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Ca- and Pt-catalysed thin-film SnO2 gas sensors for CO and CO2 detection
SE@&
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CHEMICAL
ELSEVIER
Sensors and Actuators B 24-25 (1995) 529-531
Ca- and Pt-catalysed thin-film SnOz gas sensors for CO’ and CO2 detection Klaus Steiner, Ulrich Hoefer, Fmmhofer-It&u!
Gerd Kiihner, Gerd Sulz, Elmar Wagner
flir Physikalische M&echnik,
Heidenho@nme
8, D-79110 Freibwg, Gemumy
Abstract Pt-, Ca- and Pt/Ca-catalyzed thin-film SnOz gas sensors have been structured on Si substrates. The sensors are temperature stable at 900 “C in synthetic air for 48 h. No significant outdiffusion of the sensor layers or peeling effects can be observed after annealing. Pt/Ca catalyst combinations reduce the sensor-signal drift. PtKa-catalyzed sensors show a weak CO, gas response. Ca catalysts lead to a strong NH, response. Keywords:Calcium; Gas sensors; Platinum; Tin oxide; Carbon dioxide; Carbon monoxide
1. Introduction
To enhance the selectivity, stability and sensitivity of metal-oxide gas sensors a combination of dopants, catalysts and different deposition methods are usually used [l-3]. Here we investigated Ca- and Pt-catalysed thin-film SnO, gas sensors. The stability of Pt-, Caand Pt/Ca-catalysed sensors is discussed. The CO, COz, NH, and humidity responses are described.
2. Sensor preparation A sensor cross section is illustrated in Fig. 1. The devices have been structured on Si substrates using standard thin-film technologies. The Si substrates are 100 mm in diameter. The sensor architecture also includes an integrated heater on the reverse side of the substrate to activate the SnO, gas reaction. The layer sequence is temperature stable up to 900 “C for
48 h in synthetic air. No significant o&diffusion of the sensor layers or peeling effects can be observed after annealing. The Si substrate is covered by a 1 pm thick SiO, layer for electrical isolation between the substrate and the Pt-Ta electrodes. The SnO, layer is selectively sputtered onto the interdigital electrodes using photoresist masks. The total thickness is typically 50 nm. The as-deposited SnO, is typically n-doped with a background carrier concentration in the lower 10” cmw3 range at room temperature [4]. The Pt catalysts are sputtered and Ca is evaporated directly onto the SnO, surface. The thickness of the Pt catalyst layer is nearly 5 nm, while that of the Ca layer is typically less. The sensors have been annealed in a rapid thermal annealer (RTA) to stabilize the whole layer sequence. After finishing the fabrication, the Ca presence at the sensor surface has been controlled by using energy-dispersive X-ray spectroscopy (EDXS). The active area of the device is 5 mm x 5 mm. The whole chip area is 8 mm X 8 mm. The sensors are mounted inside a TO-S package. The main advantage of this structure is the simple process technology. The power consumption is 2-3 W at temperatures between 250 and 350 “C.
3. Gas measurements
rdPt-lleater
Si02
Fig. 1. Schematic sensor cross section. 092%4005/95/$09.50 0 199.5 Elsevier Science S.A. All rights reserved SSDI 0925-4005(94)01410-J
The gas sensors were tested simultaneously in synthetic air; i.e., 20% oxygen and 80% nitrogen between 30 and 70% relative humidity. The humidity values are referred to room temperature. Additionally, commer-
530
K. Steiner et al. I Sensors and Actuakm, L3 24-25 (1995) 529-531
cially available test gases were used. The flaw rate was always 1 1 min-‘. The gas-stream direction and the sensor surface were parallel. During operation the conductance was measured once every minute. Fig. 2 shows the stability and CO, response of Pt-, Ca- and Pt/Ca-catalysed sensors. The Pt-catalysed sensor shows a basic drift towards higher resistivity. This is usually attributed to a reduced amount of oxygen vacancies after oxygen diffusion into the bulk SnO, at higher temperatures [l-3]. In contrast to the Pt-catalysed sensors, the Ca catalysts lead to an inverse drift behaviour. The resistance values are reduced significantly at 270 “C. Scanning electron microscope (SEM) investigations on the Ca-catalysed surface have shown a surface speckled with Ca clusters. Since Ca is not stable in synthetic air at higher temperatures, it is assumed here that the Ca clusters are partly oxidized. Close to the metal-oxide surface, however, the deposited Ca metallization has to take oxygen from the bulk SnO,, thereby producing bulk oxygen vacancies. Finally, a resistance reduction occurs. After several measurement cycles the signal drifts of both the Pt- and Ca-catalysed sensors have become weaker. By using Pt and Ca catalysts together at the same sensor surface, the drift of the sensors can be partly compensated. This can be seen in Fig. 2, where the Pt/Ca-catalysed sensors show the smallest signal drift. Moreover, the combination also leads to a CO, response. A weak response can be seen clearly over a wide CO, concentration range. In Ref. [5] it was assumed that the CO, gas concentration might be indirectly detected via humidity exchange between COZ and the heated Pt/Ca-catalysed SnO, surface. However, since Ca-catalysed surfaces without Pt do not show any CO, response, questions about the given mechanism arise. More investigations are necessary to clarify this point. 1.15
0
4
12
8
time
I6
20
t / 11
Fig. 2. NormaIized sensor resistance R/h vs. measurement time in response to pulses of CO,; tensor temperature T=270 ‘C, 50% relative humidity (23 “C); Rapt= 16.7 R, R,,,, = 696 0, R,,. = 1863 It, %,,~=398 a, R,,c.=S.l 0, R,-8.4 a, Rm=3.2 0.
Fig. 3. Normalized sensor resistance R/R,, vs. measurement time in response to pulses of CO, sensor temperature T= 270 “C, 50% relative humidity (23 “C); ROP,= 16.7 a, R,,,& = 696 a, Row,.-_=1863 a, R,=5.1
0, Rw,=8.4
02.
Fig. 3 shows the CO response of the Pt-, Ca- and PtKa-catalysed sensors. As expected, Pt at the metaloxide surface leads to a CO sensor response. Possible mechanisms are described elsewhere [l-3]. The Cacatalysed sensors show a weak CO response, in particular at 500 ppm. The resistance values are slightly increased. This is in contrast to the Pt-catalyscd sensor response. To the authors’ knowledge the CO reaction mechanism at Ca-catalysed surfaces is not known. At the noncatalysed SnO, surface CO reactions are always very weak [l-3]. If there is almost no Ca influence on the CO reaction, the CO response of the Ca-catalysed SnO, surface is also very weak. However, the reaction should always lead to a resistance reduction. Ca partly diffised into the SnO, bulk might also lead to an acceptor-like background concentration close to the metal-oxide surface. In such a case any CO response thus leads to a resistance increase. An inverse resistance change can also be observed on NH, reactions. This can be seen in Fig. 4. In contrast to Pt- or PtlCa-catalysed sensors, the Ca catalysts lead to a strong resistance increase. So far the mechanisms leading to the inverse response are not clarified. More detailed investigations seem to be necessary. Fig. 5 shows the normalized resistance versus time of Pt, PtlCa and Ca-catalysed SnO, sensors in synthetic air at 270 “C. During the measurements the relative humidity of the gas atmosphere was changed between 30 and 70%. At t=O h the relative humidity was 50%. As can be seen and is expected [l--3], the sensors respond to a humidity change. The sensor response is fast with all catalysts. The Ca catalyst leads to the weakest humidity response. Although the humidity response is not very large, no technology change has led to a marked humidity-sensitivity decrease. The nonannealed SnO, material showed the weakest humidity
K. Steiner et al. I Sensors and Achtators, B 24-25 (1995) 529-531
531
4. Conclusions
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Fig. 4. Normalized sensor resistance R/& vs. measurement time in response to pulses of NH,; sensor temperature T=270 ‘C; 50% relative humidity (23 ‘C); RoP,= 16.7 D ROPvc.- 696 0, R,cc. = 1863 fl, Rm=5.1 4 k=8.4 fl.
In conclusion, the CO, CO,, NH, and humidity responses of Pt-, Ca- and PtKa-catalysed SnOz sensors have been discussed. The sensors have been structured on Si substrates using standard thin-film technologies. The sensor architecture is stable in synthetic air up to 900 “C even for 48 h. No significant outdiffision of the sensor layers or peeling effects can be observed after annealing. Pt-catalysed sensors show a signal drift towards higher resistance values. The opposite is true for the Ca-catalysed sensors. The signal drift can be reduced by using Pt/Ca-catalyst combinations. The Pt/ Ca-catalysed sensors show a weak CO2 gas response. A strong NH, response can be realized with Ca catalysts. The resistance values are significantly increased. The Pt- and PtKa-catalysed sensors show a resistance reduction due to NH3.
Acknowledgement The work was partly financed by the Bundesministerium fiir Forschung und Technologie (BMJYT). We thank Professor Gbpel, University of Tiibingen and Professor Boubnov, Avangard, St. Petersburg, for helpful discussions and their cooperation.
References 03 +
0
2
4
6 time
8
IO
12
t / II
Fig. 5. Normalized sensor resistance R/R0 vs. measurement time in response to pulses of humidity; sensor temperature T=270 ‘C, 50% relative humidity (23 “C); RoP,= 16.7 fl, R,,puc. =696 a, Rope.= 1863 Q R-=5.1 a, h=8.4 l-l.
111P.T. Moseley, J.O.W. Norris and DE. Williams, Tec~ues
and Mechanisms in Gar Sensing, Adam Hilger, Bristol, 1991. Comprehensive Swey, Vol. 2,Chemical and Biochemical Sensors, Pan 1, VCH,
VI W. Giipel, J. Hesse and J.N. Zemel, Sensors, A Weioheim, 1991.
[31 T. Seiyama, Chemical Sensor Technology, Vol. 1, Kodansha,
Tokyo, 1988.
Uptmoor,W. Schweizer, H. L&v, M. Lather and K. Steiner, Ni, Io and Sb implanted Pt and V catalysed thin-film So& gas sensors, Sensors and Achutors B, 15-16 (1993) 390-395. I51 U. Hoefer, G. Ktihner, W. Schweizer, G. Sulz and K. Steiner, CO and CO2 thin-film SnOl gas sensors on Si substrates, Sensors and Actuntors B, 22 (1994) 115-119. 141 G. Sulz, G. Kiihner, H. Reiter, G.
response. However, the gas reaction was also very weak. Since the yearly local change of relative humidity is typically limited (for instance, in southwest Germany between 50 and 70%), the conductivity change might be tolerable for many applications.
T
CHEMICAL
ELSEVIER
Sensors and Actuators B 24-25 (1995) 529-531
Ca- and Pt-catalysed thin-film SnOz gas sensors for CO’ and CO2 detection Klaus Steiner, Ulrich Hoefer, Fmmhofer-It&u!
Gerd Kiihner, Gerd Sulz, Elmar Wagner
flir Physikalische M&echnik,
Heidenho@nme
8, D-79110 Freibwg, Gemumy
Abstract Pt-, Ca- and Pt/Ca-catalyzed thin-film SnOz gas sensors have been structured on Si substrates. The sensors are temperature stable at 900 “C in synthetic air for 48 h. No significant outdiffusion of the sensor layers or peeling effects can be observed after annealing. Pt/Ca catalyst combinations reduce the sensor-signal drift. PtKa-catalyzed sensors show a weak CO, gas response. Ca catalysts lead to a strong NH, response. Keywords:Calcium; Gas sensors; Platinum; Tin oxide; Carbon dioxide; Carbon monoxide
1. Introduction
To enhance the selectivity, stability and sensitivity of metal-oxide gas sensors a combination of dopants, catalysts and different deposition methods are usually used [l-3]. Here we investigated Ca- and Pt-catalysed thin-film SnO, gas sensors. The stability of Pt-, Caand Pt/Ca-catalysed sensors is discussed. The CO, COz, NH, and humidity responses are described.
2. Sensor preparation A sensor cross section is illustrated in Fig. 1. The devices have been structured on Si substrates using standard thin-film technologies. The Si substrates are 100 mm in diameter. The sensor architecture also includes an integrated heater on the reverse side of the substrate to activate the SnO, gas reaction. The layer sequence is temperature stable up to 900 “C for
48 h in synthetic air. No significant o&diffusion of the sensor layers or peeling effects can be observed after annealing. The Si substrate is covered by a 1 pm thick SiO, layer for electrical isolation between the substrate and the Pt-Ta electrodes. The SnO, layer is selectively sputtered onto the interdigital electrodes using photoresist masks. The total thickness is typically 50 nm. The as-deposited SnO, is typically n-doped with a background carrier concentration in the lower 10” cmw3 range at room temperature [4]. The Pt catalysts are sputtered and Ca is evaporated directly onto the SnO, surface. The thickness of the Pt catalyst layer is nearly 5 nm, while that of the Ca layer is typically less. The sensors have been annealed in a rapid thermal annealer (RTA) to stabilize the whole layer sequence. After finishing the fabrication, the Ca presence at the sensor surface has been controlled by using energy-dispersive X-ray spectroscopy (EDXS). The active area of the device is 5 mm x 5 mm. The whole chip area is 8 mm X 8 mm. The sensors are mounted inside a TO-S package. The main advantage of this structure is the simple process technology. The power consumption is 2-3 W at temperatures between 250 and 350 “C.
3. Gas measurements
rdPt-lleater
Si02
Fig. 1. Schematic sensor cross section. 092%4005/95/$09.50 0 199.5 Elsevier Science S.A. All rights reserved SSDI 0925-4005(94)01410-J
The gas sensors were tested simultaneously in synthetic air; i.e., 20% oxygen and 80% nitrogen between 30 and 70% relative humidity. The humidity values are referred to room temperature. Additionally, commer-
530
K. Steiner et al. I Sensors and Actuakm, L3 24-25 (1995) 529-531
cially available test gases were used. The flaw rate was always 1 1 min-‘. The gas-stream direction and the sensor surface were parallel. During operation the conductance was measured once every minute. Fig. 2 shows the stability and CO, response of Pt-, Ca- and Pt/Ca-catalysed sensors. The Pt-catalysed sensor shows a basic drift towards higher resistivity. This is usually attributed to a reduced amount of oxygen vacancies after oxygen diffusion into the bulk SnO, at higher temperatures [l-3]. In contrast to the Pt-catalysed sensors, the Ca catalysts lead to an inverse drift behaviour. The resistance values are reduced significantly at 270 “C. Scanning electron microscope (SEM) investigations on the Ca-catalysed surface have shown a surface speckled with Ca clusters. Since Ca is not stable in synthetic air at higher temperatures, it is assumed here that the Ca clusters are partly oxidized. Close to the metal-oxide surface, however, the deposited Ca metallization has to take oxygen from the bulk SnO,, thereby producing bulk oxygen vacancies. Finally, a resistance reduction occurs. After several measurement cycles the signal drifts of both the Pt- and Ca-catalysed sensors have become weaker. By using Pt and Ca catalysts together at the same sensor surface, the drift of the sensors can be partly compensated. This can be seen in Fig. 2, where the Pt/Ca-catalysed sensors show the smallest signal drift. Moreover, the combination also leads to a CO, response. A weak response can be seen clearly over a wide CO, concentration range. In Ref. [5] it was assumed that the CO, gas concentration might be indirectly detected via humidity exchange between COZ and the heated Pt/Ca-catalysed SnO, surface. However, since Ca-catalysed surfaces without Pt do not show any CO, response, questions about the given mechanism arise. More investigations are necessary to clarify this point. 1.15
0
4
12
8
time
I6
20
t / 11
Fig. 2. NormaIized sensor resistance R/h vs. measurement time in response to pulses of CO,; tensor temperature T=270 ‘C, 50% relative humidity (23 “C); Rapt= 16.7 R, R,,,, = 696 0, R,,. = 1863 It, %,,~=398 a, R,,c.=S.l 0, R,-8.4 a, Rm=3.2 0.
Fig. 3. Normalized sensor resistance R/R,, vs. measurement time in response to pulses of CO, sensor temperature T= 270 “C, 50% relative humidity (23 “C); ROP,= 16.7 a, R,,,& = 696 a, Row,.-_=1863 a, R,=5.1
0, Rw,=8.4
02.
Fig. 3 shows the CO response of the Pt-, Ca- and PtKa-catalysed sensors. As expected, Pt at the metaloxide surface leads to a CO sensor response. Possible mechanisms are described elsewhere [l-3]. The Cacatalysed sensors show a weak CO response, in particular at 500 ppm. The resistance values are slightly increased. This is in contrast to the Pt-catalyscd sensor response. To the authors’ knowledge the CO reaction mechanism at Ca-catalysed surfaces is not known. At the noncatalysed SnO, surface CO reactions are always very weak [l-3]. If there is almost no Ca influence on the CO reaction, the CO response of the Ca-catalysed SnO, surface is also very weak. However, the reaction should always lead to a resistance reduction. Ca partly diffised into the SnO, bulk might also lead to an acceptor-like background concentration close to the metal-oxide surface. In such a case any CO response thus leads to a resistance increase. An inverse resistance change can also be observed on NH, reactions. This can be seen in Fig. 4. In contrast to Pt- or PtlCa-catalysed sensors, the Ca catalysts lead to a strong resistance increase. So far the mechanisms leading to the inverse response are not clarified. More detailed investigations seem to be necessary. Fig. 5 shows the normalized resistance versus time of Pt, PtlCa and Ca-catalysed SnO, sensors in synthetic air at 270 “C. During the measurements the relative humidity of the gas atmosphere was changed between 30 and 70%. At t=O h the relative humidity was 50%. As can be seen and is expected [l--3], the sensors respond to a humidity change. The sensor response is fast with all catalysts. The Ca catalyst leads to the weakest humidity response. Although the humidity response is not very large, no technology change has led to a marked humidity-sensitivity decrease. The nonannealed SnO, material showed the weakest humidity
K. Steiner et al. I Sensors and Achtators, B 24-25 (1995) 529-531
531
4. Conclusions
I 0
2
4
6
time
8
IO
12
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Fig. 4. Normalized sensor resistance R/& vs. measurement time in response to pulses of NH,; sensor temperature T=270 ‘C; 50% relative humidity (23 ‘C); RoP,= 16.7 D ROPvc.- 696 0, R,cc. = 1863 fl, Rm=5.1 4 k=8.4 fl.
In conclusion, the CO, CO,, NH, and humidity responses of Pt-, Ca- and PtKa-catalysed SnOz sensors have been discussed. The sensors have been structured on Si substrates using standard thin-film technologies. The sensor architecture is stable in synthetic air up to 900 “C even for 48 h. No significant outdiffision of the sensor layers or peeling effects can be observed after annealing. Pt-catalysed sensors show a signal drift towards higher resistance values. The opposite is true for the Ca-catalysed sensors. The signal drift can be reduced by using Pt/Ca-catalyst combinations. The Pt/ Ca-catalysed sensors show a weak CO2 gas response. A strong NH, response can be realized with Ca catalysts. The resistance values are significantly increased. The Pt- and PtKa-catalysed sensors show a resistance reduction due to NH3.
Acknowledgement The work was partly financed by the Bundesministerium fiir Forschung und Technologie (BMJYT). We thank Professor Gbpel, University of Tiibingen and Professor Boubnov, Avangard, St. Petersburg, for helpful discussions and their cooperation.
References 03 +
0
2
4
6 time
8
IO
12
t / II
Fig. 5. Normalized sensor resistance R/R0 vs. measurement time in response to pulses of humidity; sensor temperature T=270 ‘C, 50% relative humidity (23 “C); RoP,= 16.7 fl, R,,puc. =696 a, Rope.= 1863 Q R-=5.1 a, h=8.4 l-l.
111P.T. Moseley, J.O.W. Norris and DE. Williams, Tec~ues
and Mechanisms in Gar Sensing, Adam Hilger, Bristol, 1991. Comprehensive Swey, Vol. 2,Chemical and Biochemical Sensors, Pan 1, VCH,
VI W. Giipel, J. Hesse and J.N. Zemel, Sensors, A Weioheim, 1991.
[31 T. Seiyama, Chemical Sensor Technology, Vol. 1, Kodansha,
Tokyo, 1988.
Uptmoor,W. Schweizer, H. L&v, M. Lather and K. Steiner, Ni, Io and Sb implanted Pt and V catalysed thin-film So& gas sensors, Sensors and Achutors B, 15-16 (1993) 390-395. I51 U. Hoefer, G. Ktihner, W. Schweizer, G. Sulz and K. Steiner, CO and CO2 thin-film SnOl gas sensors on Si substrates, Sensors and Actuntors B, 22 (1994) 115-119. 141 G. Sulz, G. Kiihner, H. Reiter, G.
response. However, the gas reaction was also very weak. Since the yearly local change of relative humidity is typically limited (for instance, in southwest Germany between 50 and 70%), the conductivity change might be tolerable for many applications.