Impact of research into transistor carrier mobility measurement on globally used instruments
Submitting InstitutionLiverpool John Moores University
Unit of AssessmentElectrical and Electronic Engineering, Metallurgy and Materials
Summary Impact TypeTechnological
Research Subject Area(s)
Engineering: Materials Engineering
Technology: Communications Technologies
Summary of the impact
Carrier mobility is a key parameter for the semiconductor industry, but
its measurement is characterised by poor accuracy and unreliability for
advanced transistors. The Microelectronics Research Group (RG1), working
with the Logic Devices Consortium at IMEC (Inter-University
Microelectronics Research Centre in Leuven, Belgium), developed a new
technique that overcomes these problems, implemented it on
industrial-standard equipment provided by Keithley Instruments (a US
company based in Cleveland, Ohio), and prepared the application notes and
software. This benefits test engineers in the semiconductor industry
through significant improvement in the accuracy, reliability, cost, and
efficiency of measurements. Keithley is disseminating information to its
global customer base and is highlighting it as strength of its instruments
in the promotion.
Ever since computers were invented, their speed has increased by
developing smaller and faster transistors. Transistor speed is
characterised by the electron mobility, so that its measurement is
essential for evaluating new materials and processes in the semiconductor
industry. The standard measurement technique is based on the split
capacitance-voltage (Split-CV), but it increasingly suffers from poor
accuracy and unreliability as transistors become smaller. Smaller
transistors cause problems, since they have higher gate leakage
and use materials with higher charge trapping. Moreover, the
existing techniques measure the capacitance and conduction current under
different drain voltages, and this drain voltage mismatch
introduces unacceptable errors for the small transistors operating at low
Early attempts to resolve these problems only had limited successes and
required expensive set-up and complex procedures [R2.1]. During this REF
period, a new technique has been developed and demonstrated, which
overcomes the shortcomings mentioned above without using expensive set-up,
as detailed below.
Drain voltage mismatch: To extract mobility, one needs both the
conduction current and the mobile charge. In principle, they should be
measured under the same drain voltage. The existing technique, however,
measures conduction current with non-zero drain voltage, while mobile
charge is measured with zero drain voltage, because the signal for mobile
charge would be buried by conduction current if non-zero drain voltage
were applied. This drain voltage mismatch leads to unacceptable errors for
advanced transistors [R2.1, R2.2].
An early career researcher from the Microelectronics Research Group,
Z.Ji, made a breakthrough by proposing and demonstrating, for the first
time ever, that conduction current and mobile charge could be measured
simultaneously under the same drain voltage, eliminating the drain voltage
mismatch. By raising the ramping rate of gate bias one million times over
its conventional level through applying an ultra-fast pulse, the
displacement current becomes high enough for measuring mobile charge and
will no longer be buried by conduction current [R2.1, R2.2].
Gate Leakage: As the gate oxide is downscaled close to one
nano-meter, large gate leakage buries the signal of high frequency
capacitance meter used for the split-CV [R2.3, R2.4], making the
measurement unreliable. To make signal strong enough, the frequency has to
rise to the radio frequency (RF) or even GHz range. This, however,
requires the costly RF probe station, the network analyser, and complex
test structure. In the new technique, the effect of gate leakage on
capacitance was cancelled out by subtracting the displacement current at
the two pulse edges. The measured capacitance becomes inherently
independent of the gate leakage, overcoming a major obstacle without using
Charge trapping: As high-k dielectric replaces the conventional
SiON, charge trapping in gate dielectric increases. The slow conventional
technique gives time for the trapping to take place during measurement
and, consequently, the measured mobile charge includes the trapped
charges. Since the trapped charges are immobile, they will not contribute
to the current. This leads to an underestimation of mobility. In the new
technique, the ultra-fast pulse used is too quick for trapping to occur
[R2.5, R2.6], suppressing the trapping.
The work was carried out in collaboration with the Logic Devices
Consortium at IMEC, whose roles included identifying the shortcomings of
existing techniques, supplying test samples, and verifying the proposed
technique as users. The impact, however, was delivered mainly through
Keithley Instruments, with IMEC playing a complementary role, as detailed
in section 4.
References to the research
(`**' denotes the three papers that best illustrate the quality of the
[R2.1] **Z. Ji, J. F. Zhang, and W. Zhang, "A new mobility extraction
technique based on simultaneous ultra-fast Id-Vg and Ccg-Vg measurements
in MOSFETs," IEEE Transactions on Electron Devices, vol. 59, pp.
1906-1914, 2012 (d.o.i.: 10.1109/TED.2012.2196519).
[R2.2] Z. Ji, J. Gillbert, J. F. Zhang, and W. Zhang, "A new Ultra-Fast
pulse technique (UFSP) for channel effective mobility evaluation in
MOSFETs," Proc. IEEE 26th International Conference on Microelectronic
Test Structures, Osaka, Japan, pp. 64-69, 2013 (d.o.i.: 10.1109/ICMTS.2013.6528147
[R2.3] L. Lin, Z. Ji, J. F. Zhang, W. D. Zhang, B. Kaczer, S. De Gendt,
and G. Groeseneken, "A single pulse charge pumping technique for fast
measurements of interface states," IEEE Transactions on Electron
Devices, vol. 58, pp. 1490-1498, 2011 (d.o.i.:
[R2.4] **Z. Ji, J. F. Zhang, W. Zhang, B. Kaczer, S. De Gendt, and G.
Groeseneken, "Interface states beyond band gap and their impact on charge
carrier mobility in MOSFETs," IEEE Transactions on Electron Devices,
vol. 59, pp. 783-790, 2012 (d.o.i.: 10.1109/TED.2011.2177839).
[R2.5] Z. Ji, J. F. Zhang, M. H. Chang, B. Kaczer, and G. Groeseneken,
"An analysis of the NBTI-induced threshold voltage shift evaluated by
different techniques," IEEE Transactions on Electron Devices, vol.
56, pp. 1086-1093, 2009 (d.o.i.: 10.1109/TED.2009.2016400).
[R2.6] **Z. Ji, L. Lin, J. F. Zhang, B. Kaczer, and G. Groeseneken, "NBTI
lifetime prediction and kinetics at operation bias based on ultrafast
pulse measurement," IEEE Transactions on Electron Devices, vol.
57, pp. 228-237, 2010 (d.o.i.: 10.1109/TED.2009.2037171).
Details of the impact
This case study gives an example how the RG1 provides solutions to
practical problems by direct engagement with industrial partners. The main
beneficiaries are test engineers (e.g. those in the Logic Devices
Consortium at IMEC) and equipment suppliers (e.g. Keithley Instruments).
The details of the impact for each are given in what follows.
Keithley Instruments: Keithley, as a global prime test equipment
supplier, played a leading role in disseminating the new technique. After
demonstrating the new technique on the home-made facility [R2.1], the RG1
made an effort to maximise its reach and impact by working with
Keithley. By implementing the technique on the industrial-standard
equipment, it opened a channel for its dissemination to the Keithley's
world-wide customer base.
The RG1's activities in developing new measurement techniques have been
noted by Keithley and an agreement was signed with Keithley to implement
these new techniques on Keithley's instruments in 2010 [C2.1]. After the
take-over of Keithley by Tektronix, the collaboration agreement was
renewed in 2013 [C2.2]. Following the publication of the new technique for
mobility in 2012 [R2.1], Keithley noticed its advantages and "believes
that equipping its instrument with such advanced techniques will give it a
competitive edge" [C2.2], "as it demonstrates the capability of 4200-SCS
and explores its application potential" [C2.1]. Keithley provided its most
advanced ultra-fast pulse test module 4225 to the University to implement
this technique [C2.2]. By working together with Keithley, the RG1 made the
following key contributions:
- It demonstrated that the new technique can be implemented on the
industrial standard equipment, as reported in March 2013 [C2.3].
- It prepared the application note no. 3236, authored by Z.Ji from the
RG1 [C2.1, C2.4]; the application note is available at Keithley's
website addresses given in [C2.4].
- It prepared the control software [C2.1, C2.5], available through
Keithley's website given in [C2.5].
Keithley has been disseminating information about this technique to its
world-wide customers by highlighting it in its quarterly newsletter "Test
Patterns", which was also sent to its global customers by email [C2.6]. It
is drawing attention to this technique as one of the strengths for its
award-winning 4200-SCS instruments in its sales promotion [C2.1]. These
activities provide mechanisms for a global reach and impact. According to
the data provided by Keithley, 67 customers downloaded the application
notes on the same day when they received the email notification regarding
the new feature. Each 4200 instrument costs US$ 80k, but it offers
multi-functionality and it is difficult for Keithley to tell how many
customers purchased it just because of this new feature.
The Logic Devices Consortium at IMEC: The members of this
Consortium include Intel, Global Foundries, TSMC, Sony, Panasonic, and
Xilinx. The RG1 has collaborated with it for 20 years [C2.7].
The Consortium supported the research into a new technique by providing
three sets of test samples requested by the RG1: one set (2.3-nm SiO2)
as the benchmark and two sets for testing the applicability of the new
technique to samples of high trapping (1.65-nm HfSiON/SiON) and high gate
leakage (1.28-nm HfSiON/SiON). These samples were the same as those used
by the Consortium for technology development [C2.7].
As the users of the new technique, IMEC [C2.7] corroborates that "the new
mobility measurement technique simplifies the test procedure without using
the expensive RF setup, has better accuracy and efficiency". The
significance of the improvement is supported by the following quantitative
(i) Reduced cost: The quotations received by the RG1 show
that a typical RF probe station costs approximately £130k and it also
requires network analyser, such as Agilent PNA, costing over £70k. In
addition, the test devices should have special RF structures, whose cost
is manufacturer-specific. All of these are not required for the new
(ii) Improved reliability: The commercial capacitance metre
can only tolerate gate leakage below 10 A/cm2, which is lower
than the gate leakage in advanced transistors. The capacitance measured by
the new technique is independent of gate leakage and it has been
demonstrated that mobility can be extracted with a gate leakage as high as
40 A/cm2 [R2.1, R2.2].
(iii) Better accuracy: The drain voltage mismatch induces
an error over 20% and the charge trapping can cause an additional 20%
error [R2.1, R2.2]. These errors were eliminated by the new technique.
(iv) Improved efficiency: The existing technique measures
current and mobile charges separately, whilst the new technique measures
them simultaneously, so that the number of tests is halved [R2.1, R2.2].
Sources to corroborate the impact
[C2.1] Statement from the Keithley Instrument Inc., available at
Contact Identifier Number: 1.
[C2.2] The renewed collaboration agreement between LJMU and Keithley
Instrument, available at http://www.ljmu.ac.uk/ENG/ENG_Docs/C2.2_-_Collab_-_Agreement_-_Keithley.PDF.
Contact Identifier Number: 1.
[C2.3] The paper reporting the successful implementation of the new
technique in Keithley's instruments: Z. Ji, J. Gillbert, J. F. Zhang, and
W. Zhang, "A new Ultra-Fast pulse technique (UFSP) for channel effective
mobility evaluation in MOSFETs," Proc. of IEEE 26th International
Conference on Microelectronic Test Structures, Osaka, Japan, pp.
64-69, 2013 (d.o.i: 10.1109/ICMTS.2013.6528147),
[C2.4] The Application Note, directly accessible at http://www.keithley.com/data?asset=57662
and also accessible from the list of application notes at Keithley's
[C2.5] The control software: http://www.keithley.com/data?asset=57747
[C2.6] Keithley's quarterly newsletter highlighting the new technique:
[C2.7] Statement from IMEC, available at http://www.ljmu.ac.uk/ENG/ENG_Docs/C2.7_-_Statements_from_IMEC_on_Mobility_Case.pdf.
Contact Identifier Number: 2.