Available online at www.sciencedirect.com
Procedia Engineering 47 (2012) 1466 – 1473
Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland
Transient Operation Techniques for Gas Sensor Applications
Roland Pohleaa*
a
Siemens AG, Corporate Technology, Otto-Hahn-Ring , D-81739 Munich, Germany
Abstract
This paper provides an update, how transient operation methods can improve the performance of gas sensors and
sensor systems for specific applications. Even new applications can be enabled, if existing state-of-the-art sensors are
operated in transient modes adapted to the needs of the application in mind.
Several parameters are available for transient variation, depending on the particular sensing technology:
Suspended Gate FET sensors deliver a simultaneous readout of work function and capacitive effects. By applying
pulsed gate voltages these effects can be interpreted separately and used e.g. for compensation of humidity effects
and improved selectivity.
Modulation of the operating temperature of μ-machined metal oxide sensors is creating a type of virtual sensor array.
In this case, the dynamic behavior of adsorption and desorption at changing temperature levels delivers a fingerprint
for application relevant gas mixtures.
The pulsed application of charge voltage on standard exhaust lambda probes and the recording of the related
discharge characteristic over time provide an exhaust NOx sensor based on a standard sensor setup. The charging
creates a misbalance in oxygen-related surface species not available under continuous operation conditions. The
related discharge characteristic is strongly influenced by the interaction of these oxygen species with NOx.
©
ThePublished
Authors. Published
by Elsevier
Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense
©2012
2012
by Elsevier
Ltd.
Sp. z.o.o. Open access under CC BY-NC-ND license.
Keywords: Gas sensor; transient operation; metal oxide, exhaust gas, Suspended Gate FET
1. Introduction
In the last decades, gas sensors have been more and more established in our daily live. Nevertheless,
only a few technologies are commercially used in large quantity applications. The most common ones are
* Corresponding author. Tel.: +49 89 636-48934 ; fax: +49 89 636-46881 .
E-mail address: [email protected].
1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense
Sp. z.o.o. Open access under CC BY-NC-ND license.
doi:10.1016/j.proeng.2012.09.432
Roland Pohle / Procedia Engineering 47 (2012) 1466 – 1473
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zirconia-based lambda probes for car exhaust monitoring and resistive metal oxide sensors for natural gas
alarms and air quality monitoring. High development efforts have been used to enable reliable and cost
effective production of these sensors on an industrial level with several millions of pieces per year. In
parallel, Suspended Gate FET sensors have been brought to a state of technological maturity, which
allows this new low cost, low power gas sensor technology to enter large quantity markets.
Beside these established technologies, enormous efforts are made on new sensing materials for existing
transducer platforms and on innovative transducer principles. Nevertheless there are high costs and
simultaneously high financial risks to consider in order establish a new sensor technology on the market,
even if the results of the foregoing research and development are highly promising.
Up to now, two main strategies are obvious to cope with this situation:
- Avoid the need of new gas sensors
This approach has been followed e.g. by predicting exhaust gas concentrations from signals available
in the vehicle engine management system [1,2]. Often this approach fails, since the inherently high
requirements for the quality and stability of the parameters used as input for the estimation can not be
fulfilled.
- Use existing sensors
Following this path, additional costs have to be encountered related to the fact, that a sensor not
designed for a specific application will not show the expected performance.
A third approach emerges from the second, if the sensor is respect more as a system than as a component.
Respecting the special physical, chemical and electrical properties and inherent functional mechanisms,
existing sensor technologies can be operated in a way, that allows to fulfill criteria requisited by a
targeted application. By transient manipulation of existing functional operational parameters of a gas
sensor (e.g. temperature, electrode potential), the sensing characteristics can be altered dramatically.
Since the control of these parameters is already implemented in the sensor itself and often also to in
existing driving circuits, a very cost effective way is created to adapt established sensors for additional
applications.
2. Gate pulsed readout of Floating Gate FET sensors
The readout of gas induced work function changes via hybrid suspended gate field effect devices
(HSGFET) is accepted as a promising technique for the realization of a versatile, low-cost sensor
platform since several years [3]. The industrialization of the advanced floating gate FET (FGFET) device
is already started by Micronas GmbH [4]. The freedom in choice of sensing materials is due to the fact
that the hybrid setup enables to use sensing materials produced in independent technological steps not
compatible to CMOS standards. Numerous sensing layers for a variety of gases have been developed
creating a signal in the FGFET setup by different physical and chemical effects [5].
Commonly HSGFET and FGFET sensors are considered to read out changes in work function due to
adsorption of gas molecules as a surface effect, which is only valid for conducting or thin sensing layers.
In the case of relatively thick (several μm) isolating sensing layers, a significant part of the response is
due to capacitive effects related to the volume of the sensing layer. Regarding surface induced signals,
baseline instabilities can be induced by unintended surface conductivity and charge drifts within the
transducer limiting the accuracy and the longterm stability of the sensor signal. By promoting the
capacitive components of the sensor signal, surface related drift effects can be suppressed effectively.
This approach is exemplified by means of the gate-pulsed readout of FGFET sensors [6].
Fig. 1a depicts the hybrid FGFET setup, which consists of a CMOS FET structure as readout device
and a suspended gate including the gas sensitive layer. The floating gate electrode is prolonged forming a
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Roland Pohle / Procedia Engineering 47 (2012) 1466 – 1473
capacitive element with the suspended gate electrode on the backside of the sensitive layer. The air gap is
of the size of a few microns to allow gas diffusion into the capacity. As depicted in the equivalent circuit
diagram of an FGFET type sensor (fig. 1b), the capacitive coupling of the floating gate is determined by
fixed capacities given by the CMOS process (Cwell, Cpass) and capacities related to the hybrid FGFET
setup (Clayer, Cair gap) influenced by the gas atmosphere.
Ugate
suspended gate
floating gate
Clayer
suspended gate
electrode
gas sensitive layer
air gap
Cair gap
Readout FET
Cpass
Cwell
capacitance well
substrate (p-Si)
Usource
Udrain
readout FET
Fig. 1 (a). : Scheme of the Floating Gate FET (FGFET) [4]. (b) simplified equivalent circuit diagram of the FGFET. Clayer :
Capacitance of the gas sensitive layer, Cair gap : Capacitance of the air gap, Cpass : Capacitance of the passivation layer on top of the
floating gate. Cwell : Capacitance between floating gate and the capacitance well [6].
The influence of changing humidity levels on the gate pulse response of a FGFET with polyamide as
sensing layer is shown in fig. 2a. The change in output voltage caused by the gate voltage pulse at 85 %
relative humidity (% r.h.) is significantly higher compared to the response at 40 % rel. humidity. This
correlates to a better capacitive coupling of the gate voltage to the FET and therefore to an increase in
gate capacity. This increase can be caused by swelling of the polymer sensing layer or by an increase of
the dielectric constant of the sensing layer. The response of a FGFET calibrated as described above
exhibits an accurate measurement of relative humidity comparable to the reference sensor (fig. 2b).
voltage on
suspended gate
1.0
FGFET signal
2.3
2.2
40
FGFET gate pulsed readout (% r.h.)
r.h. reference (% r.h.)
80
1.5
2.5
2.4
90
0.5
relative humidity (% r.h.)
2.6
2.0
40% r.h.
85% r.h.
voltage on suspended gate (V)
2.7
gate pulse response
at 40% rel. humidity
FGFET signal (V)
2.8
FGFET with polyamide sensing layer
gate pulse response
at 85% rel. humidity
2.9
70
60
50
40
30
20
10
50
60
(a)
time (s)
70
0.0
0
0
20
40
60
80
100
120
140
160
180
time (minutes)
(b)
Fig. 2. (a) Response curves of a FGFET sensor to gate voltage pulses at changing humidity levels. (b) comparison of the transient
response of a calibrated FGFET humidity sensor in gate pulsed operation with a reference r.h. sensor at humidity levels from 10% to
85% r.h.
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Besides increased baseline stability the gate pulsed readout eliminates cross sensitivities to numerous
gases as demonstrated for H2, NH3 and NO2 in fig. 3. This is again a validation, that continuous readout
delivers readout mainly of work function changes due to gas adsorption on the sensor layer surface, while
in pulsed readout volume effects are dominating the response due to capacitive changes.
50
r.h. reference (% r.h.)
FGFET continuous readout
FGFET gate pulsed readout
40
35
30
FGFET with polyamide sensing layer
25
100
10
1
720
780
840
50
T (°C)
H2
40
NH3
30
NO2
20
900
time (min)
960
1020
10
r.h. (%) T(°C)
[gas] (ppm)
rel. humidity (%)
45
1080
Fig. 3. Cross sensitivities of a FGFET humidity sensor to H2, NH3 and NO2 in continuous and gate pulsed readout.
3. Stepwise temperature modulation of micromachined metal oxide based gas sensors
The modulation of the operation temperature of metal oxide gas sensors is already widely investigated
as reviewed in [7] in order to increase selectivity and improve signal stability and has also been combined
with adapted data evaluation algorithms for specific applications like fire detection [8]. Conventional
metal oxide sensors have relatively long thermal time constants and are therefore limited to operation
with relatively long temperature cycle times in the range of several ten seconds ending up in long
response times and high power consumption. Therefore the combination of MEMS based sensors with
thermal time constants in the millisecond range with temperature modulated operation is obvious.
Commercial micromachined metal oxide based gas sensors (AppliedSensor type AS MLC) have been
investigated using the temperature modulated operation as depicted in fig 4a: The sensor temperature is
stepwise varied in the range from 100°C up to 400°C, while the sensor resistance is recorded in set of 90
data points with a time interval of 10 ms. Each data point corresponds to the sensor resistance at a specific
sensor temperature with a specific gas response. In this way, a virtual sensor array is created, allowing the
distinction of different gaseous substances. As an example, the signal evaluation of the temperature
modulated response has been optimized for the suppression of NO2 cross sensitivity as depicted in fig 4b.
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10k
2.0
500
gas sensor temperature modulated operation
AppliedSensor sensor in
temperature modulated operation
heater power = 5mWsec
= 0.5 mW at 10sec sample rate
300
4k
200
2k
100
0
0.0
0.2
0.4
0
0.8
0.6
1.0
H2
CO
NO2
0.5
continuous readout
evaluation of temperature modulated signal
100
70
r.h. (%)
H2 (ppm)
CO (ppm)
NO2 (ppm)
10
1
60
50
40
0.1
600
630
660
time (sec)
(a)
690
720
time (min)
750
780
30
810
(b)
Fig. 4. (a) example for the temperature profile and the related sensor resistance used for detection of human activities; (b)
Suppression of NO2 cross sensitivity by adapted signal evaluation of the sensor response in temperature modulated operation.
The ability to recognize human activities with the help of gas sensors has been investigated with
different sensor technologies and signal evaluation approaches [9,10]. For our approach we combine the
MEMS metal oxide gas sensor, which shows excellent response to human induced Volatile Organic
Compounds (VOCs) [11] with both temperature-modulated operation and statistic methods for data
evaluation. The responses of several temperature modulated gas sensors installed in different places in a
test apartment are shown in Fig. 5a. The sensors respond clearly to the number of people in the room, if
windows are opened or by cooking. The evaluation of the sensor data regarding more abstracted activities
of daily living (fig. 5b) demonstrates the capability of non invasive activity monitoring by temperaturemodulated metal oxide gas sensors.
gas sensor response in test appartment
ceiling
kit chen window
near electric cooker
desk
couch
sensor resistance (Ohm)
open kitchen
window
100000
4 persons in room
open all
windows
prediction
personal hygiene
sleeping
2 persons in room
1 persons in room
Prepare and drink
coffee
Cooking and eating
10000
12:00
Increased
physical activity
cooking
13:00
14:00
15:00
16:00
17:00
Increased
physical activity
Cooking
and
eating
Prepare
and drink
coffee
sleeping
18:00
time
(a)
(b)
Fig. 5. (a) response of 5 metal oxide sensors in temperature modulated operation in different places of a test apartment; (b)
prediction of activities of daily living based on data obtained from temperature modulated metal oxide sensors.
Personal
hygiene
rel.humidity (%)
6k
Signal (a.u.)
Tsensor
sensor
resistance
[gas] (ppm)
400
Tsensor (°C)
sensor resistance (Ohm)
1.5
8k
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Roland Pohle / Procedia Engineering 47 (2012) 1466 – 1473
4. Pulsed Polarization of Platinum Electrodes on YSZ
Electrochemical devices based on oxygen-ion-conducting yttria-stabilized-zirconia (YSZ) are
appropriate for applications as oxygen sensor at temperatures higher than 600°C. Almost any automobile
being powered by a gasoline combustion engine is equipped with at least one zirconia exhaust gas oxygen
sensor (lambda sensor) for detection of the air-to-fuel-ratio or lambda. At lower temperatures, the
chemical reactions on the surface of metal-oxide electrodes and electrolytes compete with
electrochemical reactions. These simultaneous reactions limit the selectivity to different exhaust gas
components. Regarding NOx detection, standard potentiometric sensors suffer under the opposite sign for
the emf potential of NO and NO2 which makes it very hard to monitor total NOx [12]. Therefore, a wide
range of complex material systems is under investigation in order to obtain reliable NOx detection [13,
14]. A main drawback of this approach is the insufficient knowledge about the long term stability of these
material systems. Hence, our approach is to use standard lambda probes known as robust and reliable
systems and to apply transient operation techniques to measure NOx concentrations in a reliable way.
The transient measurement cycle is shown schematically in fig. 6a [15]: after a positive charging
voltage is applied, the voltage supply is disconnected and the self-discharge voltage of the sensor is
recorded. This procedure is then repeated used a voltage with opposite sign. A positive charging voltage
is defined as a higher potential of the outer exhaust electrode with respect to the inner air electrode,
which is exposed to a reference gas with defined pO2 (fig. 6b). This pulse sequence with electrode
polarisations of opposite signs is applied permanently and the discharge curves are measured
continuously. All relevant parameters as voltage, pulse duration and pause time have been varied in order
to evaluated their influence of the discharge behavior.
porous detection layer
exhaust electrode
pO 2
pO 2ref
Uheater
US
YSZ solid electrolyte
reference electrode
rod-type heater
(a)
(b)
Fig. 6 (a): Schematic depiction of the measurement approach; (b) Schematic depiction of a thimble-type Lambda sensor.
The difference in gas response using continuous mixed potential readout (no external voltage applied)
and the evaluation of the discharge curve described above is illustrated in fig. 7. Tests with the same
gases and gas concentrations have been performed with both methods. By continuous readout of the
mixed potential nearly no response to NO was obtained (fig 7c). In contrast, the response to NO is the
most prominent compared to other gases if the pulsed discharge method with positive voltage is applied
(fig 7a). By polarisation with negative pulses the response to reducing gases is comparable to the mixed
potential measurement, while the response to NH3 and NO2 is slightly increased (fig 7b). In comparison,
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the response to ethanol is similar for continuous readout and by evaluation of the positive discharge curve
respectively.
Recent investigations indicate that changes in oxidation states of platinum induced by the
charging/discharging process plays a major role in the related NOx detection mechanism [16].
(a) after positive pulse
(b) after negative pulse
(c) continuous voltage readout
Figure 7: Comparison of the characteristic curves related to NO, NH3, ethanol, H2 and hydrocarbon mixture (a) by evaluation of the
positive discharge curve, (b) by evaluation of the negative discharge curve and (c) with continuous mixed potential readout.
5. Conclusion
It has been demonstrated that transient operation is a powerful way to increase the performance of
existing gas sensors. Very much depending on the sensor technology and the related application,
improvements in stability, selectivity and sensitivity can be obtained without expensive development on
the sensor element itself. For SGFET sensors, the separate use of surface and volume effects on the
sensing layer is realized by evaluation of signal response with pulsed gate voltage. This method can be
used either to suppress cross sensitivities or to extract two independent signals from one sensing element.
Temperature modulated operation of commercial available metal oxide sensors has proved not only to
increase selectivity, but also to differentiate complex gas mixtures arising from activities of daily living.
If pulsed voltages are applied to standard zirconia-based automotive lambda probes and the self-discharge
behavior in between the voltage pulses is investigated, a very selective response of the discharge
characteristic to different NOx concentrations have been obtained. Despite these promising experimental
results, detailed investigations of the physical and chemical effects related to the transient operation are
crucial for optimization of the operation parameters for specific requirements.
Roland Pohle / Procedia Engineering 47 (2012) 1466 – 1473
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