nanostructured oxides on porous silicon microhotplates for nh3 sensing

Nanostructured oxides on porous silicon microhotplates
for NH
3
sensing
R. Triantafyllopoulou
a,
*
, X. Illa
b
, O. Casals
b
, S. Chatzandroulis
a
, C. Tsamis
a
,
A. Romano-Rodriguez
b
, J.R. Morante
b
a
NCSR ‘‘Demokritos’’, Institute of Microelectronics, 15310, Aghia Paraskevi, Athens, Greece
b
7-EME/CeRMAE/IN[2]UB, Department of Electronics, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain
Received 5 October 2007; received in revised form 18 December 2007; accepted 27 December 2007
Available online 2 January 2008
Abstract
Low power micromachined gas sensors based on suspended micro-hotplates are presented in this work. The sensors were fabricated
using Porous Silicon Technology. Two different metal-modified nanostructured sensitive materials were deposited on top of the active
area of the micro-hotplates, using the micro-dropping technique: SnO
2

:Pd and WO
3
:Cr. For the characterization of both gas sensors,
measurements in NH
3
ambient took place, in isothermal mode of operation. Improved sensors characteristics were obtained for SnO
2
:Pd
sensors, compared to WO
3
:Cr, for these operating conditions.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Gas sensors; Porous silicon; Micro-hotplates; Nanostructured metal oxides; NH
3
sensing
1. Introduction
The last four decades the sensitivity of semiconductive
metal oxides in gas sensing under atmospheric conditions,
has been intensively studied. Solid state chemical sensors
are one of the most common devices employed for the
detection of hazardous gases. The detection of NH
3
is of
high interest for application in various areas such as agri-
culture, industrial chemistry, environmental quality, auto-
motive and medical applications. Ammonia sensing can
be achieved by using conductometric gas sensors. The sens-
ing mechanism is based on conductivity changes of the sen-
sitive material, which is deposited on the top of the active
area of the sensors, and corresponds to electrical modifica-

tions caused both by ammonia and the by-products of the
oxidation reaction of ammonia at the surface. Moreover, it
has been demonstrated that the sensitivity towards various
gases can be increased by using metal additives and by
decreasing the crystallite size of the catalytic material [1].
In this work, we present measurements of low power gas
sensors in NH
3
ambient. The sensors are based on
suspended Porous Silicon micro-hotplates, for low power
consumption. In order to enhance sensor sensitivity,
additive-modified nan ostructured metal oxides were used
as sensitive materials (SnO
2
:Pd and WO
3
:Cr), fabricated
by a sol-gel process and deposited via micro-dropping.
Taking into account that the occupational exposure limit
for NH
3
is 50 ppm, the measurements occurred mainly in
low concentrations of NH
3
, in order to detect the gas below
this limit.
2. Experimental
The fabrication process of suspended Porous Silicon
micro-hotplates has been reported in detail elsewhere [2].
The micromachined sensors consisted of Porous Silicon

membranes and a heater of doped polysilicon, which was
embedded between two insulating layers. Ti/Pt layers were
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.mee.2007.12.038
*
Corresponding author. Tel.: +30 210 650 3113; fax: +30 210 651 1723.
E-mail address: (R. Triantafyllopoulou).
www.elsevier.com/locate/mee
Available online at www.sciencedirect.com
Microelectronic Engineering 85 (2008) 1116–1119
deposited and patterned to serve as electrodes and contact
pads, while the release of the devices was performed in a
High Density Plasma reactor. After the fabrication of the
micro-hotplates, the deposition of two different sensitive
materials took place, using the micro-dropping technique
[3]. Additive-modified nanostructured metal oxides were
prepared by a sol-gel solution and then were deposited by
micro-dropping on the suspended devices, as shown in
Fig. 1. The sol-gel process for the preparation of the
SnO
2
:Pd and WO
3
:Cr sensitive materials, is reported else-
where [4,5]. The two sensitive materials were deposited
on the micro-hotplates as follows: at first, nanopowders
were mixed with an organic solvent, in order to obtain
good adhesion to the substrate. A meniscus is formed
and then, when the meniscus reaches the micro-hotplate,
the paste is deposited by capillarity. Finally, the paste is

heated up in order to remove the organic solvent. Both
materials were thermally treated at a temperature of
300 °C, using the device’s heater, for 12 hours, in order
to modulate and activate the sensitive material before the
gas measurements. The use of micro-hotplates gives the
opportunity to fabricate gas sensor arrays that incorporate
varying sensitive materials, operating with very low power
consumption. Fig. 2 shows an array of gas sensors with
various sensitive materials, deposited by the micro-drop-
ping technique. The fabrication of micro-dropped sensors
has been mainly reported in closed type membranes, while
in this work we focus on suspended Porous Silicon micro-
hotplates.
3. Results
Characterization of the gas sensors was performed at
isothermal mode of operation, by keeping constant the
power supplied to the heater. For the micro-hotplates used
in the present work, a temperature increase rate of $ 21 °C/
mW has been estimated, based on combined electrical
results as well as IR measurements. As a consequence, high
operating temperatures can be achieved with low power
consumption, e.g. a temperature of 300 °C is achieved with
a supply of about 13 mW. The sensors were introduced
into the test chamber and were exposed in various concen-
trations of NH
3
in dry air, both at low (2–15 ppm) and at
high (100–500 ppm) concentrations, while the working
temperature of the sensors ranges from 200 °C to 350 °C.
The exposure an d recovery time of the sensors was 15

min and 30 min respectively.
The sensitivity of the sensors was defined as the ratio
R
air
=R
NH
3
, where R
air
is the sensor resistance in dry air
and R
NH
3
the electrical resistance in the targeted gas.
Fig. 3, shows the response of the sensors with SnO
2
:Pd
metal oxide, deposited by micro-dropping, for two different
temperatures. We notice that the sensitivity of the sensors
Fig. 1. SEM image of a micro-hotplate on top of its active area SnO
2
:Pd is
deposited by micro-dropping.
Fig. 2. SEM image of sensors array of various sensitive materials
(SnO
2
:Pd and WO
3
:Cr).
Fig. 3. Comparison of the sensitivity of gas sensors with undoped

sputtered SnO
2
sensitive material and sensors with micro-dropped
SnO
2
:Pd sensitive material.
R. Triantafyllopoulou et al. / Microelectronic Engineering 85 (2008) 1116–1119 1117
increases with gas concentration and temperature. In the
same figure, measurements of sensors using undoped
SnO
2
deposited by RF sputtering are also reported for
comparison. We notice that the sensitivity of the sensors
increases with gas concentration and temperature. In the
same figure, measurements of sensors using undoped
SnO
2
deposited by RF sputtering are also reported for
comparison. We notice that the response of sputtered
SnO
2
sensors towards NH
3
, is lower compared to micro-
dropped sensors, even when they are operated at higher
temperatures (400 °C) in our case. Such a behavior is
expected due to the nanostructured nature of the micro-
dropped materials.
Fig. 4 shows the resistance of SnO
2

:Pd and WO
3
:Cr sen-
sors for various gas pulses, as NH
3
concentration increases
from 2 ppm to 15 ppm. We notice that a good saturation
level is obtained for both materials, for the exposure and
the recovery phases as well as a good baseline, when no
gas is present. In Fig. 5 the sensitivity of the gas sensors
with SnO
2
:Pd and WO
3
:Cr is reported, for different operat-
ing temperatures. SnO
2
:Pd gas sensors exhibit higher sensi-
tivity in detecting NH
3
compared to WO
3
:Cr sensors, in
agreement with the literature [6], with low power consump-
tion. We notice different temperature dependence for the
response for each material. In the case of WO
3
:Cr, sensor
response is increased as the temperature raises from
290 °C to 350 °C. However, in the case of SnO

2
:Pd the sig-
nal obtained at 280 °C is significantly higher than that
obtained at 350 °C. Such a behavior is not unusual for
metal oxide sensors and is attributed to the mechanisms
of gas adsorption and desorption on the surface of the cat-
alytic material. In principle, a metal oxide can adsorb oxy-
gen from the atmosphere both as O
2
À
and O
À
species. The
adsorption of O
À
is more reactive and thus makes the
material more sensitive to the presence of a reducing gas,
such as NH
3
. At relatively low temperatures the surface
preferentially adsorbs O
2
À
and the sensor response is con-
sequently low. As the tempe rature increases the dominant
process becomes the adsorption of O
À
and hence the sensi-
tivity of the material increases. When the temperature
increases too much, then desorption of all the oxygen ionic

species adsorbed previously occurs and the sensitivity
decreases again [7].
4. Conclusions
In this work, low power micromachined gas sensors
based on suspended micro-hotplates were fabricated and
characterized. Two different metal-modified nanostruc-
tured sensitive materials were deposited on top of the active
area of the micro-hotplates, using the micro-dropping tech-
nique: SnO
2
:Pd and WO
3
:Cr. Porous Silicon micro-hot-
plates were used for low power consumption.
Characterization of gas sensors was performed for various
NH
3
concentrations and operating temperatures, using iso-
thermal mode of operation. Improved charact eristics were
obtained for SnO
2
:Pd sensors, compared to WO
3
:Cr, for
these operating conditions.
Acknowledgments
This work was partially supported by the Greek General
Secretariat of Research and Technology (PENED, Con-
tract 04ED630), by the Spanish Ministry of Educa tion
and Science through the CROMINA project (TEC2004-

06854-C03-01) and by the European Union through the
GOODFOOD project (IST-1-508774-IP).
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Fig. 4. Typical graph of the resistance of both gas sensors with SnO
2
:Pd
and WO
3
:Cr sensitive materials, for various pulses of low concentrations
of NH
3
: 2-15 ppm.
Fig. 5. Sensitivity of gas sensors with SnO
2
:Pd and WO
3
:Cr micro-
dropped sensitive materials, in various temperatures, for low concentra-
tions on NH
3.
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