- Email: [email protected]
Pt-doped SnO2 thin film based micro gas sensors with high selectivity to toluene and HCHO
Accepted Manuscript Title: Pt-doped SnO2 thin film based micro gas sensors with high selectivity to toluene and HCHO Authors: Jun-gu Kang, Joon-Shik Park, Hoo-Jeong Lee PII: DOI: Reference:
S0925-4005(17)30421-5 http://dx.doi.org/doi:10.1016/j.snb.2017.03.010 SNB 21922
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
15-10-2016 28-2-2017 3-3-2017
Please cite this article as: Jun-gu Kang, Joon-Shik Park, Hoo-Jeong Lee, Pt-doped SnO2 thin film based micro gas sensors with high selectivity to toluene and HCHO, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.03.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Pt-doped SnO2 thin film based micro gas sensors with high selectivity to toluene and HCHO Jun-gu Kang1,2, Joon-Shik Park2*, Hoo-Jeong Lee1,3**
1
School of Advanced Materials Sciences & Engineering, Sungkyunkwan University, Suwon Gyeonggi,
Republic of Korea, 2
Smart Sensor Research Center, Korea Electrical Technology Institute (KETI), Seongnam Gyeonggi,
Republic of Korea, 3
SKKU Advanced Institute of Nanotechnology (SAINT), Suwon, Gyeonggi, Republic of Korea
Corresponding authors: *E-mail: [email protected] **
E-mail: [email protected]
Highlights
-
Thin film based MEMS gas sensors (in other words, thin film micro gas sensors) with welldefined sensing surface area and low power consumption were successfully fabricated and characterized to indoor pollutant gases such as HCHO, Toluene, and CO.
-
Operating conditions such as operation power consumption and sensing layer thickness for micro gas sensors to have good selectivities to HCHO and toluene gases were optimized.
-
Sensitivities of micro gas sensors with SnO2 thin films were enhanced with adding Pt on the surfaces of SnO2 thin films.
-
Microstructures of SnO2 and Pt doped SnO2 thin films using TEM techniques were characterized and investigated regarding the co-relationship between sensing properties and microstructures.
ABSTRACT: Thin film-based micro gas sensors using undoped and Pt-doped SnO2 thin films with thicknesses of 50 and 120 nm are deposited using RF sputtering on MEMS structures and their sensing characteristics to 25 ppm CO, 25 ppm toluene, and 1 ppm HCHO gases were investigated at 300-440℃. The Pt-doped SnO2 gas sensors with the thickness of 120 nm showed the selectivity to 1 ppm HCHO gas at 31.5 mW power consumption and to 25 ppm toluene gas at 45 mW. The control of gas selectivity by tuning thickness of sensing film and Pt catalyst loading was discussed. The results of this study suggest the possibility of employing thin film-based micro gas sensors for real applications with mass productivity and cost effectiveness.
Keywords : MEMS gas sensors, Pt doped, SnO2 thin film, indoor pollutant gases
1. Introduction Nanomaterials such as nanowires [1], and nanoparticles[2, 3] with high surface area to volume ratio [4] have been researched due to their high gas responses in the metal oxide semiconductor chemiresistors. However, the slurry-based drop coating of nanomaterials on small sensing area fabricated by MEMS technology is a difficult task, which hampers the sensor fabrication in a large scale. [5, 6] On the other hand, thin film-based gas sensors are better suited for mass production, since semiconductor fabrication processing ensures a well-defined sensing area and thus a low sample-tosample deviation in sensitivities. In addition, for mobile phone applications of gas sensors,
much
attention has recently been given to the development of gas sensors which can be operated at a low power consumption.[7] The micro gas sensors fabricated on micro platform via MEMS processing is a good strategy to realize low-power gas detection.[8] In the present study, we developed micro gas sensors capable of working under a power condition of tens-milliwatt, much lower than that of the conventional ones operating at hundreds-of-milliwatt. We used SnO2 thin films as sensing materials to fabricate micro gas sensors on MEMS structures in order
to predict sensing area and reduce the deviation of the sensitivities of fabricated gas sensors. We also employed Pt-doped SnO2 thin films as sensing materials for micro gas sensors to compare with undoped SnO2 gas sensors and enhance sensitivities to CO, toluene and HCHO gases. In addition, we measured and analyzed sensitivities to CO, toluene and HCHO gases at various working temperatures. We characterized the microstructure of the SnO2 films using transmission electron microscopy (TEM) and discussed in connection with the sensor performance.
2. Experiments Fabrication process for micro-platform for thin film-based gas sensors used in this study is similar to that of our previous work.[8] A SiNx film with the thickness of 2 µm was deposited on a P type Si (1 0 0) substrate by using low pressure chemical vapor deposition process (LPCVD). A platinum thin film with the thickness of 200 nm, used for micro heater, was deposited on the SiNx film. The Pt heater layer was patterned and then etched using a dry etching process with deep reactive ion etcher (DRIE). Insulating layers of SiO2/SiNx/SiO2 with a thickness of 1 µm was deposited on the patterned heater layer. A sensing layer of SnO2 thin film with thickness of 50 or 120 nm was deposited using a RF sputtering process with 100 W RF power at room temperature. For some samples, we deposited a thin layer (~0.5nm) of Pt as catalyst on the SnO2 film using E-beam evaporation. A sensing electrode of Ti/Pt film was deposited using sputtering on a patterned photo-resist layer, and then the photoresist layer was removed by lift off process. After finishing the front side process of the wafer, we etched the Si wafer from the backside using KOH wet etching to produce SiNx membrane, a device structure suitable for low power consumption. As shown in Fig. 1(a) and (b), the chip size was 2828 × 2828 µm; the membrane size was 1202 µm × 1202 µm, following a guidance suggested by a previous study on the ratio of the membrane size and heater size [9]. The fabricated micro gas sensors were then attached on TO-5 package by wire bonding, as shown in Figure 2.
Gas sensing properties to 25 ppm CO, 25 ppm toluene, and 1 ppm HCHO gases were investigated using a gas measurement system. The gas concentrations were selected following the guidance
provided by Occupational Safety and Health Administration (OSHA). First, micro gas sensors were annealed at 42 mW power consumption for 2 hours by using joule heating. Gas sensing properties to CO, toluene and HCHO gases and temperature dependence of micro gas sensors were characterized over various ranges of power consumption from 25 to 45 mW. The target gases at different concentrations were controlled by a flow control system. We obtained 25 ppm CO gas by mixing 475 SCCM ambient air and 25 SCCM CO gas (500 ppm in air balance), 25 ppm toluene gas by mixing 475 SCCM ambient air and 25 SCCM toluene gas (500 ppm in air balance), and 1 ppm HCHO gas by mixing 475 SCCM ambient air and 25 SCCM HCHO gas (20 ppm in air balance). Total flow was kept as 500 SCCM to remove flow speed effect. We also confirmed the temperature of the micro heater in micro platform, which was measured in power by an IR camera (M8, MobIR, China, emissivity = 0.1). Figure 3 shows the variation of the micro heater temperature with respect to power consumption, displaying a linear relation.
3. Results and discussion
We characterized the microstructure of SnO2 films using TEM. Figure 4 shows bright field (BF) and high resolution transmission electron microscopy (HRTEM) images of undoped SnO2 thin films with thicknesses of 50 and 120 nm. In the BF image, the 50 nm film shows a microstructure with nearly a monotonous contrast (except some regions with a dark contrast at the top part) (Figs. 4a and 4b), suggesting a low crystallinity or amorphous. A closer look into the film using HRTEM reveals that the film is composed of grains with the size of only several nanometers (Figs. 4c and 4d). Another interesting feature of the microstructure is that a number of pores appear roughly aligned vertically (some denoted with the arrows), an appearance giving a misleading impression that the microstructure is columnar. The 120 nm film shows a microstructure with some distinctive contrasts, suggesting a higher crystallinity. In the high angle annular dark field (HAADF) image of Pt-doped SnO2 thin film shown in Fig. 5, evaporated Pt shows an island-like morphology on the surface of the SnO2 film.
We used the SnO2 films (undoped and Pt-doped, 50nm- and 120nm-thick) as sensing materials to investigate the effects of Pt doping and film thickness. For SnO2-based sensors, it is well-known that such injected gases as CO, toluene, HCHO, react with SnO2 thin films according to the following reactions [10, 11, 12]:
CO + O-ad(SnO2-x) → CO2 + (SnO2-x)+ + e-
(1)
C6H5CH3 + 18O-ad(SnO2-x) → 7CO2 + 4H2O + 18e-
(2)
HCHO + O-ad(SnO2-x) → HCOOH + eHCOOH + O-ad(SnO2-x) → CO2 + H2O + e-
(3)
Because the injected gases are reducing gases, they combine with O- ions adsorbing on the SnO2 surface to form CO2 and, in turn, generate electrons, which decreases the resistance of SnO2. [13] Using gas sensors fabricated using the four different SnO2 films, we checked how the sensors performed for the three gases (CO, toluene, and HCHO). Figure 6 shows gas responses The graph displays gas responses — defined as Ra/Rg (Ra = Resistance of gas sensor in ambient air, Rg = Resistance of gas sensor in reactive gas) — of the four gas sensors at various powers (24.5 to 45mW). For HCHO gas (1ppm), the Pt-doped 120 nm SnO2 gas sensor shows a selectivity at 31.5 mW power consumption, as shown in Figure 6(d). For toluene gas (25ppm), the Pt-doped 120 nm SnO2 gas sensor exhibits a selectivity at 44 mW power consumption, as shown in Figure 6(h). However, no sample shows selectivity to CO gas (25ppm). In addition, it should be noted that adding Pt brought about improvement in response for all gases. A study done by Yamazoe et al. reported a similar Pt effect, explaining it using the spillover effect of the reactant gases. [14]
To understand the selectivity of the sensors to HCHO and toluene, we plotted the data in terms of
power consumption (that is, temperature), as shown in Fig. 7. The graphs show that the temperature dependence is somewhat different for the gases, hinting that the selectivity among the gases arises from the differences in sensing temperature. For HCHO, first, the response increases with the rise of power consumption from 24.5 to 39 mW, and then decreased at 44 mW. Such volcano-type dependence with operation temperature is consistent with reports in other studies on n-type semiconductor gas sensors to reducing gases [15]. According to the reports, the sensitivity increases as oxygen ions and reactants react more readily with the temperature increasing, and then decreases at higher temperatures as the amount of oxygen adsorbates decreases due to high thermal energy.
For toluene, the response continues to increase without showing any decline (except for the undoped 50nm-thick SnO2). This data suggests that the sensors (in particular, Pt-doped 120 nm sample) show a much better response to toluene at higher temperatures, implying that a similar volcano-type dependence might appear at a temperature range higher than employed in this study. Gas response to CO remains low across the entire temperature range for all the sensors, as shown in Fig. 7 (c). Comparison of the three graphs reveals that the temperature range in which the sensors respond highly is uniquely different for the HCHO and toluene gases (HCHO for 39mW and toluene for 44mW), lending a selectivity for the temperature range. It should be noted that the gas response is, for all the samples, higher for a thicker film (120nm), as shown in Fig. 6. According to the TEM analysis on the films, shown in Fig. 4, the main difference is crystallinity: the thicker film shows a higher crystallinity. Several previous studies [16, 17] report on the influence of crystallinity on gas response that a film with a higher crystallinity shows a better gas response.
4. Conclusions In this study, we fabricated thin film-based micro gas sensors using SnO2 thin films with the thicknesses of 50 and 120 nm as a sensing material, which were deposited using RF sputtering process on MEMS structures. Using undoped and Pt-doped samples, we characterize the gas response to 25 ppm CO, 25 ppm toluene, and 1 ppm HCHO gases with power consumption from 24.5 mW (300℃)
to 45 mW (440℃). The sensors displayed some selectivity to HCHO and toluene gases (Pt-doped 120 nm SnO2 gas sensors to 1 ppm HCHO gas at 31.5 mW power consumption; Pt-doped 120 nm SnO2 gas sensor to 25 ppm toluene gas) but no response to CO gas. We further discussed on the mechanism of the selectivity and the effects of Pt doping. The results of this study suggest the possibility of employing thin film-based micro gas sensor, which have significant advantages on mass productivity and cost effectiveness.
Acknowledgements This research was supported by the Project No. 10043800, of ―S/W converged components technology development program‖ by KEIT and MOTIE in Korea. The authors appreciate for research funding. J.S.Park and J.G.Kang also would like to acknowledge the partial support from the R&D Convergence Program of MSIP and NST of Republic of Korea (Grant CAP-13-1-KITECH).
References [1] F. Hernandez-Ramirez, J.D. Prades, J.R. Morante, Metal oxide nanowire gas sensors, Sens. Mater. 21(2009) 219-27. [2] T.K. Starke, G.S. Coles, Laser-ablated nanocrystalline SnO2 material for low-level CO detection, Sens. Actuators B 88 (2003) 227-33. [3] S. Habibzadeh, A.A. Khodadadi, Y. Mortazavi, CO and ethanol dual selective sensor of Sm2O3doped SnO2 nanoparticles synthesized by microwave-induced combustion, Sens. Actuators B 144 (2010) 131-8. [4] H.-S. Woo, C.W. Na, J.-H. Lee, Design of Highly Selective Gas Sensors via Physicochemical Modification of Oxide Nanowires: Overview, Sensors, 16 (2016) 1531.
[5] F. Cháveza, G.F. Pérez-Sáncheza, O. Goizb, P. Zaca-Morána, R. Peña-Sierrab, A. MoralesAcevedob, C. Felipec, and M. Soledad-Priegod, Sensing performance of palladium-functionalized WO 3 nanowires by a drop-casting method, Applied Surface Science, 275 (2013) 28-35. [6] J. Wu, K. Tao, J. Miao, L.K. Norford, Improved selectivity and sensitivity of gas sensing using a 3D reduced graphene oxide hydrogel with an integrated microheater, ACS appl. Mater. Interfaces, 7 (2015) 27502-10. [7] J.M. Azzarelli, K.A. Mirica, J.B. Ravnsbæk, T.M. Swager, Wireless gas detection with a smartphone via rf communication, Proc. Natl. Acad. Sci. USA, 111 (2014) 18162-18166. [8] K.-Y. Choi, J.-S. Park, K.-B. Park, H.J. Kim, H.-D. Park, S.-D. Kim, Low power micro-gas sensors using mixed SnO2 nanoparticles and MWCNTs to detect NO2, NH3, and xylene gases for ubiquitous sensor network applications, Sens. Actuators B 150 (2010) 65-72. [9] S.-I. Park, J.-S. Park, M.-H. Lee, K.-B. Park, S.-D. Kim, H.-D. Park, et al., Design and Fabrication of a Micro Gas Sensor Using Nano Sensing Materials on Multi-layer Type Micro Platform with Low Power Consumption, IEMEK J. Embed. Sys. Appl. 2(2007) 76-81. [10] S. Bai, W. Guo, J. Sun, J. Li, Y. Tian, A. Chen, et al., Synthesis of SnO2–CuO heterojunction using electrospinning and application in detecting of CO, Sens. Actuators B 226 (2016) 96–103. [11] L. Liu, Y. Zhang, G.G. Wang, S.C. Li, L.Y. Wang,Y. Han, X.X. Jiang, A.G. Wei, High toluene sensing properties of NiO-SnO2 composite nanofiver sensors operating at 330℃, Sens. Actuators B 166-167 (2012), 519-525. [12] J.A. Dirksen, K. Duval, T.A. Ring, NiO thin-film formaldehyde gas sensor, Sens. Actuators B 80 (2001), 106-115. [13] C. Li, M. Lv, J. Zuo, X. Huang, SnO2 highly sensitive CO gas sensor based on quasi-molecularimprinting mechanism design, Sensors, 15 (2015), 3789–3800. [14] N. Yamazoe, Y. Kurokawa, T. Seiyama, Effects of additives on semiconductor gas sensors, Sens. Actuators, 4(1983) 283-9.
[15] Y. Shimizu, SnO2 gas sensor, in: Encyclopedia of Applied Electrochemistry, Springer New York, (2014) pp. 1974-1982. [16] A. Katoch, Z.U. Abideen, J.H. Kim, S.S. Kim, Crystallinity dependent gas-sensing abilities of ZnO hollow fibers, Met. Mater. Int., 22, 5 (2016), 942-946 [17] B. Karunagaran, P. Uthirakuma, S.J. Chung, S. Velumani, E.K. Suh, TiO2 thin film gas sensor for monitoring ammonia, Mater. Charact., 58 (2007), 680–684
Figure captions Figure 1 The Design of gas sensor (a) and its fabricated optical microscope (OM) image (b). Figure 2 (a) The OM images of undoped SnO2 micro gas sensor packaged on TO-5 package using wire bonding process, (b) magnified OM image of (a), (c) OM image of undoped SnO2 sensing surface area, (d) OM images of Pt-doped SnO2 micro gas sensor on TO-5 package, (e) magnified OM image of (d), and (f) OM image of Pt-doped SnO2 sensing surface area. Figure 3 Temperature vs power consumption of the micro gas sensor measured by the IR camera (MobIR M8, Emissivity = 0.1). Figure 4 The TEM cross sectional images of SnO2 thin film with thicknesses of (a) 50 nm, (b) 120 nm, HRTEM images of SnO2 thin film with thicknesses of (c) 50 nm and (d) 120 nm. Figure 5 The HAADF image of Pt-doped SnO2 thin film.
Figure 6 Gas responses to 25 ppm CO, 25 ppm toluene and 1 ppm HCHO gases of undoped and Ptdoped SnO2 gas sensors at various ranges of power consumption (a, b) 24.5 mW, (c, d) 31.5 mW, (e, f) 39 mW, and (g, h) 45 mW. Figure 7 Gas responses of undoped and Pt-doped SnO2 gas sensors to (a) 1 ppm HCHO, (b) 25 ppm toluene and (c) 25 ppm CO gases.
S0925-4005(17)30421-5 http://dx.doi.org/doi:10.1016/j.snb.2017.03.010 SNB 21922
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
15-10-2016 28-2-2017 3-3-2017
Please cite this article as: Jun-gu Kang, Joon-Shik Park, Hoo-Jeong Lee, Pt-doped SnO2 thin film based micro gas sensors with high selectivity to toluene and HCHO, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.03.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Pt-doped SnO2 thin film based micro gas sensors with high selectivity to toluene and HCHO Jun-gu Kang1,2, Joon-Shik Park2*, Hoo-Jeong Lee1,3**
1
School of Advanced Materials Sciences & Engineering, Sungkyunkwan University, Suwon Gyeonggi,
Republic of Korea, 2
Smart Sensor Research Center, Korea Electrical Technology Institute (KETI), Seongnam Gyeonggi,
Republic of Korea, 3
SKKU Advanced Institute of Nanotechnology (SAINT), Suwon, Gyeonggi, Republic of Korea
Corresponding authors: *E-mail: [email protected] **
E-mail: [email protected]
Highlights
-
Thin film based MEMS gas sensors (in other words, thin film micro gas sensors) with welldefined sensing surface area and low power consumption were successfully fabricated and characterized to indoor pollutant gases such as HCHO, Toluene, and CO.
-
Operating conditions such as operation power consumption and sensing layer thickness for micro gas sensors to have good selectivities to HCHO and toluene gases were optimized.
-
Sensitivities of micro gas sensors with SnO2 thin films were enhanced with adding Pt on the surfaces of SnO2 thin films.
-
Microstructures of SnO2 and Pt doped SnO2 thin films using TEM techniques were characterized and investigated regarding the co-relationship between sensing properties and microstructures.
ABSTRACT: Thin film-based micro gas sensors using undoped and Pt-doped SnO2 thin films with thicknesses of 50 and 120 nm are deposited using RF sputtering on MEMS structures and their sensing characteristics to 25 ppm CO, 25 ppm toluene, and 1 ppm HCHO gases were investigated at 300-440℃. The Pt-doped SnO2 gas sensors with the thickness of 120 nm showed the selectivity to 1 ppm HCHO gas at 31.5 mW power consumption and to 25 ppm toluene gas at 45 mW. The control of gas selectivity by tuning thickness of sensing film and Pt catalyst loading was discussed. The results of this study suggest the possibility of employing thin film-based micro gas sensors for real applications with mass productivity and cost effectiveness.
Keywords : MEMS gas sensors, Pt doped, SnO2 thin film, indoor pollutant gases
1. Introduction Nanomaterials such as nanowires [1], and nanoparticles[2, 3] with high surface area to volume ratio [4] have been researched due to their high gas responses in the metal oxide semiconductor chemiresistors. However, the slurry-based drop coating of nanomaterials on small sensing area fabricated by MEMS technology is a difficult task, which hampers the sensor fabrication in a large scale. [5, 6] On the other hand, thin film-based gas sensors are better suited for mass production, since semiconductor fabrication processing ensures a well-defined sensing area and thus a low sample-tosample deviation in sensitivities. In addition, for mobile phone applications of gas sensors,
much
attention has recently been given to the development of gas sensors which can be operated at a low power consumption.[7] The micro gas sensors fabricated on micro platform via MEMS processing is a good strategy to realize low-power gas detection.[8] In the present study, we developed micro gas sensors capable of working under a power condition of tens-milliwatt, much lower than that of the conventional ones operating at hundreds-of-milliwatt. We used SnO2 thin films as sensing materials to fabricate micro gas sensors on MEMS structures in order
to predict sensing area and reduce the deviation of the sensitivities of fabricated gas sensors. We also employed Pt-doped SnO2 thin films as sensing materials for micro gas sensors to compare with undoped SnO2 gas sensors and enhance sensitivities to CO, toluene and HCHO gases. In addition, we measured and analyzed sensitivities to CO, toluene and HCHO gases at various working temperatures. We characterized the microstructure of the SnO2 films using transmission electron microscopy (TEM) and discussed in connection with the sensor performance.
2. Experiments Fabrication process for micro-platform for thin film-based gas sensors used in this study is similar to that of our previous work.[8] A SiNx film with the thickness of 2 µm was deposited on a P type Si (1 0 0) substrate by using low pressure chemical vapor deposition process (LPCVD). A platinum thin film with the thickness of 200 nm, used for micro heater, was deposited on the SiNx film. The Pt heater layer was patterned and then etched using a dry etching process with deep reactive ion etcher (DRIE). Insulating layers of SiO2/SiNx/SiO2 with a thickness of 1 µm was deposited on the patterned heater layer. A sensing layer of SnO2 thin film with thickness of 50 or 120 nm was deposited using a RF sputtering process with 100 W RF power at room temperature. For some samples, we deposited a thin layer (~0.5nm) of Pt as catalyst on the SnO2 film using E-beam evaporation. A sensing electrode of Ti/Pt film was deposited using sputtering on a patterned photo-resist layer, and then the photoresist layer was removed by lift off process. After finishing the front side process of the wafer, we etched the Si wafer from the backside using KOH wet etching to produce SiNx membrane, a device structure suitable for low power consumption. As shown in Fig. 1(a) and (b), the chip size was 2828 × 2828 µm; the membrane size was 1202 µm × 1202 µm, following a guidance suggested by a previous study on the ratio of the membrane size and heater size [9]. The fabricated micro gas sensors were then attached on TO-5 package by wire bonding, as shown in Figure 2.
Gas sensing properties to 25 ppm CO, 25 ppm toluene, and 1 ppm HCHO gases were investigated using a gas measurement system. The gas concentrations were selected following the guidance
provided by Occupational Safety and Health Administration (OSHA). First, micro gas sensors were annealed at 42 mW power consumption for 2 hours by using joule heating. Gas sensing properties to CO, toluene and HCHO gases and temperature dependence of micro gas sensors were characterized over various ranges of power consumption from 25 to 45 mW. The target gases at different concentrations were controlled by a flow control system. We obtained 25 ppm CO gas by mixing 475 SCCM ambient air and 25 SCCM CO gas (500 ppm in air balance), 25 ppm toluene gas by mixing 475 SCCM ambient air and 25 SCCM toluene gas (500 ppm in air balance), and 1 ppm HCHO gas by mixing 475 SCCM ambient air and 25 SCCM HCHO gas (20 ppm in air balance). Total flow was kept as 500 SCCM to remove flow speed effect. We also confirmed the temperature of the micro heater in micro platform, which was measured in power by an IR camera (M8, MobIR, China, emissivity = 0.1). Figure 3 shows the variation of the micro heater temperature with respect to power consumption, displaying a linear relation.
3. Results and discussion
We characterized the microstructure of SnO2 films using TEM. Figure 4 shows bright field (BF) and high resolution transmission electron microscopy (HRTEM) images of undoped SnO2 thin films with thicknesses of 50 and 120 nm. In the BF image, the 50 nm film shows a microstructure with nearly a monotonous contrast (except some regions with a dark contrast at the top part) (Figs. 4a and 4b), suggesting a low crystallinity or amorphous. A closer look into the film using HRTEM reveals that the film is composed of grains with the size of only several nanometers (Figs. 4c and 4d). Another interesting feature of the microstructure is that a number of pores appear roughly aligned vertically (some denoted with the arrows), an appearance giving a misleading impression that the microstructure is columnar. The 120 nm film shows a microstructure with some distinctive contrasts, suggesting a higher crystallinity. In the high angle annular dark field (HAADF) image of Pt-doped SnO2 thin film shown in Fig. 5, evaporated Pt shows an island-like morphology on the surface of the SnO2 film.
We used the SnO2 films (undoped and Pt-doped, 50nm- and 120nm-thick) as sensing materials to investigate the effects of Pt doping and film thickness. For SnO2-based sensors, it is well-known that such injected gases as CO, toluene, HCHO, react with SnO2 thin films according to the following reactions [10, 11, 12]:
CO + O-ad(SnO2-x) → CO2 + (SnO2-x)+ + e-
(1)
C6H5CH3 + 18O-ad(SnO2-x) → 7CO2 + 4H2O + 18e-
(2)
HCHO + O-ad(SnO2-x) → HCOOH + eHCOOH + O-ad(SnO2-x) → CO2 + H2O + e-
(3)
Because the injected gases are reducing gases, they combine with O- ions adsorbing on the SnO2 surface to form CO2 and, in turn, generate electrons, which decreases the resistance of SnO2. [13] Using gas sensors fabricated using the four different SnO2 films, we checked how the sensors performed for the three gases (CO, toluene, and HCHO). Figure 6 shows gas responses The graph displays gas responses — defined as Ra/Rg (Ra = Resistance of gas sensor in ambient air, Rg = Resistance of gas sensor in reactive gas) — of the four gas sensors at various powers (24.5 to 45mW). For HCHO gas (1ppm), the Pt-doped 120 nm SnO2 gas sensor shows a selectivity at 31.5 mW power consumption, as shown in Figure 6(d). For toluene gas (25ppm), the Pt-doped 120 nm SnO2 gas sensor exhibits a selectivity at 44 mW power consumption, as shown in Figure 6(h). However, no sample shows selectivity to CO gas (25ppm). In addition, it should be noted that adding Pt brought about improvement in response for all gases. A study done by Yamazoe et al. reported a similar Pt effect, explaining it using the spillover effect of the reactant gases. [14]
To understand the selectivity of the sensors to HCHO and toluene, we plotted the data in terms of
power consumption (that is, temperature), as shown in Fig. 7. The graphs show that the temperature dependence is somewhat different for the gases, hinting that the selectivity among the gases arises from the differences in sensing temperature. For HCHO, first, the response increases with the rise of power consumption from 24.5 to 39 mW, and then decreased at 44 mW. Such volcano-type dependence with operation temperature is consistent with reports in other studies on n-type semiconductor gas sensors to reducing gases [15]. According to the reports, the sensitivity increases as oxygen ions and reactants react more readily with the temperature increasing, and then decreases at higher temperatures as the amount of oxygen adsorbates decreases due to high thermal energy.
For toluene, the response continues to increase without showing any decline (except for the undoped 50nm-thick SnO2). This data suggests that the sensors (in particular, Pt-doped 120 nm sample) show a much better response to toluene at higher temperatures, implying that a similar volcano-type dependence might appear at a temperature range higher than employed in this study. Gas response to CO remains low across the entire temperature range for all the sensors, as shown in Fig. 7 (c). Comparison of the three graphs reveals that the temperature range in which the sensors respond highly is uniquely different for the HCHO and toluene gases (HCHO for 39mW and toluene for 44mW), lending a selectivity for the temperature range. It should be noted that the gas response is, for all the samples, higher for a thicker film (120nm), as shown in Fig. 6. According to the TEM analysis on the films, shown in Fig. 4, the main difference is crystallinity: the thicker film shows a higher crystallinity. Several previous studies [16, 17] report on the influence of crystallinity on gas response that a film with a higher crystallinity shows a better gas response.
4. Conclusions In this study, we fabricated thin film-based micro gas sensors using SnO2 thin films with the thicknesses of 50 and 120 nm as a sensing material, which were deposited using RF sputtering process on MEMS structures. Using undoped and Pt-doped samples, we characterize the gas response to 25 ppm CO, 25 ppm toluene, and 1 ppm HCHO gases with power consumption from 24.5 mW (300℃)
to 45 mW (440℃). The sensors displayed some selectivity to HCHO and toluene gases (Pt-doped 120 nm SnO2 gas sensors to 1 ppm HCHO gas at 31.5 mW power consumption; Pt-doped 120 nm SnO2 gas sensor to 25 ppm toluene gas) but no response to CO gas. We further discussed on the mechanism of the selectivity and the effects of Pt doping. The results of this study suggest the possibility of employing thin film-based micro gas sensor, which have significant advantages on mass productivity and cost effectiveness.
Acknowledgements This research was supported by the Project No. 10043800, of ―S/W converged components technology development program‖ by KEIT and MOTIE in Korea. The authors appreciate for research funding. J.S.Park and J.G.Kang also would like to acknowledge the partial support from the R&D Convergence Program of MSIP and NST of Republic of Korea (Grant CAP-13-1-KITECH).
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Figure captions Figure 1 The Design of gas sensor (a) and its fabricated optical microscope (OM) image (b). Figure 2 (a) The OM images of undoped SnO2 micro gas sensor packaged on TO-5 package using wire bonding process, (b) magnified OM image of (a), (c) OM image of undoped SnO2 sensing surface area, (d) OM images of Pt-doped SnO2 micro gas sensor on TO-5 package, (e) magnified OM image of (d), and (f) OM image of Pt-doped SnO2 sensing surface area. Figure 3 Temperature vs power consumption of the micro gas sensor measured by the IR camera (MobIR M8, Emissivity = 0.1). Figure 4 The TEM cross sectional images of SnO2 thin film with thicknesses of (a) 50 nm, (b) 120 nm, HRTEM images of SnO2 thin film with thicknesses of (c) 50 nm and (d) 120 nm. Figure 5 The HAADF image of Pt-doped SnO2 thin film.
Figure 6 Gas responses to 25 ppm CO, 25 ppm toluene and 1 ppm HCHO gases of undoped and Ptdoped SnO2 gas sensors at various ranges of power consumption (a, b) 24.5 mW, (c, d) 31.5 mW, (e, f) 39 mW, and (g, h) 45 mW. Figure 7 Gas responses of undoped and Pt-doped SnO2 gas sensors to (a) 1 ppm HCHO, (b) 25 ppm toluene and (c) 25 ppm CO gases.