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Microfabrication of flexible gas sensing devices based on nanostructured semiconducting metal oxides
Sensors and Actuators A 219 (2014) 88–93
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Microfabrication of flexible gas sensing devices based on nanostructured semiconducting metal oxides S. Vallejos a,b,∗ , I. Gràcia a , E. Figueras a , J. Sánchez a , R. Mas a , O. Beldarrain a , C. Cané a a b
Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), Campus UAB, 08193 Barcelona, Spain SIX Research Center, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technicka 12, CZ-61600 Brno, Czech Republic
a r t i c l e
i n f o
Article history: Received 20 May 2014 Received in revised form 18 August 2014 Accepted 2 September 2014 Available online 10 September 2014 Keywords: Flexible gas sensors Nanostructures Tungsten oxide AACVD
a b s t r a c t Flexible gas sensor devices comprised of heating and transducing elements are produced by directly integrating multilayer polymeric-based platforms and highly crystalline semiconducting metal oxide nanostructures grown via vapour-phase method, as main improvement over other methods for fabricating flexible gas sensors. Thermal simulations and characterizations of the heating element demonstrate these devices provide uniform temperature distribution at the sensing active area, and the electrical properties of the sensing film and electrodes indicate the networked-nanostructures are ohmically connected. Validation of the sensing device shows repeatable and satisfactory responses towards ethanol, demonstrating this fabrication method, with potential in a cost effective production for large-scale applications, is an attractive route for developing next generation of gas sensing devices provided of flexibility and functionality. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Over the last decade, the interest in sensing technologies based on non-conventional substrates such as plastics, paper or textiles has increased with the idea to introduce these systems to new settings, reducing production cost and adding new functionalities. The availability of flexible sensing technologies, which combine the flexibility of organic materials and the functionality of inorganic materials (e.g. semiconductors), could offer a gateway to breakthroughs in distinct areas, for instance in gas sensors, in which several vapours of industrial, health, law enforcement, and security interest are relevant, and in which wearable, light, cheap, and low power consumption monitoring systems, yet not available in the market, are required [1,2]. In particular, chemo-resistive gas sensors based on semiconducting metal oxides (SMOx) have shown great advantages due to their sensitivity to a number of gaseous species, compact size, simple architecture, and low cost production. Although flexible materials offer many attractive properties for the fabrication of new microsystems, they also place severe limitations (e.g. thermal and chemical resistance) on the quality of semiconductors that can be integrated onto these materials. To
∗ Corresponding author at: SIX Research Center, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technicka 12, CZ-61600 Brno, Czech Republic. Tel: +420 541 146 153 ; fax: +420 541 146 298. E-mail addresses: [email protected], [email protected] (S. Vallejos). http://dx.doi.org/10.1016/j.sna.2014.09.001 0924-4247/© 2014 Elsevier B.V. All rights reserved.
overcome these restrictions various methods for the integration of semiconductors and flexible transducers have been described in the literature, but in general they could be grouped in two approaches: post-transfer (indirect) and direct methods. Post-transfer methods are the most commonly reported and involve the transfer of nanostructured materials (previously prepared at high temperatures) on flexible substrates by drop-cast (wet-transfer method) or by using stamps or soluble glues (dry-transfer methods) [1]. Direct methods, in contrast, are infrequently reported and generally involve the grown of nanostructured materials by hydrothermal processes at relatively low temperatures [3,4] or by thermal oxidation-based processes using catalyst seeds [5]. The literature shows that several SMOx are suitable for chemoresistive gas sensors (e.g. SnO2 , WO3 , In2 O3 , ZnO, TiO2 , V2 O5 ) and recently these SMOx, in the form of quasi-one-dimensional nanostructures (e.g. nanowires, nanotubes, nanorods, nanoneedles), have demonstrated promising sensing properties due to their high surface-area-to-volume ratio [6]. In particular, the literature related to flexible gas sensors report the use of zinc oxide nanostructures, mostly via direct integration approaches [3,4,5,7], whereas materials such as tin oxide, tungsten oxide and indium oxide have been less used, with only few works reporting the use of these materials as thick (SnO2 , WO3 ) [8] or thin (InOx ) [9] films integrated via wet-transfer or sputtering method, respectively. Similarly, most of the works on carbon nanofibers [10] and carbon nanotubes [11,12] integrated in flexible devices report the use of post-transfer methods.
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Fig. 1. Optical microscope imaging of the polymeric-based multilayer platforms, including the heating and transducing elements prior to the sensing film integration (a and b), and schematic view of the layers comprising the sensing device (c).
Despite potential advantages of gas-phase methods, for direct growth of nanostructures on different substrates, these methods have not been reported for the fabrication of flexible gas sensors yet. Current advances in CVD and its variants, such as aerosol assisted CVD [13], provide the possibility to overcome the temperature restrictions for the integration of nanostructures, either via appropriate selection or design of precursors [14] or by the utilization of chemically active solvents to transport these precursors [15], and recently aerosol assisted CVD has been adapted to integrate nanostructures on fragile silicon-based MEMS platforms [16]. Here, we report the fabrication and characterization of flexible gas sensing devices comprised of heating and transducing elements, which, in addition, incorporate highly crystalline tungsten oxide nanostructures directly integrated via aerosol assisted CVD. 2. Sensor fabrication 2.1. Processing steps Polymeric transducing platforms were fabricated on a commercial high heat resistant polyimide foil (Upilex-S, 125 m, UBE). Fabrication started with the surface activation of a 4 in. polyimide foil wafer in oxygen plasma to promote adherence of the metallic layers (ALCATEL AMS 110, 600 W, 3 min). Subsequently, patterning of a double loop heater was achieved using a reversible photoresist layer (AZ 5214E, 1.7 m) followed by the deposition of a Ti/Pt (25 nm/250 nm) layer via sputtering (ALCATEL 610); the use of a reversible photoresist, providing negative wall angles of the pattern, improved the lift-off process of the metallic layers. Platinum was selected as heating material for its good properties and its linearity between electrical resistance and temperature. Then, an insulation layer was formed onto the heater by spin coating (4500 rpm, 40 s) of a solution of polyimide precursorpolyamic acid (U-Varnish, UBE), which afterwards was cured to accelerate the imidization reaction at 350 ◦ C following the temperature pattern suggested by the manufacturer. This thermal process ensures obtaining the parameters reported by the manufacturer, including the long-term heat resistance and low outgassing. Opening of the heater contacts was achieved by dry etching with oxygen plasma (ALCATEL AMS 110, 600 W, 8 min), and the electrodes of Ti/Pt (25 nm/250 nm), with a gap of 5 m,
were patterned by lift-off following the same procedure used to structure the heater. Finally, tungsten oxide nanostructures were grown at 350 ◦ C on the top of the electrodes via aerosol assisted CVD of tungsten hexacarbonyl (40 mg, W(CO)6 , Sigma–Aldrich, ≥97%) dissolved in methanol (10 ml, Sigma–Aldrich, ≥99.6%) [17]. Briefly, the adjustment of conditions for growing tungsten oxide nanostructures from W(CO)6 involved a screening of deposition temperatures, solvents and solution concentrations; it is worth noting that in this work, W(CO)6 was used as AACVD precursor due to its lower decomposition temperature [18] compared to tungsten hexaphenoxide W(OPh)6 (the precursor studied in our previous works) [16]. A shadow mask was used during the deposition process, in order to protect the contacts and confine the film deposition to the electrode area. Fig. 1a and b shows the processed wafer and a single element prior to the sensing material deposition, and Fig. 1c depicts the layers comprising the whole device. Bending of the device, before and after AACVD deposition, showed no visible detachment of the Ti/Pt layers or the tungsten oxide films, indicating the layers comprising the structure have relatively strong adherence to the polymer; the reliability of sputtered Ti/Pt layers on polymeric foils to circular bending has also been demonstrated recently [19]. After deposition, the structures were annealed in air at 375 ◦ C for 1 h, and then fixed on a TO-8 package to facilitate the characterization steps. 2.2. Simulation and characterization Electrothermal simulations of the heaters were carried out using the Joule Heating and Thermal Expansion model of COMSOL Multiphysics 4.3a, and the electrical characterizations, for both heaters and sensing film, were achieved using an electrometer (Keithley 2400) controlled by Labview. The cross section of the device, the morphology of the sensing film and its elemental composition were examined using Scanning Electron Microscopy (SEM and EDX – Carl Zeiss, Auriga Series, 3 kV), and the sensing material structure using X-Ray Diffraction (XRD – Bruker, AXS D8-Advance, Cu K␣ radiation operated at 40 kV and 40 mA). Gas sensors were tested in a continuous flow test chamber provided of mass flow controllers (Brooks 5850E) to regulate the mixture of pure synthetic air and ethanol (C2 H5 OH, Praxair, 100 ppm) [20]. The sensors were exposed to ethanol and subsequently the
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chamber was purged with air until initial baseline resistance was recovered. The whole testing period comprised of 100 h during which sensors were tested to ethanol at operating temperatures between 150 and 250 ◦ C, performing up to five replicates for each condition. The sensor response (R) was defined as R = Ra /Rg , where Ra is the sensor resistance in air at stationary state and Rg represents the sensor resistance after 10 min of ethanol exposure.
Intermittent measurements of the microheater electrical resistance during the testing period indicated no deviation of its initial value, showing good medium-term stability of the microheater. This is attributed to the thermal processes to which the device is subjected during fabrication (the thermal processes thermodynamically relax and stabilize the multiple layers comprising the device and their associated resistances, including the microheaters).
3. Results and discussion
3.2. Electrical properties
3.1. Thermal properties
The current (I)–voltage (V) characteristics of the sensing device measured in a continuous flow of dry air at various temperatures are displayed in Fig. 3, and indicate a n-type semiconductor behaviour of the sensing film (i.e. decreasing electrical resistance as temperature increases). From the curves, we observed the sensing film deposited onto the electrodes present an ohmic behaviour, which indicates that not only the metal-semiconductor junction between the platinum electrodes and the tungsten oxide nanostructures, but also the semiconductor-semiconductor junction among the networked tungsten oxide nanoneedles are ohmic. The type of contacts formed in the networked film is important in chemo-resistive gas sensors as the contacts modulate the ratio of electrical resistance changes during analyte detection [22]. Estimation of the activation energy for electrical conduction based on Arrhenius equation revealed an apparent energy of 0.21 eV, which is in agreement with earlier results reported for tungsten oxide films [23,24].
Electro-thermal simulations to evaluate the temperature distribution of the device were performed biasing the heating element at various voltages. Overall, results obtained from the simulations showed uniform thermal distribution at the active area of the sensing device with symmetric temperature profile as a result of the double loop design of the microheater. Fig. 2a and b displays the simulation results obtained from biasing the microheater at 10 V. The temperature coefficient of resistance (TCR) for the platinum microheater was calculated between 200 ◦ C and room temperature, leading to a value of 1.15 × 10−3 ◦ C−1 , which is less than half the value reported for bulk platinum resistors (3.85 × 10−3 ◦ C−1 ), but consistent with other reports for sputtered Pt/Ti thin films [21]. The electrical characterization of the microheater displayed a linear power-temperature tendency, showing a very low power drift (∼2%) with decreasing temperature (Fig. 2c), likely related to the temperature inertia of the polymeric structure, and a power consumption of ∼100 mW at 250 ◦ C. In addition, the dynamic response of the microheaters indicated the device reach the desired temperature in less than 0.3 s (Fig. 2d).
3.3. Material properties Cross-section SEM imaging of the device prior to the sensing film deposition (Fig. 4a and b) showed the layers comprising the
Fig. 2. Simulated thermal profile (a) and temperature distribution (b), and experimental power consumption (c) and dynamic response of the heater at different bias voltage (d).
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Fig. 3. I–V characteristics of the sensing nanostructures grown onto the electrodes at various operating temperatures.
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device are compact and uniform, with the electrodes and heating elements separated by a polyimide layer of approximately 1 m thick. Similarly, cross section and top view of the device after deposition of the sensing film (Fig. 4c and d) displayed nonaligned nanoneedles of approximately 100 nm thick and 10 m length (aspect ratio: 100) connected between the electrode gaps (EG), and EDX of the sensing film indicated consistently O/W ratios of ∼2.7 in different points of the films indicating the formation of partially reduced tungsten oxide structures. XRD analysis of the active area of the sensing device revealed the presence of ˚ b = 7.540 A, ˚ monoclinic phase WO3 (P21/n space group, a = 7.306 A, ˚ and ˇ = 90.88 ; ICCD card no. 72-0677) and indicated c = 7.692 A, a strong preferred orientation in the [0 1 0] direction, showing ˚ and 48.2◦ 2 intense diffraction peaks at 23.5◦ 2 (d = 3.77 A) ˚ corresponding to the (0 2 0) and (0 4 0) reflections of (d = 1.88 A), this monoclinic phase, which is consistent with previous results for AACVD of W(OPh)6 [24]. The pattern also displayed diffractions from platinum described with a face-centered cubic phase ˚ ICCD card no. 04-0802), indicating the film forming (a = 3.923 A; the electrodes and heater reach a stable crystalline phase after the various thermal processes involved in the fabrication of the device. The morphology and structural properties of the tungsten oxide nanostructures synthetized in this work (i.e. from W(CO)6 ) are comparable to those synthetized from W(OPh)6 on conventional substrates [15,16,24,25]. No visible degradation of the transducing sensing platform was observed after deposition of the sensing material with the polyimide keeping its initial shape and aspect.
3.4. Gas sensing properties
Fig. 4. SEM of the sensing device section before (a), (b) and after (c) sensing film deposition, and top view of the sensing film (d).
Tungsten oxide has been intensively studied for gas sensing application, demonstrating exceptional properties to NOx [26–28] and O3 [26,27,29], and also high potential for the detection of EtOH [24,30–32] H2 [24,32,33], H2 S [26,34,35] and NH3 [31,35], including other gases, which typically showed less affinity for tungsten oxide, such as CO [24] and Benzene [36]. In this work, the devices were validated for ethanol detection, by using dc resistance measurements at various operating temperatures from 150 to 250 ◦ C. The dependence of sensor response with operating temperature to ethanol indicated a bell-shaped variation of the response and temperature with a maximum value at 220 ◦ C (Fig. 5a), whereas the response and recovery time described an inverse relation with the operating temperature, showing their lower values at 250 ◦ C (Fig. 5b). These results can be understood by the cooperation of two opposite effects: an increasing probability of activated detection reactions when the operating temperature rises (i.e. from 150 ◦ C to 220 ◦ C) and an increasing probability of adsorbed gas molecules to desorb when the sensor operating temperature increases (i.e. over 220 ◦ C) [37]. The film-resistance changes towards ethanol at 250 ◦ C are displayed in Fig. 6 and demonstrate a typical decrease of electrical resistance when exposed to ethanol. Fig. 6 also indicates the reproducibility of the response showing similar characteristics (e.g. change of electrical resistance and time of response and recovery) for each ethanol injection with a maximum standard errors of the response about ±0.1%. At this operating temperature the response reached a stationary state after 600 s of analyte exposure with complete recovery of the baseline resistance, although with a low drift (< 0.8%). A comparison of the gas sensing properties of these devices and those from the literature is complex as sensor performance depends on various fabrication and test parameters (e.g. microstructure and morphology of the SMOx, geometrical features of the heaters and transducers, gas flow and sensor operating temperature). However, we noticed the sensing properties of the sensors fabricated in this
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Fig. 5. Response (a) and response-recovery times of the sensing device as function of the operating temperature.
References
Fig. 6. Replicates of the sensing film-resistance changes at 250 ◦ C to 100 ppm of ethanol.
work are consistent with the results reported in the literature of flexible gas sensors [4,5,7,12].
4. Conclusions Flexible gas sensor devices comprised of heating and transducing elements were produced by directly integrating polymericbased multilayer platforms and highly crystalline tungsten oxide nanostructures grown via aerosol assisted CVD. The microfabrication processes were adjusted to prevent exceeding the glass transition temperature of commercial polyimides, ensuring thermal compatibility at each processing step. Thermal analysis of the device showed uniform temperature distribution at the sensing active area of the sensor, electrical tests indicated the networked tungsten oxide nanoneedles contacts are ohmic, and the sensing results towards ethanol showed satisfactory responses, which are consistent with those reported in the literature of flexible gas sensors. The method reported here, which includes the direct integration of the complete gas sensing device, shows itself as an attractive route for developing next generation of gas sensors, provided of flexibility and functionality.
Acknowledgements The support of the ‘Ministerio de Economia y Competitividad’ and the ‘South Moravian Programme – SoMoPro’ via grants TEC2010-21357, TEC2013 - 48147 and 4SGA8678 are gratefully acknowledged.
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I. Gràcia received her Ph.D. degree in physics in 1993 from the Autonomous University of Barcelona, Spain, working on chemical sensors. She joined the National Microelectronics Center (CNM) working on photolithography, currently she is full time senior researcher in the Micro-Nano Systems department of the CNM and her work is focused on gas sensing technologies and MEMS reliability.
E. Figueras graduated in Physics in 1983 at Universitat Autonoma de Barcelona and received his Ph.D. Physics in 1988. Since 1989 is Tenured Scientist (Científico Titular) at the Microelectronic Institute of Barcelona (IMB-CNM, CSIC). He spent ten years as head of the clean room, carrying out the standardization of the new technologies developed at the Institute. Since 2000 his research is in the field of micro/nanostructures for gas sensors.
J. Sánchez received his MSc in Chemistry in 2003 from the University of Barcelona (UB). In 2004, he joined the National Centre for Microelectronics (IMB-CNM, CSIC) working as a process engineer in the wet etching area and later in the photolithography area in the Clean Room. Since 2012 he is responsible of the Photolithography area. His main area of activity is focused in the development of resins and polymers and selects the most suitable optical lithographic method to succeed.
R. Mas received her MSc in Chemistry Sciences in the Autonomous University of Barcelona in 1998. She is currently working as process engineer in the clean room (CNM-CSIC). She is responsible of dry etch area. Her main research focuses in silicon technology for the manufacture of integrated circuits, micro-electro-mechanical systems, microelectronic sensors and dry etching of news materials.
O. Beldarrain obtained her degree in Electronics Engineering in 2008 in the University of the Basque Country, (UPV-EHU). In 2011, she received her MSc in Micro and Nanoelectrónic Engineering in the Autonomous University of Barcelona, (UAB). In 2009, she joined the National Centre for Microelectronics, (IMB-CNM, CSIC) and currently she is working as a process engineer in the Clean Room. She is responsible of the Thermal Process and Chemical Vapor Deposition area. Her main area of activity is focused in the development of different thermal processes, the deposition of materials such as Silicon Oxide, Nitride and Polysilicon and the study of techniques for the deposition of high-k dielectrics, particularly via Atomic Layer Deposition.
Biographies
S. Vallejos is at present SoMoPro – Marie Curie fellow at Brno University of Technology. She received her Ph.D. degree from the Universitat Rovira i Virgili and has been working earlier at the Instituto de Microelectrónica de Barcelona and the University College London. Her current research is generally focused in gas sensing technologies and nanoparticle research. She is interested in exploring scalable synthesis methods able to tailor and engineer the sensing properties of nanomaterials, as well as in the development of a new generation of gas sensing microsystems for applications in safety, security and air quality monitoring.
C. Cané is Telecommunications Engineer and he received his Ph.D. in 1989. Since 1990 he is full time senior researcher at CNM and has been working in the development of CMOS technologies and also on mechanical and chemical sensors and microsystems. He is member of the technical committee of EURIMUS-EUREKA programme since 1999. Over the years he has been coordinator of several R&D projects, both at national and international level in the MST field. He has performed management activities as head of the Microsystems and Silicon Technologies Department of CNM and as vice-director of CNM in Barcelona. He is the coordinator of the GoodFood Integrated Project from the 6th Framework Programme (FP6-IST-508774-IP).
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Microfabrication of flexible gas sensing devices based on nanostructured semiconducting metal oxides S. Vallejos a,b,∗ , I. Gràcia a , E. Figueras a , J. Sánchez a , R. Mas a , O. Beldarrain a , C. Cané a a b
Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), Campus UAB, 08193 Barcelona, Spain SIX Research Center, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technicka 12, CZ-61600 Brno, Czech Republic
a r t i c l e
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Article history: Received 20 May 2014 Received in revised form 18 August 2014 Accepted 2 September 2014 Available online 10 September 2014 Keywords: Flexible gas sensors Nanostructures Tungsten oxide AACVD
a b s t r a c t Flexible gas sensor devices comprised of heating and transducing elements are produced by directly integrating multilayer polymeric-based platforms and highly crystalline semiconducting metal oxide nanostructures grown via vapour-phase method, as main improvement over other methods for fabricating flexible gas sensors. Thermal simulations and characterizations of the heating element demonstrate these devices provide uniform temperature distribution at the sensing active area, and the electrical properties of the sensing film and electrodes indicate the networked-nanostructures are ohmically connected. Validation of the sensing device shows repeatable and satisfactory responses towards ethanol, demonstrating this fabrication method, with potential in a cost effective production for large-scale applications, is an attractive route for developing next generation of gas sensing devices provided of flexibility and functionality. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Over the last decade, the interest in sensing technologies based on non-conventional substrates such as plastics, paper or textiles has increased with the idea to introduce these systems to new settings, reducing production cost and adding new functionalities. The availability of flexible sensing technologies, which combine the flexibility of organic materials and the functionality of inorganic materials (e.g. semiconductors), could offer a gateway to breakthroughs in distinct areas, for instance in gas sensors, in which several vapours of industrial, health, law enforcement, and security interest are relevant, and in which wearable, light, cheap, and low power consumption monitoring systems, yet not available in the market, are required [1,2]. In particular, chemo-resistive gas sensors based on semiconducting metal oxides (SMOx) have shown great advantages due to their sensitivity to a number of gaseous species, compact size, simple architecture, and low cost production. Although flexible materials offer many attractive properties for the fabrication of new microsystems, they also place severe limitations (e.g. thermal and chemical resistance) on the quality of semiconductors that can be integrated onto these materials. To
∗ Corresponding author at: SIX Research Center, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technicka 12, CZ-61600 Brno, Czech Republic. Tel: +420 541 146 153 ; fax: +420 541 146 298. E-mail addresses: [email protected], [email protected] (S. Vallejos). http://dx.doi.org/10.1016/j.sna.2014.09.001 0924-4247/© 2014 Elsevier B.V. All rights reserved.
overcome these restrictions various methods for the integration of semiconductors and flexible transducers have been described in the literature, but in general they could be grouped in two approaches: post-transfer (indirect) and direct methods. Post-transfer methods are the most commonly reported and involve the transfer of nanostructured materials (previously prepared at high temperatures) on flexible substrates by drop-cast (wet-transfer method) or by using stamps or soluble glues (dry-transfer methods) [1]. Direct methods, in contrast, are infrequently reported and generally involve the grown of nanostructured materials by hydrothermal processes at relatively low temperatures [3,4] or by thermal oxidation-based processes using catalyst seeds [5]. The literature shows that several SMOx are suitable for chemoresistive gas sensors (e.g. SnO2 , WO3 , In2 O3 , ZnO, TiO2 , V2 O5 ) and recently these SMOx, in the form of quasi-one-dimensional nanostructures (e.g. nanowires, nanotubes, nanorods, nanoneedles), have demonstrated promising sensing properties due to their high surface-area-to-volume ratio [6]. In particular, the literature related to flexible gas sensors report the use of zinc oxide nanostructures, mostly via direct integration approaches [3,4,5,7], whereas materials such as tin oxide, tungsten oxide and indium oxide have been less used, with only few works reporting the use of these materials as thick (SnO2 , WO3 ) [8] or thin (InOx ) [9] films integrated via wet-transfer or sputtering method, respectively. Similarly, most of the works on carbon nanofibers [10] and carbon nanotubes [11,12] integrated in flexible devices report the use of post-transfer methods.
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Fig. 1. Optical microscope imaging of the polymeric-based multilayer platforms, including the heating and transducing elements prior to the sensing film integration (a and b), and schematic view of the layers comprising the sensing device (c).
Despite potential advantages of gas-phase methods, for direct growth of nanostructures on different substrates, these methods have not been reported for the fabrication of flexible gas sensors yet. Current advances in CVD and its variants, such as aerosol assisted CVD [13], provide the possibility to overcome the temperature restrictions for the integration of nanostructures, either via appropriate selection or design of precursors [14] or by the utilization of chemically active solvents to transport these precursors [15], and recently aerosol assisted CVD has been adapted to integrate nanostructures on fragile silicon-based MEMS platforms [16]. Here, we report the fabrication and characterization of flexible gas sensing devices comprised of heating and transducing elements, which, in addition, incorporate highly crystalline tungsten oxide nanostructures directly integrated via aerosol assisted CVD. 2. Sensor fabrication 2.1. Processing steps Polymeric transducing platforms were fabricated on a commercial high heat resistant polyimide foil (Upilex-S, 125 m, UBE). Fabrication started with the surface activation of a 4 in. polyimide foil wafer in oxygen plasma to promote adherence of the metallic layers (ALCATEL AMS 110, 600 W, 3 min). Subsequently, patterning of a double loop heater was achieved using a reversible photoresist layer (AZ 5214E, 1.7 m) followed by the deposition of a Ti/Pt (25 nm/250 nm) layer via sputtering (ALCATEL 610); the use of a reversible photoresist, providing negative wall angles of the pattern, improved the lift-off process of the metallic layers. Platinum was selected as heating material for its good properties and its linearity between electrical resistance and temperature. Then, an insulation layer was formed onto the heater by spin coating (4500 rpm, 40 s) of a solution of polyimide precursorpolyamic acid (U-Varnish, UBE), which afterwards was cured to accelerate the imidization reaction at 350 ◦ C following the temperature pattern suggested by the manufacturer. This thermal process ensures obtaining the parameters reported by the manufacturer, including the long-term heat resistance and low outgassing. Opening of the heater contacts was achieved by dry etching with oxygen plasma (ALCATEL AMS 110, 600 W, 8 min), and the electrodes of Ti/Pt (25 nm/250 nm), with a gap of 5 m,
were patterned by lift-off following the same procedure used to structure the heater. Finally, tungsten oxide nanostructures were grown at 350 ◦ C on the top of the electrodes via aerosol assisted CVD of tungsten hexacarbonyl (40 mg, W(CO)6 , Sigma–Aldrich, ≥97%) dissolved in methanol (10 ml, Sigma–Aldrich, ≥99.6%) [17]. Briefly, the adjustment of conditions for growing tungsten oxide nanostructures from W(CO)6 involved a screening of deposition temperatures, solvents and solution concentrations; it is worth noting that in this work, W(CO)6 was used as AACVD precursor due to its lower decomposition temperature [18] compared to tungsten hexaphenoxide W(OPh)6 (the precursor studied in our previous works) [16]. A shadow mask was used during the deposition process, in order to protect the contacts and confine the film deposition to the electrode area. Fig. 1a and b shows the processed wafer and a single element prior to the sensing material deposition, and Fig. 1c depicts the layers comprising the whole device. Bending of the device, before and after AACVD deposition, showed no visible detachment of the Ti/Pt layers or the tungsten oxide films, indicating the layers comprising the structure have relatively strong adherence to the polymer; the reliability of sputtered Ti/Pt layers on polymeric foils to circular bending has also been demonstrated recently [19]. After deposition, the structures were annealed in air at 375 ◦ C for 1 h, and then fixed on a TO-8 package to facilitate the characterization steps. 2.2. Simulation and characterization Electrothermal simulations of the heaters were carried out using the Joule Heating and Thermal Expansion model of COMSOL Multiphysics 4.3a, and the electrical characterizations, for both heaters and sensing film, were achieved using an electrometer (Keithley 2400) controlled by Labview. The cross section of the device, the morphology of the sensing film and its elemental composition were examined using Scanning Electron Microscopy (SEM and EDX – Carl Zeiss, Auriga Series, 3 kV), and the sensing material structure using X-Ray Diffraction (XRD – Bruker, AXS D8-Advance, Cu K␣ radiation operated at 40 kV and 40 mA). Gas sensors were tested in a continuous flow test chamber provided of mass flow controllers (Brooks 5850E) to regulate the mixture of pure synthetic air and ethanol (C2 H5 OH, Praxair, 100 ppm) [20]. The sensors were exposed to ethanol and subsequently the
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chamber was purged with air until initial baseline resistance was recovered. The whole testing period comprised of 100 h during which sensors were tested to ethanol at operating temperatures between 150 and 250 ◦ C, performing up to five replicates for each condition. The sensor response (R) was defined as R = Ra /Rg , where Ra is the sensor resistance in air at stationary state and Rg represents the sensor resistance after 10 min of ethanol exposure.
Intermittent measurements of the microheater electrical resistance during the testing period indicated no deviation of its initial value, showing good medium-term stability of the microheater. This is attributed to the thermal processes to which the device is subjected during fabrication (the thermal processes thermodynamically relax and stabilize the multiple layers comprising the device and their associated resistances, including the microheaters).
3. Results and discussion
3.2. Electrical properties
3.1. Thermal properties
The current (I)–voltage (V) characteristics of the sensing device measured in a continuous flow of dry air at various temperatures are displayed in Fig. 3, and indicate a n-type semiconductor behaviour of the sensing film (i.e. decreasing electrical resistance as temperature increases). From the curves, we observed the sensing film deposited onto the electrodes present an ohmic behaviour, which indicates that not only the metal-semiconductor junction between the platinum electrodes and the tungsten oxide nanostructures, but also the semiconductor-semiconductor junction among the networked tungsten oxide nanoneedles are ohmic. The type of contacts formed in the networked film is important in chemo-resistive gas sensors as the contacts modulate the ratio of electrical resistance changes during analyte detection [22]. Estimation of the activation energy for electrical conduction based on Arrhenius equation revealed an apparent energy of 0.21 eV, which is in agreement with earlier results reported for tungsten oxide films [23,24].
Electro-thermal simulations to evaluate the temperature distribution of the device were performed biasing the heating element at various voltages. Overall, results obtained from the simulations showed uniform thermal distribution at the active area of the sensing device with symmetric temperature profile as a result of the double loop design of the microheater. Fig. 2a and b displays the simulation results obtained from biasing the microheater at 10 V. The temperature coefficient of resistance (TCR) for the platinum microheater was calculated between 200 ◦ C and room temperature, leading to a value of 1.15 × 10−3 ◦ C−1 , which is less than half the value reported for bulk platinum resistors (3.85 × 10−3 ◦ C−1 ), but consistent with other reports for sputtered Pt/Ti thin films [21]. The electrical characterization of the microheater displayed a linear power-temperature tendency, showing a very low power drift (∼2%) with decreasing temperature (Fig. 2c), likely related to the temperature inertia of the polymeric structure, and a power consumption of ∼100 mW at 250 ◦ C. In addition, the dynamic response of the microheaters indicated the device reach the desired temperature in less than 0.3 s (Fig. 2d).
3.3. Material properties Cross-section SEM imaging of the device prior to the sensing film deposition (Fig. 4a and b) showed the layers comprising the
Fig. 2. Simulated thermal profile (a) and temperature distribution (b), and experimental power consumption (c) and dynamic response of the heater at different bias voltage (d).
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Fig. 3. I–V characteristics of the sensing nanostructures grown onto the electrodes at various operating temperatures.
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device are compact and uniform, with the electrodes and heating elements separated by a polyimide layer of approximately 1 m thick. Similarly, cross section and top view of the device after deposition of the sensing film (Fig. 4c and d) displayed nonaligned nanoneedles of approximately 100 nm thick and 10 m length (aspect ratio: 100) connected between the electrode gaps (EG), and EDX of the sensing film indicated consistently O/W ratios of ∼2.7 in different points of the films indicating the formation of partially reduced tungsten oxide structures. XRD analysis of the active area of the sensing device revealed the presence of ˚ b = 7.540 A, ˚ monoclinic phase WO3 (P21/n space group, a = 7.306 A, ˚ and ˇ = 90.88 ; ICCD card no. 72-0677) and indicated c = 7.692 A, a strong preferred orientation in the [0 1 0] direction, showing ˚ and 48.2◦ 2 intense diffraction peaks at 23.5◦ 2 (d = 3.77 A) ˚ corresponding to the (0 2 0) and (0 4 0) reflections of (d = 1.88 A), this monoclinic phase, which is consistent with previous results for AACVD of W(OPh)6 [24]. The pattern also displayed diffractions from platinum described with a face-centered cubic phase ˚ ICCD card no. 04-0802), indicating the film forming (a = 3.923 A; the electrodes and heater reach a stable crystalline phase after the various thermal processes involved in the fabrication of the device. The morphology and structural properties of the tungsten oxide nanostructures synthetized in this work (i.e. from W(CO)6 ) are comparable to those synthetized from W(OPh)6 on conventional substrates [15,16,24,25]. No visible degradation of the transducing sensing platform was observed after deposition of the sensing material with the polyimide keeping its initial shape and aspect.
3.4. Gas sensing properties
Fig. 4. SEM of the sensing device section before (a), (b) and after (c) sensing film deposition, and top view of the sensing film (d).
Tungsten oxide has been intensively studied for gas sensing application, demonstrating exceptional properties to NOx [26–28] and O3 [26,27,29], and also high potential for the detection of EtOH [24,30–32] H2 [24,32,33], H2 S [26,34,35] and NH3 [31,35], including other gases, which typically showed less affinity for tungsten oxide, such as CO [24] and Benzene [36]. In this work, the devices were validated for ethanol detection, by using dc resistance measurements at various operating temperatures from 150 to 250 ◦ C. The dependence of sensor response with operating temperature to ethanol indicated a bell-shaped variation of the response and temperature with a maximum value at 220 ◦ C (Fig. 5a), whereas the response and recovery time described an inverse relation with the operating temperature, showing their lower values at 250 ◦ C (Fig. 5b). These results can be understood by the cooperation of two opposite effects: an increasing probability of activated detection reactions when the operating temperature rises (i.e. from 150 ◦ C to 220 ◦ C) and an increasing probability of adsorbed gas molecules to desorb when the sensor operating temperature increases (i.e. over 220 ◦ C) [37]. The film-resistance changes towards ethanol at 250 ◦ C are displayed in Fig. 6 and demonstrate a typical decrease of electrical resistance when exposed to ethanol. Fig. 6 also indicates the reproducibility of the response showing similar characteristics (e.g. change of electrical resistance and time of response and recovery) for each ethanol injection with a maximum standard errors of the response about ±0.1%. At this operating temperature the response reached a stationary state after 600 s of analyte exposure with complete recovery of the baseline resistance, although with a low drift (< 0.8%). A comparison of the gas sensing properties of these devices and those from the literature is complex as sensor performance depends on various fabrication and test parameters (e.g. microstructure and morphology of the SMOx, geometrical features of the heaters and transducers, gas flow and sensor operating temperature). However, we noticed the sensing properties of the sensors fabricated in this
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Fig. 5. Response (a) and response-recovery times of the sensing device as function of the operating temperature.
References
Fig. 6. Replicates of the sensing film-resistance changes at 250 ◦ C to 100 ppm of ethanol.
work are consistent with the results reported in the literature of flexible gas sensors [4,5,7,12].
4. Conclusions Flexible gas sensor devices comprised of heating and transducing elements were produced by directly integrating polymericbased multilayer platforms and highly crystalline tungsten oxide nanostructures grown via aerosol assisted CVD. The microfabrication processes were adjusted to prevent exceeding the glass transition temperature of commercial polyimides, ensuring thermal compatibility at each processing step. Thermal analysis of the device showed uniform temperature distribution at the sensing active area of the sensor, electrical tests indicated the networked tungsten oxide nanoneedles contacts are ohmic, and the sensing results towards ethanol showed satisfactory responses, which are consistent with those reported in the literature of flexible gas sensors. The method reported here, which includes the direct integration of the complete gas sensing device, shows itself as an attractive route for developing next generation of gas sensors, provided of flexibility and functionality.
Acknowledgements The support of the ‘Ministerio de Economia y Competitividad’ and the ‘South Moravian Programme – SoMoPro’ via grants TEC2010-21357, TEC2013 - 48147 and 4SGA8678 are gratefully acknowledged.
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I. Gràcia received her Ph.D. degree in physics in 1993 from the Autonomous University of Barcelona, Spain, working on chemical sensors. She joined the National Microelectronics Center (CNM) working on photolithography, currently she is full time senior researcher in the Micro-Nano Systems department of the CNM and her work is focused on gas sensing technologies and MEMS reliability.
E. Figueras graduated in Physics in 1983 at Universitat Autonoma de Barcelona and received his Ph.D. Physics in 1988. Since 1989 is Tenured Scientist (Científico Titular) at the Microelectronic Institute of Barcelona (IMB-CNM, CSIC). He spent ten years as head of the clean room, carrying out the standardization of the new technologies developed at the Institute. Since 2000 his research is in the field of micro/nanostructures for gas sensors.
J. Sánchez received his MSc in Chemistry in 2003 from the University of Barcelona (UB). In 2004, he joined the National Centre for Microelectronics (IMB-CNM, CSIC) working as a process engineer in the wet etching area and later in the photolithography area in the Clean Room. Since 2012 he is responsible of the Photolithography area. His main area of activity is focused in the development of resins and polymers and selects the most suitable optical lithographic method to succeed.
R. Mas received her MSc in Chemistry Sciences in the Autonomous University of Barcelona in 1998. She is currently working as process engineer in the clean room (CNM-CSIC). She is responsible of dry etch area. Her main research focuses in silicon technology for the manufacture of integrated circuits, micro-electro-mechanical systems, microelectronic sensors and dry etching of news materials.
O. Beldarrain obtained her degree in Electronics Engineering in 2008 in the University of the Basque Country, (UPV-EHU). In 2011, she received her MSc in Micro and Nanoelectrónic Engineering in the Autonomous University of Barcelona, (UAB). In 2009, she joined the National Centre for Microelectronics, (IMB-CNM, CSIC) and currently she is working as a process engineer in the Clean Room. She is responsible of the Thermal Process and Chemical Vapor Deposition area. Her main area of activity is focused in the development of different thermal processes, the deposition of materials such as Silicon Oxide, Nitride and Polysilicon and the study of techniques for the deposition of high-k dielectrics, particularly via Atomic Layer Deposition.
Biographies
S. Vallejos is at present SoMoPro – Marie Curie fellow at Brno University of Technology. She received her Ph.D. degree from the Universitat Rovira i Virgili and has been working earlier at the Instituto de Microelectrónica de Barcelona and the University College London. Her current research is generally focused in gas sensing technologies and nanoparticle research. She is interested in exploring scalable synthesis methods able to tailor and engineer the sensing properties of nanomaterials, as well as in the development of a new generation of gas sensing microsystems for applications in safety, security and air quality monitoring.
C. Cané is Telecommunications Engineer and he received his Ph.D. in 1989. Since 1990 he is full time senior researcher at CNM and has been working in the development of CMOS technologies and also on mechanical and chemical sensors and microsystems. He is member of the technical committee of EURIMUS-EUREKA programme since 1999. Over the years he has been coordinator of several R&D projects, both at national and international level in the MST field. He has performed management activities as head of the Microsystems and Silicon Technologies Department of CNM and as vice-director of CNM in Barcelona. He is the coordinator of the GoodFood Integrated Project from the 6th Framework Programme (FP6-IST-508774-IP).