- Email: [email protected]
CMOS Integrated Tungsten Oxide Nanowire Networks for ppb-level H2S Sensing
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 272 – 275
30th Eurosensors Conference, EUROSENSORS 2016
CMOS integrated tungsten oxide nanowire networks for ppb-level H2S sensing J. Krainera, M. Delucaa, E. Lacknera, R. Wimmer-Teubenbachera, F. Sosadaa, C. Gspanb, K. Rohracherc, E. Wachmannc, A. Koecka* a Materials Center Leoben Forschung GmbH, Roseggerstr. 12, Leoben 8700, Austria Institute for Electron Microscopy and Fine Structure Research, Steyrerg. 17/III, Graz 8010, Austria c ams AG, Tobelbader Str. 30, Premstaetten 8141, Austria
b
Abstract Tungsten oxide is an intensively studied semiconductor with the relevant application as H 2S gas sensor. In this work we report on H2S gas sensor devices based on tungsten oxide nanowire networks, which are integrated on a CMOS fabricated microhotplate chip. Tungsten oxide gas sensors were prepared by the deposition of nanowire networks onto interdigitated electrodes on CMOS. The material properties of the tungsten oxide nanowires were investigated by TEM and Raman spectroscopy, confirming their nonstoichiometric state. Utilising WO3-x nanowire networks as gas sensing material we obtained extraordinary sensitivity to H2S: concentrations down to 10 ppb have been detected, even in the presence of 50% relative humidity. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: CMOS integrated; nanotechnology; gas sensor; nanowire network; tungsten oxide nanowires; CMOS microhotplate
1. Introduction The demand on monitoring harmful and toxic gases extremely increased in the last decades. In particular in industrial processes there is a great interest in detection of gases to prevent unintended exposure. A highly toxic and flammable gas such as hydrogen sulphide can be found in various technical areas. It is considered as broad-spectrum
*Corresponding author. Tel.: +43-3842-45922-505 E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.189
J. Krainer et al. / Procedia Engineering 168 (2016) 272 – 275
poison, which means that it can affect different systems in the human body. Depending on the gas concentration and the exposure time, humans can suffer from unconsciousness, nausea, headache or even death. Therefore H 2S concentration has to be monitored. At the moment there are many methods of monitoring harmful gases including H2S, but most sensors suffer from high cost, high power consumption and inconvenient handling. The integration of gas sensitive materials like metal oxide semiconductors onto CMOS (Complementary Metal Oxide Semiconductors) enables high volume production of low-power and low-cost smart gas sensor devices for consumer market applications [1]. However, the realisation of CMOS gas sensor systems is a highly challenging task. The gas sensing material has to be implemented on CMOS microhotplates considering all restrictions of the microchip and its electronic components itself (i.e. thermal and mechanical stability). Herein, we report the implementation of a tungsten oxide nanowire network on a prefabricated CMOS microhotplate chip (Fig. 1, left) via drop-coating. We investigated the H2S gas sensing behavior of the implemented nanowire network under environmental conditions with relative humidity levels of 25%, 50% and 75% (at 20°C). The sensors were exposed to 10 ppb, 30 pbb and 60 ppb H2S, concentrations well below the critical threshold value for human exposure of 5 ml/m3 (ppm) [2]. 2. Experimental 2.1. Synthesis and characterisation of tungsten oxide nanowires The tungsten oxide nanowires were synthesised by a hydrothermal method. Tungstic acid (0.85 g; H 2WO4; Sigma Aldrich, 99% purity) was dissolved in 30 ml deionised water, 40 g potassium sulphate (K2SO4; Sigma Aldrich, t99% purity) was added to the suspension and agitated to form a paste-like mixture. The paste was transferred to a Teflonlined autoclave with a capacity of 45 ml. The thermal treatment was performed at 180°C for 12 h. After completion of the hydrothermal reaction, the autoclave was naturally cooled down to room temperature and the product was centrifuged and washed several times with deionized water and ethanol. The resulting nanowire powder was dried at 80°C overnight [3]. The tungsten oxide nanowires were characterised using transmission electron microscopy (TEM, FEI Titan, 300 kV). The product composition was investigated by Raman spectroscopy (LabRAM HR800, Horiba Jobin Yvon). 2.2. Sensor processing The fabrication of the CMOS microhotplate chip has been realised in a 0.35 μm standard CMOS technology from ams AG; the microhotplate is released in a post process MEMS-etching step. The microhotplate consists of an active area with dimensions of 74 x 74 µm2 suspended in air and connected to the rest of the silicon chip by four arms (length 150 µm, width 12 µm) [4]. An e-beam lithography process was implemented to fabricate Ti/Au interdigitated electrodes with spaces of 2 µm in between the electrodes to ensure a good electrical contact between the nanowire network and the electrical contacts of the CMOS microhotplate chip. Ti and Au (5 nm and 200 nm thickness) electrode layers were deposited by thermal evaporation on the CMOS microhotplate (Fig. 1, right). The nanowires were deposited by drop-coating of a nanowire suspension onto the prefabricated Ti/Au interdigitated electrodes by the use of an inkjet-printer (FUJIFILM – Dimatix DMP2831). The sensors were annealed at 400°C for 12 h in ambient air. 2.3. Gas sensor evaluation The sensor was tested in an automated gas measurement setup, where carrier gas, target gas concentration and relative humidity were precisely controlled by digital mass flow controllers (constant flow rate of 1000 sccm). Synthetic air (80% N2, 20% O2) was used as background gas. Relative humidity (rh) was adjusted separately. The electrical resistance of the sensor device was measured by a Keithley 2400 SourceMeter in constant voltage operation. The gas measurements were conducted toward H2S in concentrations of 10 ppb, 30 ppb and 60 ppb in humid synthetic air with levels of 25%, 50% and 75% rh (at 20°C) with sensor operation temperatures in the range of 200 to 400°C. The sensors were exposed to the H2S gas pulses for 5 min with breaks of 15 min synthetic air in between. The sensing
273
274
J. Krainer et al. / Procedia Engineering 168 (2016) 272 – 275
Fig. 1. CMOS chip with various microhotplates (left) and SEM image of CMOS microhotplate with IDES electrodes (right).
behaviour was analysed in terms of sensor resistance, sensor response and response time. The sensor response was defined as the relative difference between the resistance value in the background gas before the gas pulse and the resistance value during the gas pulse expressed in percentage.The response time is determined as the time the sensor resistance takes to reach 90% of its saturation value during the gas pulse. 3. Results and discussion Fig. 2 (left) shows a TEM image of the dense tungsten oxide nanowire network. The close-up in Fig. 2 (right) shows that the nanowires possess a crystalline and fibrous structure with diameters in the range of 15 to 30 nm. The elemental composition was examined by room-temperature Raman spectroscopy as shown in Fig. 3 using both green and red lasers as excitation source. The broad appearance of bands 1 and 2 (related to long and short W-O-W bonds) is due to a wide range of bonding lengths, which is associated with non-stoichiometry [5]. Band 3 corresponds to WO stretching modes in distorted WO4 tetrahedra [6]. Band 4 at ~960 cm-1 is due to the vibration of terminal oxygen atoms and is visible only in materials with high effective surface area [7]. Thus, its presence here is likely related to the fibrous structure of the nanowires. After the extensive gas measurement programme with the variation of sensor operation temperature and different relative humidity levels (25%, 50% and 75% rh) an optimum operation temperature of 250°C could be determined for the WO3-x nanowire gas sensor, which is in accord with literature [8]. Fig. 3 shows a gas measurement sequence at 250°C with 50% rh where the high sensitivity is demonstrated by a noticeable sensor response up to 7% even at an ultra-low concentration of 10 ppb H2S. The sensor showed a constant response time to all three gas pulses (Table 1).
Fig. 2. TEM images of the dense tungsten oxide nanowire network (left) and single tungsten oxide nanowires with fibrous texture (right).
J. Krainer et al. / Procedia Engineering 168 (2016) 272 – 275
275
Fig. 3. (a) Raman spectra of WO3-x nanowire network; (b) H2S response of WO3-x nanowire network CMOS sensor (50% rh; 250°C). Table 1. Gas response and response time of WO3-x nanowire network CMOS sensor at 250°C and 50% rh. Gas concentration [ppb]
Gas response [%]
Response time [min]
10
7
3.04
30
23
3.25
60
33
3.37
4. Conclusion Nanowire networks of non-stoichiometric tungsten oxide could be successfully integrated on a CMOS microhotplate chip by a simple drop-coating technique. It could be confirmed that 250°C is the optimum temperature for WO3-x nanowire network sensors. Remarkably, even concentrations down to 10 ppb H2S could be detected at a humidity level of 50% rh. Thus, it could be shown that a CMOS integrated WO 3-x nanowire sensor represents a promising candidate for the realisation of a smart sensor device. Acknowledgements This work has been performed within the projects “MSP - Multi Sensor Platform for Smart Building Management” (FP7-ICT-2013-10 Collaborative Project, No. 611887) and “RealNano - Industrial Realisation of Innovative CMOS based Nanosensors” (Austrian Research Promotion Agency, No. 843598). References [1] J.W. Gardner, S. Member, P.K. Guha, F. Udrea, J.A. Covington, CMOS Interfacing for Integrated Gas Sensors : A Review, IEEE Sens. J. 10 (2010) 1833–1848. [2] SUVA, Grenzwerte am Arbeitsplatz 2016 MAK-Werte, BAT-Werte, Grenzwerte für physikalische Einwirkungen, 2016. [3] X.C. Song, Y.F. Zheng, E. Yang, Y. Wang, Large-scale hydrothermal synthesis of WO3 nanowires in the presence of K2SO4, Mater. Lett. 61 (2007) 3904–3908. [4] M. Siegele, C. Gamauf, A. Nemecek, G.C. Mutinati, S. Steinhauer, A. Koeck, et al., Optimized integrated micro-hotplates in CMOS technology, New Circuits Syst. Conf. IEEE 11th (2013) 1–4. [5] D.Y. Lu, J. Chen, J. Zhou, S.Z. Deng, N.S. Xu, J.B. Xu, Raman spectroscopic study of oxidation and phase transition in W18O49 nanowires, J. Raman Spectrosc. 38 (2007) 176–180. [6] J.A. Horsley, E. Wachs, J.M. Brown, G.H. Via, F.D. Hardcastlex, Structure of Surface Tungsten Oxide Species in the WO3/Al2O3 Supported Oxide System from X-ray Absorption Near-Edge Spectroscopy and Raman Spectroscopy, J. Phys. Chem. 91 (1987) 4014–4020. [7] A. Baserga, V. Russo, F. Di Fonzo, A. Bailini, D. Cattaneo, C.S. Casari, et al., Nanostructured tungsten oxide with controlled properties: Synthesis and Raman characterization, Thin Solid Films. 515 (2007) 6465–6469. [8] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, et al., Ultrasensitive and highly selective gas sensors using threedimensional tungsten oxide nanowire networks, Appl. Phys. Lett. 88 (2006) 203101.
ScienceDirect Procedia Engineering 168 (2016) 272 – 275
30th Eurosensors Conference, EUROSENSORS 2016
CMOS integrated tungsten oxide nanowire networks for ppb-level H2S sensing J. Krainera, M. Delucaa, E. Lacknera, R. Wimmer-Teubenbachera, F. Sosadaa, C. Gspanb, K. Rohracherc, E. Wachmannc, A. Koecka* a Materials Center Leoben Forschung GmbH, Roseggerstr. 12, Leoben 8700, Austria Institute for Electron Microscopy and Fine Structure Research, Steyrerg. 17/III, Graz 8010, Austria c ams AG, Tobelbader Str. 30, Premstaetten 8141, Austria
b
Abstract Tungsten oxide is an intensively studied semiconductor with the relevant application as H 2S gas sensor. In this work we report on H2S gas sensor devices based on tungsten oxide nanowire networks, which are integrated on a CMOS fabricated microhotplate chip. Tungsten oxide gas sensors were prepared by the deposition of nanowire networks onto interdigitated electrodes on CMOS. The material properties of the tungsten oxide nanowires were investigated by TEM and Raman spectroscopy, confirming their nonstoichiometric state. Utilising WO3-x nanowire networks as gas sensing material we obtained extraordinary sensitivity to H2S: concentrations down to 10 ppb have been detected, even in the presence of 50% relative humidity. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: CMOS integrated; nanotechnology; gas sensor; nanowire network; tungsten oxide nanowires; CMOS microhotplate
1. Introduction The demand on monitoring harmful and toxic gases extremely increased in the last decades. In particular in industrial processes there is a great interest in detection of gases to prevent unintended exposure. A highly toxic and flammable gas such as hydrogen sulphide can be found in various technical areas. It is considered as broad-spectrum
*Corresponding author. Tel.: +43-3842-45922-505 E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.189
J. Krainer et al. / Procedia Engineering 168 (2016) 272 – 275
poison, which means that it can affect different systems in the human body. Depending on the gas concentration and the exposure time, humans can suffer from unconsciousness, nausea, headache or even death. Therefore H 2S concentration has to be monitored. At the moment there are many methods of monitoring harmful gases including H2S, but most sensors suffer from high cost, high power consumption and inconvenient handling. The integration of gas sensitive materials like metal oxide semiconductors onto CMOS (Complementary Metal Oxide Semiconductors) enables high volume production of low-power and low-cost smart gas sensor devices for consumer market applications [1]. However, the realisation of CMOS gas sensor systems is a highly challenging task. The gas sensing material has to be implemented on CMOS microhotplates considering all restrictions of the microchip and its electronic components itself (i.e. thermal and mechanical stability). Herein, we report the implementation of a tungsten oxide nanowire network on a prefabricated CMOS microhotplate chip (Fig. 1, left) via drop-coating. We investigated the H2S gas sensing behavior of the implemented nanowire network under environmental conditions with relative humidity levels of 25%, 50% and 75% (at 20°C). The sensors were exposed to 10 ppb, 30 pbb and 60 ppb H2S, concentrations well below the critical threshold value for human exposure of 5 ml/m3 (ppm) [2]. 2. Experimental 2.1. Synthesis and characterisation of tungsten oxide nanowires The tungsten oxide nanowires were synthesised by a hydrothermal method. Tungstic acid (0.85 g; H 2WO4; Sigma Aldrich, 99% purity) was dissolved in 30 ml deionised water, 40 g potassium sulphate (K2SO4; Sigma Aldrich, t99% purity) was added to the suspension and agitated to form a paste-like mixture. The paste was transferred to a Teflonlined autoclave with a capacity of 45 ml. The thermal treatment was performed at 180°C for 12 h. After completion of the hydrothermal reaction, the autoclave was naturally cooled down to room temperature and the product was centrifuged and washed several times with deionized water and ethanol. The resulting nanowire powder was dried at 80°C overnight [3]. The tungsten oxide nanowires were characterised using transmission electron microscopy (TEM, FEI Titan, 300 kV). The product composition was investigated by Raman spectroscopy (LabRAM HR800, Horiba Jobin Yvon). 2.2. Sensor processing The fabrication of the CMOS microhotplate chip has been realised in a 0.35 μm standard CMOS technology from ams AG; the microhotplate is released in a post process MEMS-etching step. The microhotplate consists of an active area with dimensions of 74 x 74 µm2 suspended in air and connected to the rest of the silicon chip by four arms (length 150 µm, width 12 µm) [4]. An e-beam lithography process was implemented to fabricate Ti/Au interdigitated electrodes with spaces of 2 µm in between the electrodes to ensure a good electrical contact between the nanowire network and the electrical contacts of the CMOS microhotplate chip. Ti and Au (5 nm and 200 nm thickness) electrode layers were deposited by thermal evaporation on the CMOS microhotplate (Fig. 1, right). The nanowires were deposited by drop-coating of a nanowire suspension onto the prefabricated Ti/Au interdigitated electrodes by the use of an inkjet-printer (FUJIFILM – Dimatix DMP2831). The sensors were annealed at 400°C for 12 h in ambient air. 2.3. Gas sensor evaluation The sensor was tested in an automated gas measurement setup, where carrier gas, target gas concentration and relative humidity were precisely controlled by digital mass flow controllers (constant flow rate of 1000 sccm). Synthetic air (80% N2, 20% O2) was used as background gas. Relative humidity (rh) was adjusted separately. The electrical resistance of the sensor device was measured by a Keithley 2400 SourceMeter in constant voltage operation. The gas measurements were conducted toward H2S in concentrations of 10 ppb, 30 ppb and 60 ppb in humid synthetic air with levels of 25%, 50% and 75% rh (at 20°C) with sensor operation temperatures in the range of 200 to 400°C. The sensors were exposed to the H2S gas pulses for 5 min with breaks of 15 min synthetic air in between. The sensing
273
274
J. Krainer et al. / Procedia Engineering 168 (2016) 272 – 275
Fig. 1. CMOS chip with various microhotplates (left) and SEM image of CMOS microhotplate with IDES electrodes (right).
behaviour was analysed in terms of sensor resistance, sensor response and response time. The sensor response was defined as the relative difference between the resistance value in the background gas before the gas pulse and the resistance value during the gas pulse expressed in percentage.The response time is determined as the time the sensor resistance takes to reach 90% of its saturation value during the gas pulse. 3. Results and discussion Fig. 2 (left) shows a TEM image of the dense tungsten oxide nanowire network. The close-up in Fig. 2 (right) shows that the nanowires possess a crystalline and fibrous structure with diameters in the range of 15 to 30 nm. The elemental composition was examined by room-temperature Raman spectroscopy as shown in Fig. 3 using both green and red lasers as excitation source. The broad appearance of bands 1 and 2 (related to long and short W-O-W bonds) is due to a wide range of bonding lengths, which is associated with non-stoichiometry [5]. Band 3 corresponds to WO stretching modes in distorted WO4 tetrahedra [6]. Band 4 at ~960 cm-1 is due to the vibration of terminal oxygen atoms and is visible only in materials with high effective surface area [7]. Thus, its presence here is likely related to the fibrous structure of the nanowires. After the extensive gas measurement programme with the variation of sensor operation temperature and different relative humidity levels (25%, 50% and 75% rh) an optimum operation temperature of 250°C could be determined for the WO3-x nanowire gas sensor, which is in accord with literature [8]. Fig. 3 shows a gas measurement sequence at 250°C with 50% rh where the high sensitivity is demonstrated by a noticeable sensor response up to 7% even at an ultra-low concentration of 10 ppb H2S. The sensor showed a constant response time to all three gas pulses (Table 1).
Fig. 2. TEM images of the dense tungsten oxide nanowire network (left) and single tungsten oxide nanowires with fibrous texture (right).
J. Krainer et al. / Procedia Engineering 168 (2016) 272 – 275
275
Fig. 3. (a) Raman spectra of WO3-x nanowire network; (b) H2S response of WO3-x nanowire network CMOS sensor (50% rh; 250°C). Table 1. Gas response and response time of WO3-x nanowire network CMOS sensor at 250°C and 50% rh. Gas concentration [ppb]
Gas response [%]
Response time [min]
10
7
3.04
30
23
3.25
60
33
3.37
4. Conclusion Nanowire networks of non-stoichiometric tungsten oxide could be successfully integrated on a CMOS microhotplate chip by a simple drop-coating technique. It could be confirmed that 250°C is the optimum temperature for WO3-x nanowire network sensors. Remarkably, even concentrations down to 10 ppb H2S could be detected at a humidity level of 50% rh. Thus, it could be shown that a CMOS integrated WO 3-x nanowire sensor represents a promising candidate for the realisation of a smart sensor device. Acknowledgements This work has been performed within the projects “MSP - Multi Sensor Platform for Smart Building Management” (FP7-ICT-2013-10 Collaborative Project, No. 611887) and “RealNano - Industrial Realisation of Innovative CMOS based Nanosensors” (Austrian Research Promotion Agency, No. 843598). References [1] J.W. Gardner, S. Member, P.K. Guha, F. Udrea, J.A. Covington, CMOS Interfacing for Integrated Gas Sensors : A Review, IEEE Sens. J. 10 (2010) 1833–1848. [2] SUVA, Grenzwerte am Arbeitsplatz 2016 MAK-Werte, BAT-Werte, Grenzwerte für physikalische Einwirkungen, 2016. [3] X.C. Song, Y.F. Zheng, E. Yang, Y. Wang, Large-scale hydrothermal synthesis of WO3 nanowires in the presence of K2SO4, Mater. Lett. 61 (2007) 3904–3908. [4] M. Siegele, C. Gamauf, A. Nemecek, G.C. Mutinati, S. Steinhauer, A. Koeck, et al., Optimized integrated micro-hotplates in CMOS technology, New Circuits Syst. Conf. IEEE 11th (2013) 1–4. [5] D.Y. Lu, J. Chen, J. Zhou, S.Z. Deng, N.S. Xu, J.B. Xu, Raman spectroscopic study of oxidation and phase transition in W18O49 nanowires, J. Raman Spectrosc. 38 (2007) 176–180. [6] J.A. Horsley, E. Wachs, J.M. Brown, G.H. Via, F.D. Hardcastlex, Structure of Surface Tungsten Oxide Species in the WO3/Al2O3 Supported Oxide System from X-ray Absorption Near-Edge Spectroscopy and Raman Spectroscopy, J. Phys. Chem. 91 (1987) 4014–4020. [7] A. Baserga, V. Russo, F. Di Fonzo, A. Bailini, D. Cattaneo, C.S. Casari, et al., Nanostructured tungsten oxide with controlled properties: Synthesis and Raman characterization, Thin Solid Films. 515 (2007) 6465–6469. [8] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, et al., Ultrasensitive and highly selective gas sensors using threedimensional tungsten oxide nanowire networks, Appl. Phys. Lett. 88 (2006) 203101.