Fabrication of Palladium Functionalized Sol-Gel
Based SnO2 Gas Sensor for H2 and CO Detection
Meitham Amereh1, Pouria Mehrabi1, Reza Nadafi2, Mina Hoorfar1
1
School of Engineering, University of British Columbia, Kelowna, Canada
2
Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran
mina.hoorfar@ubc.ca
Abstract—This study presents design, micro-fabrication and The proposed design configuration presented here consists
neural network calibration of a low cost micro-gas sensor. This of six main elements which includes silicon wafer substrate,
sensor is able to identify two gases (CO and H2) and their electrical and thermal insulators, micro-heater, sensing layer
concentrations in a mixture. Using the micro-fabrication process and Au electrodes. The Tin-Oxide solution is prepared by Sol-
the sensitivity of the sensors has increased while decreasing the Gel method [18], and is deposited using the Pechini rout
fabrication cost. Using the Pechini route technique, the sensing
technique [19] for formation of the sensing layer. The low
layer is fabricated based on a layer of metal oxide semiconductor
(SnO2) which is doped with palladium. Sensing layer surface pressure chemical vapor deposition (LPCVD) method [20] is
morphology has been characterized (using Differential Scanning then used to deposit N2O3 and SiO2, and electrodes and pads are
Calorimetry (DSC), Scanning Electron Microscopy (SEM), and X- then patterned using photolithography. To detect the target
Ray Diffraction (XRD)) to study (i) the molecular structure of the analytes within a mixture, a neural network calibration
sensing layer fabricated under various conditions, and (ii) the technique is employed. In this technique, 1000 predetermined
effect of palladium doping on the crystalline structure. Following data is used to apply the back-propagation training algorithm
our previous study, the neural network calibration is employed to by which the behavior of the sensor is trained to detect target
perform identification process. Four different concentrations of gases in a complex compound. Appropriate responses have
Co and H2 are exposed to the fabricated sensor and the results of
been reported in our previous study [1].
output voltages show the high sensitivity of the sensing layer.
II. FABRICATION
Keywords— gas sensor, metal oxide semiconductor, micro
fabrication, chemical vapor deposition. As the first step in the micro fabrication process, a
300µm300µm silicon die made of a double-sided polished
I. INTRODUCTION wafer with the orientation of <100> and thickness of 375µm is
Gas sensors have widely been used in many applications due used as the substrate. The substrate is placed in an environment
to their small size, low cost, and high sensitivity. Surface saturated with Oxygen with the temperature of 1000ºC until the
acoustic wave (SAW) devices [2, 3], micro-machined 450nm of oxide layer is thermally created on both sides. Then,
cantilevers [4], conducting polymer sensors [5-7], and metal 280nm of nitrogen oxide is deposited on both sides at 770ºC,
oxide semiconductor sensors [8-10] are examples of developed using LPCVD. As the next step, 210nm of nickel is deposited
gas sensors for different applications. Chiou et al. [11] designed using the sputtering process to form the heater. As an electrical
SnO2 gas sensor with a TaN micro-heater. They measured the insulator, 325nm of silicon oxide is deposited on the nickel
sensor responses at various operating temperatures in order to layer using the plasma-etched chemical vapor deposition
determine the optimum operating temperature for detection of (PECVD) method. In order to prevent formation of cracks or
H2S. Also, Pin et al. [12] studied a micro gas sensor with SnO2- bubbles during the fabrication process, the layer is annealed
NiO sensitive film fabrication to detect indoor formaldehyde. under the temperature of 700ºC in an environment saturated
with nitrogen for 24 hours. Afterwards, 10nm of Titanium seed
A prodigious number of research has been conducted on the
layer is sputtered on the structure followed by Au sputtering. A
fabrication of SnO2-based gas sensors using different methods
480nm layer of Au electrodes and pads are then patterned using
[13]. It has been shown that deposition of sol-gel based thin film
the photolithography process. Finally, the Sol-Gel solution is
is faster, simpler and cost-effective [14], [15] as compared to
deposited using the Pechini rout method to fabricate sensing
methods such as electrospinning and chemical vapor deposition
layer. This layer is then doped with palladium by chemical
[16]. Also, palladium catalytic effects and artificial micro
inoculation. In order to apply the Au pads connection, the right
structural changes occurring as the result of this type of
and left sides are etched through the wet etching process.
fabrication leads to high sensitivity toward specific gases [17].
978-1-5090-1012-7/17/$31.00 ©2017 IEEE
A B
A B
A
Fig. 3. (A) Schematic of heat transfer in entire sensor. (B) Thermal resistive
circuit of sensor.
Fig. 1. (A), (B) Overview of packaging and wiring of fabricated
sensor.
A B
A B
Fig. 2. (A) Photo of the inter-digitated Au electrodes and pads. (B) Photo
of the fabricated nickel layer (the heater).
Fig. 1 shows the sensor including pins, device, and wire
bonding. Also, Fig. 2 shows the photo of the Au electrodes/pads
and the nickel layer. The schematic of the heat transfer through
the entire sensor in addition to the thermal resistive circuit are Fig. 4. (A) XRD result for pure tin oxide. (B) XRD result for tin oxide with additive
shown in Figs. 3.A and 3.B, respectively. Thermal resistances palladium. (C) DSC results on the specimen. Red represents the pure tin oxide and
black represents the tin oxide layer doped with the additive Pd.
and rates of heat transfer through different layers are presented
in Table 1. Based on these values the efficiency and sensitivity
A B
of the sensor, ζthermal and Sthermal, are respectively calculated as
89.3% and 15.33 (K/mW). Considering these values, it is
expected that materials selected for different layers and their
dimensions lead to high performance of this sensor.
III. RESULTS
After deposition of the sensing pallet, a thorough study was
conducted to characterize the surface molecular structure of the
fabricated sensing layer. Three methods have been used:
Differential Scanning Calorimetry (DSC), Scanning Electron . C D
Microscope (SEM) and X-Ray Diffraction (XRD). As the first
step, X-Ray Diffraction tests were performed on two forms of
fabricated sensing layers, i.e., pure tin oxide and doped tin
oxide with palladium (Pd). The results are shown in Figs. 4.A
and 4.B, respectively. The presence of tin oxide and palladium
in the sample was verified after checking the peaks in the
diagrams. To compare the difference in the required reaction
heat, DSC was used for both deposited sensing layers (see the
results in Fig. 4.C). These results verify that doped layer needs Fig. 5. SEM images of specimens cured at different temperatures
and different periods. (A) Curing temperature: 700ºC, curing
less heat to start chemical reaction. Also, SEM was used to time: 3h with magnification of 500. (B) Curing temperature:
image the crystalline structure and dimensions of particles 700ºC, curing time: 6h with magnification of 20000. (C) Curing
under different preparation conditions, e.g. curing time period temperature: 900ºC, curing time: 3h with magnification of
and curing temperature (see Fig. 5). SEM images illustrate that 20000. (D) Curing temperature: 900ºC, curing time: 9h with
the higher the curing temperature and time, the more ordered magnification of 20000.
the molecular form that can be achieved. In high magnification
(Fig. 5. D), crystalline structure of the particles has a cylindrical The sensor is exposed to two gases (H2 and CO). Figs. 6.A and
shape. 6.B show the response of the sensor to different concentrations
TABLE I. THERMAL RESISTANCES AND RATES OF HEAT show the presence of Pd in doped sensing layer, but also prove
TRANSFER THROUGH SENSOR that the doped layer needs less time to start the chemical
reaction. Also, SEM images depict that more ordered molecular
Rate of
Thermal Value
heat Value (mW)
form is achieved by increasing the curing time and temperature.
resistance (K/W)
transfer Finally, the sensor was exposed to two target gases with
different concentrations. The sensor’s response showed better
R1 0.7 𝑞̇ 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑣𝑒 32.6 recovery time for H2. The size of grain crystalline is directly
proportional to the fabricated layer thickness, however the
R2 5.1 𝑈̇𝐻𝑒𝑎𝑡𝑒𝑟 24.2 sensitivity has the opposite behavior and is inversely
R3 4.3×105 𝑈̇𝑒𝑙−𝑖𝑠 3.1
proportional with crystalline size and thickness.
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