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Optimization of physical filtering for selective high temperature H2 sensors
Sensors and Actuators B 68 Ž2000. 146–150 www.elsevier.nlrlocatersensorb
Optimization of physical filtering for selective high temperature H 2 sensors T. Weh ) , M. Fleischer, H. Meixner Siemens AG, ZT MS 2, Otto-Hahn-Ring 6, D-81739 Munchen, Germany ¨
Abstract This paper investigates the improvement in selectivity by means of applying a physical gas filter. Looking for a selective and highly sensitive hydrogen sensor, various filter systems, single, dual and buried filter systems, were deposited on top of a basic high-temperature semiconducting Ga 2 O 3 thin film sensor. Sensor respondence on a big variety of gases present in domestic ambience and especially hydrogen and ethane, which is the hardest test on cross sensitivity, is shown. Using a single filter SiO 2 layer a selective hydrogen sensor is realized, which is able to detect 50 ppm hydrogen at an operation temperature of 7008C in humid air without any interference caused by other gases. Dual SiO 2rmetal oxide filter systems and buried metal oxide layer filters show behaviors varying from selective ethane to selective hydrogen detection. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Hydrogen; Ethane; Filter; Metal oxide; Bias
1. Introduction Aiming at applications like fire warning, industrial process control systems or leakage-control systems for hydrogen-driven vehicles, a highly sensitive and selective hydrogen sensor is required with easy handling abilities and low cost requirements. This sensor should be able to detect low concentrations of hydrogen, 10 to 50 ppm H 2 , in a realistic environment, where other gases in different concentrations are present during the appearance of hydrogen. Therefore a standard high temperature gas sensor was taken, with detection mechanism based on the gas-dependent resistance of the n-type semiconductor Ga 2 O 3 w1,2x. This basic Ga 2 O 3 sensor is sensitive to many gases including methane, CO, NO, propane, ethane and hydrogen, but its handling is easy and the signals can be caught with low-level electronics. So, a modification has to be made for eliminating the influence of all other gases except hydrogen, and, if possible, increasing the sensitivity on hydrogen. The most effective way of transforming this basic Ga 2 O 3 sensor to a highly selective and sensitive hydrogen sensor is to apply a physical filter located directly on top of the
)
Corresponding author. E-mail address: [email protected] ŽT. Weh..
sensor layer w3,4x. By diffusion control a filter like this impedes the reactive gases from reaching the Ga 2 O 3 but lets the hydrogen pass through.
2. Experimental The high-temperature sensor used here is a Ga 2 O 3 thin film sensor with a heating structure on the back side. Below the Ga 2 O 3 sensor layer an interdigital structure for sensing the resistance of the n-type metal oxide is placed; the operating temperature of the sensor is 7008C. A fully computer-controlled gas mixing unit was used to apply the gases to the sensor; the gas-dependent resistance was recorded with this equipment, too. The physical filter used in this type of application is a 200-nm-thick SiO 2 layer w5x. For the first time a BIAS power was applied during reactive sputtering of the SiO 2 layer, a 99.999% pure Si target was used, and the gas mixture during the sputter process was 50 sccm argon and 10 sccm oxygen. The SiO 2 layer was stabilized afterwards at a temperature of 10508C for 10 h. The additionally applied Al 2 O 3 and Ga 2 O 3 filter layers were RF sputtered and separately stabilized, too. Investigations about joint annealed layers with SiO 2 and Ga 2 O 3 , not shown here, showed no further step up in performance of the sensors.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 7 5 - 5
T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
147
3. Results and discussion 3.1. Measurements on hydrogen with single filter sensors With this filter on top of the sensor the sensitivity and selectivity to hydrogen and the interference resistance compared with the basic Ga 2 O 3 sensor has raised dramatically, the sensor is only slightly deranged by ethane and propane, as shown in Fig. 1. As shown in this figure nearly all given gases are suppressed. With concentrations of 5000 ppm methane or 1000 ppm CO no signal at all appears, if the sensor is in contact with propane the signal is down to 3% of the signal a basic Ga 2 O 3 sensor generates. Only ethane is deranging the sensor in a serious way, not shown here, because some hydrogen atoms are cracked off the ethane molecule producing a false hydrogen signal on the sensor. With ethane having one of the smallest binding energies on hydrogen atoms it is the hardest test for high-temperature hydrogen detection sensors. Fig. 2 shows the increase in sensitivity to hydrogen, particularly at low concentrations. Fig. 3 shows the influence of the BIAS power level during the sputter process of the SiO 2 on the sensitivity on hydrogen. If the BIAS power level is smaller than 15 W the behavior of the sensor layer is quite similar to a non BIAS-sputtered filter layer. With 30-W BIAS or more the filter layer is improved in its performance on hydrogen, reaching the best sensitivity to hydrogen at a BIAS power level of 50 W, which represents the maximum BIAS power level allowed in the RF sputter machine used. The recovery time of the sensors depends on the signal height; therefore, not all sensors are at zero level at the end of the experiment. In exactly the same steps of BIAS power level the selectivity is increasing. The SiO 2 layer seems to be more compact with higher BIAS power level during sput-
Fig. 1. Direct comparison between a basic Ga 2 O 3 sensor and an SiO 2 modified sensor at a temperature of 7008C in humid air on different gases.
Fig. 2. Sensitivity to hydrogen of a BIAS-sputtered SiO 2 modified sensor, a non BIAS-sputtered SiO 2 modified sensor and a basic Ga 2 O 3 sensor at a temperature of 7008C in humid air.
tering of the SiO 2 , a necessary parameter for getting a useful filter layer on the basic Ga 2 O 3 sensor transforming it into a hydrogen sensor. On the other hand, no significant influence on the sensitivity by varying the layer thickness of the SiO 2 could be found. If the SiO 2 layer is closed and compact, the sensor will act correctly, but if there are any holes or uncovered edges on the sensor top, the selectivity is gone. Fig. 4 shows the sensitivity to hydrogen of identically prepared sensors with varying SiO 2 layer thickness. 3.2. Dual filter metal oxide layer on top of SiO2 To further improve sensing behavior dual Ga 2 O 3rSiO 2 and Al 2 O 3rSiO 2 layer systems, see Fig. 5, on top of the basic Ga 2 O 3 sensor have been investigated since single filter layers show no more potential for optimization.
Fig. 3. Sensitivity to hydrogen of SiO 2 modified sensors with different BIAS power level during RF sputtering at temperature of 7008C in humid air.
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T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
Fig. 4. Comparison of SiO 2 modified sensors with different thick layers of SiO 2 at a temperature of 7008C in humid air.
Typical filter layers were between 80 and 300 nm thickness, all layers were separately stabilized after the sputter process. All sensors are at an operating temperature of 7008C; lower temperatures decrease the reaction time on changing the gas concentrations, higher temperatures lower the selectivity, particularly ethane makes more confusion on the sensor. As shown in Fig. 6, the Al 2 O 3 layer and the SiO 2 layer are combining the effects of the single filter characteristics, but this modification transforms a basic Ga 2 O 3 sensor into a high selective ethane sensor: With this filter system on top of the sensor only ethane is able to pass both filters layers. As a side effect SiO 2 takes over the separation of the materials to avoid formation of Al x Ga 2yx O 3 . A second dual-layer configuration for detecting ethane is sputtering a Ga 2 O 3 layer on top of the SiO 2 modified sensor. In direct comparison to Fig. 5 the Al 2 O 3 is replaced by RF sputtered Ga 2 O 3 . As shown in Fig. 7 no big difference on sensing ethane can be seen compared to the Al 2 O 3rSiO 2 modified sensor shown above. Similar to the first given dual-layer sensor, ethane is detected with good signal height, hydrogen is suppressed even at high concentrations. After all this result is a little
Fig. 5. Schematics of a dual layer system with Al 2 O 3 and SiO 2 applied to a basic Ga 2 O 3 sensor.
Fig. 6. Hydrogen and ethane contact of an SiO 2 modified sensor and SiO 2 qAl 2 O 3 dual filter sensors at a temperature of 7008C in humid air.
bit surprising compared to the experiments done in Ref. w6x: Applying a Ga 2 O 3 thick film on top of the sensor the ethane is burned never reaching the surface of the sensor. With a Ga 2 O 3 sputtered layer having a lot smaller surface the ethane not only reaches the surface; as an expected effect, it transforms the sensor in an ethane sensor blocking the hydrogen very effectively. A difference between the SiO 2rAl 2 O 3 and the SiO 2rGa 2 O 3 sensor can be seen in applying propane or CO to the sensors; they are slightly differently deranged on these gases. In the case of ethane no difference in the behavior of the two different modifications can be found. 3.3. Buried layer sensors Using Ga 2 O 3 or Al 2 O 3 layers as top layers for filtering resulted in strong ethane sensitivity. To use these layers without catalytic influences of their surfaces these layers have to be buried. As shown in Fig. 1, SiO 2 is a good choice to bury other materials, because it does no harm to hydrogen and keeps other gases away from any buried layers.
Fig. 7. Hydrogen and ethane contact of SiO 2 qGa 2 O 3 modified sensors at a temperature of 7008C in humid air.
T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
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Fig. 8. SEM photograph of a buried layer Ga 2 O 3 sensor.
Fig. 9. SEM photograph of a buried layer Cr-doped SrTiO 3 .
Considering the impact of ethane on the hydrogen sensor a way has to be found to eliminate the disturbance of the sensor caused by H atoms cracked off the ethane, otherwise no reliable hydrogen detection can be made. Going on the search for materials to be buried blocking the ethane a n-type and a p-type metal oxide were used. Ga 2 O 3 , the n-type metal oxide used in this case, was buried between two SiO 2 layers, all separately sputtered and annealed at a temperature of 9008C, see Fig. 8. The p-type metal oxide at an operating temperature of 7008C used here is SrTiO 3 doped with Cr2 O5 . At lower temperatures SrTiO 3 is an n-type metal oxide, but at temperatures higher than 5008C it changes to a p-type metal oxide. This layer was again placed between two SiO 2 layers, all layers were separately sputtered and annealed at a temperature of 9008C, see Fig. 9. First the buried layer Ga 2 O 3 sensors were investigated to hydrogen and ethane, see Fig. 10. Comparing this Ga 2 O 3 buried layer sensor to the one using Ga 2 O 3 as the top layer, where the top layer Ga 2 O 3 is fully exposed to the used gases, the buried layer sensor is still responding
on ethane, but the sensor is still responding to hydrogen. Burying the Ga 2 O 3 layer enables the sensor to detect the total amount of both gases. All the other gases like propane, CO or NO, not shown here, are suppressed similar to the SiO 2 single filter sensor.
Fig. 10. Hydrogen and ethane contact of buried layer Ga 2 O 3 sensors at a temperature of 7008C in humid air.
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T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
als than above with different parameters during the RF sputter process have to be screened and characterized.
Acknowledgements This study was partly supported by a BMBF research grant ŽForderungskennzeichen 16 V 777r0. from the Ger¨ man ministry of education and research.
References Fig. 11. Hydrogen and ethane contact of buried layer Cr-doped SrTiO 3 sensors at a temperature of 7008C in humid air.
Applying the Cr-doped SrTiO 3 buried layer sensors to the same gas program, ethanerhydrogen gas exposure, a different behavior can be seen, see Fig. 11. This sensor type performs well on hydrogen and it suppresses ethane even at high concentrations. The sensitivity to hydrogen is as high as shown in Fig. 3, which shows the SiO 2 coated sensor with 50 W of BIAS power level during sputtering of the SiO 2 layer, so there seems to be no influence of the buried SrTiO 3 layer on detecting hydrogen. Exposing the sensor to a lot of other gases used in technical environments, like propane, methane, CO, CO 2 or NO, shows a similar suppression as the SiO 2 single filter sensor does Žnot shown here.. The important quality of this type of sensor modification is the very good suppression of ethane without bothering the detection of hydrogen.
4. Conclusion Optimizing filter systems can not only increase selectivity using a given sensor, it is also possible to increase the sensitivity without changing the qualities of the basic sensor. If dual layer filters are applied, filter characteristics can be improved further on, sometimes totally new characteristics can be found. Additionally new parameters occur like joint annealing or choosing combinations of materials in multilayer systems. Optimizing on parameters during RF sputtering of an SiO 2 filter layer gives an increase in sensitivity and selectivity on hydrogen, the single filter SiO 2 modified sensor is able to detect 50 ppm hydrogen ignoring the presence of other technical gases. On the search for further improvements dual filter systems were presented, an Al 2 O 3rSiO 2 dual-filter layer sensor and a Ga 2 O 3rSiO 2 dual-filter layer sensor for sensing ethane with high disturbance resistance. To gain the limits of optimization further investigations are necessary, other materials and combinations of materi-
w1x M. Fleischer, Sensing reducing gases at high temperatures using long-term stable Ga 2 O 3 thin films, Sens. Actuators, B 6–7 Ž1998. 257–261. w2x M. Fleischer, H. Meixner, Characterization and crystallite growth of semiconducting high temperature stable Ga 2 O 3 Thin Films. w3x M. Seth, Oberflachenmodifikationen an Ga 2 O 3 -Dunnschichten, The¨ ¨ sis, University of Gießen, 1996. w4x M. Fleischer, H. Meixner, Selectivity in high temperature operated semiconductor gas sensors, Sens. Actuators, B 52 Ž1998. 179–187. w5x A. Katsuki, K. Fukui, H 2 selective gas sensor based on SnO 2 , Sens. Actuators, B 52 Ž1998. 30–37. w6x G.K. Flingelli, M. Fleischer, H. Meixner, Selective detection of methane in domestic environments using a catalyst sensor system based on Ga 2 O 3 , Sens. Actuators, B Ž1998. 258–262.
Biographies
Thomas Weh was born in Innsbruck, Austria, on May 30, 1970. He received his Dipl-Ing degree in common electrical engineering in 1996 from the Technical University Graz, Austria. His current research interest is in developing a high selective and sensitive hydrogen sensor for technical use. Maximilian Fleischer was born in Munich on May 7, 1961. He received his doctoral degree in physics from the Technical University in Munich in 1992. Since 1992 he has been in the employ of the corporate R&D of Siemens and is engaged in the development of new type of gas sensors based on high temperature stable semiconducting metal oxides. Since 1996 he has been the responsible project manager for the gas project group. In 1998, he received the doctor habil. from the Technical University of Budapest for work about gasrmetal oxide interactions. His research interests include new types of semiconducting metal oxides for gas sensors, application of laser diodes for selective gas sensors, application of work function methods for the realisation of low-power gas FETs and ultrasonic motors. He has been engaged in national and European research projects and collaborations with German and European Universities. Hans Meixner was born in Marl, Germany, in 1939. He received the Dr rer nat. in Physics from the technical University of Munich in 1972. In 1973 he joined the research laboratories of Siemens in Munich where he is currently Department Head: Sensors- und Actuator-Technologies. He is a member of steering committees of national and international sensor conferences and works as referee in various subjects. Since 1992 he has been associate professor of the Technical University of Budapest.
Optimization of physical filtering for selective high temperature H 2 sensors T. Weh ) , M. Fleischer, H. Meixner Siemens AG, ZT MS 2, Otto-Hahn-Ring 6, D-81739 Munchen, Germany ¨
Abstract This paper investigates the improvement in selectivity by means of applying a physical gas filter. Looking for a selective and highly sensitive hydrogen sensor, various filter systems, single, dual and buried filter systems, were deposited on top of a basic high-temperature semiconducting Ga 2 O 3 thin film sensor. Sensor respondence on a big variety of gases present in domestic ambience and especially hydrogen and ethane, which is the hardest test on cross sensitivity, is shown. Using a single filter SiO 2 layer a selective hydrogen sensor is realized, which is able to detect 50 ppm hydrogen at an operation temperature of 7008C in humid air without any interference caused by other gases. Dual SiO 2rmetal oxide filter systems and buried metal oxide layer filters show behaviors varying from selective ethane to selective hydrogen detection. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Hydrogen; Ethane; Filter; Metal oxide; Bias
1. Introduction Aiming at applications like fire warning, industrial process control systems or leakage-control systems for hydrogen-driven vehicles, a highly sensitive and selective hydrogen sensor is required with easy handling abilities and low cost requirements. This sensor should be able to detect low concentrations of hydrogen, 10 to 50 ppm H 2 , in a realistic environment, where other gases in different concentrations are present during the appearance of hydrogen. Therefore a standard high temperature gas sensor was taken, with detection mechanism based on the gas-dependent resistance of the n-type semiconductor Ga 2 O 3 w1,2x. This basic Ga 2 O 3 sensor is sensitive to many gases including methane, CO, NO, propane, ethane and hydrogen, but its handling is easy and the signals can be caught with low-level electronics. So, a modification has to be made for eliminating the influence of all other gases except hydrogen, and, if possible, increasing the sensitivity on hydrogen. The most effective way of transforming this basic Ga 2 O 3 sensor to a highly selective and sensitive hydrogen sensor is to apply a physical filter located directly on top of the
)
Corresponding author. E-mail address: [email protected] ŽT. Weh..
sensor layer w3,4x. By diffusion control a filter like this impedes the reactive gases from reaching the Ga 2 O 3 but lets the hydrogen pass through.
2. Experimental The high-temperature sensor used here is a Ga 2 O 3 thin film sensor with a heating structure on the back side. Below the Ga 2 O 3 sensor layer an interdigital structure for sensing the resistance of the n-type metal oxide is placed; the operating temperature of the sensor is 7008C. A fully computer-controlled gas mixing unit was used to apply the gases to the sensor; the gas-dependent resistance was recorded with this equipment, too. The physical filter used in this type of application is a 200-nm-thick SiO 2 layer w5x. For the first time a BIAS power was applied during reactive sputtering of the SiO 2 layer, a 99.999% pure Si target was used, and the gas mixture during the sputter process was 50 sccm argon and 10 sccm oxygen. The SiO 2 layer was stabilized afterwards at a temperature of 10508C for 10 h. The additionally applied Al 2 O 3 and Ga 2 O 3 filter layers were RF sputtered and separately stabilized, too. Investigations about joint annealed layers with SiO 2 and Ga 2 O 3 , not shown here, showed no further step up in performance of the sensors.
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 7 5 - 5
T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
147
3. Results and discussion 3.1. Measurements on hydrogen with single filter sensors With this filter on top of the sensor the sensitivity and selectivity to hydrogen and the interference resistance compared with the basic Ga 2 O 3 sensor has raised dramatically, the sensor is only slightly deranged by ethane and propane, as shown in Fig. 1. As shown in this figure nearly all given gases are suppressed. With concentrations of 5000 ppm methane or 1000 ppm CO no signal at all appears, if the sensor is in contact with propane the signal is down to 3% of the signal a basic Ga 2 O 3 sensor generates. Only ethane is deranging the sensor in a serious way, not shown here, because some hydrogen atoms are cracked off the ethane molecule producing a false hydrogen signal on the sensor. With ethane having one of the smallest binding energies on hydrogen atoms it is the hardest test for high-temperature hydrogen detection sensors. Fig. 2 shows the increase in sensitivity to hydrogen, particularly at low concentrations. Fig. 3 shows the influence of the BIAS power level during the sputter process of the SiO 2 on the sensitivity on hydrogen. If the BIAS power level is smaller than 15 W the behavior of the sensor layer is quite similar to a non BIAS-sputtered filter layer. With 30-W BIAS or more the filter layer is improved in its performance on hydrogen, reaching the best sensitivity to hydrogen at a BIAS power level of 50 W, which represents the maximum BIAS power level allowed in the RF sputter machine used. The recovery time of the sensors depends on the signal height; therefore, not all sensors are at zero level at the end of the experiment. In exactly the same steps of BIAS power level the selectivity is increasing. The SiO 2 layer seems to be more compact with higher BIAS power level during sput-
Fig. 1. Direct comparison between a basic Ga 2 O 3 sensor and an SiO 2 modified sensor at a temperature of 7008C in humid air on different gases.
Fig. 2. Sensitivity to hydrogen of a BIAS-sputtered SiO 2 modified sensor, a non BIAS-sputtered SiO 2 modified sensor and a basic Ga 2 O 3 sensor at a temperature of 7008C in humid air.
tering of the SiO 2 , a necessary parameter for getting a useful filter layer on the basic Ga 2 O 3 sensor transforming it into a hydrogen sensor. On the other hand, no significant influence on the sensitivity by varying the layer thickness of the SiO 2 could be found. If the SiO 2 layer is closed and compact, the sensor will act correctly, but if there are any holes or uncovered edges on the sensor top, the selectivity is gone. Fig. 4 shows the sensitivity to hydrogen of identically prepared sensors with varying SiO 2 layer thickness. 3.2. Dual filter metal oxide layer on top of SiO2 To further improve sensing behavior dual Ga 2 O 3rSiO 2 and Al 2 O 3rSiO 2 layer systems, see Fig. 5, on top of the basic Ga 2 O 3 sensor have been investigated since single filter layers show no more potential for optimization.
Fig. 3. Sensitivity to hydrogen of SiO 2 modified sensors with different BIAS power level during RF sputtering at temperature of 7008C in humid air.
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T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
Fig. 4. Comparison of SiO 2 modified sensors with different thick layers of SiO 2 at a temperature of 7008C in humid air.
Typical filter layers were between 80 and 300 nm thickness, all layers were separately stabilized after the sputter process. All sensors are at an operating temperature of 7008C; lower temperatures decrease the reaction time on changing the gas concentrations, higher temperatures lower the selectivity, particularly ethane makes more confusion on the sensor. As shown in Fig. 6, the Al 2 O 3 layer and the SiO 2 layer are combining the effects of the single filter characteristics, but this modification transforms a basic Ga 2 O 3 sensor into a high selective ethane sensor: With this filter system on top of the sensor only ethane is able to pass both filters layers. As a side effect SiO 2 takes over the separation of the materials to avoid formation of Al x Ga 2yx O 3 . A second dual-layer configuration for detecting ethane is sputtering a Ga 2 O 3 layer on top of the SiO 2 modified sensor. In direct comparison to Fig. 5 the Al 2 O 3 is replaced by RF sputtered Ga 2 O 3 . As shown in Fig. 7 no big difference on sensing ethane can be seen compared to the Al 2 O 3rSiO 2 modified sensor shown above. Similar to the first given dual-layer sensor, ethane is detected with good signal height, hydrogen is suppressed even at high concentrations. After all this result is a little
Fig. 5. Schematics of a dual layer system with Al 2 O 3 and SiO 2 applied to a basic Ga 2 O 3 sensor.
Fig. 6. Hydrogen and ethane contact of an SiO 2 modified sensor and SiO 2 qAl 2 O 3 dual filter sensors at a temperature of 7008C in humid air.
bit surprising compared to the experiments done in Ref. w6x: Applying a Ga 2 O 3 thick film on top of the sensor the ethane is burned never reaching the surface of the sensor. With a Ga 2 O 3 sputtered layer having a lot smaller surface the ethane not only reaches the surface; as an expected effect, it transforms the sensor in an ethane sensor blocking the hydrogen very effectively. A difference between the SiO 2rAl 2 O 3 and the SiO 2rGa 2 O 3 sensor can be seen in applying propane or CO to the sensors; they are slightly differently deranged on these gases. In the case of ethane no difference in the behavior of the two different modifications can be found. 3.3. Buried layer sensors Using Ga 2 O 3 or Al 2 O 3 layers as top layers for filtering resulted in strong ethane sensitivity. To use these layers without catalytic influences of their surfaces these layers have to be buried. As shown in Fig. 1, SiO 2 is a good choice to bury other materials, because it does no harm to hydrogen and keeps other gases away from any buried layers.
Fig. 7. Hydrogen and ethane contact of SiO 2 qGa 2 O 3 modified sensors at a temperature of 7008C in humid air.
T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
149
Fig. 8. SEM photograph of a buried layer Ga 2 O 3 sensor.
Fig. 9. SEM photograph of a buried layer Cr-doped SrTiO 3 .
Considering the impact of ethane on the hydrogen sensor a way has to be found to eliminate the disturbance of the sensor caused by H atoms cracked off the ethane, otherwise no reliable hydrogen detection can be made. Going on the search for materials to be buried blocking the ethane a n-type and a p-type metal oxide were used. Ga 2 O 3 , the n-type metal oxide used in this case, was buried between two SiO 2 layers, all separately sputtered and annealed at a temperature of 9008C, see Fig. 8. The p-type metal oxide at an operating temperature of 7008C used here is SrTiO 3 doped with Cr2 O5 . At lower temperatures SrTiO 3 is an n-type metal oxide, but at temperatures higher than 5008C it changes to a p-type metal oxide. This layer was again placed between two SiO 2 layers, all layers were separately sputtered and annealed at a temperature of 9008C, see Fig. 9. First the buried layer Ga 2 O 3 sensors were investigated to hydrogen and ethane, see Fig. 10. Comparing this Ga 2 O 3 buried layer sensor to the one using Ga 2 O 3 as the top layer, where the top layer Ga 2 O 3 is fully exposed to the used gases, the buried layer sensor is still responding
on ethane, but the sensor is still responding to hydrogen. Burying the Ga 2 O 3 layer enables the sensor to detect the total amount of both gases. All the other gases like propane, CO or NO, not shown here, are suppressed similar to the SiO 2 single filter sensor.
Fig. 10. Hydrogen and ethane contact of buried layer Ga 2 O 3 sensors at a temperature of 7008C in humid air.
150
T. Weh et al.r Sensors and Actuators B 68 (2000) 146–150
als than above with different parameters during the RF sputter process have to be screened and characterized.
Acknowledgements This study was partly supported by a BMBF research grant ŽForderungskennzeichen 16 V 777r0. from the Ger¨ man ministry of education and research.
References Fig. 11. Hydrogen and ethane contact of buried layer Cr-doped SrTiO 3 sensors at a temperature of 7008C in humid air.
Applying the Cr-doped SrTiO 3 buried layer sensors to the same gas program, ethanerhydrogen gas exposure, a different behavior can be seen, see Fig. 11. This sensor type performs well on hydrogen and it suppresses ethane even at high concentrations. The sensitivity to hydrogen is as high as shown in Fig. 3, which shows the SiO 2 coated sensor with 50 W of BIAS power level during sputtering of the SiO 2 layer, so there seems to be no influence of the buried SrTiO 3 layer on detecting hydrogen. Exposing the sensor to a lot of other gases used in technical environments, like propane, methane, CO, CO 2 or NO, shows a similar suppression as the SiO 2 single filter sensor does Žnot shown here.. The important quality of this type of sensor modification is the very good suppression of ethane without bothering the detection of hydrogen.
4. Conclusion Optimizing filter systems can not only increase selectivity using a given sensor, it is also possible to increase the sensitivity without changing the qualities of the basic sensor. If dual layer filters are applied, filter characteristics can be improved further on, sometimes totally new characteristics can be found. Additionally new parameters occur like joint annealing or choosing combinations of materials in multilayer systems. Optimizing on parameters during RF sputtering of an SiO 2 filter layer gives an increase in sensitivity and selectivity on hydrogen, the single filter SiO 2 modified sensor is able to detect 50 ppm hydrogen ignoring the presence of other technical gases. On the search for further improvements dual filter systems were presented, an Al 2 O 3rSiO 2 dual-filter layer sensor and a Ga 2 O 3rSiO 2 dual-filter layer sensor for sensing ethane with high disturbance resistance. To gain the limits of optimization further investigations are necessary, other materials and combinations of materi-
w1x M. Fleischer, Sensing reducing gases at high temperatures using long-term stable Ga 2 O 3 thin films, Sens. Actuators, B 6–7 Ž1998. 257–261. w2x M. Fleischer, H. Meixner, Characterization and crystallite growth of semiconducting high temperature stable Ga 2 O 3 Thin Films. w3x M. Seth, Oberflachenmodifikationen an Ga 2 O 3 -Dunnschichten, The¨ ¨ sis, University of Gießen, 1996. w4x M. Fleischer, H. Meixner, Selectivity in high temperature operated semiconductor gas sensors, Sens. Actuators, B 52 Ž1998. 179–187. w5x A. Katsuki, K. Fukui, H 2 selective gas sensor based on SnO 2 , Sens. Actuators, B 52 Ž1998. 30–37. w6x G.K. Flingelli, M. Fleischer, H. Meixner, Selective detection of methane in domestic environments using a catalyst sensor system based on Ga 2 O 3 , Sens. Actuators, B Ž1998. 258–262.
Biographies
Thomas Weh was born in Innsbruck, Austria, on May 30, 1970. He received his Dipl-Ing degree in common electrical engineering in 1996 from the Technical University Graz, Austria. His current research interest is in developing a high selective and sensitive hydrogen sensor for technical use. Maximilian Fleischer was born in Munich on May 7, 1961. He received his doctoral degree in physics from the Technical University in Munich in 1992. Since 1992 he has been in the employ of the corporate R&D of Siemens and is engaged in the development of new type of gas sensors based on high temperature stable semiconducting metal oxides. Since 1996 he has been the responsible project manager for the gas project group. In 1998, he received the doctor habil. from the Technical University of Budapest for work about gasrmetal oxide interactions. His research interests include new types of semiconducting metal oxides for gas sensors, application of laser diodes for selective gas sensors, application of work function methods for the realisation of low-power gas FETs and ultrasonic motors. He has been engaged in national and European research projects and collaborations with German and European Universities. Hans Meixner was born in Marl, Germany, in 1939. He received the Dr rer nat. in Physics from the technical University of Munich in 1972. In 1973 he joined the research laboratories of Siemens in Munich where he is currently Department Head: Sensors- und Actuator-Technologies. He is a member of steering committees of national and international sensor conferences and works as referee in various subjects. Since 1992 he has been associate professor of the Technical University of Budapest.