Accepted Manuscript

Accepted Manuscript

Accepted Manuscript Title: Scalable fabrication of SnO2 thin films sensitized with CuO islands for enhanced H2 S gas sensing performance Author: Nguyen Van Toan Nguyen Viet Chien Nguyen Van Duy Dang Duc Vuong Nguyen Huu Lam Nguyen Duc Hoa Nguyen Van Hieu Nguyen Duc Chien PII: DOI: Reference: S0169-4332(14)02400-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.134 APSUSC 28999 To appear in: APSUSC Received date: Revised date: Accepted date: 19-8-2014 1-10-2014 24-10-2014 Please cite this article as: N Van Toan, N.V Chien, N Van Duy, D.D Vuong, N.H Lam, N.D Hoa, N Van Hieu, N.D Chien, Scalable fabrication of SnO2 thin films sensitized with CuO islands for enhanced H2 S gas sensing performance, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.134 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 Highlight ►CuO island-sensitized SnO2 thin film sensors were fabricated at waferscale t ►SnO2-CuO island sensors significantly enhanced H2S gas response Ac ce pt ed M an us cr ip ►The thickness of CuO islands strongly affected on H2S gas sensing performance Page of 20 Scalable fabrication of SnO2 thin films sensitized with CuO islands for enhanced H2S gas sensing performance us cr ip t Nguyen Van Toan1, Nguyen Viet Chien1, Nguyen Van Duy1, Dang Duc Vuong2, Nguyen Huu Lam2, Nguyen Duc Hoa1,*, Nguyen Van Hieu1,*, Nguyen Duc Chien1,2,* International Training Institute for Materials Science (ITIMS), Hanoi University of Science an 1) and Technology (HUST), Dai Co Viet Road, Hanoi, Vietnam School of Engineering Physics (SEP), Hanoi University of Science and Technology (HUST), ce pt ed No Dai Co Viet Road, Hanoi, Vietnam M 2) Ac Corresponding authors *Nguyen DucChien, Professor, *Nguyen Van Hieu, Associate Professor, * Nguyen DucHoa, PhD International Training Institute for Materials Science, Hanoi University of Science and Technology, No.1, Dai Co Viet Road, Hanoi, Vietnam Phone: 84 38680787 Fax: 84 38692963 E–mail: hieu@itims.edu.vn/chien.nguyenduc@hust.edu.vn No Dai Co Viet, Hanoi, Vietnam Postal address: Page of 20 Abstract: The detection of H2S, an important gaseous molecule that has been recently marked as t a highly toxic environmental pollutant, has attracted increasing attention We fabricate a wafer- us cr ip scale SnO2 thin film sensitized with CuO islands using microelectronic technology for the improved detection of the highly toxic H2S gas The SnO2–CuO island sensor exhibits significantly enhanced H2S gas response and reduced operating temperature The thickness of CuO islands strongly influences H2S sensing characteristics, and the highest H2S gas response is an observed with 20 nm-thick CuO islands The response value (Ra/Rg) of the SnO2–CuO island sensor to ppm H2S is as high as 128 at 200 °C and increases nearly 55-fold compared with that M of the bare SnO2 thin film sensor Meanwhile, the response of the SnO2–CuO island sensor to H2 (250 ppm), NH3 (250 ppm), CO (250 ppm), and LPG (1000 ppm) are low (1.3 to 2.5) The ed enhanced gas response and selectivity of the SnO2–CuO island sensor to H2S gas is explained by the sensitizing effect of CuO islands and the extension of electron depletion regions because of pt the formation of p–n junctions ce Keywords: H2S gas sensor, reactive sputtering, SnO2, CuO, wafer-scale fabrication Ac Introduction H2S is an extremely toxic and irritating gas that has been recently identified as an emergent air pollutant with the increase in industrial activities such as petroleum or natural gas drilling and refining [1] The permissible exposure limit for H2S is very low; thus, the detection and monitoring of H2S concentration is crucial to protect human lives [2] Gas chromatographybased methods can detect such harmful pollutants with high precision [3] However, these methods are not suitable and effective for real-time monitoring as compared with metal oxide Page of 20 semiconductor-based sensors [1] Resistive sensors have advantages such as small size, simple construction, low weight, low power consumption, and low cost [4] The principal working mechanism of resistive gas sensors is based on variations in the electrical resistance us cr ip t (conductance) of the metal oxide semiconductor sensing layer upon exposure to analytic gas [5,6] Nanostructured materials of thin film, nanoparticles, nanowires, nanofibers, and nanorods of different metal oxide semiconductors have been investigated for their H2S gas sensing capabilities [7–9] For instance, bare Fe2O3 thin films prepared by the electron beam evaporation of Fe followed by thermal oxidation were used for the ppm-level detection of H2S [10] an However, bare materials not exhibit a high sensitivity to H2S gas [11] Doping of p-type and/or noble metals to n-type metal oxide semiconductors has been recently reported to M significantly enhance the H2S sensing characteristics of such bare materials [12,13] This ed technique has been recently applied to improve the sensitivity to H2S gas of SnO2 nanowires by decoration with NiO nanoparticles [14] Modifying low-dimensional materials such as pt nanowires, nanofibers, and nanorods to enhance sensing performance is suitable for proving the concept but not effective for large-scale operations [15] Gupta et al [16,17] have devoted ce considerable efforts to develop different metal oxides in the form of thin films for gas sensing Ac applications CuO thin films were prepared for sub-ppm H2S sensing at room temperature; the sensor exhibited the highest response to H2S, followed by Cl2, NH3, NO, CO, and CH4 [18] The sensing mechanism was claimed by the conversion of CuO into CuS upon exposure to high H2S concentrations (>50 ppm) and the decrease in sensor resistance Multilayered SnO2–CuO thin films were also fabricated for highly sensitive H2S sensing [19] Loading CuO islands on the top surface of SnO2 thin films significantly enhances the response to H2S gas [17] However, recently reported experiments have been mostly limited to preparing and investigating the gas Page of 20 sensing properties of–CuO thin films [17,18] Proper design and synthesis processes to realize the large-scale fabrication of compacted devices are of important issue, and stills a challenge in practical applications A previous study deposited CuO islands through shadow masks as large as us cr ip t 600 μm, making the device size impossible to reduce [17] The H2S sensing characteristics of SnO2 thin films sensitized with CuO islands are strongly dependent on CuO layer behavior and sensor fabrication Thus, the technological development and wafer-scale fabrication of gas sensors are important concerns an We report in detail the fabrication of H2S gas sensors using SnO2 thin films sensitized with CuO islands The islands were designed to have diameters as small as μm to realize micro-sized gas M sensor fabrication The wafer-scale fabrication of H2S gas sensors was realized using microelectronic technology The thickness of the CuO islands was optimized to enhance the gas pt Experimental ed sensing performance of the sensors The schematic for the wafer-scale fabrication of H2S sensor arrays based on CuO island- ce sensitized SnO2 thin films (noted as CuO–SnO2 thin films) is shown in Figure The sensor Ac design involves a microheater, a pair of electrodes composed of Pt/Cr layers deposited on a thermally oxidized silicon wafer, and a sensing layer composed of CuO–SnO2 thin films [Figure 1(A)] A gas sensing layer comprising CuO–SnO2 thin films was prepared through reactive sputtering Thicknesses of the SnO2 and CuO thin films were measured using a Veeco Dektak 150 Surface Profilometer (Veeco Instruments Inc., USA), with accuracy of 0.6 nm A inch Sn target was used to deposit SnO2 thin films (~40 nm) under the following sputtering conditions: pressure, 10−6 torr; working pressure, 5×10−3 torr; and Ar/O2 flow ratio, 50:50 Cu Page of 20 islands were deposited using Cu as the target and Ar/O2 as the sputter gases Sputtering conditions were similar to that of the SnO2 deposition The deposition rate of CuO is nm/min, thus by controlling the deposition time of 1, 2, 3, and min, we could control the thickness of us cr ip t CuO islands to be about 5, 10, 15, and 20 nm, respectively The size of the sensing area was 150 μm × 150 μm, whereas the diameter and distance between CuO islands were both μm A silicon backside was etched into the SiO2 membrane to reduce heat loss from the microheater and accordingly reduce power consumption [4] The fabrication of sensor wafers involved the following process flow: (1) thermal oxidation of Si wafer; (2) photolithography for the an deposition of the Pt/Cr electrode and (3) the microheater by sputtering; (4) – (5) lift off; (6)–(9) patterned deposition of SnO2 thin films as a sensing layer; (10)–(11) deposition of sensitized M CuO islands; and (12) silicon backside etching to reduce the power consumption of the device stability of the sensors ed [Figure 1(B)] Finally, heat treatment was conducted at 400 °C for h in air to ensure the pt Sensor measurements were taken using a flow-through technique Details about the gas sensing ce measurement system can be found in Ref [20] Briefly, the sensing system is a chamber of about liter in volume Indie the sensing chamber, two tungsten needles were used for electrical Ac connection to the device for gas-sensing measurement A series of mass flow controlled were used to control the injection of analytic gas into the sensing chamber Prior to these measurements, dry air was blown through the sensing chamber until the desired stability of sensor resistance was reached Sensor resistance was continuously measured using a Keithley (model 2602) instrument connected to a computer while switching dried air and analytic gases on and off during each cycle The total gas flow rate was 400 sccm The sensor response is defined as S=Ra/Rg, where Ra and Rg are the resistances of the sensor in dry air and analytic gas, Page of 20 respectively Details about the gas mixing system can be found elsewhere [20] In this experiment, we used the standard gas concentration of 1000 ppm H2S balanced in nitrogen and mixed with dry air as carrier using a series of mass flow controllers to obtain a lower us cr ip t concentration The gas concentration was calculated as follows: C(ppm)=Cstd(ppm)f/(f+F), where f and F are the flow rates of analytic gas and dry air, respectively, and Cstd(ppm) is the concentration of the standard gas used in the experiment The selectivity of the fabricated sensor against other gases such as CO, NH3, H2, and LPG was also studied by separately measuring the an variation in sensor resistance upon exposure to each gas Results and discussion M A photo of the fabricated sensor wafer is shown in Figure 2(A), where up to 350 sensor chips can be obtained in a 4-inch silicon wafer Each sensor chip can be cut into mm × mm A ed magnified SEM image of the center of a chip is shown in Figure 2(B) The sensing area was pt surrounded by a 20 µm-wide meander wire heater The sensing area was marked by a white square in Figure 2(C) The thin film has a porous structure and shows many rifts [Figure 2(D)] ce The porous thin film was formed from ~10 nm nanocrystals The SnO2 and CuO areas were hardly distinguished in the SEM image [Figure 2(D)] because the low difference in contrast of Ac SnO2 and CuO at this high magnification observation EDS analysis of the CuO area [white circle in Figure 2(E)] shows the existence of C, O, Cu, Si, Pt, and Sn [Figure 2(F)] The peaks of C and Si originated from contaminated carbon on the surface and silicon substrate, respectively The presence of Pt was ascribed to Pt-coating for SEM measurement O, Cu, and Sn were components of the prepared material Estimation of the composition of the area is shown in the inset of Figure 2(F) Page of 20 Figure The crystal structure of the fabricated CuO–SnO2 thin film sensor was characterized by XRD t [Figure 3(A)] All diffraction peaks can be perfectly indexed to the tetragonal rutile structure of us cr ip SnO2, coinciding with the reported data from JCPDS (card No 41-1445) No detectable peak of the CuO phase can be observed in the XRD pattern, possibly because of the low signal of the very thin catalytic layer [21] The diffraction peaks of SnO2 are very broad because of the nanocrystallinity of the fabricated thin film The average crystalline size of the SnO2 thin films was an approximately 11 nm, as calculated by the Scherrer formula using the (110) peak This value is smaller than that of previously reported SnO2 thin films fabricated by rheotaxial growth thermal M oxidation [22] Yamazoe et al [23] studied the H2S gas sensing properties of SnO2 thin films and found that the response to H2S abruptly increases for thin films prepared from sol with ed crystalline sizes larger than 10 nm Further characterization of the CuO–SnO2 thin films by Raman spectroscopy is shown in Figure 3(B) The spectrum shows two Raman modes of CuO pt (~619 cm−1) [24] and non-stoichiometric SnO2 (~669 cm−1) [25] The peak at ~669 cm-1 could ce not find in the bulk stoichiometric SnO2 where the Raman modes centered at 123 (B1g), 476 (Eg), 634 (A1g), and 778 cm-1 (B2g)) [26], but in the mixture phases of Sn and SnO2 (or non- Ac stoichiometric SnO2-δ), as reported by Wang et al [27] The Raman results indicate that the CuO islands were successfully fabricated on the non-stoichiometric SnO2 thin films Figure The transient H2S response of the bare SnO2 and nm SnO2–CuO island-sensitized thin film sensors measured at different temperatures is shown in Figure Both sensors showed similar responses; that is, their resistance decreased upon exposure to H2S gas and then normalized when Page of 20 H2S was turned off The results agree with the typical sensing characteristics of n-type semiconductor gas sensors upon exposure to a reducing gas [11] The Ra/Rg of the bare SnO2 thin film sensor increased as the operating temperature increased from 250 °C to 400 °C The us cr ip t maximum response was as low as 3.3, 5.6, and 8.5 to 1, 2.5, and ppm H2S, respectively, at 400 °C [Figures 4(A, B)] By contrast, the response of the SnO2–CuO island sensor decreased as the operating temperature increased from 250 °C to 400 °C This result indicates that CuO island sensitization reduces optimal working temperature and consequently decreases device power consumption The response of the sensor also significantly improved when sensitized with the an CuO islands [Figures 4(C, D)] The maximum responses of the SnO2–CuO island sensor were 17.3, 30.1, and 44.9 respectively for 1, 2.5, and ppm H2S at 250 ° C Conversely, the response M to ppm H2S (at 270 °C) of a SnO2–CuO multi-layer thin film microsensor is only 1.68 (or 68%) ed [26], whereas the response to 20 ppm H2S of a nanoporous CuO–SnO2 film sensor is only 39 at 250 °C [27] The SnO2–CuO island sensor also had better H2S sensing characteristics than pt recently reported ZnO, SnO2, WO3, Au–WO3, and Pt–WO3 thin film sensors [9] Similarly, Chowdhuri et al [17] significantly enhanced the H2S sensing performance of a SnO2 thin film ce sensor by sensitization with CuO islands The improved sensitivity and fast response can be Ac explained by the spill-over mechanism The p–n junctions formed between the SnO2 thin film and the CuO islands are also responsible for the enhancement of H2S sensing characteristics [28] Figure The effects of CuO island thickness on the H2S sensing characteristics of the SnO2 thin film sensor were also studied The performance of the SnO2–CuO island-sensitized thin film sensor with different CuO island thicknesses (10, 15, and 20 nm) was compared with that of the bare SnO2 thin film sensor under similar conditions The transient responses of the sensors are shown Page of 20 in Figures 5(A–C) The SnO2–CuO island sensor exhibited good response and recovery characteristics regardless of island thickness For a given island thickness, the sensor response was measured at different H2S concentrations (1, 2.5, and ppm) and temperatures (250 °C to us cr ip t 400 °C) The responses of the sensor to H2S gas at a gas concentration of 2.5 ppm and an operating temperature of 250 °C are shown in Figures 5(D) and 5(E), respectively The response to H2S gas increased with increasing CuO thickness from nm to 20 nm The maximum response of 128 (to ppm H2S) was achieved for the 20 nm CuO sample at 250 °C This result indicates that CuO sensitization can increase the gas response to ppm H2S by up to 55-fold an The relationship between gas response and operating temperature of all CuO–SnO2 thin film sensors reveals that the response increases with decreasing operating temperature regardless of M CuO island thickness The response and recovery times at the given temperature were ed approximately and 80 s, respectively Decreasing operating temperature and increasing CuO island thickness to improve sensor response further increased the response and recovery times pt (data not shown) Such attempts to further enhance the response are not compulsory because the Figure Ac [1,15,29] ce 20 nm SnO2–CuO sensor can be efficiently used to monitor H2S gas in polluted air at 250 °C Selectivity was studied as another sensor parameter We tested the selectivity of the 20 nm-thick CuO island sensor to different gases such as H2S, H2, LPG, CO, and NH3 Figure shows the response to different gases of the bare SnO2 and 20 nm CuO island-sensitized thin film sensors CuO island sensitization significantly enhanced sensor selectivity Although the concentration of interfering gases was approximately a hundred times higher than that of H2S, the sensor response to H2S gas was approximately five times higher than that to other gases The results suggest that Page 10 of 20 the CuO island-sensitized thin film sensor can be fabricated at a wafer scale to monitor highly toxic H2S in polluted air [1,4,9] us cr ip t Figure Conclusion H2S gas sensors based on SnO2 thin film sensitized with CuO islands were fabricated using conventional microelectronic technique The technique enables wafer-scale sensor array fabrication through reactive sputtering and photolithography lift-off The thickness of CuO an islands was controlled to enhance H2S sensing performance The best sensor fabricated can M detect sub-ppm level H2S concentration with high sensitivity and fast response–recovery time The sensor also exhibits good selectivity against interfering gases such as H2, LPG, CO, and Acknowledgments: pt air ed NH3, and demonstrates high potential for 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