Raman thermography – Enabling semiconductor companies to improve the reliability, performance and lifetimes of their devices
Submitting InstitutionUniversity of Bristol
Unit of AssessmentPhysics
Summary Impact TypeTechnological
Research Subject Area(s)
Physical Sciences: Other Physical Sciences
Engineering: Materials Engineering
Summary of the impact
Raman thermography, a new technique for measuring channel temperature in
semiconductor electronic devices developed at the University of Bristol,
has been used by many companies to characterise their semiconductor
devices. The technology has enabled companies to develop more robust,
reliable, higher performing devices and circuits for high-end space,
radar, communication and power conversion applications. This is
illustrated here in detail on the example of the company, United
Monolithic Semiconductor (UMS) (Germany-France), which used the technique
to improve the lifetime of its Gallium Nitride (GaN) and Gallium Arsenide
(GaAs) semiconductor devices so that they meet customer requirements for
product qualification. Corresponding impact resulted for the companies
TriQuint (USA), Northrup Grumman (USA), QinetiQ (UK), Selex Galileo (UK
& Italy), NXP (UK & Netherlands), Thales Alenia Spaciale (France),
Sharp (Japan) and Hitachi Cable (Japan).
Reliable operation of microwave and switching devices made of the
semiconductor materials Gallium Nitride (GaN) and Gallium Arsenide (GaAs)
is essential for their deployment in space, radar, communication, and
power conversion applications, however device lifetime is exponentially
inversely related to device temperature.
Detailed knowledge of the device channel temperature resulting from
Joule's heating, ie the temperature in the device region that determines
its performance and lifetime, is therefore essential for qualifying these
technologies for commercial and military standards. As critical device
dimensions in this semiconductor technology range from the sub-micron to
only a few micron range, accurate temperature measurements with sub-micron
resolutions is essential. This, however, has not been achievable with
current common thermography techniques such as infra-red thermography with
its only many micron spatial resolution.
In 2002  Prof. Kuball's group demonstrated that Raman scattering
spectroscopy can be applied to measure channel temperature in these and
other semiconductor devices with sub-micron spatial resolution throughout
the device structure .
The technique of Raman thermography developed in Bristol is fundamentally
based on the fact that the energy of atomic lattice vibrations, so-called
phonons, in the semiconductor is dependent on the device temperature.
Temperature can therefore be determined with 0.5 micron resolution by
measuring changes in phonon frequencies between when a device is switched
on and off. Such spatial resolution can be achieved using standard
microscope objectives, if even smaller dimensions (100-300nm) are
required, then solid immersion lenses can be employed.
A key challenge in the development of the Raman thermography technique
was the separation of temperature effects and stress/strain effects that
influence the phonon energy in the semiconductor devices . This
involved gaining a detailed understanding of how temperature and
stress/strain affect phonon modes, and how this can be taken advantage of
in a semiconductor device context. This understanding was used to develop
the technique to accurately quantify channel temperature in semiconductor
devices, and also heat transfer across interfaces inside the semiconductor
devices, including the impact of material imperfections such as
dislocations . This also included the implementation of the ability to
measure temperature transients in semiconductor devices with nanosecond
time resolution, which is essential both for microwave and for power
switching devices . The research was subsequently used to thermally
optimize semiconductor structure for optimized devices of high reliability
Apart of Professor Kuball (in Bristol from 1997), PhD students
and postdoctoral researchers of his research group contributed to this
development and subsequent impact: Dr Hayes / PhD student
2002-2005, Dr Pomeroy / PhD student 2003-2006 & postdoctoral
researcher 2006 - present; Dr Sarua / postdoctoral researcher
& subsequently research fellow and lecturer, 2003 - present; Dr G.
Riedel / PhD student 2006-2009; Dr. T Batten / PhD student
2007-2010; Dr R. Simms / PhD student 2007-2010; Dr N. Killat
/ PhD student 2009-2012 & postdoctoral researcher 2012-2013.
References to the research
 *M. Kuball, J.M. Hayes, M.J. Uren, T. Martin, J.C.H. Birbeck, R.S.
Balmer, and B.T. Hughes, Measurement of temperature in high-power
AlGaN/GaN HFETs using Raman scattering, IEEE Electron Dev. Lett. 23,
7 (2002), doi: 10.1109/55.974795
 J.W. Pomeroy, M. Kuball, D. J. Wallis, A. M. Keir, K. P. Hilton, R.
S. Balmer, M. J. Uren, T. Martin, and P.J. Heard, Thermal mapping of
defects in AlGaN/GaN heterostructure field-effect transistors using
micro-Raman spectroscopy, Appl. Phys. Lett. 87, 103508
(2005), doi: 10.1063/1.2041823
 A. Sarua, Hangfeng Ji, M. Kuball, M. J. Uren, T. Martin, K.
J. Nash, K. P. Hilton, and R. S. Balmer, Piezoelectric strain in
AlGaN/GaN heterostructure field effect transistors under bias, Appl.
Phys. Lett. 88, 103502 (2006), doi: 10.1063/1.2182011
 G.J. Riedel, J.W. Pomeroy, K.P. Hilton, J.O. Maclean, D.J. Wallis,
M.J. Uren, T. Martin, U. Forsberg, A. Lundskog, A. Kakanakova-Georgieva,
G. Pozina, E. Janzén, R. Lossy, R. Pazirandeh, F. Brunner, J. Würfl, and
M. Kuball, Reducing Thermal Resistance of AlGaN/GaN Electronic Devices
Using Novel Nucleation Layers, IEEE Electron Dev. Lett. 30,
103 (2009), doi: 10.1109/LED.2008.2010340
 *G.J. Riedel, J.W. Pomeroy, K.P. Hilton, J.O. Maclean, D.J. Wallis,
M. J. Uren, T. Martin, M. Kuball, Nanosecond Time-scale Thermal
Dynamics of AlGaN/GaN Electronic Devices, IEEE Electron Dev. Lett. 29,
416 (2008), doi: 10.1016/j.sse.2010.11.002.
 *A. Manoi, J.W. Pomeroy, N. Killat, and M. Kuball, Benchmarking
of Thermal Boundary Resistance in AlGaN/GaN HEMTs on SiC Substrates:
Implications of the Nucleation Layer Microstructure, IEEE Electron
Dev. Lett 31, 1395 (2010), doi: 10.1109/LED.2010.2077730
Grants supporting this research and its application include 11 grants
with value in excess of £3M from the UK Technology Strategy Board (TSB)
and Engineering and Physical Sciences Research Council (EPSRC), the
European Defense Agency (EDA), European Space Agency (ESA), EC Framework
Programme 7 (FP7), the US Office of Naval Research (ONR) and the Defense
Advanced Research Projects Agency (DARPA). Highlight grants include:
Next Generation GaN-on-Diamond RF Power Amplifier (PI Kuball, DARPA
(USA), $150k, 2011-2013)
MANGA (manufacturable GaN): SiC substrates and GaN epi wafers
supply chain (PI Kuball, EDA (EC), £300k, 2010-2014).
Design for Reliability of Future Technologies (DRIFT) (PI
Kuball, ONR (USA), $500k, 2008-2013).
GaN Reliability and Technology Transfer Initiative (PI Kuball,
ESA (EC), Euro 150k, 2008-2010).
Novel time-resolved thermal imaging: AlGaN/GaN heterostructure
field effect transistors (PI Kuball, EPSRC, £400k,
Thermal imaging of active AIGaN/GaN field effect transistors using
micro-Raman spectroscopy (PI Kuball, EPSRC, 2004-2007, £
Details of the impact
Linking the research to impact
Raman thermography technology developed at Bristol was made available to
UK, European, US and Asian companies to characterize their semiconductor
electronic devices, from 2007 onwards. The companies were made aware of
the new technique developed at Bristol at European Space Agency (ESA) —
Ministry of Defense (MoD) workshops in Europe, CS-Mantech in the USA and
other industry meetings. As result of this, measurements were performed in
Bristol on 100s of industry supplied devices, paid by industry (total in
excess of £200k), to thermally optimize device structures, determine
device lifetime, and ultimately for the qualification of new technology to
fulfil industry and/or military standards to generate industrial impact.
This work also resulted in a patent [A].
Nature, extent and beneficiaries of the impact
The research enabled companies to develop more robust, reliable, higher
performing devices and circuits for high-end space, radar and
communication applications, in particular using GaN and GaAs technology.
Channel temperatures determined on the devices were correlated against
device failure rates, to predict device lifetime. The results were used by
the companies to optimize processing and device design for enabling better
and more reliable devices. Typically lifetimes of these devices in the
early stage of our interaction with the companies were in the 1000s of
hours, however, at least 106 hours in many cases 109
hours are needed for most commercial applications which most companies we
work with now achieve.
A typical example of how this technology has been implemented by industry
is United Monolithic Semiconductor (UMS), a joint venture between THALES
(France) and EADS GmbH (Germany) [B]. UMS is seen by European defence and
Space agencies and companies as the European supplier of US International
Traffics and Arms Regulation (ITAR) free GaN technology, as the export of
GaN technology from the US is heavily restricted.
Dr. H. Blanck from UMS states [B]: "As a result of these channel
temperature measurements we were able to freeze and qualify our GaN
technology, in addition support the further development of our current
commercial GaAs products, also providing customers with device
implementation critical technology information", ie UMS was able to
implement defined new production processes (`freeze') that are now used
for providing commercial GaN devices and circuits, which meet the needed
reliability criteria, and to support GaAs customer needs. There is no
other experimental technique other than Raman thermography available that
can achieve the required accuracy in channel temperature information for
the qualification of GaN and GaAs technology [C].
Similar work to characterize channel temperature in semiconductor devices
was carried out for TriQuint (USA) [D], Selex Galileo (UK & Italy)
[E], furthermore for Thales Alenia Spaciale (France), NXP (UK &
Netherlands), QinetiQ (UK), Northrop Grumman (USA), Sharp (Japan) and
Hitachi Cable (Japan). Raman thermography was for example used by TriQuint
USA in 2010 [D] to validate its thermal simulation models to enable
accurate channel temperature quantification for its GaN technology, to
enable its commercial GaN technology development. It also was used to
enable the development of the TriQuint GaN-diamond integrated transistor
in 2012/2013 which featured in recent TriQuint and DARPA press releases
[F] developed for high end US defense applications.
Due to the uniqueness of the Bristol developed equipment, the
measurements were performed in the laboratories of the University of
Bristol, with devices sent by the companies or in cases Research &
Development engineers of the companies travelled with the devices to
Bristol due to their sensitive nature (including from the USA). The
University of Bristol is in process of licensing the IP of Raman
thermography to the semiconductor equipment manufacturer Quantum Focus
Instruments (QFI), San Diego, USA, also to develop a low cost version of
the Raman thermography system.
Evidence to corroborate the impact
UMS states [B] "For reasons of commercial confidentiality it is not
possible to give sales figures or any equivalent. However, I can confirm
that UMS have invested significantly in commercialising the GaN
technology, and that it now forms a strategic part of our business
generating significant revenue. In addition, GaAs RF technology already
is a large part of our sales figures. The technology developed at the
University of Bristol and its use on our technology significantly
benefited us financially. In 2009, the headcount at UMS was 255 people,
for sales of 47M€."
It is clear that the work done at Bristol has enabled the company to
enter the GaN market earlier than would otherwise have been possible,
giving it a competitive advantage over its rivals.
As due to confidential reasons sales figures can not be disclosed by UMS,
the large commercial impact of the Bristol development for UMS is
demonstrated by its willingness to directly pay for contract measurements
on their devices in Bristol. These measurements were performed on a daily
rate charge basis of £1200 + VAT, typically within 3-5 measurement days in
each measurement campaign. UMS states [B] "our direct financial
contribution to the University of Bristol for contract measurements well
exceeded £50,000, in addition to the direct benefits to us from
measurements performed by Bristol on our technology paid to Bristol by
ESA under the GREAT2 contracts, and by the
European Defence Agency (EDA) under the MANGA (manufacturable GaN)
contract." Direct UMS contract measurements were performed 2007 -
present. Financial contributions to Bristol for contract measurements from
ESA (`High power GaN HFET for ESA applications — GREAT2 [G])
amounted to Euro 150k (2008-2014), and by the European Defense Agency
(MANGA [H]) to £300k (2010-2014).
Sources to corroborate the impact
[A] Patent Measurement of semiconductor temperature by Raman
scattering, M. Kuball and J.M. Hayes, US Patent 6,786,637. Describes
the experimental methodology for temperature measurement in semiconductor
devices, to measure temperature with submicron spatial resolution which
was used for this Impact Study and describes the Bristol IP rights of this
[B] Support letter from United Monolithic Semiconductor, dated 15 April
2013, Dr. Herve Blanck, Manager Technology Research and Development, UMS.
Dr. H. Blanck verifies that UMS was able to implement new production
processes and for the commercial impact of the Bristol work for UMS.
[C] "Bristol University and Quantum Focus Set to Revolutionise Thermal
Imaging", III-V Review 18 (7), 30 (2005). Describes the
joined development of a commercial Raman thermography tool for
semiconductor industry, by Quantum Focus Instruments, San Diego, USA and
the University of Bristol.
[D] Contact name: Dr. Jose Jimenez, Development Engineering Fellow,
TriQuint USA; N. Killat, M. Kuball, T. Chou, U. Chowdury, and J, Jimenez,
Temperature assessment of AlGaN/GaN HEMTs: A comparative study by
Raman, electrical and IR thermography, Proceedings of International
Reliability Physics Symposium (IRPS) 2010. Raman thermography has been
used by TriQuint to validate its thermal simulation models, for its
[E] Contact name: Dr. Graham Morrison, Senior Engineer, Selex Galileo UK;
J.W. Pomeroy, M. Bernardoni, D.M. Craig, G.D. Morrison, B. Wilkinson, and
M. Kuball, Comprehensive Thermal Analysis of Pulsed GaAs HPAs for
Lifetime Estimation, Proceedings of International Microwave
Symposium (IMS) 2013. Characterization of channel temperature in
semiconductor devices to validate Selex Galileo thermal device models to
support and enable advanced circuit design for product development.
Development of the TriQuant GaN-diamond integrated transistor, as part its
line of future products, funded as part of the USA Defense Advanced
Research Projects Agency (DARPA) project NJTT.
Impact of GaN technology for European Space Industry as result of the
European Space Agency (ESA) project GREAT2, and therefore Bristol impact
as part of this project for European development of US Import and Export
Restriction (ITAR) GaN technology.
[H] M. Mikulla, S. Storm, N. Helenius, M. Poisson, E. Janzen, E. Zanoni,
and M. Kuball, Manga: Manufacturable GaN, Proceedings of Microwave
Integrated Circuits Conference 2011. Description of Bristol contributions
to the ESA project GREAT2 to develop European ITAR free GaN technology.