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Material properties and the influence of metallic catalysts at the surface of highly dense SnO2 films
Sensors and Actuators B 70 Ž2000. 196–202 www.elsevier.nlrlocatersensorb
Material properties and the influence of metallic catalysts at the surface of highly dense SnO 2 films a a J. Wollenstein , H. Bottner , M. Jaegle a , W.J. Becker b, E. Wagner a,) ¨ ¨ b
a Fraunhofer Institute of Physical Measurement Techniques, Heidenhofstr. 8, D-79110 Freiburg, Germany Department of Electrical Engineering, UniÕersity of Kassel, Wilhelmshoher ¨ Allee 71, D-34121 Kassel, Germany
Received 25 October 1999; accepted 3 March 2000
Abstract We present an approach to optimize the specific response to gases by using specially prepared nanosized platinum on highly dense sputtered polycrystalline SnO 2 . Structural and morphological analyses of the SnO 2 and platinum thin films were performed. Gas measurements were carried out with single chip thin-film SnO 2 sensor arrays on silicon substrates. Pt nanoclusters covering the sensitive layer significantly affect the O 3 , CO and NO 2 sensitivities and the corresponding dynamic response. q 2000 Elsevier Science B.V. All rights reserved. Keywords: SnO 2 ; Thin film; Gas sensor; Surface catalyst; Platinum
1. Introduction Dispersions of small catalyst particles on metal oxide sensitive layers are commonly used as catalytic activators in gas-sensing devices. Platinum is a well known catalyst, but its capability to increase the selectivity of metal oxide gas sensors is however far from being well understood and is thus still a matter of investigation w1,2x. It is desirable that the catalyst be dispersed on the surface of gas-sensing metal oxide, rather than used as a continuous film, so that the sensing film will not be shorted out by the metal. The physical, electronic, and chemical properties of the Ptrmetal oxide interface depend strongly on the preparation of the substrate, the deposition of the metal particles and the used treatments w3x. We investigate how specially prepared Pt nanoclusters significantly affect the CO, NO 2 and O 3 sensitivities and the corresponding dynamic response. 2. Experimental details The sensor used in the experiments is a single chip thin-film SnO 2 sensor array with four sensing elements. )
Corresponding author. E-mail address: [email protected] ŽE. Wagner..
The array shown in Fig. 1 is structured using conventional photolithography, sputtering and evaporation techniques. A TarPt resistance layer Ž25:200 nm thick. for heating the device to its operating temperature Ž100–4008C. and interdigital electrodes are deposited and structured on a silicon substrate which is covered by a 1 mm SiO 2 insulating layer. Polycrystalline n-type SnO 2 Ž60-nm thick. is sputtered onto the electrodes. The morphological characteristics of the naked SnO 2 layer was investigated with an atomic force microscope ŽAFM.. AFM scans of the sensor surface of the sintered Ž7008C, air. SnO 2 reveal that the roughness is below 1 nm. This takes us to conclude that sputtered SnO 2 is highly dense in comparison to tin dioxide layers which are made with other common deposition techniques like sol–gel or rheotaxial growth thermal oxidation ŽRGTO. w4,5x. Immediately after the SnO 2 deposition followed the catalyst preparation. Platinum was deposited by using two methods: evaporation Žnominally 1.5-nm thick. and sputtering Žnominally 5-nm thick.. Fig. 2 shows equal scaled SEM images of the morphology of sputtered Žabove. and evaporated Žbelow. platinum on the SnO 2 surface before and after annealing at 7008C, 8008C and 9008C in synthetic air for 1 h, respectively. After deposition the platinum layer completely covers the SnO 2 and thus causes a short circuit. Before annealing, both the tin dioxide as well as the platinum surface are very smooth. Starting with annealing temperatures above 6008C the
0925-4005r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 5 6 9 - 4
J. Wollenstein et al.r Sensors and Actuators B 70 (2000) 196–202 ¨
Fig. 1. Schematic top view of the tin oxide thin film array with four sensing elements. The chip size is 9 mm2 . The layout comprises interdigital structures with symmetrical Pt electrodes Žblack.. The SnO 2 layers Žgrey. are set up either as areas on one side or in parallel stripes on the other side of the chip.
platinum begins to tear open, forms clusters and finally crystallites on the surface. The thicker sputtered layer shows this behaviour very clearly. At 7008C the platinum
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resembles a liquid drawn together by surface tension, at 8008C it is shaped like drops and at 9008C like crystals. The thinner evaporated platinum layer at 7008C already shows small crystallites which increase in size with higher temperatures. AFM scans of the evaporated platinum cluster covered SnO 2 surfaces after annealing at 7008C in synthetic air result in a roughness higher than 5 nm. The size of sputtered and evaporated crystallites is in the same range at 9008C. It should be noticed that the Pt morphology on the SnO 2 surface after 1 h of annealing at temperatures below 9008C depends strongly on the preparation and film thickness. But also, as expected, the tin dioxide is affected by the annealing process. Its grains are getting more distinct with higher annealing temperatures. The samples annealed at 9008C show a grain size of about 30 nm as can be seen in Fig. 2. Our systematic study of a large number of devices which were annealed at 7008C revealed that, despite the partial short-circuits of the well conducting platinum clusters, the resistance of sensors with Pt catalyst is significantly higher. For our sensor device the mean resistance values at 3008C in synthetic air are: 20.3 k V Žsputtered Pt catalyst., 37.7 k V Ževaporated Pt catalyst. and 12.6 k V Žwithout catalyst.. This can be conclusively attributed to Schottky barriers forming depletion regions beneath the Pt clusters. The work function of SnO 2 is 4.7 eV. As Pt has a higher work function of 5.7 eV Žboth at room temperature. the formation of a depletion zone is probable and this increases the resistance of the device.
Fig. 2. SEM images of the morphology of evaporated Ž1.5 nm. and sputtered Ž5 nm. Pt on the SnO 2 surface as grown and after annealing at different temperatures.
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3. Results and discussion 3.1. Material and electrical properties Every overgrowth is ruled by the structural and morphological properties of the substrate. Thus it is important to know its characteristics for the overgrowth of SnO 2 on top of the underlying TarPt contact, which itself is determined by the properties of the SirSiO 2 surface. For the formation of the contact TarPt electrodes, two different methods were used: sputtering and e-beam evaporation. The structural characterisation was performed by X-ray analysis. The common Qr2Q plot Žnot shown here. indicates a significantly textured grown for the ²111: direction for sputtered as well as for the evaporated TarPt layers. Fig. 3 shows the pole figures accordingly. It is obvious, that in both cases the contact layers grow highly ²111: fiberlike textured. Surprisingly, the texture is more distinctive for the sputtered layer Žpolar spreadf 2.88., than for the evaporated ones Žpolar spread f 108.. We explain this behaviour by the argon pre-etching process
prior to the contact-layer growth. This may lead to a special surface formation which causes such nearly perfectly textured sputtered TarPt layers. Prior to the e-beam TarPt evaporation, no precleaning was performed. A similar X-ray analysis was performed for the sensitive SnO 2 layer. Fig. 4 shows the Qr2Q plot for our sputtered SnO 2 films together with some important relative X-ray intensities for the usual SnO 2 phase ACassiteriteB w6x. The figure demonstrates that even the growth of the sputtered thin film SnO 2 is significantly textured, in contrast to typical thick film SnO 2 which shows the standard ACasseritite or tin stoneB characteristics w7x. Due to the textured growth, which can be taken as an intermediate state between nanopolycrystalline material on one hand and single crystal on the other hand, we expect a different sensing behaviour especially for sensitivity Ždifferent number of possible reactive surface statesrplaces, due to the different specific surface area. and response time Ždue to highly dense textured sputtered material.. The resistivity measurement of the sensitive material SnO 2 and derived from the sensitivity is also determined
Fig. 3. Pole figure of evaporated and sputtered TarPt metallization ŽTa 25 nm, Pt 200 nm. on silicon substrate, which is covered by a thermal 1 mm SiO 2 layer.
J. Wollenstein et al.r Sensors and Actuators B 70 (2000) 196–202 ¨
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Fig. 4. X-ray spectra of our sputtered SnO 2 layer on SiO 2 rSi substrate. The dashed lines are the relative X-ray intensities for the common ACassiteriteB SnO 2 phase Žtheoretical intensities: 100% w26.68x, 75% w33.88x, 21% w37.98x, 57% w51.78x..
by the SnO 2rPt contact with its Schottky barrier. As it is known Žsee also Fig. 2. that the shape and surface roughness of Pt changes under annealing procedures, it is important to examine the recrystallisation of Pt under the SnO 2 layer. Fig. 5 shows an AFM picture of a PtrSnO 2 contact area after annealing at 7008C for 4 h. The annealing process had no significant effect on the SnO 2 morphology on SiO 2 substrate. But there is recognizable increase of the Pt roughness and a small but also significant increase of the SnO 2 roughness at the underlying Pt electrode. Mainly,
Fig. 5. AFM picture of a PtrSnO 2 contact area after annealing at 7008C for 4 h.
we do not expect an impact of the Pt recrystallisation on the height of the contact Schottky barrier, but we believe that it is worth mentioning, that for the adhesion of both layer and with the long-term reliability, the necessary annealing procedures have to be chosen very carefully. 3.2. CO and NO2 response The response of the sensors to different concentrations of CO and NO 2 was measured at a constant working temperature of 3608C. These gases were chosen because of their importance for ventilation control of air in rooms or automobiles. Fig. 6 shows the response of such sensors to different concentrations of CO and NO 2 in synthetic air Ž50% r.H., 258C.. The selectivity and the dynamic behaviour of the signal to NO 2 and CO depend significantly on the presence of Pt clusters at the SnO 2 surface. The SnO 2 layers with Pt catalyst show a very low sensitivity to NO 2 . The sensitivity of the naked SnO 2 to CO is higher and is similar for both types of Pt sensors. The mean CO sensitivity values R 0rR for our sensor device to 100 ppm at 3608C in synthetic air are: 2.3 Žsputtered Pt catalyst., 1.7 Ževaporated Pt catalyst. and about 5 Žwithout catalyst.. However, the sensors with Pt clusters have a much shorter transient time in case of CO concentration steps.
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Fig. 6. Comparison of the normalized response ŽT s 3608C, 40% r.H.. to CO and NO 2 of a SnO 2 thin-film sensor with a sensor with Pt clusters at the surface.
In order to investigate the temperature dependence of the CO response of AnakedB and AplatinisedB tin dioxide, a set of gas measurements were carried out. In these experiments, the sensor operation temperature was stepped from 2608C to 4608C. During each 6-h interval in which the working temperature was kept constant, the sensors were exposed 2 h with 50 ppm CO. Fig. 7 shows the results as a function of working temperature in synthetic air Ž50% r.H... Diagrammed are the mean sensitivity values R 0rR to 50 ppm CO determined from the response of 15 sensors. The error bars result from the standard deviation representing the
production tolerances of the sensors which were taken from eight wafers within four productions runs. For the naked tin dioxide layers, a sensitivity maximum was found at 3408C. This behaviour of the sensor response is similar to the reported temperature dependence of the CO response for other polycrystalline SnO 2 thin films w8,9x. In case of CO exposure, the response and recovery time constants increase up to several hours with lower temperatures. The layers with small Pt particles at the surface show a different characteristic. In the analysed temperature range, the sensitivity increases strongly with lower temperatures with no significant decrease on the
Fig. 7. The sensitivity to 50 ppm CO of naked thin film SnO 2 and with Pt clusters at the surface as a function of working temperature in synthetic air Ž50% r.H... Diagrammed is the sensor response of the element Ža. of the gas sensor array shown in Fig. 1.
J. Wollenstein et al.r Sensors and Actuators B 70 (2000) 196–202 ¨
response and recovery time constants. This high sensitivity of the platinised tin dioxide at low temperatures may result from the modulation of the space charge region near the platinum clusters by the gas induced change of the surface potential of the platinum. Adsorbed CO molecules are considered to change the work-function of the platinum catalyst. The more the resulting change of the depletion depth dominates the conductivity of the tin dioxide layer, the higher the ratio between gas induced depth modulation and the layer thickness becomes. Since the depth of the depletion zone decreases with rising carrier concentrations, i.e., temperatures, this ratio and thus the sensitivity decreases with rising temperatures. In case of NO 2 , the investigation of the temperature dependence of the response of AplatinisedB tin dioxide results into some surprising aspects. We measured the sensor response to 2 and 5 ppm in the range of 240–4608C. At working temperatures above 3008C, the sensors with Pt catalyst show a very low resistivity increase to NO 2 ŽFig. 6.. At surface temperatures below 3008C, the reaction mechanism obviously changes. We observed a resistance decrease with short response times, whereas the sensitivity increases as the temperature is decreased. Remarkably, at low temperature, the AplatinisedB sensors do not react with the widely observed common oxidising interaction w10x. It is unlikely that this behaviour is directly attributed to a disintegration of NO 2 into NO and Oy at the hot Pt surface. It is also unlikely that the reaction of NO x is not ruled by the thermodynamic equilibrium of NO 2rNO in ambient air. At working temperature only a few percent NO 2 is dissociated to NO. It is more probable that NO 2 converts into different nitrogen-containing species, which results different competitive interactions depending at sev-
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eral parameters Žsurface temperature, gas concentration, availability of surface oxygen, etc... 3.3. Ozone response Pure thin film SnO 2 is highly sensitive to ozone w1x. The effect of ozone may be generic for thin film metal oxides: InO 3 , WO 3 and MoO 3 show large signals to ozone exposure w1,11,12x. The interaction of solar radiation and air pollution caused by urban traffic is the largest source of ozone emission in ambient air. We performed measurements with our sensors in a national air monitoring station in Freiburg, Germany. The comparison of the sensor signal with the results from the official gas measurement equipment indicated that the sensor response is mainly due to changes of the ozone concentration in the ambient atmosphere w13x. This dominating ozone sensitivity is in accordance with results of measurements with WO 3 devices performed in United Kingdom in a comparable way w1x. Thus for air-quality measurements in times with high solar radiation Že.g., summer., the elimination of the ozone cross sensitivity is indispensable. For this application we investigate the possibility of using catalyst supported SnO 2 gas sensors. Fig. 8 shows the response of our sensors to 100 ppb of ozone in synthetic air Ž50% r.H., 258C.. The sensor response to O 3 depends significantly on the presence of Pt clusters at the SnO 2 surface. The mean sensitivity value RrR 0 of the pure SnO 2 for our sensor device to 100 ppb O 3 at 3608C working temperature in synthetic air is 30. The SnO 2 layers with Pt catalyst show a very low sensitivity to O 3 . It is probable that this behaviour is attributed to a disintegration of ozone at the hot surface of the Pt clusters, but the reaction mechanism is still unknown in
Fig. 8. Comparison of the response to 100 ppb ozone of a SnO 2 nanofilm sensor with a sensor with Pt clusters at the surface ŽT s 3808C, 40% r.H... The sensors were exposed three times to ozone Ž1 h. followed by 1 h pure synthetic air, respectively.
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detail and a matter of our future investigations. As well as the question how the sensor reacts in a O 3 , NO x and CO gas mixture.
4. Conclusions and outlook Our experiments show that Pt nanoclusters on Ahighly densedB thin film SnO 2 significantly affect the CO, NO 2 and O 3 sensitivities and the corresponding dynamic response. The measurement of the temperature dependence of the CO response reveals that the sensitivity of platinised sensors raised with lower temperatures without a distinctive influence at dynamic response constants. This behaviour maybe the key to future low temperature CO sensors based on SnO 2 . In case of NO 2 and O 3 , dispersions of small Pt particles on the sensitive layer eliminate the gas reaction to these oxidising gases at working temperature Žaround 3408C.. This behaviour enters in a new design for a CO sensor array with improved selectivity, which is still under development. Also, other metallic catalysts are currently under investigation.
w8x D.-D. Lee, W.-Y. Chung, Gas-sensing characteristics of SnO 2y x thin film with added Pt, Sens. Actuators 20 Ž1989. 301–305. w9x M. Guilio et al., Properties of reactively sputtered thin oxide film as CO gas sensors, Sens. Actuators, B 23 Ž1995. 193–195. w10x W. Gopel, K.D. Schierbaum, D. Schmeisser, H.D. Wiemdorfer, ¨ ¨ Prototype chemical sensors for the detection of O 2 and NO 2 in gases, Sens. Actuators 17 Ž1989. 377–384. w11x A. Gurlo, N. Barsan, M. Ivanoskaya, U. Weirmar, W. Gopel, In 2 O 3 ¨ and MoO 3-In 2 O 3 thin films semiconductor sensors: interaction with NO 2 and O 3 , Sens. Actuators, B 47 Ž1998. 92–99. w12x T. Doll, A. Fuchs, I. Eisele, G. Faglia, S. Groppelli, G. Sberveglieri, Conductivity and work function ozone sensors based on indium oxide, Sens. Actuators, B 49 Ž1–2. Ž1998. 63–67. w13x U. Hoefer, H. Bottner, G. Kuhner, G. Sulz, F. Volz, J. Wollenstein, ¨ ¨ ¨ Multifunctional air-quality system based on advanced SnO 2 -sensorarray structures, Proceedings Cimtec, 14–19 June 1998, Florence, 1998.
Biographies Jurgen Wollenstein received his diploma in Electrical Engineering from ¨ ¨ the University of Kassel, Germany. Currently he is finishing his PhD thesis at the same university. In 1994 he joined the Fraunhofer Institute of Physical Measurement, Freiburg, Germany, where he is engaged in developing semiconductor gas sensors. The focus of his work is on field effect gas sensors devices.
Acknowledgements This study is supported partly by the German BMBF within the MISCHGAS project. The authors wish to thank Dr. G. Muller, Daimler Chrysler, Munich for the ozone ¨ measurements.
References w1x Williams, D., Semiconducting oxides as gas-sensitive resistors, presented as a plenary paper at Eurosensors XII Southampton, Sept. 1998. w2x M. Sauvan, C. Pijolat, Selectivity improvement of SnO 2 films by superficial metallic films, Proceedings Eurosensors XII Southampton, Sept. 1998. w3x S. Semancik, T.B. Fryberger, Model studies of SnO 2 -based gas sensors: vacancy defects and Pd additive effects, Sens. Actuators, B 1 Ž1990. 97–102. w4x Z. Jin et al., Application of nano-crystalline porous tin oxide thin film for CO sensing, Sens. Actuators, B 52 Ž1998. 188–194. w5x G. Sberveglieri, Classical and novel technique for preparation of SnO 2 thin-film gas sensors, Sens. Actuators, B 6 Ž1992. 239–247. w6x A. Winchell, H. Winchell, Microscopic Character of Artificial Inorganic Solid Substances, 1964, p. 69. w7x Weimar, U., Oxidgassensoren und Multikomponentenanalyse, Thesis, Universitat Fakultat ¨ Tubingen, ¨ ¨ f ur ¨ Chemie und Pharmazie, 1996.
Martin Jaegle received his diploma degree in Physics from the University of Freiburg, Germany in 1995. Currently he works at the Fraunhofer Institute of Physical Measurement, Freiburg, Germany, on semiconductor chemical gas sensors and thermoelectrical devices. Before joining the Fraunhofer Institute, he has worked for Litef, Freiburg on integrated optical devices. Harald Bottner graduated with a diploma degree in Chemistry from the ¨ University of Munster, Germany in 1974 and received his PhD in 1977 ¨ from the same university. In 1980 he joined the Fraunhofer Institut f ur ¨ Physikalische Messtechnik, Freiburg, Germany. From 1980 to 1995 he developed IV–VI semiconductor lasers. Currently he is group leader for development of semiconductor chemical gas sensors and thermoelectrical devices. Wolf Jurgen Becker is professor and head of the Department of Engineer¨ ing Measurement at University of Kassel, Germany since 1984. He graduated with a diploma degree in Physics of the Technical University of Darmstadt, Germany in 1966 and received his PhD in 1969 from the same university. From 1970 to 1976 he worked at Bayer Leverkusen and from 1976 at Spanner Pollux, Ludwigshafen. Elmar Wagner is director of the Fraunhofer Institute of Physical Measurement Techniques in Freiburg, Germany. He received his diploma degree in Physics and his PhD degree from the Technical University of Munich. Before joining the Fraunhofer-Institute he has worked for the Max-Planck Institute of Solid-State Research in Stuttgart, Hewlett-Packard in Palo Alto, and AEG-Telefunken in Heilbronn.
Material properties and the influence of metallic catalysts at the surface of highly dense SnO 2 films a a J. Wollenstein , H. Bottner , M. Jaegle a , W.J. Becker b, E. Wagner a,) ¨ ¨ b
a Fraunhofer Institute of Physical Measurement Techniques, Heidenhofstr. 8, D-79110 Freiburg, Germany Department of Electrical Engineering, UniÕersity of Kassel, Wilhelmshoher ¨ Allee 71, D-34121 Kassel, Germany
Received 25 October 1999; accepted 3 March 2000
Abstract We present an approach to optimize the specific response to gases by using specially prepared nanosized platinum on highly dense sputtered polycrystalline SnO 2 . Structural and morphological analyses of the SnO 2 and platinum thin films were performed. Gas measurements were carried out with single chip thin-film SnO 2 sensor arrays on silicon substrates. Pt nanoclusters covering the sensitive layer significantly affect the O 3 , CO and NO 2 sensitivities and the corresponding dynamic response. q 2000 Elsevier Science B.V. All rights reserved. Keywords: SnO 2 ; Thin film; Gas sensor; Surface catalyst; Platinum
1. Introduction Dispersions of small catalyst particles on metal oxide sensitive layers are commonly used as catalytic activators in gas-sensing devices. Platinum is a well known catalyst, but its capability to increase the selectivity of metal oxide gas sensors is however far from being well understood and is thus still a matter of investigation w1,2x. It is desirable that the catalyst be dispersed on the surface of gas-sensing metal oxide, rather than used as a continuous film, so that the sensing film will not be shorted out by the metal. The physical, electronic, and chemical properties of the Ptrmetal oxide interface depend strongly on the preparation of the substrate, the deposition of the metal particles and the used treatments w3x. We investigate how specially prepared Pt nanoclusters significantly affect the CO, NO 2 and O 3 sensitivities and the corresponding dynamic response. 2. Experimental details The sensor used in the experiments is a single chip thin-film SnO 2 sensor array with four sensing elements. )
Corresponding author. E-mail address: [email protected] ŽE. Wagner..
The array shown in Fig. 1 is structured using conventional photolithography, sputtering and evaporation techniques. A TarPt resistance layer Ž25:200 nm thick. for heating the device to its operating temperature Ž100–4008C. and interdigital electrodes are deposited and structured on a silicon substrate which is covered by a 1 mm SiO 2 insulating layer. Polycrystalline n-type SnO 2 Ž60-nm thick. is sputtered onto the electrodes. The morphological characteristics of the naked SnO 2 layer was investigated with an atomic force microscope ŽAFM.. AFM scans of the sensor surface of the sintered Ž7008C, air. SnO 2 reveal that the roughness is below 1 nm. This takes us to conclude that sputtered SnO 2 is highly dense in comparison to tin dioxide layers which are made with other common deposition techniques like sol–gel or rheotaxial growth thermal oxidation ŽRGTO. w4,5x. Immediately after the SnO 2 deposition followed the catalyst preparation. Platinum was deposited by using two methods: evaporation Žnominally 1.5-nm thick. and sputtering Žnominally 5-nm thick.. Fig. 2 shows equal scaled SEM images of the morphology of sputtered Žabove. and evaporated Žbelow. platinum on the SnO 2 surface before and after annealing at 7008C, 8008C and 9008C in synthetic air for 1 h, respectively. After deposition the platinum layer completely covers the SnO 2 and thus causes a short circuit. Before annealing, both the tin dioxide as well as the platinum surface are very smooth. Starting with annealing temperatures above 6008C the
0925-4005r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 5 6 9 - 4
J. Wollenstein et al.r Sensors and Actuators B 70 (2000) 196–202 ¨
Fig. 1. Schematic top view of the tin oxide thin film array with four sensing elements. The chip size is 9 mm2 . The layout comprises interdigital structures with symmetrical Pt electrodes Žblack.. The SnO 2 layers Žgrey. are set up either as areas on one side or in parallel stripes on the other side of the chip.
platinum begins to tear open, forms clusters and finally crystallites on the surface. The thicker sputtered layer shows this behaviour very clearly. At 7008C the platinum
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resembles a liquid drawn together by surface tension, at 8008C it is shaped like drops and at 9008C like crystals. The thinner evaporated platinum layer at 7008C already shows small crystallites which increase in size with higher temperatures. AFM scans of the evaporated platinum cluster covered SnO 2 surfaces after annealing at 7008C in synthetic air result in a roughness higher than 5 nm. The size of sputtered and evaporated crystallites is in the same range at 9008C. It should be noticed that the Pt morphology on the SnO 2 surface after 1 h of annealing at temperatures below 9008C depends strongly on the preparation and film thickness. But also, as expected, the tin dioxide is affected by the annealing process. Its grains are getting more distinct with higher annealing temperatures. The samples annealed at 9008C show a grain size of about 30 nm as can be seen in Fig. 2. Our systematic study of a large number of devices which were annealed at 7008C revealed that, despite the partial short-circuits of the well conducting platinum clusters, the resistance of sensors with Pt catalyst is significantly higher. For our sensor device the mean resistance values at 3008C in synthetic air are: 20.3 k V Žsputtered Pt catalyst., 37.7 k V Ževaporated Pt catalyst. and 12.6 k V Žwithout catalyst.. This can be conclusively attributed to Schottky barriers forming depletion regions beneath the Pt clusters. The work function of SnO 2 is 4.7 eV. As Pt has a higher work function of 5.7 eV Žboth at room temperature. the formation of a depletion zone is probable and this increases the resistance of the device.
Fig. 2. SEM images of the morphology of evaporated Ž1.5 nm. and sputtered Ž5 nm. Pt on the SnO 2 surface as grown and after annealing at different temperatures.
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3. Results and discussion 3.1. Material and electrical properties Every overgrowth is ruled by the structural and morphological properties of the substrate. Thus it is important to know its characteristics for the overgrowth of SnO 2 on top of the underlying TarPt contact, which itself is determined by the properties of the SirSiO 2 surface. For the formation of the contact TarPt electrodes, two different methods were used: sputtering and e-beam evaporation. The structural characterisation was performed by X-ray analysis. The common Qr2Q plot Žnot shown here. indicates a significantly textured grown for the ²111: direction for sputtered as well as for the evaporated TarPt layers. Fig. 3 shows the pole figures accordingly. It is obvious, that in both cases the contact layers grow highly ²111: fiberlike textured. Surprisingly, the texture is more distinctive for the sputtered layer Žpolar spreadf 2.88., than for the evaporated ones Žpolar spread f 108.. We explain this behaviour by the argon pre-etching process
prior to the contact-layer growth. This may lead to a special surface formation which causes such nearly perfectly textured sputtered TarPt layers. Prior to the e-beam TarPt evaporation, no precleaning was performed. A similar X-ray analysis was performed for the sensitive SnO 2 layer. Fig. 4 shows the Qr2Q plot for our sputtered SnO 2 films together with some important relative X-ray intensities for the usual SnO 2 phase ACassiteriteB w6x. The figure demonstrates that even the growth of the sputtered thin film SnO 2 is significantly textured, in contrast to typical thick film SnO 2 which shows the standard ACasseritite or tin stoneB characteristics w7x. Due to the textured growth, which can be taken as an intermediate state between nanopolycrystalline material on one hand and single crystal on the other hand, we expect a different sensing behaviour especially for sensitivity Ždifferent number of possible reactive surface statesrplaces, due to the different specific surface area. and response time Ždue to highly dense textured sputtered material.. The resistivity measurement of the sensitive material SnO 2 and derived from the sensitivity is also determined
Fig. 3. Pole figure of evaporated and sputtered TarPt metallization ŽTa 25 nm, Pt 200 nm. on silicon substrate, which is covered by a thermal 1 mm SiO 2 layer.
J. Wollenstein et al.r Sensors and Actuators B 70 (2000) 196–202 ¨
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Fig. 4. X-ray spectra of our sputtered SnO 2 layer on SiO 2 rSi substrate. The dashed lines are the relative X-ray intensities for the common ACassiteriteB SnO 2 phase Žtheoretical intensities: 100% w26.68x, 75% w33.88x, 21% w37.98x, 57% w51.78x..
by the SnO 2rPt contact with its Schottky barrier. As it is known Žsee also Fig. 2. that the shape and surface roughness of Pt changes under annealing procedures, it is important to examine the recrystallisation of Pt under the SnO 2 layer. Fig. 5 shows an AFM picture of a PtrSnO 2 contact area after annealing at 7008C for 4 h. The annealing process had no significant effect on the SnO 2 morphology on SiO 2 substrate. But there is recognizable increase of the Pt roughness and a small but also significant increase of the SnO 2 roughness at the underlying Pt electrode. Mainly,
Fig. 5. AFM picture of a PtrSnO 2 contact area after annealing at 7008C for 4 h.
we do not expect an impact of the Pt recrystallisation on the height of the contact Schottky barrier, but we believe that it is worth mentioning, that for the adhesion of both layer and with the long-term reliability, the necessary annealing procedures have to be chosen very carefully. 3.2. CO and NO2 response The response of the sensors to different concentrations of CO and NO 2 was measured at a constant working temperature of 3608C. These gases were chosen because of their importance for ventilation control of air in rooms or automobiles. Fig. 6 shows the response of such sensors to different concentrations of CO and NO 2 in synthetic air Ž50% r.H., 258C.. The selectivity and the dynamic behaviour of the signal to NO 2 and CO depend significantly on the presence of Pt clusters at the SnO 2 surface. The SnO 2 layers with Pt catalyst show a very low sensitivity to NO 2 . The sensitivity of the naked SnO 2 to CO is higher and is similar for both types of Pt sensors. The mean CO sensitivity values R 0rR for our sensor device to 100 ppm at 3608C in synthetic air are: 2.3 Žsputtered Pt catalyst., 1.7 Ževaporated Pt catalyst. and about 5 Žwithout catalyst.. However, the sensors with Pt clusters have a much shorter transient time in case of CO concentration steps.
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Fig. 6. Comparison of the normalized response ŽT s 3608C, 40% r.H.. to CO and NO 2 of a SnO 2 thin-film sensor with a sensor with Pt clusters at the surface.
In order to investigate the temperature dependence of the CO response of AnakedB and AplatinisedB tin dioxide, a set of gas measurements were carried out. In these experiments, the sensor operation temperature was stepped from 2608C to 4608C. During each 6-h interval in which the working temperature was kept constant, the sensors were exposed 2 h with 50 ppm CO. Fig. 7 shows the results as a function of working temperature in synthetic air Ž50% r.H... Diagrammed are the mean sensitivity values R 0rR to 50 ppm CO determined from the response of 15 sensors. The error bars result from the standard deviation representing the
production tolerances of the sensors which were taken from eight wafers within four productions runs. For the naked tin dioxide layers, a sensitivity maximum was found at 3408C. This behaviour of the sensor response is similar to the reported temperature dependence of the CO response for other polycrystalline SnO 2 thin films w8,9x. In case of CO exposure, the response and recovery time constants increase up to several hours with lower temperatures. The layers with small Pt particles at the surface show a different characteristic. In the analysed temperature range, the sensitivity increases strongly with lower temperatures with no significant decrease on the
Fig. 7. The sensitivity to 50 ppm CO of naked thin film SnO 2 and with Pt clusters at the surface as a function of working temperature in synthetic air Ž50% r.H... Diagrammed is the sensor response of the element Ža. of the gas sensor array shown in Fig. 1.
J. Wollenstein et al.r Sensors and Actuators B 70 (2000) 196–202 ¨
response and recovery time constants. This high sensitivity of the platinised tin dioxide at low temperatures may result from the modulation of the space charge region near the platinum clusters by the gas induced change of the surface potential of the platinum. Adsorbed CO molecules are considered to change the work-function of the platinum catalyst. The more the resulting change of the depletion depth dominates the conductivity of the tin dioxide layer, the higher the ratio between gas induced depth modulation and the layer thickness becomes. Since the depth of the depletion zone decreases with rising carrier concentrations, i.e., temperatures, this ratio and thus the sensitivity decreases with rising temperatures. In case of NO 2 , the investigation of the temperature dependence of the response of AplatinisedB tin dioxide results into some surprising aspects. We measured the sensor response to 2 and 5 ppm in the range of 240–4608C. At working temperatures above 3008C, the sensors with Pt catalyst show a very low resistivity increase to NO 2 ŽFig. 6.. At surface temperatures below 3008C, the reaction mechanism obviously changes. We observed a resistance decrease with short response times, whereas the sensitivity increases as the temperature is decreased. Remarkably, at low temperature, the AplatinisedB sensors do not react with the widely observed common oxidising interaction w10x. It is unlikely that this behaviour is directly attributed to a disintegration of NO 2 into NO and Oy at the hot Pt surface. It is also unlikely that the reaction of NO x is not ruled by the thermodynamic equilibrium of NO 2rNO in ambient air. At working temperature only a few percent NO 2 is dissociated to NO. It is more probable that NO 2 converts into different nitrogen-containing species, which results different competitive interactions depending at sev-
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eral parameters Žsurface temperature, gas concentration, availability of surface oxygen, etc... 3.3. Ozone response Pure thin film SnO 2 is highly sensitive to ozone w1x. The effect of ozone may be generic for thin film metal oxides: InO 3 , WO 3 and MoO 3 show large signals to ozone exposure w1,11,12x. The interaction of solar radiation and air pollution caused by urban traffic is the largest source of ozone emission in ambient air. We performed measurements with our sensors in a national air monitoring station in Freiburg, Germany. The comparison of the sensor signal with the results from the official gas measurement equipment indicated that the sensor response is mainly due to changes of the ozone concentration in the ambient atmosphere w13x. This dominating ozone sensitivity is in accordance with results of measurements with WO 3 devices performed in United Kingdom in a comparable way w1x. Thus for air-quality measurements in times with high solar radiation Že.g., summer., the elimination of the ozone cross sensitivity is indispensable. For this application we investigate the possibility of using catalyst supported SnO 2 gas sensors. Fig. 8 shows the response of our sensors to 100 ppb of ozone in synthetic air Ž50% r.H., 258C.. The sensor response to O 3 depends significantly on the presence of Pt clusters at the SnO 2 surface. The mean sensitivity value RrR 0 of the pure SnO 2 for our sensor device to 100 ppb O 3 at 3608C working temperature in synthetic air is 30. The SnO 2 layers with Pt catalyst show a very low sensitivity to O 3 . It is probable that this behaviour is attributed to a disintegration of ozone at the hot surface of the Pt clusters, but the reaction mechanism is still unknown in
Fig. 8. Comparison of the response to 100 ppb ozone of a SnO 2 nanofilm sensor with a sensor with Pt clusters at the surface ŽT s 3808C, 40% r.H... The sensors were exposed three times to ozone Ž1 h. followed by 1 h pure synthetic air, respectively.
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detail and a matter of our future investigations. As well as the question how the sensor reacts in a O 3 , NO x and CO gas mixture.
4. Conclusions and outlook Our experiments show that Pt nanoclusters on Ahighly densedB thin film SnO 2 significantly affect the CO, NO 2 and O 3 sensitivities and the corresponding dynamic response. The measurement of the temperature dependence of the CO response reveals that the sensitivity of platinised sensors raised with lower temperatures without a distinctive influence at dynamic response constants. This behaviour maybe the key to future low temperature CO sensors based on SnO 2 . In case of NO 2 and O 3 , dispersions of small Pt particles on the sensitive layer eliminate the gas reaction to these oxidising gases at working temperature Žaround 3408C.. This behaviour enters in a new design for a CO sensor array with improved selectivity, which is still under development. Also, other metallic catalysts are currently under investigation.
w8x D.-D. Lee, W.-Y. Chung, Gas-sensing characteristics of SnO 2y x thin film with added Pt, Sens. Actuators 20 Ž1989. 301–305. w9x M. Guilio et al., Properties of reactively sputtered thin oxide film as CO gas sensors, Sens. Actuators, B 23 Ž1995. 193–195. w10x W. Gopel, K.D. Schierbaum, D. Schmeisser, H.D. Wiemdorfer, ¨ ¨ Prototype chemical sensors for the detection of O 2 and NO 2 in gases, Sens. Actuators 17 Ž1989. 377–384. w11x A. Gurlo, N. Barsan, M. Ivanoskaya, U. Weirmar, W. Gopel, In 2 O 3 ¨ and MoO 3-In 2 O 3 thin films semiconductor sensors: interaction with NO 2 and O 3 , Sens. Actuators, B 47 Ž1998. 92–99. w12x T. Doll, A. Fuchs, I. Eisele, G. Faglia, S. Groppelli, G. Sberveglieri, Conductivity and work function ozone sensors based on indium oxide, Sens. Actuators, B 49 Ž1–2. Ž1998. 63–67. w13x U. Hoefer, H. Bottner, G. Kuhner, G. Sulz, F. Volz, J. Wollenstein, ¨ ¨ ¨ Multifunctional air-quality system based on advanced SnO 2 -sensorarray structures, Proceedings Cimtec, 14–19 June 1998, Florence, 1998.
Biographies Jurgen Wollenstein received his diploma in Electrical Engineering from ¨ ¨ the University of Kassel, Germany. Currently he is finishing his PhD thesis at the same university. In 1994 he joined the Fraunhofer Institute of Physical Measurement, Freiburg, Germany, where he is engaged in developing semiconductor gas sensors. The focus of his work is on field effect gas sensors devices.
Acknowledgements This study is supported partly by the German BMBF within the MISCHGAS project. The authors wish to thank Dr. G. Muller, Daimler Chrysler, Munich for the ozone ¨ measurements.
References w1x Williams, D., Semiconducting oxides as gas-sensitive resistors, presented as a plenary paper at Eurosensors XII Southampton, Sept. 1998. w2x M. Sauvan, C. Pijolat, Selectivity improvement of SnO 2 films by superficial metallic films, Proceedings Eurosensors XII Southampton, Sept. 1998. w3x S. Semancik, T.B. Fryberger, Model studies of SnO 2 -based gas sensors: vacancy defects and Pd additive effects, Sens. Actuators, B 1 Ž1990. 97–102. w4x Z. Jin et al., Application of nano-crystalline porous tin oxide thin film for CO sensing, Sens. Actuators, B 52 Ž1998. 188–194. w5x G. Sberveglieri, Classical and novel technique for preparation of SnO 2 thin-film gas sensors, Sens. Actuators, B 6 Ž1992. 239–247. w6x A. Winchell, H. Winchell, Microscopic Character of Artificial Inorganic Solid Substances, 1964, p. 69. w7x Weimar, U., Oxidgassensoren und Multikomponentenanalyse, Thesis, Universitat Fakultat ¨ Tubingen, ¨ ¨ f ur ¨ Chemie und Pharmazie, 1996.
Martin Jaegle received his diploma degree in Physics from the University of Freiburg, Germany in 1995. Currently he works at the Fraunhofer Institute of Physical Measurement, Freiburg, Germany, on semiconductor chemical gas sensors and thermoelectrical devices. Before joining the Fraunhofer Institute, he has worked for Litef, Freiburg on integrated optical devices. Harald Bottner graduated with a diploma degree in Chemistry from the ¨ University of Munster, Germany in 1974 and received his PhD in 1977 ¨ from the same university. In 1980 he joined the Fraunhofer Institut f ur ¨ Physikalische Messtechnik, Freiburg, Germany. From 1980 to 1995 he developed IV–VI semiconductor lasers. Currently he is group leader for development of semiconductor chemical gas sensors and thermoelectrical devices. Wolf Jurgen Becker is professor and head of the Department of Engineer¨ ing Measurement at University of Kassel, Germany since 1984. He graduated with a diploma degree in Physics of the Technical University of Darmstadt, Germany in 1966 and received his PhD in 1969 from the same university. From 1970 to 1976 he worked at Bayer Leverkusen and from 1976 at Spanner Pollux, Ludwigshafen. Elmar Wagner is director of the Fraunhofer Institute of Physical Measurement Techniques in Freiburg, Germany. He received his diploma degree in Physics and his PhD degree from the Technical University of Munich. Before joining the Fraunhofer-Institute he has worked for the Max-Planck Institute of Solid-State Research in Stuttgart, Hewlett-Packard in Palo Alto, and AEG-Telefunken in Heilbronn.