nanostructured oxides on porous silicon microhotplates for nh3 sensing

Moranteb 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

Nanostructured oxides on porous silicon microhotplates

R Triantafyllopouloua,*, X Illab, O Casalsb, S Chatzandroulisa, C Tsamisa,

A Romano-Rodriguezb, J.R Moranteb

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: SnO2:Pd and WO3:Cr For the characterization of both gas sensors, measurements in NH3ambient took place, in isothermal mode of operation Improved sensors characteristics were obtained for SnO2:Pd sensors, compared to WO3: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

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

suspended Porous Silicon micro-hotplates, for low power consumption In order to enhance sensor sensitivity, additive-modified nanostructured metal oxides were used

by a sol-gel process and deposited via micro-dropping Taking into account that the occupational exposure limit

this limit

2 Experimental The fabrication process of suspended Porous Silicon

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: roubini@imel.demokritos.gr (R Triantafyllopoulou).

www.elsevier.com/locate/mee 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

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

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

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

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

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

a supply of about 13 mW The sensors were introduced into the test chamber and were exposed in various

high (100–500 ppm) concentrations, while the working

The exposure and recovery time of the sensors was 15 min and 30 min respectively

The sensitivity of the sensors was defined as the ratio

Rair=RNH3, where Rair is the sensor resistance in dry air

Fig 3, shows the response of the sensors with SnO2: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 :Pd sensitive material.

increases with gas concentration and temperature In the

same figure, measurements of sensors using undoped

comparison We notice that the sensitivity of the sensors

increases with gas concentration and temperature In the

same figure, measurements of sensors using undoped

comparison We notice that the response of sputtered

micro-dropped sensors, even when they are operated at higher

expected due to the nanostructured nature of the

micro-dropped materials

Fig 4shows the resistance of SnO2:Pd and WO3:Cr

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

consump-tion We notice different temperature dependence for the

response is increased as the temperature raises from

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

material more sensitive to the presence of a reducing gas,

con-sequently low As the temperature increases the dominant process becomes the adsorption of O 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

Characterization of gas sensors was performed for various

iso-thermal mode of operation Improved characteristics were

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 Education and Science through the CROMINA project (TEC2004-06854-C03-01) and by the European Union through the GOODFOOD project (IST-1-508774-IP)

References

[1] D.G Rickerby, M.C Horrillo, J.P Santos, P Serrini, Nanostructured Materials 9 (1997) 43–52.

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 : 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.

[2] R Triantafyllopoulou, S Chatzandroulis, C Tsamis, A Tserepi,

Microelectronics Engineering 83 (4–9) (2006) 1189–1191.

[3] J Cerda`, A Cirera, A Vila`, A Cornet, J.R Morante, Thin Solid

Films 391 (2) (2001) 265–269.

[4] I Jimenez, M.A Centeno, R Scotti, F Morazzoni, J Arbiol, A.

Cornet, J.R Morante, J Mater Chem 14 (2004) 2412–2420.

[5] A Cirera, A Vila, A Cornet, J.R Morante, Mater Sci Eng C 15 (2001) 203–205.

[6] A.M Ruiz, X Illa, R Diaz, A Romano-Rodriguez, J.R Morante, Sensors and Actuators B 118 (2006) 318–322.

[7] B Karunagaran, Periyayya Uthirakumar, S.J Chung, S Velumani, E.-K Suh, Materials Characterization 58 (2007) 680–684.