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ScienceDirect Procedia Engineering 87 (2014) 787 – 790
EUROSENSORS 2014, the XXVIII edition of the conference series
Optimization of CMOS integrated nanocrystalline SnO2 gas sensor devices with bimetallic nanoparticles G. C. Mutinatia, E. Bruneta, A. Koeckb*, S. Steinhauerb, O. Yurchenkoc, E. Laubenderc, G. Urbanc, J. Siegertd, K. Rohracherd, F. Schrankd, M. Schremsd a Molecular Diagnostics, AIT Austrian Institute of Technology GmbH, Donau-City-Strasse 1, 1220 Vienna, Austria Materials for Microelectronics, Materials Center Leoben Forschung GmbH, Roseggerstrasse 12, 8700 Leoben, Austria c The Freiburg Materials Research Center, Albert-Ludwigs-Universität Freiburg, Stefan-Meier Str. 21, 79104 Freiburg, Germany d ams AG, Tobelbaderstrasse 30, 8141 Unterpremstaetten, Austria b
Abstract We present gas sensor devices based on nanocrystalline SnO2 films, which are integrated on CMOS fabricated micro-hotplate (μhp) chips. Bimetallic nanoparticles (NPs) such as PdAu, PtAu, and PdPt have been synthesized for optimizing the sensing performance of these sensors. We demonstrate that proper functionalization with PdAu-NPs leads to a strongly improved sensitivity to the toxic gas carbon monoxide while the cross sensitivity to humidity and carbon dioxide is almost completely suppressed, which is of high importance for real life environmental conditions. We also present μhp chips employing Through-Silicon-Via (TSV) technology, which are capable for flexible 3D-integration of different types of gas sensors to a multi-parameter nanosensor system. Such CMOS integrated systems are promising candidates for realizing smart sensor devices for consumer market applications. © 2014 2014The TheAuthors. Authors.Published Published Elsevier © byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the scientific committee of Eurosensors 2014. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: SnO2 nanocrystalline films; gas sensors; bimetallic nanoparticles; micro-hotplates; CMOS integration
1. Introduction Gas sensors are of high importance for many applications ranging from indoor air quality monitoring and personal safety systems for CO detection in household heating systems to outdoor environmental monitoring. All these applications require reliable, compact and efficient gas sensor systems.
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1877-7058 © 2014 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/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.679
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Realization of smart gas sensor systems for daily life applications requires solving of highly challenging issues: x Nanocomponents such as nanocrystalline films or NPs have to be employed as gas sensing components in order to improve and optimize sensor performance, x The gas sensing components have to be implemented on CMOS microchips by means of heterogeneous integration. This is indispensable when developing smart devices for consumer electronic markets. We have developed gas sensors based on ultrathin SnO2-films fabricated by a spray pyrolysis process, which are able to detect the toxic gas CO down to a concentration of less than 5 ppm [1]. The spray pyrolysis technique is a simple and flexible process and is suitable for cost efficient fabrication of gas sensing devices. We have demonstrated that this process is suitable for heterogeneous integration of SnO2 gas sensing layers with CMOS fabricated μhp chips as System-On-Chip [2, 3]. In this paper we show that nanocrystalline SnO2 gas sensors can be optimized by proper functionalization with bimetallic NPs which is a most promising approach for the realization of multi-parameter sensing devices. We describe fabrication of bimetallic NPs and the related functionalization process. The presented solution meets economic demands for mass production due to the fully standard CMOS realization of the μhp chips and additional post-backend processes. 2. Experimental Results 2.1. Gas sensor fabrication The fabrication of the μhp chip is realized using standard 0.35μm CMOS technology from the industrial partner ams AG. It consists of a fully released 70 x 70 μm2 plate with 4 suspension arms connected to the rest of the silicon chip (Fig. 1a). Holes in the active area of the μhp are realized by standard CMOS etching of the passivation layer for contacting the deposited SnO2 layer by the underlying top metal (aluminum) in 4-point configuration. The nanocrystalline SnO2 film with thickness around 50 nm is deposited by spray pyrolysis as post-backend process on the finished CMOS chip, patterned by photolithography and Ar ion etching. A final etching step with an isotropic XeF2 vapor phase process fully releases the μhp. Fig. 1b shows the typical resistance changes of the gas sensor to CO at operating temperature 300°C for 10, 30, 50, 70, and 90 ppm CO concentrations in dry and humid (rh = 40%) synthetic air. Although in dry air the sensor response is high, the signal is strongly reduced in humid ambient, which demonstrates the problem with gas sensors when measuring in real life conditions.
Fig. 1. (a) SEM image of CMOS implemented SnO2 thin film gas sensor device designed for 4-point measurements; (b) sensor response to 10, 30, 50, 70, and 90 ppm CO in dry and humid synthetic air.
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Fig. 2. (a) Gas sensor response to CO of SnO2-thin film reference sensor in dry synthetic air; (b) Gas sensor response to CO of SnO2-thin film functionalized with PdAu-NPs in dry synthetic air.
2.2. Nanoparticle fabrication and sensor functionalization Bimetallic NPs have been synthesized using the microemulsion method according to a procedure described by Habrioux et al. [4] and Hussein et al. [5]. NPs prepared by this procedure exist in form of a water-in-oil emulsion, where the NPs are located in surfactant stabilized water droplets, which are dispersed in the oil phase. The non-ionic surfactant Brij30© was used as stabilizer, n-heptane as oil-phase and NaBH4 as reducing agent of noble metal salts (HAuCl4 and K2PdCl4). The molar ratio w0 = [H2O]/[surfactant] was 4. The selected molar ratio of Pd to Au in PdAu-NPs was 7 to 3; total metal concentration in the emulsion was 1 mmol/l. The bimetallic composition of the NP surfaces was verified by means of XPS. The gas sensors have been functionalized by drop coating of the sensor area with NPs emulsion followed by washing of the coated surface with isopropanol and annealing at 400°C for 30 min.
Fig. 3. (a) Gas sensor response to CO of SnO2-thin film functionalized with PdAu-NPs in real-life environmental conditions; (b) prototype chip containing 16 different μhp on 2,4 x 2,4 mm2 area
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The gas sensor response of a SnO2-thin film sensor to 10, 30, and 50 ppm CO in dry synthetic air is shown in Fig. 2a. Response of a SnO2-thin film functionalized with PdAu-NPs in dry synthetic air is shown in Fig. 2b. The overall response is strongly increased; the functionalized sensor shows a high response of almost 40% at 200°C operating temperature reaching a maximum response of more than 60% at 300°C. The gas sensor response of a SnO2-thin film functionalized with PdAu-NPs at 300°C operation temperature in humid synthetic air (25%, 35%, and 70% rh) is shown in Fig. 3a. In order to simulate practical conditions, measurements have also been performed at 35% rh in presence of 2000 ppm CO2. The sensor response is almost equivalent for all environmental conditions. 3. Discussion and Outlook Generally, the response of SnO2 sensors increases with increasing temperature. The measurements presented in Fig. 2b show that PdAu-NPs strongly increase the sensor response. Thus PdAu-NPs enable lower operating temperatures and reduced power consumption of the device. The measurements shown in Fig.3a demonstrate that cross sensitivities to humidity and CO2 are almost completely suppressed by the PdAu-NPs. These results are of high importance for practical applications, because the conditions –35% RH plus CO2 – correspond to real-life settings. We interpret this as follows: The functionalization with bimetallic NPs enhances the sensitivity towards CO because of the spillover effect, which is explained as activation of molecular oxygen dissociation on NPs surface followed by diffusion of atomic oxygen to the SnO2 surface [13]. Synergistic effects caused by the bimetallic PdAu composition might explain the suppression of cross sensitivity to humidity. This, however, will be investigated in detail in order to fully understand this important behavior. Presently we are working on chips containing arrays of μhp, which employ Through-Silicon-Via (TSV) technology and are capable for 3D-System-in-Package integration. Fig. 3b shows a prototype chip containing 16 different μhp on 2,4 x 2,4 mm2 area, which has been fabricated for optimizing the μhp performance. By implementing different SnO 2 thin film sensors and nanowire sensors each one specifically functionalized with NPs, several different target gases relevant for environmental monitoring should become distinguishable. Ultimate goal is a fully integrated nanosensor system containing all required electronics and circuitry. Such a multi-parameter smart sensor device could be implemented as safety feature in smart phones in order to enable personal monitoring of the surrounding air and screening potential environmental health hazards. This work is presently under progress in the FP7 project “MSP – Multi-Sensor-Platform for Smart Building Management” (www.multisensorplatform.eu). Acknowledgements The authors thank Dr. Laith Hussein for introducing the microemulsion technology for NP fabrication. This work was done within the projects “NanoSmart - Nanosensor system for smart gas sensing applications” (MNT-Eranet, No. 828691, FFG - Austrian Research Promotion Agency) and “MSP - Multi Sensor Platform for Smart Building Management” (FP7-ICT-2013-10 Collaborative Project, No. 611887). References [1] [2] [3] [4] [5] [6]
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