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OCR for page 65
7
Conclusions and Recommendations
There are great opportunities for wide bandgap
semiconductors to improve the performance of many
nonelectronic technologies. Major benefits to system
architecture would result if cooling systems for
components could be eliminated without compromising
system performance (e.g., power, efficiency, speed). The
existence of commercially available high-temperature
semiconductor devices would provide significant benefits
in such areas as:
· sensors and controls for automobiles and aircraft;
· high-power switching devices for the electric
power industry, electric vehicles, etc.; and
· control electronics for the nuclear power
industry.
With the possible exception of LEDs, however,
present commercial demand for wide bandgap
semiconductor materials is limited. While there are few
pressing applications that cannot be achieved without wide
bandgap materials, the vast array of applications, and
hence the value, will only be realized once these materials
have evolved to such an extent that off-the-shelf devices
are available.
This chapter is divided into two sections. The first
section presents general conclusions and recommendations
about future research priorities to accelerate the
acceptance of high-temperature semiconductor materials.
This section discusses the temperature ranges for the
different materials to be used, the competitiveness of U.S.
research versus foreign competition, the systems in which
high-temperature electronic materials should initially be
introduced, and the government/industry/un~versity
collaborations required to forward the development of
high-temperature semiconductor materials. The second
section discusses the barriers to the successful
65
development, manufacture, packaging, and integration of
wide bandgap materials into existing systems and presents
the key research and development priorities to overcome
these barriers.
GENERAL CONCLUSIONS AND
RECOMMENDATIONS
Temperature Ranges
Silicon and silicon-on-insulator (SOI) electronics may
be sufficient for some applications for temperatures up to
300 °C. Such applications include digital logic, some
memory technologies, and some aerated analog and power
applications. Silicon-based technology will not be
sufficient for many applications operating in the 200-
300 °C range, however, such as power-conditioning
devices in higher-temperature control systems. These
devices will have to be produced from another material
system. Devices based on SiC are well positioned to meet
this need, particularly e-channel enhancemer~t-mode
MOSFETs. However, significant technological barriers,
such asmicropipes, oxide qualify, contacts, metallization,
packaging, and reliability evaluation still need to be
further addressed.
As a result of fundamental limitations, silicon-based
technologies will not be useful at temperatures above
300 °C. Other materials must be used for these
temperature ranges, but the choices are somewhat less
clear. Technology based on GaAs might be used for
systems operating up to 400 °C. Just working at elevated
temperatures is not the only concern, however. It is also
essential that the devices reliably function over a wide
range from very cold (i.e., -20 °C) to very hot (i.e.,
400 °C). Based on the evidence presented in this report,
OCR for page 66
Materials for High-Temperature Semiconductor Devices
devices based on e-type SiC are the only type that
currently appear to meet the temperature-range and
reliability requirements, but additional development is
needed. Eventually, high-temperature electronic
technology could be developed for reliable operation even
for temperatures above 600 °C.
U.S. Competitiveness
As described in the Preface, considerable international
resources are currently being devoted to developing
electronic technologies either tailored for or supportive of
high-temperature operation. The United States is focusing
most of its efforts on high-temperature applications and
currently has a slight lead in SiC research.
Europe appears to be increasing its effort in wide
bandgap materials, especially for power electronics. This
research area is synergistic with high-temperature
applications because the generation of internal heat is a
limiting factor in power devices and can be mitigated by
larger bandgap and higher thermal conductivity materials.
The dedication of European resources to this area is seen
in the founding of the collaborative organization HITEN,
which was established in 1992 to coordinate nascent
European efforts in high-temperature electronics.
Japan is emphasizing the use of wide bandgap
materials for opto-electronics and leads in the use of
nitrides for light sources. Japan is also becoming
interested in power and high-temperature applications.
Unfortunately, the closed nature of Japanese industry
made it difficult for the committee to determine the true
level of interest in wide bandgap materials research. The
increased interest in high-power, high-temperature
applications is evident in Japan's annual domestic SiC
conference, however. The Third Domestic (Japan) SiC
Conference convened in Osaka on October 27-28, 1994,
with approximately 160 experts in attendance. Contrary to
Japan's previous two conferences, there was a greater
emphasis at the Osaka conference on high-power, high-
temperature applications than on LEDs.
The Commonwealth of Independent States had a
number of major programs in SiC development, but the
current iFinanciall difficulties of most of the
Commonwealth's institutions are preventing many
laboratories from continuing their research. There is a
wealth of expertise and information available for
leveraging by other countries, however. For instance, the
66
European Community is planning on supporting a SiC
growth effort in St. Petersburg (Y.M. Tairov and V.E.
Chelnekov, personal communication, 1994~.
The committee believes that the U.S. wide bandgap
materials research community is currently very
competitive in the international research community. To
remain competitive in the international research
community, the committee recommends that demonstration
technologies be pursued to motivate further research and
increase interest in high-temperature semiconductor
applications.
Demonstration Technologies
To increase interest and motivate further research in
wide bandgap materials, a realistic, inspiring application
focus must be found that can make system designers aware
of the benefits of high-temperature electronics. A wide
bandgap transistor that operates at 150 °C will not drive
the technology because it will be in direct competition
with the more economically efficient silicon technologies.
The demonstration technologies must be system circuits
(i.e., not an individual device) that can be inserted into
essentially nonelectronic systems (e.g., turbine engine,
nuclear reactor, chemical refinery, or metallurgical mill)
with the goal of measurably increasing system
performance.
As discussed in Chapter 1, the committee believes
that there eventually will be a niche market for
semiconductors with temperature capabilities higher than
that of silicon, and that this market will be sufficiently
large to justify the cost of development. However, this
belief is tempered by the recognition that because such
electronics will be used in new ways there is little
immediate demand. The market will grow only in synergy
with the availability of components. This suggests that
development of high-temperature electronics not be
undertaken in isolation. Instead, such development can
and should be leveraged from development of other
technologies with more immediate applications, thus
reducing the costs and the risks of both. Three suitable
application areas are high-power electronics, nuclear
reactor electronics, and opto-electronics.
Power switching devices, for example, would be a
good demonstration technology for high-temperature
semiconductor materials. High-voltage, high-power
electronics, while not necessarily used as high-temperature
OCR for page 67
Conclusions and Recommendations
devices, nevertheless need wide bandgap semiconductors
because of their superior breakdown voltages and high
thermal conductivities. There is already considerable
research being pursued in this area because (1) improved
high-power switching devices could save an estimated $6
billion in the cost of construction of additional
transmission lines; and (2) the smoother, more efficient
use of the transmission system would reduce the need for
new generating capacity, which the Electric Power
Research Institute estimates would be a savings of $50
billion in North America alone over the next 25 years
(Spitznagel, 1994~.
The pursuit of demonstration technologies would not
only increase interest in wide bandgap materials, it would
also provide significant testbeds for the application of the
technology and enhance our understanding of the generic
technologies recluired to further high-temperature device
operation (e.g., materials etching and implantation;
degradation modes of metallic gates, contacts, and
interconnects at high temperatures; packaging behavior at
high temperatures; and accelerated-testing and reliability-
testing methodologies to ensure proper functioning). The
ability to grow a reasonably defect-free material is not the
only requirement for the realization of a successful
technology. The development of demonstration
technologies would also help identify other factors that
must be resolved for high-temperature electronics to be
incorporated into existing systems.
Funding Strategy
The need for new development funds for
demonstration technologies and future wide bandgap
materials is not necessary in the comunittee's opinion.
Government funding currently exists for long-range
research in wide bandgap materials, although additional
funding would certainly allow more options to be
evaluated within a shorter period of time. Industry has
also demonstrated a willingness to commercialize new
developments if the projected payback to their investments
can occur within the short term (NRC, 1993~. The
committee believes that the high-temperature research
community should leverage the research funding for wide
bandgap materials that Is currently being provided by the
high-power and optics markets, where no viable
alternatives to wide bandgap materials currently exist.
67
Building on the funding for other areas dependent on wide
bandgap materials reduces the need for potential users of
high-temperature devices to fund the required materials
development exclusively and, thus, may render it cost
effective.
The committee recommends the following strategy for
the development of wide bandgap materials:
develop precompetitive alliances and integrated
programs (national laboratories, universities, and
industries) for coordinating research, technical
skills, and capabilities to expedite research in the
most efficient manner;
direct research at a technology demonstrator that
has definite applications (i.e., is a product) and
addresses the usually neglected areas of
packaging, assembly, testing, and reliability
(e.g., high-power switches; integrated motor
control; power phase shifter);
concurrently develop materials, design, testing,
and packaging; and
build and test the demonstration component on a
cost-share basis that encourages teaming, ensures
adequate funds, and requires periodic deliveries.
The committee believes that the founding of a
newsletter that provides a summary of published
worldwide developments in high-temperature
semiconductor research would assist the establishment,
development, am maintenance of (1) a fundamental
long-term materials effort, (2) an infrastructure within the
industry, (3) a group to monitor interrzational
development, arid (4) a U.S. information group for
highlighting advances.
MATERIALS-SPECIFIC CONCLUSIONS
AND RECOMMENDATIONS
The first three parts of this section concentrate on the
major wide bandgap materials discussed in this report:
SiC, nitrides, and diamond. The final part of this section
concerns the generic problems in packaging that will
affect the production of all high-temperature electronic
O
c .evlces.
OCR for page 68
Silicon Carbide
SiC is an indirect bandgap semiconductor and has
enjoyed the longest history and greatest development with
regard to both materials growth and device realization. As
such, SiC is currently the most advanced of the wide
bandgap semiconductor materials and in the best position
for near-term commercial application. Its main application
will be in high-power, high-temperature, high-frequency,
and high-radiation environments. It will not be suitable for
blue lasers or ultraviolet light emitters, however, except
as a potential substrate material. The specific technical
issues for SiC that require further research are
summarized in the box, Technical Issues for SiC. The
three key research efforts for the development of
commercially viable SiC devices are
Wafer production: The 1- and 2-inch SiC wafers
now in production are rapidly approaching device
quality where they might be used for commercial
production of devices and circuits with
acceptable yield. It could be argued that such
small wafers are entirely sufficient for what will
be a relatively small market (compared with
silicon) with a very high-price premium, and
therefore an early investment in larger wafers is
not justified. However, the entire commercial
infrastructure for electronics manufacture is
based on a wafer size of at least 3 inches, and
Technical Issues for Nitrides
Substrate development
Nitride substrates for CVD homo-epitaxy
High thermal conductivity, quasi-lattice matching substrates
(bow high electrical conductivity and semi-insulating)
Further improvement of crystal perfection and doping (CVD
grown)
Reduce defect density and background impurities
Better control of n- and pipe doping
New technologies for epitaxial growth
Improve surface morphology
Improve processing (CVD growth)
Ohmic contacts
Low contact resistance
High-temperature contacts
High-temperature packaging
Improve understanding of basic properties and knowledge of
design parameters
Materials for High-Temperature Semiconductor Devices
Technical Issues for SiC
Purger improvement of crystal perfection (boule and CVD
growth)
Eliminate micropipes
Reduce defect density
Reduce background impurities
Improve surface morphology
Further improvement of doping (boule and CVD growth)
New n- and p-type dopants
New mid-gap impurities for semi-insulating substrates
Introduce rare earth elements in growth
Improve processing (boule and CVD growth)
Improve oxides/passivation
Find alternative insulators (nitrides)
Reduce contact resistivin,r for pipe material
Develop high-temperature n- and p-type contacts
High-temperature packaging
Improve understanding of basic properties and knowledge
of design parameters
preferably 4 inches, as a minimum.
Reconstructing a small-wafer infrastructure that
became obsolete over 30 years ago will be both
an expense and an obstacle to the introduction of
commercial SiC electronics. The committee
believes that the development of larger SiC
wafers is viewed as the more cost-effective
approach to commercial development.
Film growth: Chemical vapor deposition,
molecular-beam epitaxy, and other film-growth
technologies and chemistries require refinement
to produce epitaxial films with n- and p-type
doping ranges from 10~3 to 102° cm~3 for
nitrogen, aluminum, boron, gallium, transition
metals, and rare earth elements.
Manufacturing processes: Lower-cost device-
production methods are required to make the
manufacture of SiC devices more competitive
with the silicon technologies.
.
Nitrides
Interest in the direct bandgap nitride materials (i.e.,
GaN, A1N, AlGaN, and InGaN) has dramatically
increased recently because of their optical properties. The
materials show great promise and are likely to dominate
the visible and ultraviolet opto-electronics market.
Nichia's recent bright blue LEDs have already stimulated
increased industrial effort (e.g., Hewlett Packard, Spectra
68
OCR for page 69
Conclusions and Recommendations
Technical Issues for Diamond
Improvement of grown, crystal perfection, and growth
Reduce and control impurities of bulk synthetic diamonds
Produce large-area (hetero-epitaxy) single-crystal films of
diamond on nondiamond substrates at reasonable cost
Synthetically produce larger bulk diamond at reasonable cost
Improve n- and p-type doping
Improve implantation for doping
Improve processing
Ohmic contacts
Low contact resistance
High-temperature contacts
Hydrogen passivation
Improved understanding
Dopant diffusion
Knowledge of design parameters
Diode Laboratories, Xerox PARC) in materials growth,
contact metallurgy and reliability, and device reliability
and testing, although the materials have defect densities of
greater than 10~°/cm2 and the mechanism of photo
emission is currently unknown. Heterojunctions in the
nitrides also hold promise for higher-speed devices
compared with SiC.
Their applicability for power
development and nlgn-~requency devices is unproven at
this time, and the technologies for wafer production,
doping, and etching are currently less developed than SiC
and require more longer-term research before they will be
competitive with other electronic materials. However, as
development of photonic applications for wide bandgap
materials progresses, the opto-electronic market may
provide an effective way to leverage the development of
these materials for high-temperature device applications.
The specific technical issues for nitrides research is
summarized in the box, Technical Issues for Nitrides. The
committee identified the following three research efforts
as being key to the development of nitride devices:
· Compatible substrates: Better-matched substrates
are required for nitride wafer production to be
commercially tenable.
· Wafer production: Growth of quasi-crystalline
films of GaN, AlGaN, and A1N should be
pursued on substrates such as SiC to gain
thermal advantages.
· Doping: Methods for both n- and p-type doping
of Group III nitrides are required.
69
Diamond
Diamond is a well-understood material, but its use for
active electronic device applications is not feasible at this
time because of the difficulties associated with its
economical growth and doping. While diamond transistors
have been designed, fabricated, and tested, their
performance is also orders of magnitude less than that
which is expected from the electrical properties intrinsic
to diamond. The poor performance is thought to result
from excessive nitrogen impurities and from as yet not
fully explained surface-depletion effects. The current
prognosis for diamond is primarily as a protective
coating, a thermal management film, and a material for
electron-emitting cathodes. The specific technical issues
for diamond research are summarized in the box,
Technical Issues for Diamond.
Packaging
Much more research is required in the area of high-
temperature packaging. For high-temperature electronics
to be commercially viable and provide true performance
advantages, interconnection and packaging technologies
are required that can reliably operate at temperatures up
to 600 °Cfor 1(74 hours. To attain these goals, innovative
packaging techniques will be required. The specific
technical issues for packaging research are summarized in
the box, Technical Issues for Packaging. The three key
research efforts for the development of high-temperature
packages are
· Metallization: Contacts are required in the 10-6 to
10-7 Q/cm2 range that have lon~-term durability
at temperatures
up to 600 °C. Greater
understanding Is needed of the long-term effects
Technical Issues for Packaging
Improve reliability of high-temperature contacts
Improve metallization
Improve device development tools
Improve process-control tools
Improve polishing, cutting, mounting, and etching mends
Develop reliability and aging tests
Develop computer-aided design tools
OCR for page 70
Materials for High-Temperature Semiconductor Devices
of high temperatures on contact and interconnect
metallurgy, degradation and failure modes,
reliability, and interfaces.
Device reliability and aging testing: Existing
methods of accelerated, environmental life testing
of packages must be adapted for high
70
temperature applications to ensure the accurate
assessment of device reliability and aging.
· Computer-aided design tools: Computer-aided
design tools are required that incorporate
electrical and mechanical simulation of high-
temperature electronic systems.
Representative terms from entire chapter:
bandgap materials
