Bonds Single-crystal Diamond and SiC at Room Temperature; Enables Boost to Radar Performance
2017年12月7日 - 11:18AM
JCN Newswire (英)
Fujitsu Limited and Fujitsu Laboratories Ltd. today announced
development of the world's first technology for bonding
single-crystal diamond to a silicon carbide (SiC)(1) substrate at
room temperature. Using this technology for heat dissipation in a
high-power gallium nitride (GaN)(2) high electron-mobility
transistor (HEMT)(3) enables stable operations at high power
levels. Application of this technology is expected to significantly
enhance the performance of weather radars and wireless
communications.
Boosting range and power in radar and wireless communications also
increases the heat produced by devices, which adversely affects
their performance and reliability. This creates a need to
efficiently carry device heat to a cooling structure.
Single-crystal diamond is known to have good thermal conductivity,
but with existing technologies, the argon (Ar) beams used to remove
impurities in the manufacturing process create a low-density
damaged layer on the surface, which weakens bonding strength. Also,
bonding with an insulating film such as silicon nitride (SiN)
impairs thermal conductivity due to SiN's thermal resistance.
Now, by protecting the surface of the diamond with an extremely
thin metallic film, Fujitsu and Fujitsu Laboratories succeeded in
preventing the formation of the damaged layer and bonding
single-crystal diamond to a SiC substrate at "room-temperature
bonding"(4). Simulations using actual measurements of thermal
parameters have confirmed that devices using this technology would
lower thermal resistance to 61% of existing ones.
This technology promises GaN-HEMT power amps for transmitters to
operate at higher power, and increase the observable range by
roughly 1.5 times when applied to systems such as weather
radar.
This research was conducted in part with support from the
Innovative Science and Technology Initiative for Security,
established by the Acquisition, Technology & Logistics Agency
(ALTA), Japan Ministry of Defense.
Details of this technology are being presented at the IEEE
Semiconductor Interface Specialists Conference (SISC2017), running
December 6-9 in San Diego, U.S.
Background
In recent years, high-frequency GaN-HEMT power amps have widely
been used for long-range radio applications, such as radar and
wireless communications. They are also expected to be used in
weather radar that observes localized heavy rains, for example, or
in the forthcoming 5G millimeter-band mobile communications
protocols. For these types of radars or wireless communications
using the microwave to millimeter-wave bands, by raising the output
of the GaN-HEMT power amps used for transmissions, the distance
that radio waves can propagate will allow the expansion of the
observational range of radar while enabling longer and higher
capacity communications. This is why GaN-HEMT power amps with
higher power output are desirable.
http://www.acnnewswire.com/topimg/Low_FujitsuSiCFig1.jpg
Figure 1: Structure of conventional GaN-HEMT power amp
Issues
In GaN-HEMT power amps, some of the input power is converted to
heat (Figure 1). This heat is dispersed into the SiC substrate, and
is carried away by a cooling structure (heat sink). Although the
SiC substrate has relatively high thermal conductivity, a material
with even better thermal conductivity will be needed for devices
with increasingly higher power output to efficiently carry device
heat to the cooling structure.
Single-crystal diamond has extremely good thermal
conductivity-almost five times that of a SiC substrate-and is known
as a material that can efficiently spread heat. In order to bond a
single-crystal diamond to a device as a cooling material, normal
production processes use an Ar beam to remove impurities, which
produces a low-density damaged surface that weakens the bonds the
single-crystal diamond can form. Moreover, using SiN or other
insulating films for bonding interferes with thermal conductivity
due to SiN's thermal resistance.
About the Technology
Fujitsu and Fujitsu Laboratories have succeeded in developing the
world's first technology for room-temperature bonding onto a SiC
substrate single-crystal diamond, used for high-efficiency cooling
of GaN-HEMT power amps. These two hard materials have different
coefficients of thermal expansion (Figure 2).
To prevent the Ar beam from forming a damaged layer on the diamond
surface, the companies developed a technique that protects the
surface with an extremely thin metallic film before it is exposed
to the Ar beam. In order to ensure the surface is planar, for good
bonding at room temperature, the metallic film is held to a
thickness of 10 nm or less. This technology was confirmed to
prevent the formation of the damaged layer on the diamond surface
after Ar beam exposure (Figure 3), resulting in improved bonding
strength and single-crystal diamond bonded at room temperature to a
SiC substrate for GaN-HEMT.
http://www.acnnewswire.com/topimg/Low_FujitsuSiCFig2.jpg
Figure 2: Structure of GaN-HEMT power amp with bonded diamond
http://www.acnnewswire.com/topimg/Low_FujitsuSiCFig3.jpg
Figure 3: Diamond cross section after Ar beam exposure
http://www.acnnewswire.com/topimg/Low_FujitsuSiCFig4.jpg
Figure 4: A GaN-HEMT/SiC substrate with diamond bonded using this
technology
Results
Thermal resistance, which expresses how difficult it is for heat to
pass through something, was measured in samples that were bonded at
room temperature, and the SiC/diamond interface was found to have
an extremely low thermal resistance of 6.7 x 10-8 m2K/W
(square-meter kelvins per watt). Simulations using this measured
parameter showed that this technology would significantly reduce
thermal resistance of 200W-class devices, to 61%(5) (Figure 5). Use
of this technology promises GaN-HEMT power amps for transmitters
with even higher power output. When used in systems such as weather
radars, GaN-HEMT power amps for transmitters could be expected to
increase the radar's observable range by a factor of 1.5. This
would allow for quicker detection of the cumulonimbus clouds that
can produce sudden rainstorms, and contribute to a safer and more
secure society in terms of disaster readiness.
http://www.acnnewswire.com/topimg/Low_FujitsuSiCFig5.jpg
Figure 5: Simulated comparison of heat in 200W-class GaN-HEMT power
amps
Future Plans
Fujitsu and Fujitsu Laboratories plan to assess the thermal
resistance and output performance of GaN-HEMT power amps that use
this technology, and aim to implement it in high-output,
high-frequency GaN-HEMT power amps in fiscal 2020, with use in
applications for weather radars and 5G wireless communications
systems.
(1) Silicon Carbide (SiC)
A material with relatively good thermal conductivity, about 420
watts per meter per kelvin (W/mK), used for substrates on which
GaN-HEMT crystals are grown.
(2) Gallium nitride (GaN)
A wide band-gap semiconductor material that operates with a higher
breakdown-voltage than semiconductor technologies based on previous
materials, such as silicon (Si)- or gallium-arsenide (GaAs)-based
technologies.
(3) High electron mobility transistor (HEMT)
A field-effect transistor that takes advantage of operation of the
electron layer at the boundary between semiconductor materials with
different bandgaps, which is relatively rapid compared to that
within conventional semiconductors. Invented in 1980 by Fujitsu,
this technology is currently used in a number of IT applications,
including satellite transceivers, cellular equipment, GPS-based
navigation systems, and broadband wireless networking systems.
(4) Room-temperature bonding
A technology in which surfaces of different materials are cleaned
in a vacuum by an argon beam and bonded at room temperature. Also
known as surface activated bonding. This can bond materials that
have different coefficients of thermal expansion.
(5) Thermal resistance reduced to 61%
Equivalent to an 80degC reduction in surface temperature in a 200-W
class device.
About Fujitsu Laboratories
Founded in 1968 as a wholly owned subsidiary of Fujitsu Limited,
Fujitsu Laboratories Ltd. is one of the premier research centers in
the world. With a global network of laboratories in Japan, China,
the United States and Europe, the organization conducts a wide
range of basic and applied research in the areas of Next-generation
Services, Computer Servers, Networks, Electronic Devices and
Advanced Materials. For more information, please see:
http://www.fujitsu.com/jp/group/labs/en/.
About Fujitsu Ltd
Fujitsu is the leading Japanese information and communication
technology (ICT) company, offering a full range of technology
products, solutions, and services. Approximately 155,000 Fujitsu
people support customers in more than 100 countries. We use our
experience and the power of ICT to shape the future of society with
our customers. Fujitsu Limited (TSE:6702) reported consolidated
revenues of 4.5 trillion yen (US$40 billion) for the fiscal year
ended March 31, 2017. For more information, please see
http://www.fujitsu.com.
* Please see this press release, with images, at:
http://www.fujitsu.com/global/about/resources/news/press-releases/
Source: Fujitsu Ltd
Contact:
Fujitsu Laboratories Ltd.
Devices & Materials Laboratory
E-mail: next-press@ml.labs.fujitsu.com
Fujitsu Limited
Public and Investor Relations
Tel: +81-3-6252-2176
URL: www.fujitsu.com/global/news/contacts/
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