Accepted Manuscript

Accepted Manuscript

Accepted Manuscript

Title: Scalable fabrication of SnO2 thin films sensitized with

CuO islands for enhanced H2S 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

(2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.134

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Accepted Manuscript

Highlight

►CuO island-sensitized SnO2 thin film sensors were fabricated at

wafer-scale.

►SnO2-CuO island sensors significantly enhanced H2S gas response.

►The thickness of CuO islands strongly affected on H2S gas sensing

performance.

Accepted Manuscript

Scalable fabrication of SnO2 thin films sensitized with CuO islands

for enhanced H2S gas sensing performance

Nguyen Van Toan 1 , Nguyen Viet Chien 1 , Nguyen Van Duy 1 ,

Dang Duc Vuong 2 , Nguyen Huu Lam 2 , Nguyen Duc Hoa 1,* ,

Nguyen Van Hieu 1,* , Nguyen Duc Chien 1,2,*

1)

International Training Institute for Materials Science (ITIMS), Hanoi University of Science

and Technology (HUST), Dai Co Viet Road, Hanoi, Vietnam

2)

School of Engineering Physics (SEP), Hanoi University of Science and Technology (HUST),

No 1 Dai Co Viet Road, Hanoi, Vietnam

Corresponding authors

*Nguyen DucChien, Professor,

*Nguyen Van Hieu, Associate Professor,

84 4 38680787

84 4 38692963 hieu@itims.edu.vn/chien.nguyenduc@hust.edu.vn

No 1 Dai Co Viet, Hanoi, Vietnam

Accepted Manuscript

Abstract: The detection of H2S, an important gaseous molecule that has been recently marked as

a highly toxic environmental pollutant, has attracted increasing attention We fabricate a

wafer-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

observed with 20 nm-thick CuO islands The response value (R a /R g) of the SnO2–CuO island

sensor to 5 ppm H2S is as high as 128 at 200 °C and increases nearly 55-fold compared with that

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

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

the formation of p–n junctions

Keywords: H2S gas sensor, reactive sputtering, SnO2, CuO, wafer-scale fabrication

1 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

chromatography-based 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

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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

(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]

However, bare materials do 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

significantly enhance the H2S sensing characteristics of such bare materials [12,13] This

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

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

considerable efforts to develop different metal oxides in the form of thin films for gas sensing

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

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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

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

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 5 μm to realize micro-sized gas

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

sensing performance of the sensors

2 Experimental

The schematic for the wafer-scale fabrication of H2S sensor arrays based on CuO

island-sensitized SnO2 thin films (noted as CuO–SnO2 thin films) is shown in Figure 1 The sensor

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 2 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

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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 5 nm/min,

thus by controlling the deposition time of 1, 2, 3, and 4 min, we could control the thickness of

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 5 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

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

CuO islands; and (12) silicon backside etching to reduce the power consumption of the device

[Figure 1(B)] Finally, heat treatment was conducted at 400 °C for 2 h in air to ensure the

stability of the sensors

Sensor measurements were taken using a flow-through technique Details about the gas sensing

measurement system can be found in Ref [20] Briefly, the sensing system is a chamber of about

1 liter in volume Indie the sensing chamber, two tungsten needles were used for electrical

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=R a /R g , where R a and R g are the resistances of the sensor in dry air and analytic gas,

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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

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

variation in sensor resistance upon exposure to each gas

3 Results and discussion

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 4 mm × 4 mm A

magnified SEM image of the center of a chip is shown in Figure 2(B) The sensing area was

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)]

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

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)

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Figure 2

The crystal structure of the fabricated CuO–SnO2 thin film sensor was characterized by XRD

[Figure 3(A)] All diffraction peaks can be perfectly indexed to the tetragonal rutile structure of

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

nano-crystallinity of the fabricated thin film The average crystalline size of the SnO2 thin films was

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

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

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

(~619 cm−1) [24] and non-stoichiometric SnO2 (~669 cm−1) [25] The peak at ~669 cm-1 could

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-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 3.

The transient H2S response of the bare SnO2 and 5 nm SnO2–CuO island-sensitized thin film

sensors measured at different temperatures is shown in Figure 4 Both sensors showed similar

responses; that is, their resistance decreased upon exposure to H2S gas and then normalized when

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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 R a /R g of the bare SnO2 thin

film sensor increased as the operating temperature increased from 250 °C to 400 °C The

maximum response was as low as 3.3, 5.6, and 8.5 to 1, 2.5, and 5 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

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 5 ppm H2S at 250 ° C Conversely, the response

to 1 ppm H2S (at 270 °C) of a SnO2–CuO multi-layer thin film microsensor is only 1.68 (or 68%)

[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

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

sensor by sensitization with CuO islands The improved sensitivity and fast response can be

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 4.

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