Covington, James A. (2001) CMOS and SOI CMOS FET-based gas sensors. PhD thesis, University of Warwick.

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Abstract

In recent years, there has been considerable interest in the use of gas/vapour monitors and
electronic nose instruments by the environmental, automotive and medical industries. These
applications require low cost and low power sensors with high yield and high reproducibility,
with an annual prospective market of 1 million pounds. Present device and sensor
technologies suffer a major limitation, their incompatibility with a standard silicon CMOS
process. These technologies have either operating/annealing temperatures unsuited for
MOSFET operation or an inappropriate sensing mechanism. The aim of this research is the
development of CMOS compatible gas/vapour sensors, with a low cost of fabrication, high
device repeatability and, in the future, transducer sensor amalgamation. Two novel
approaches have been applied, utilising bulk CMOS and SOI BiCMOS. The bulk CMOS
designs use a MOSFET sensing structure, with an active polymeric gate material, operating at
low temperatures (<100°C), based on an array device of four elements, with channel lengths
of 10 μm or 5 μm. The SOI designs exploit a MOSFET heater with a chemoresistive or
chemFET sensing structure, on a thin membrane formed by the epi-taxial layer. By applying
SOI technology, the first use in gas sensor applications, operating temperatures of up to 300
°C can be achieved at a power cost of only 35 mW (simulated). Full characterisation of the
bulk CMOS chemFET sensors has been performed using electrochemically deposited (e.g.
poly(pyrrole)/BSA)) and composite polymers (e.g. poly(9-vinylcarbazole)) to ethanol and
toluene vapour in air. In addition, environmental factors (humidity and temperature) on the
response and baseline were investigated. This was carried out using a newly developed flow
injection analysis test station, which conditions the test vapour to precise analyte (<15 PPM
of toluene) and water concentrations at a fixed temperature (RT to 105°C +- 0.1), with the
sensor characterised by either I-V or constant current instrumentation. N-channel chemFET
sensors operated at constant current (10 μA) with electrochemically deposited and composite
polymers showed sensitivities of up to 1.1 μV/PPM and 4.0 μV/PPM to toluene vapour and to
1.1 μV/PPM and 0.4 μV/PPM for ethanol vapour, respectively, with detection limits of <20
PPM and <100 PPM to toluene and <20 PPM and 10+ PPM to ethanol vapour (limited by
baseline noise), respectively. These responses followed either a power law (composite
polymers) or a modified Langmuir isotherm model (electrochemically deposited polymers)
with analyte concentration. It is proposed that this reaction-rate limited response is due to an
alteration in polymers work function by either a partial charge transfer from the analyte or a
swelling effect (polymer expansion). Increasing humidity caused, in nearly all cases a
reduction in relative baseline, possible by dipole formation at the gate oxide surface. For the
response, increasing humidity had no effect on sensors with composite polymers and an
increase for sensors with electrochemically-deposited polymers. Higher temperatures caused
a reduction in baseline signal, by a thermal expansion of the polymer, and a reduction in
response explained by the analyte boiling point model describing a reduction in the bulk
solubility of the polymer. Electrical and thermal characterisation of the SOI heaters,
fabricated by the MATRA process, has been performed. I-V measurements show a reduction
in drain current for a MOSFET after back-etching, by a degradation of the carrier mobility.
Dynamic measurement showed a two stage thermal response (dual exponential), as the
membrane reaching equilibrium (100-200 ms) followed by the bulk (1-2 s). A temperature
coefficient of 8 mW/°C was measured, this was significantly higher than expected from
simulations, explained by the membrane being only partially formed. Diode and resistive
temperature sensors showed detection limits under 0.1°C and shown to measure a modulated
heater output of less than 1°C at frequencies higher than 10Hz. The principal research
objectives have been achieved, although further work on the SOI device is required. The
results and theories presented in this study should provide a useful contribution to this
research area.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Library of Congress Subject Headings (LCSH):	Gas detectors, Metal oxide semiconductors, Complementary, Metal oxide semiconductor field-effect transistors
Official Date:	September 2001
Dates:	Date Event September 2001 Submitted
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Gardner, J. W. (Julian W.), 1958-
Sponsors:	Engineering and Physical Sciences Research Council (EPSRC)
Extent:	xxxi, 312 p.
Language:	eng
URI:	http://wrap.warwick.ac.uk/id/eprint/3589

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