MNE 16 (2022) 100156
Contents lists available at ScienceDirect
Micro and Nano Engineering
journal homepage: www.sciencedirect.com/journal/micro-and-nano-engineering
MEMS based metal oxide semiconductor carbon dioxide gas sensor
H.R. Shwetha a, *, S.M. Sharath a, B. Guruprasad c, S.B. Rudraswamy b
a
Department of Electronics & Communication Engineering, J. N. N. College of Engineering, Shivamogga 577204, India
Department of Electronics & Communication Engineering, Sri Jayachamarajendra College of Engineering, Mysuru 570006, India
c
Department of Electronics & Communication Engineering, Alva’s Institute of Engineering Technology, Moodbidri 574225, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
COMSOL Multiphysics
Gas sensor
Low power
Metal oxide semiconductor
Microheater
Carbon dioxide
This paper describes the design and development of low power Micro Electro Mechanical Systems (MEMS)
microheater and metal oxide semiconductor CO2 sensor. To achieve low power, suspended plasma enhanced
chemical vapour deposited SiO2 diaphragm is used. BaTiO3-CuO is considered as metal oxide doped with 1% Ag
and will be used as a sensing material to sense the CO2 gas. To get the required temperature for the sensing film,
three different metals namely, Platinum (Pt), Titanium (Ti) and Tungsten (W) are simulated by using COMSOL
Multiphysics 5.6. The proposed microheater structure is shown to have a good temperature consistency
throughout the heater’s active region while consuming low power. The microheater geometry of 100 μm × 100
μm with its electro-thermal temperature results is presented here. For an applied voltage, we report a maximum
average temperature of Pt i.e. ~99.51%, Ti ~ 97.12% and W ~ 89.78% for 300 ◦ C respectively. Fabrication of
CO2 sensor along with MEMS microheater had been designed and demonstrated. Energy consumed by the
proposed platinum microheater geometry is 4.8 mW at 250 ◦ C and 5.8 mW at 300 ◦ C. The sensitivity charac­
teristic is based on resistance sensing which has been found to be 21% for 400 ppm CO2 gas concentration and
70% for 1000 ppm. Comparatively capacitive based sensitivity is found to be ~54% for 400 ppm and 95% for
1000 ppm.
1. Introduction
Man-made activities such as manufacturing industries, vehicles and
chemical-intensive agriculture produce hazardous substances and have
a substantial impact on human health and the green environment.
Hazardous gases include carbon monoxide, carbon dioxide, methane,
nitrogen dioxide, and many others that negatively impact the environ­
ment. Carbon Dioxide (CO2) makes a harmful impact on the ecosystem
by causing acid rain, rising global temperature and eventually affecting
human health. Therefore, carbon dioxide has traditionally been recog­
nized as one of the most serious atmospheric pollutants. Indoor air
quality monitoring and the Internet of Things (IoT) in smart homes,
cities and the healthcare sector are creating a demand for miniaturised,
low-cost CO2 gas sensors in the marketplace [1–4].
Infrared and optical-based carbon dioxide gas sensors have been
examined as recent breakthroughs. All the current solutions are
designed for indoor use and they can detect CO2 concentration levels
ranging from 30 to 70 ppm. We are working on a sensor for environ­
mental applications that can detect CO2 levels up to 1000 ppm. Table 1
gives the comparison of commercially available CO2 sensors [5]. When
comparing conventional gas sensors to Metal Oxide Semiconductor
(MOS) based gas sensors, the most prevalent issue is that other types of
sensors consume more power and are more expensive.
Therefore, reliable and cost-effective low power CO2 gas sensors are
desirable. In this regard, various research groups have been working on
different sensing technologies in order to develop low-cost, low-power,
and miniaturised sensors. MOS-based sensors have shown to be effective
in meeting the parameters listed above. Researchers have to come up
with the research findings on modelling of gas sensors, selection of
material for sensing film, morphologies of the sensing film and other
various aspects to improve the sensitivity and selectivity of the gas
sensors [6–11]. Shravanti Joshi et al. observed a significant improve­
ment in selective CO2 gas detection by utilising p-type CaO and n-type
ZnO heterostructure. The structure is synthesised at 50 ◦ C from the
conversion of Zn5(CO3)2(OH)6 using Ca(OH)2. In dry and humidified
circumstances, the enhanced gas sensing performance is obtained at an
optimum temperature of 150 ◦ C for 500 ppm of CO2 gas concentration
[12]. Abhishek Ghosh et al. describes the better CO2 detecting features
using a ZnO dopped with calcium thin film. The cross-sensitivity of
target gas was observed with H2 and CO gases. The best CO2 sensor
* Corresponding author.
E-mail addresses: shwethahr@jnnce.ac.in (H.R. Shwetha), sharathsm@jnnce.ac.in (S.M. Sharath), rudra.swamy@sjce.ac.in (S.B. Rudraswamy).
https://doi.org/10.1016/j.mne.2022.100156
Received 6 September 2021; Received in revised form 22 May 2022; Accepted 23 June 2022
Available online 4 July 2022
2590-0072/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
response (25,000 ppm) was observed at a temperature of 400 ◦ C [13].
The authors Kim, M.Y et al. has explained the CO2 sensitivity with La2O3
as an doping element with the sensing film. The sensor sensitivity has
been increased from 1.14 to 1.52 at 400 ◦ C as the La2O3 doping con­
centration was increased to 2.2 mol%, and then dropped as the amount
of La2O3 additive was further increased [14].
Mandayo, G. et al. and G Herran, J et al. have used thick sensing films
to sense the CO2 gas. In comparing the different metal oxides, thick
BaTiO3-CuO film shows the highest sensitivity toward CO2 gas. A
comprehensive investigation of various additives has been carried out,
where Ag doped with BaTiO3-CuO has been proven to be the most
effective additive in increasing the CO2 sensitivity. These thick films,
however, require a high operating temperature (>600 ◦ C) [15,16].
A carbon dioxide gas sensor based on BaTiO3-CuO metal oxide was
developed and characterised by the authors [17]. The greatest sensor
response was obtained at an ideal temperature of 250 ◦ C, with a sensi­
tivity of about 19% for 350 ppm and 80% for 1000 ppm, which is the
best result previously recorded for Ag doped BaTiO3-CuO based CO2
sensors [17].
The impact of CO2 gas on a nanostructured sensing layer composed
of BaTiO3 spheroids coated over CuO microleaves and subsequently
mixed with Ag catalyst was investigated by the authors. The equimolar
mixture of nanocomposite CuO-BaTiO3 doped with 1% Ag has a high
sensitivity toward CO2 gas. The proposed sensor shows an outstanding
selectivity (against CO, NO2, and SO2) [18] and long-term stability (180
days) at 120 ◦ C. With an increase in CO2 gas concentration from 100
ppm to 1000 ppm, the response magnitude increased linearly from 10%
to 60% [19].
In the humid air, the reaction occurs between BaTiO3 and CO2
resulting in carbonation which results in an increase in sensor conduc­
tivity. The authors [20] also mentioned that BaTiO3 sensors have a high
degree of reproducibility when it comes to 1500 ppm of CO2 gas expo­
sure at 280 ◦ C with 50% RH and the sensor sensitivity increased from
1.03 to 1.73. From the experimental results, the authors proved that the
concentration of CO2 gas is directly related to the change in resistance of
the sensing material and the sensor has a relatively stable baseline. (The
baseline is the time before the signal response at which steady state is
said to be attained) [20].
The sensing technique depends on the heterojunction created be­
tween p-type BaTiO3 and n-type CuO metal oxides has been discussed by
Tatsumi Ishihara and his team. However, adding Ag as an additive
creates a junction between metal and semiconductor which improves
the conductivity of the sensor [21]. Permittivity (ε), bandgap (Eg), work
function (φ) and affinities (χ) are all the physical characteristics of
semiconductors that differ from one to another. The performance of the
gas sensor is investigated using physical parameters of the sensing film
and changes in electrical properties of the p–n heterojunction, such as
resistance and capacitance. At thermal equilibrium conditions, a
depletion region is formed between p-type BaTiO3 and n-type CuO
heterojunction and there is a creation of capacitance in that region.
In the majority of cases, the metal oxide sensing film is sandwiched
between the interdigitated electrodes which can form a capacitive
structure. Meyer and Haeusler have patented a highly selective CO2
sensor based on the device capacitance measurement using interdigi­
tated electrodes [22]. However, in most of these situations, the effect of
nanostructures on capacitive detection of test species was not described
by the authors. According to Dutta et al., the capacitance measurement
is mostly dependent on the sensor sensing film physical parameters like
spacing, density, and vertical alignment [23–25].
As known that capacitance can be expressed as,
(1)
C = ε A/d
where ε = dielectric constant.A = Aread = distance between the elec­
trodes or thickness of the sensing filmd can also be defined as depletion
width (d = x1 + x2).x1- Depletion width of BaTiO3.x2- Depletion width of
CuO.
The mechanism of gas detection in capacitive mode is defined by the
variation in any of those parameters (ε, A, or d) when the device was
exposed to the test environment [23]. Except for the humidity sensor,
changes in electrode area are unusual. Gas molecules interacting with
the sensing film might induce changes in dielectric constant and sensing
film thickness. Changes in dielectric layer thickness (d) and dielectric
constant (ε) are thus the most promising for gas detection through
capacitance fluctuation as shown in Fig. 1 [21].
Because the change in the effective dielectric constant is the primary
concern in capacitive sensing, a similar nanostructure should produce a
similar sensing performance for a given gas; thus, regardless of the ptype or n-type of conductivity. The change should be the same if proper
nanoarchitecture is used. The fluctuation in capacitance induced by gas
exposure in p-n junction type sensors is influenced by the physical
mixing of gases rather than a change in the depletion width. (23,24).
The relationship between capacitance and the work function is,
(2)
C ≈ Φ− 1/2
Φ = Φ(BaTiO3) + Φ(CuO) = barrier voltage (Vbi )
Capacitance is inversely proportional to the work function between
the p and n-type heterojunction as a result an increase of Φ would mean
a decrease in capacitance value [7].
The key parameter in the metal oxide semiconductor based gas
sensor behaviour is the adsorption of the gas molecules on the sensing
film [26,27]. Hence, the majority of commercially available gas sensors
utilize either thick or thin metal oxide sensing film to be deposited on
the heater structure. Gas molecules can easily diffuse through the entire
thin sensing film with a porous metal oxide layer and even reach the
bottom electrodes layer. Whereas in thick sensing film gas detection
processes may only influence the outer surface of a sensing layer.
Adsorption of target gas onto the film is typically a surface phenomenon
for films with low porosity and hence the type and concentration of the
gas species only modify the surface resistance. Also, adsorption of target
gas can take place on the bulk surface when a high porous sensing layer
is used. G. Korotcenkov et al. [28] investigated the effect of sensing film
thickness on the sensor performance of a SnO2 gas sensor. From their
experiments, they have concluded that the thin film thickness influences
the sensor response, response time, and operating temperature.
The chemical reactions take place between the sensing film and the
gas molecules. The number of free electrons on the sensor film is
determined by this redox process, which happens at a high temperature.
These free electrons improve or reduce the conductivity of the sensor.
Hence integrated microheater plays a vital role in achieving the
Table 1
Comparison of available gas CO2 gas sensors.
Sl. No.
Sensor model
Technology
Measuring range (ppm)
Sensitivity (%)
Power consumption (mW)
Cost (Rs/− )
1.
2.
3.
4.
5.
6.
T6615
TGS 822
MGSL-MG811
SCD40
SCD-30
Ambient Gas Detector
Non-dispersive infrared (NDIR)
MOS
Solid Electrolyte
Non-dispersive infrared (NDIR)
NDIR
Photoacoustic Spectroscopy (PAS)
2000 ppm
50–5000
400–5000
400–5000
400–10,000
0–50,000
90
32
63
52
64
75
165
660
77.6
88
67.5
20.3
3500/−
750/−
4099/−
5369/−
6299/−
53,250/−
2
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Fig. 1. MOS sensor model based on capacitive mechanism a) without target gas b) with target gas.
adequate temperature in the reaction. The parameters like geometry, the
type of material, the stability of the material and power consumption are
necessary to consider in the design of a microheater. Various researchers
have come up with different geometries to ensure low power. Table 2
depicts the survey of different researchers on the geometry of micro­
heaters and their power consumption.
The power efficiency (mmo2*K/mW) is defined as the temperature
increases when 1 mW of power is applied to a microheater with an
equivalent area of 1mm2. The power efficiency of recently surveyed
microheaters is summarized in Table 2 and Table 3. To calculate the
efficiency (mmo2*K/mW) multiply the heater area by the temperature
increase from room temperature, then divide by the power consump­
tion. Tables 2 and 3 compare the performance of microheaters of
different heater areas and of the same dimension 100 μm × 100 μm.
On comparing the above performance of the microheater, Table 2
shows that there is a general tendency toward higher power efficiency in
microheaters of a large area. Siegele et al. [36] and Ali et al. [39] are
good examples of this. Due to convective heat loss to the ambient air, a
larger heater area results in more energy loss. This could be explained by
the fact that heaters with a higher surface area have a lower circum­
ference to surface ratio.
In today’s technology, we require smaller and lower power devices.
In this connection, to achieve a miniaturised sensor device the area of
the heater area must be reduced, resulting in a lower-power device.
Hence, trade-offs exist between power efficiency and power consump­
tion [31]. The uniformity of temperature distribution is affected by the
line widths and distance between lines. Improved power homogeneity
and decreased power dissipation can be achieved by reducing separa­
tion. According to the previous survey, the proposed microheater design
has a higher power efficiency and consumes less power than prior heater
designs with the same dimensions of 100 μm × 100 μm as shown in
Table 3.
Winncy Y et al. reported the issue in resistive based metal oxide
sensing mechanism and found a good prospect in the detection of CO2
gas by using capacitor metal oxide structure. Good sensor response,
selectivity and miniaturization can be achieved if we use capacitive
structures [40]. When BaTiO3 and PbO were used alone as sensing layers
and subjected to varied quantities of CO2 gas molecules, there were no
changes in capacitance values. When BaTiO3 and PbO metal oxides are
mixed in an equimolar nature with a parallel Ag electrode, the capaci­
tance value changes significantly when the sensor is exposed to 2% CO2
(20,000 ppm) and remaining 98% is inert nitrogen gas [41].
The mixed oxide CuO-BaTiO3 exhibit high sensitivity toward CO2 gas
with an optimum operating temperature is about 800 ◦ K. On the target
gas, the influence of composition CuO)x (BaTiO3)1-x has been investi­
gated. If the molar concentration is below 0.3 (x < 0.3), the capacitance
value of the device falls, and if it is >0.3 (x > 0.3), the capacitance of the
device increases. If the molar concertation is at 0.7 (x = 0.7) and above,
then it is hard to measure the capacitance value of the device. Thus, the
equimolar mixture (CuO)0.5 (BaTiO3)0.5 is optimum for the detection of
CO2 gas. In addition, the effect of catalytic Ag on the selectivity of CO2
gas with other interface gases (CO, CH4, H2, and H2O) has been observed
[42]. Forming a heterojunction of p and n-type metal oxide semi­
conductors will increase the number of sensing points. Sensor sensitivity
can be improved in this way. As a result, mixed oxides are valuable and
it is an effective material for developing the capacitive based gas sensor.
Hence, we are integrating Ag-doped with n-p heterojunction BaTiO3
and CuO metal oxide semiconductor to design an efficient, low-power,
highly sensitive, and selective MOS based sensor for the pollution
monitoring system to detect CO2 gas. Both resistive and capacitive
sensing mechanisms are used to measure the sensitivity and selectivity
of the gas sensor.
2. Experimental work
2.1. Microheater design & simulations
The operating temperature of the sensing layer determines the per­
formance of semiconductor metal oxide gas sensors. Operating tem­
perature influences the redox reactions and response rates on the surface
Table 2
Microheater geometries of different areas and their power consumption.
Geometry of microheater
Length 1
(mm)
Length 2
(mm)
Heater area
(mm2)
Achieved temperature
(oc)
Power consumption
(mW)
Efficiency (mmo2*K)/
mW
References
Meander
Serpentine Layout1Membrane A
Suspended beam
Pulsed heater
1.4
0.9
1.6
0.9
2.24
0.81
303
300
62.2
~35.8
10.19
6.33
[29]
[30]
0.3
0.35
0.3
0.054
0.09
0.0189
388
~57
7.88
9
4.20
0.077
[31]
[32]
3
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Table 3
Microheater geometries of the same area and their power consumption.
Geometry of
microheater
Length 1
(mm)
Length 2
(mm)
Heater area
(mm2)
Achieved temperature
(oc)
Power consumption
(mW)
Efficiency (mmo2*K)/
mW
References
Modified spiral
Meander
Meander
Plate
Meander
Meander
Proposed microheater
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.01
0.01
0.01
0.01
0.01
0.01
0.01
446
480
300
400
400
600
300
49.1
20.1
27.5
24
10
33
5.8
0.14
0.23
0.10
0.16
0.38
0.18
0.48
[33]
[34]
[35]
[36]
[37]
[38]
of sensing film for various target gases. Therefore, microheaters are the
key constituent in miniature metal oxide semiconductor gas sensors
because detecting the electrical properties such as a change in resistance
or capacitance of sensing film requires a high temperature. The design of
the microheater is essential to restrain the distribution of temperature
along with the active area and it should possess good heat confinement,
high stability, good fabrication yield as well as low power consumption.
To fulfil the above said key factors, one must choose proper microheater
geometry and the performance of the heater can be made effective by
choosing an ideal high resistive material.
The spiral, meander, double meander and double spiral are the most
commonly used microheater structures. In these structures, the active
area temperature remains an obstacle to gas sensor performance
although they provide average temperature uniformity across the
sensing area [43,44]. To work out these issues, we designed, simulated
and analysed the three different metals for the proposed MEMS based
microheater geometry with the aim of optimized low power and
improving their temperature uniformity. Electro-thermal simulation
using COMSOL Multiphysics 5.6 is used to design the proposed micro­
heater. The uniform distribution of temperature over the heater area and
power dissipation is briefly discussed in this paper.
Designing a heater geometry is of great importance to achieve a
preferred temperature uniformity and overall performance of a metal
oxide semiconductor sensor. Hence, heating element geometry designs
and the heating material must be chosen carefully. Temperature uni­
formity can be achieved throughout the heater area by using materials
with a high thermal conductivity [45]. Three different conductive
metals Platinum, Titanium and Tungsten are used individually as heater
elements, with a thickness of 100 nm. To reduce the conduction losses a
very thin (20 nm) silicon dioxide membrane of size 100 × 100 μm is used
as a high temperature electrical insulator, which supports the heating
film [46]. The performance of the heater is directly affected by the
electrical insulators [47]. Good homogeneity and required operating
temperature reveal the use of heater material.
The proposed microheater geometry pattern is shown in Fig. 2 and its
specifications are tabulated in Table 4.
Table 4
Dimensions of the microheater design.
Materials used
Pt/Ti/W
Area
Thickness
Width
Finger gap
100 *100 μm
100 nm
2.5 μm
5 μm
Platinum is one of the special elements in the Earth’s crust, and it has
a variety of implications, including a low density, high thermal capacity,
and high electrical conductivity. Because of its high melting point, it has
excellent temperature stability, allowing it to tolerate high temperatures
and achieve the highest temperature for the given smallest voltage.
Tungsten (W) can operate at high temperatures and has good mechan­
ical strength [48]. A titanium (Ti) layer is usually used to enhance the
adherence of the heater’s thin film [49].
2.2. Microheater simulation with mathematical formulation
In MOS based sensor, the microheater itself acts as a temperature
sensor and provides the required temperature to the sensing film.
Microheaters are used to detect the change in electrical properties of the
sensing film, which in turn detects the target gas concentration. The
following Eqs. (3) to (7) define the resistance of microheater material
and its geometry [50].
R=
ρL
A
(3)
where, R-resistance of material, which is used as a heater element,
ρ-resistivity of a metal, L-Length of the heater, A-surface area of the
heater.
The resistance of a heater and attained temperature can be related
using Eq. (4) [51].
RT = Ro [1 + α(T − To ) ]
(4)
where, RT - Resistance is determined for changed Temperature.Ro Resistance calculated at Room Temperature To.α - Microheater Tem­
perature Coefficient of Resistance (TCR).T - Temperature calculated in
o
C.
Electrostatics, Heat transfer statics, and Mechanical statics are the
three sets of major laws, which are involved in the modelling and
simulating of the microheater structure [52].
By applying joule-heating power Qj, in the below Eq. (5) & (6), the
temperature uniformity over the heater geometry can be calculated.
ρCp ∇T = ∇(k∇T) + Qj
(5)
Qj = σ|∇V|2
(6)
where, σ is the electrical conductivity of the metal, which is used as a
heating element. Cp is the heat capacity at constant pressure, k is the
thermal conductivity of the heater material and ∇T is the difference in
temperature over the microheater structure [52].
Fig. 2. Proposed microheater design geometry.
4
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
The power consumption is described as:
P=
V2
R
(7)
where P is the power, and V is the applied voltage.
For the detailed evaluation of joule heating and good thermal physics
in the heater design, COMSOL Multiphysics is utilized with all the Eqs.
(3) to (7) and the material properties of microheaters as shown in
Table 5 [52].
3. Gas sensor fabrication
For fabricating the full gas sensing device, a silicon bulk micromachined method with six mask levels had devised and refined. Fig. 3
depicts the gas sensor fabrication process flow. For sensor fabrication, a
p-type 3′′ single side polished 1–10 Ω-cm resistive silicon wafer with
crystal orientation 〈100〉 and thickness 500 μm was employed. A crosssectional and top view of the final developed gas sensor device is shown
in Fig. 4.
On both sides of the silicon wafer, SiO2 (of 1 μm thickness) is
deposited by using Plasma Enhanced Chemical Vapour Deposition
(PECVD) process. For the microheater structure, Pt of (100 nm) thick­
ness is sputtered and patterned. Again, a very thin layer of 20 nm of
PECVD SiO2 is deposited over the microheater which acts as an insulator
between sensing electrodes and the microheater. TECPORT RF Sput­
tering tool is used to deposit the BaTiO3-CuO-Ag sensing film over an
oxide silicon samples. Sensing electrodes made up of Pt or Ti are
deposited on top of the sensing materials by a sputtering process. In
order to increase the yield in the fabrication process, etch the silicon
from the backside with the help of the Deep Reactive Ion Etching (DRIE)
method. Deep silicon etching on the rear of a silicon wafer is solely done
to ensure that the microheater heats the top sensor layer rather than the
bottom substrate. This ensures that no power is lost or absorbed by the
substrate, resulting in a higher device yield.
Fig. 3. Design flow of proposed gas sensor.
stability and good adhesion properties allowing it to be used at high
temperatures for a long period of time [53]. Titanium has high resistivity
and poor heat conductivity, hence temperature distribution over the
geometry remains low. Due to the low resistivity and strong thermal
conductivity, tungsten has the highest power consumption, and this
material assures a high temperature. According to the previous litera­
ture review [29–38], it takes about 7 to 10 mW power to raise the
temperature of a microheater to 300 ◦ C. Based on the geometry of the
microheater that we have designed is compared with the geometry of
the previous literature papers as shown in Tables 2 and 3. The proposed
microheater geometry consumes less power i.e. 5.8 mW power to raise
the temperature to 300 ◦ C.
4. Results and discussion
4.1. Microheater material selection effects on MOS gas sensors
The temperature uniformity of the microheater was simulated and
compared for three different voltages: 0.1, 0.5, and 1 V respectively. The
three different heater materials (Pt, Ti and W) of the same heater ge­
ometry are simulated and the results are recorded. The heater geometry
of different materials is simulated against different voltage levels and
their corresponding temperature and power consumptions are tabulated
in Table 6.
Fig. 5 shows the simulation results of temperature distribution over
the platinum heater structure, and the heater power consumption.
On comparing the simulated outcomes of several heating materials,
platinum is chosen as the best heating metal for a microheater as it gives
a uniform distribution of temperature over the active area (300 ◦ C for
~99.51% of heater area). Platinum has a high sheet resistance, thermal
4.2. Material characterization
After the deposition of BaTiO3-CuO-Ag sensing film, X-ray diffraction
has been carried out to check the structure and phases of the sensing
film. As sensing film is made up of three different phases XRD shows the
average grain size of tens of nanometres. The structure of BaTiO3 is
tetragonal, CuO is monoclinic, and Ag is a cubic crystal system. The XRD
pattern is shown in Fig. 6. The grain sizes are calculated by using the
Debye Scherrer equation and are in the range of 30 to 50 nm.
The chemical compositions of the BaTiO3-CuO-Ag sensing film have
been analysed using an X-ray photoelectron spectroscope (XPS). The
XPS pattern for Ba, Ti, Cu, O and Ag is as shown in Fig. 7 and found that
the low intense carbon is the only contamination on the sensing film
surface.
The Titanium 2p area with peak electron volt is depicted in Fig. 8 (a)
[54]. Binding energy of Barium is 779.8 eV for 3d5/2 peak is as shown in
Fig. 8 (b) [55]. The Oxygen 1 s line shows the two peaks, one with
BaTiO3 at 526.7 eV and another with CuO at 528.6 eV as shown in Fig. 8
(c) [56–58]. Copper 2p have a peak at 933.8 eV which corresponds to
CuO as shown in Fig. 8 (d) [59]. The silver 3d plot is depicted in Fig. 8 (e)
and corresponds to elemental Ag, with the Ag 3d5/2 situated at a binding
energy of 368.4 eV [60].
Scanning Electron Micrographs (SEM) images of sputter-deposited
BaTiO3-CuO doped with 1% Ag sensing film is as shown in Fig. 9.
Table 5
Material Properties for COMSOL Simulations.
Parameter
Titanium
(Ti)
Tungsten
(W)
Platinum
(Pt)
Heat Capacity [J/kg*K]
Young’s modulus(E) [Pa]
Thermal expansion coefficient(α)
[1/K]
Thermal conductivity(k) [W/
(m*K)
Density (ρv) [kg/m^3]
Electric conductivity(σ) [S/m]
Electric resistivity[Ωm]
522
115.7e9
8.60e-6
132
0.036e9
4.5e-6
133
168e9
8.80e-6
21.9
175
71.6
4506
2.6e6
42e-8
17,800
1.5e6
5e-8
21,450
8.9e6
0.34e-8
5
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Fig. 4. Developed MOS gas sensor (a) cross-sectional view (b) top view.
capacitance of sensing film respectively. Ra and Ca denote the resistance
and capacitance of sensing film in air respectively. Rg and Cg denote the
resistance and capacitance values of sensing film after exposure to the
test gas respectively [63]. The Matlab R2010a application is used to
record real-time monitoring of the sensor’s signal, with data acquisition
occurring every second. The response time is determined by calculating
the time taken by the sensor from 10% to 90% to reach the steady state
resistance or capacitance value when CO2 gas is given.
The response of the CO2 sensor is observed with the variation in
temperature which has been shown in Fig. 11. The changes that occur in
the capacitance of the sensing film are used to measure the response of
the sensor. The CO2 concentration was set at 500 ppm and Ag catalytic
doping was varied between 1% to 5%. CO2 sensitivity is shown to be
strongly dependent on the operating temperature. At low temperatures,
the activation energy for the reaction of the sensing film is lowered. At
high temperatures, rather than CO2 adsorption desorption may become
dominant. As a result, when the operating temperature rises above an
ideal threshold, CO2 sensitivity reduces. The temperature at which
maximum sensitivity is achieved is found to be 300 ◦ C.
In order to see the resistance behaviour of the proposed platinum
microheater, the current and voltage graph is analysed with respect to
temperature this has been shown in Fig. 12.
The real-time temperature of the of the proposed platinum micro­
heater is monitored by using closed feedback temperature controlled
cascade probe station with a ± 1 ◦ C of accuracy. The resistance change
factor per degree Celsius of temperature change (TCR) (α) is found to be
1.45 × 10− 3/◦ C.
Fig. 13 describes the correlation between the power needed to raise
the proposed microheater temperature and which only requires 5.85
mW to reach 300 ◦ C. As a result, using the cascade probe station the
microheater can be approximately configured to reach the desired
temperature.
When exposed to increasing CO2 concentrations, the sensor may
recover quite well, except for the baseline drift, as shown in Fig. 14. The
greatest sensitivity was recorded for resistance sensing is 21% for 400
ppm CO2 gas concentration and 70% for 1000 ppm (CO2 gas concen­
tration), comparatively capacitive based sensitivity is found to be ~54%
for 400 ppm (CO2 gas concentration) and 95% for 1000 ppm (CO2 gas
concentration).
MOS based gas sensors are typically responsive to several stimuli and
exhibit cross sensitivity over different gasses. Therefore, selectivity is a
critical parameter that may be calculated by comparing selectivity or
cross sensitivity or the response to interfering gases. The crosssensitivity of the CO2 sensor in the presence of additional gas such as
CO, NO2, SO2 species is shown in Fig. 15. Single test gas was let into the
testing chamber and compared the sensitivity of each tested gas. The bar
graphs depict the sensor selective responses to several target gases.
Under the typical concentration of interfering gases for ambient air
quality monitoring applications, the sensor response to other gases is
extremely low, implying that the BaTiO3-CuO-based gas sensor has
excellent selectivity for CO2.
Table 6
COMSOL simulation results of microheater geometry.
Microheater material
Power consumption(mW)
Temperature (◦ C)
Platinum
3
4.8
5.8
7.1
4.3
6.1
7.6
9.7
10.4
13.6
17.5
21.1
200
250
300
350
200
250
300
350
200
250
300
350
Titanium
Tungsten
Below the figure reveals no defective zones and the surface with
spherically shaped grains of homogeneous metal oxides. It also shows
that the sensing film is made of fine crystallites with higher porosity.
Before the integration of the gas sensor on the device platform, the
sensing material deposition process parameters and film thickness are
fine-tuned to achieve selectively high responsiveness to each of the
target gases [61,62]. For BaTiO3-CuO with 1% Ag dopant, the film
thickness has been optimized for a high and selective response of 150
nm.
4.3. Gas sensing results
A gas calibration chamber is utilized to integrate the gas sensor. The
developed gas sensor is shown in Fig. 10(a). Fig. 10(b) shows the sensor
system, which encompasses computer-controlled measuring equipment,
an electrometer from Keithley Instruments in the United States, and
mass flow controllers from Bronkhorst F-201CV. To take real-time
sensor measurements, the electrometer communicated with a com­
puter running Labview 2020 software and a GPIB card from National
Instruments. A temperature controller with a K-type thermocouple and a
temperature range of 300 ◦ C was used to monitor the operating tem­
perature of the sensor sensing film. To introduce and release the gas, the
gas detecting chamber has one inlet and outlet. For CO2 sensing, the LGS
SAW resonator baseline resonant frequency is first stabilised in a steady
flow of synthetic air at a specific operating temperature. Then, utilising
mass flow controllers (MFCs), a precise amount of CO2 is introduced into
the chamber.
The sensitivity of the gas sensor has been characterised as,
R(%) =
Rg − Ra
ΔR
X100 (or)R(%) =
X100
Rair
Ra
(8)
C(%) =
Cg − Ca
ΔC
X100 (or)C(%) =
X100
Cair
Ca
(9)
where R and C denote the sensitivity due to the changes in resistance and
6
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Fig. 5. (a) Temperature profile of platinum microheater (b) Power profile for platinum microheater at 250 ◦ C (c) Power profile for platinum microheater at 300 ◦ C.
Fig. 6. X-ray diffraction pattern of BaTiO3-CuO-Ag thins.
Fig. 7. X-ray photoelectron spectroscope spectrum of BaTiO3-CuO-Ag.
The sensitivity characteristics based on resistance sensing are found
to be 21% for 400 ppm and 70% for 1000 ppm, whereas capacitive based
sensitivity is found to be ~54% for 400 ppm and 95% for 1000 ppm as
shown in the above figure. Based on the characteristics, we can conclude
that capacitance-based sensitivity is superior to resistive-based sensi­
tivity because sensitivity is almost 35% higher than resistive based CO2
gas sensor. Chemical sensors of the capacitive type are noted for their
exceptional stability, selectivity, fast response and sensitivity to a wide
range of analytes and capacitive based MOS sensors require low DC
power [61,62]. As a result, we have attempted to enhance the perfor­
mance of chemical sensors by combining the properties of chem­
icapacitor sensors with the benefits of low-dimensional materials with a
high surface-to-volume ratio [63].
To study the effect of humidity on the response of the fabricated gas
sensor, CO2 is introduced in the gas chamber with the change in relative
humidity from 10% to 80% RH. A commercial ultrasonic humidifier JR
7
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Fig. 8. XPS regions of a) The titanate b) The barium c) The oxygen d) Copper and e) silver.
Fig. 9. SEM micrographs of sputtered thin sensing metal-oxide film.
model JRG provides the necessary humidity for calibration. The hu­
midifier uses a 1.6 MHz oscillator with a 100 V output voltage. After the
pump has worked for 12 h to achieve a stable RH in the test chamber, the
measurement procedure begins. By supplying a DC voltage to the inte­
grated micro-heater, the temperature of the sensor can be adjusted from
room temperature to 300 ◦ C. Fig. 16 shows the reaction of the BaTiO3CuO-Ag sample to 500 ppm CO2 gas as a function of applied RH. Here,
the optimum operating temperature of 300 ◦ C was applied. The effect of
humidity on the conductance of metal oxide semiconductors is accepted
as the conductance increases with RH and the process is reversible [64].
H2O molecules provide the necessary conditions for oxygen adsorption,
electrons, and oxygen vacancies.
Thus, due to the surface distribution and coverage of hydroxyl
groups and the presence of oxygen species, the effect of RH may enhance
or reduce the conductance with response to the target gases [65].
When humidity increases from 10% to about 50% RH, the change in
the hydroxyl groups is smaller than that of the oxygen on the surface.
Therefore, it continues to increase the response. When the humidity is
higher than 50% RH, the hydroxyl group OH covers almost the surface
and limits the oxygen adsorption. Therefore, in high humidity, the
number of electrons released due to water molecule absorption is
considerably high with respect to the effect of electrons due to the
adsorption of CO2 gas being less observed. The humidity effect on the
performance of the sensor can be controlled by the temperature.
The relation between Temperature and humidity is given in the
below equation
Td = T − ((100 − RH)/5 )
(10)
whereTd= New point of temperature in degree Celsius.T = Observed
temperature in degree Celsius.RH = Relative humidity.
Humidity and temperature have an inverse relationship. When the
temperature rises, the relative humidity falls, causing the air to become
drier and when the temperature drops, the air becomes wet causing the
relative humidity to rise [66]. To maintain the operating temperature of
the sensor as a constant, we can adjust the DC voltage supply to the
integrated microheater.
8
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Fig. 10. MOS gas sensor (a) fabricated sensor device (b) gas sensor system.
Fig. 13. Temperature power relationship of the platinum microheater.
Fig. 11. Sensitivity v/s operating temperature of sensing film.
yield. The integrated platinum microheater geometry showed good
temperature uniformity and uses 4.8 mW of power to reach the tem­
perature of 250 ◦ C and 5.8 mW to reach a temperature of 300 ◦ C. The
resistive based sensitivity for CO2 gas was found to be 21% for 400 ppm
of CO2 gas concentration and 70% for 1000 ppm and the capacitive
based sensitivity for CO2 gas was found to be around 54% for 400 ppm
and 95% for 1000 ppm.
SEM examinations revealed three phases in the sputter-deposited
films, indicating the formation of p-n heterojunction between BaTiO3
and CuO. The existence of three phases in the deposited films is evident
from XRD and XPS examination, revealing that there is an increase in the
overall performance of the sensing mechanism which is due to the cre­
ation of heterojunctions between BaTiO3 and CuO by adding (1%) Ag as
a catalyst. The selectivity of CO2 gas over CO, NO2, and SO2 gases is
explored using Ag-doped BaTiO3-CuO thin films, and it has been
discovered that CO2 gas is resistant to interference from other gases.
Finally, our work has resulted in highest sensitivity till date, and
capacitive based measurement gave the best results over the resistive
based measurements.
Fig. 12. Temperature-dependent I-V plot of platinum microheater.
5. Conclusions
CRediT authorship contribution statement
The current study has focused on the design, simulation of micro­
heater and fabrication of a metal oxide semiconductor gas sensor with
1% Ag doped BaTiO3-CuO sensing film for selective CO2 gas detection.
In comparison to the thermally grown SiO2 diaphragm, fabricated sensor
devices using the PECVD SiO2 diaphragm showed a good fabrication
H.R. Shwetha: Formal analysis, Investigation, Methodology, Soft­
ware, Writing – original draft, Writing – review & editing, Validation,
Conceptualization. S.B. Rudraswamy: Project administration, Super­
vision, Writing – review & editing.
9
H.R. Shwetha et al.
Micro and Nano Engineering 16 (2022) 100156
Fig. 14. BaTiO3-CuO-Ag (1%) metal oxide response to change in CO2 gas concentration at 300 ◦ C (a) Resistive Response (b) Capacitive Response.
BaTiO3_CuO_Ag(1%) & Operating Temp: 300oC
70
CO2
10
CO
6
NO2
Sensor Capacitive based
Selectivity (%)
Sensor Resistive based
Selectivity (%)
BaTiO3_CuO_Ag(1%) & Operating Temp: 300oC
80
70
60
50
40
30
20
10
0
3
SO2
1000ppm
3ppm
0.1ppm
0.1ppm
Different test gases and its concentration
100
95
80
60
40
20
0
10
CO2
6
CO
1000ppm 3ppm
NO2
3
SO2
0.1ppm 0.1ppm
Different test gases and its concentration
(a)
(b)
Fig. 15. Sensor selectivity/cross-sensitivity response for interfering at 300 ◦ C (a) Resistive based sensor (b) Capacitive based sensor.
Science’s Centre for Nano Science and Engineering (CeNSE), Bangalore,
India.
Sensor Response (%)
Sensing Film: BaTiO3_CuO_Ag (1%)
Target Gas : 500 ppm of CO2 gas
References
70
60
50
40
30
20
10
0
[1] E. Larson, D. Abrahamson, P. Ciborowski, Effects of atmospheric carbon dioxide on
U.S. peak electrical generating capacity, IEEE Technol. Soc. 3 (1984) 4, https://doi.
org/10.1109/MTAS.1984.5009891.
[2] J.A. Bick, K.E. Holbert, An analysis of atmospheric carbon dioxide content and
impacts of global generation supply changes, North Am. Power Symp. (2012),
https://doi.org/10.1109/NAPS.2012.6336349.
[3] Y. Xie, X. Feng, J. Pang, Research about CO2 emission under the current energy
structure in Tianjin, Int. Conf. E-Product E-Service E-Entertainment (2010),
https://doi.org/10.1109/ICEEE.2010.5660365.
[4] H.G. Abadin, C.H.S.J. Chou, F.T. Llados, Health effects classification and its role in
the derivation of minimal risk levels: immunological effects, Regul. Toxicol.
Pharmacol. 47 (2007) 249–256.
[5] https://www.gassensing.co.uk/.
[6] Z. Hua, Z. Qiu, Y. Li, Y. Zeng, Y. Wu, X. Tian, M. Wang, E. Li, A theoretical
investigation of the power-law response of metal oxide semiconductor gas sensors
II: size and shape effects, Sensors Actuators B Chem. 255 (2018) 3541–3549,
https://doi.org/10.1016/j.snb.2017.09.189.
[7] J. Herran, G.G. Mandayo, E. Castano, Physical behaviour of BaTiO3-CuO thin-film
under carbon dioxide atmospheres, Sensors Actuators B 127 (2007) 370–375,
https://doi.org/10.1016/j.snb.2007.04.035.
[8] B. Liao, Q. Wei, K. Wang, Y. Liu, Study on CuO-BaTiO3 semiconductor CO2 sensor,
Sensors Actuators B Chem. 80 (2001) 208–214, https://doi.org/10.1016/S09254005(01)00892-9.
[9] C.A. Papadopoulos, D.S. Vlachos, J.N. Avaritsiotis, Comparative study of various
metal-oxide-based gas-sensor architectures, Sensors Actuators B 32 (1996) 61–69.
10 20 30 40 50 60 70 80
Humidity (RH%)
Fig. 16. Humidity Response of the CO2 Gas Sensor.
Declaration of Competing Interest
None.
Acknowledgements
The authors further appreciate the technical assistance provided by
the National Nano Fabrication Facility (NNFC), Micro and Nano Char­
acterization Facility (MNCF), and Packaging lab at the Indian Institute of
10
Micro and Nano Engineering 16 (2022) 100156
H.R. Shwetha et al.
[10] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, Capacitive-type gas sensor for the
selective detection of carbon dioxide, Sensors Actuators B Chem. 13 (1993)
470–472.
[11] P. Shankar, J.B.B. Rayappan, Gas sensing mechanism of metal oxides: the role of
ambient atmosphere, type of semiconductor and gases – a review, Sci. Lett. J. 4
(2015) 126.
[12] S. Joshi, A. Jones Lathe, Y.M. Sabri, K. Suresh, A. Bhargava, S.J.S. Ippolito, Facile
conversion of zinc hydroxide carbonate to CaO-ZnO for selective CO2 gas
detection, J. Colloid Interface Sci. 558 (2020) 310–322, https://doi.org/10.1016/j.
jcis.2019.09.103.
[13] A. Ghosh, C. Zhang, S. Shi, H. Zhang, High temperature CO2 sensing and its crosssensitivity towards H2 and CO gas using calcium doped ZnO thin film coated
langasite SAW sensor, Sensors Actuators B Chem. (2019) 301, https://doi.org/
10.1016/j.snb.2019.126958.
[14] M.Y. Kim, Y.N. Choi, J.M. Bae, T.S. Oh, Carbon dioxide sensitivity of La-doped
thick film tin oxide gas sensor, Ceram. Int. 38 (2012) 657–660, https://doi.org/
10.1016/2Fj.ceramint.2011.05.129.
[15] G.G. Mandayo, F. Gonzalez, I. Rivas, I. Ayerdi, J. Herran, BaTiO3- CuO sputtered
thin-film for carbon dioxide detection, Sensors Actuators B Chem. 118 (2006)
305–310, https://doi.org/10.1016/j.snb.2006.04.056.
[16] J. Herran, G.G. Mandayo, I. Ayerdi, E. Castano, Influence of silver as an additive on
BaTiO3-CuO thin film for CO2 monitoring, Sensors Actuators B Chem. 129 (2008)
386–390.
[17] S.B. Rudraswamy, Navakanta Bhat, Optimization of RF sputtered Ag-doped
BaTiO3-CuO mixed oxide thin film as carbon dioxide sensor for environmental
pollution monitoring application, IEEE Sensors J. (2016) 13–18, https://doi.org/
10.1109/JSEN.2016.2567220.
[18] S. Samuel, J. Ippolito, et al., Efficient heterostructures of Ag@CuO/BaTio3 for lowtemperature CO2 gas detection: assessing the role of nanointerfaces during sensing
by operando drifts technique, ACS Appl. Mater. Interfaces 9 (32) (2017)
27014–27026, https://doi.org/10.1021/acsami.7b07051.
[19] Shravanti Joshi, Samuel J. Ippolito, Efficient heterostructures of Ag@CuO/BaTiO3
for lowtemperature CO2 gas detection: assessing the role of nanointerfaces during
sensing by operando DRIFTS technique, ACS Appl. Mater. Interfaces (2021),
https://doi.org/10.1021/acsami.7b07051.
[20] Fabien Le Pennec, El Halabi, et al., BaTiO3 sensitive film enhancement for CO2
detection, IEEE Sens. (2020), https://doi.org/10.1109/
SENSORS47125.2020.9278726.
[21] Tatsumi Ishihara, Shogo Matsubara, Capacitive type gas sensors, J. Electroceram. 2
(4) (1998) 215–228.
[22] B. Mondal, B. Basumatari, J. Das, et al., ZnO-SnO2 based composite type gas sensor
for selective hydrogen sensing, Sensors Actuators B Chem 194 (2014) 389–396,
https://doi.org/10.1016/j.snb.2013.12.093.
[23] K. Dutta, N. Banerjee, H. Mishra, P. Bhattacharyya, Performance improvement of
Pd/ZnO-NR/ Si MIS gas sensor device in capacitive mode: correlation with
equivalent-circuit elements, IEEE Trans. Electron Dev. 63 (2016) 1266–1273,
https://doi.org/10.1109/TED.2016.2520020.
[24] K. Dutta, A. Hazra, P. Bhattacharyya, Ti/TiO2 nanotube array/Ti capacitive device
for nonpolar aromatic hydrocarbon detection, IEEE Trans. Device Mater. Reliab. 16
(2016) 235–242, https://doi.org/10.1109/TDMR.2016.2564447.
[25] K. Dutta, B. Bhowmik, P. Bhattacharyya, Resonant frequency tuning technique for
selective detection of alcohols by TiO2 nanorod based capacitive device, IEEE
Trans. Nanotechnol. (2017), https://doi.org/10.1109/TNANO.2017.2670661.
[26] T. Simon, N. Barsan, M. Bauer, U. Weimar, Micromachined metal oxide gas sensors:
opportunities to improve sensor performance, Sens. Actuator B Chem. 73 (2001) 1.
[27] G. Korotcenkov, V. Brinzari, A. Cerneavschi, M. Ivanov, V. Golovanov, A. Cornet,
J. Morante, A. Cabot, J. Arbiol, The influence of film structure on In2O3 gas
response, Thin Solid Films 460 (2004) 315.
[28] G. Korotcenkov, B.K. Cho, Thin film SnO2-based gas sensors: film thickness
influence, Sensors Actuators B Chem. 142 (2009) 321–330.
[29] Bedoui Souhir, Gomri Sami, et al., Design, simulation, and optimization of a
meander micro hotplate for gas sensors, Trans. Electr. Electron. Mater. 17 (4)
(2016) 189–195. August 25.
[30] Andrea Gaiardo, David Novel, et al., Optimization of a low-power chemoresistive
gas sensor: predictive thermal modelling and mechanical failure analysis, Sensors
21 (3) (2021) 783, https://doi.org/10.3390/s21030783.
[31] E. Nakamura, K. Matsumoto, A. Fasoli, L. Bozano, H. Mori, Study of design
optimization method for ultra-low power micro gas sensor, in: 2019 IEEE 69th
Electronic Components and Technology Conference (ECTC), 2019, pp. 417–422,
https://doi.org/10.1109/ECTC.2019.00070.
[32] M. Ahmed, W. Xu, S. Mohamad, F. Boussaid, Y. Lee, A. Bermak, Fully integrated
bidirectional CMOS-MEMS flow sensor with low power pulse operation, IEEE
Sensors J. 19 (9) (2019) 3415–3424.
[33] Jagdeep Rahul, Kurmendra, Analysis of MEMS based micro-hot plate for gas
sensor, J. Semiconductor Dev. Circuits 6 (2) (2019).
[34] P.B. Deo, J. Technol. Adv. Sci. Res. 2 (1) (2016) 79.
[35] M.Y. Afridi, et al., IEEE Sensors J. 2 (6) (2002) 644.
[36] M. Siegele, et al., Proc. 11th NEWCAS, 2013.
[37] M.Y. Afridi, J. Geist, Proc. IEEE Sens. 2159 (2010).
[38] C. Tsamis, et al., Sensors Actuators B Chem. 95 (1–3) (2003) 78.
[39] S.Z. Ali, et al., J. Microelectromech. Syst. 17 (6) (2008) 1408.
[40] Y. Du Winncy, W. Scott, Resistive and capacitive based sensing technologies,
Yelich, Sensors Actuators 90 (2019) 4–8. https://citeseerx.ist.psu.edu/viewdo
c/download?doi=10.1.1.405.9708&rep=rep1&type=pdf.
[41] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, Sensors Actuators B 5 (1991) 97,
https://doi.org/10.1023/A:1009970405804.
[42] Ishihara Tatsumi, Kometani Kazuhiro, Mizuhara Yukako, et al., Mixed Oxide
Capacitor of CuO—BaTiO3 as a New Type CO2 Gas Sensor, J. Am. Ceramic Soc. 75
(3) (1992) 613–618, https://doi.org/10.1111/j.1151-2916.1992.tb07850.x. In this
issue.
[43] P. Kahroba, I. Mirzaee, P. Sharifi, H. Shirvani, The microcavity-based micro-heater:
an optimum design for micro-heaters, Microsyst. Technol. 14 (2008) 705–710,
https://doi.org/10.5555/3110946.3111260.
[44] J.M.B.U. Moon, C.H. Lee, M.B. Shim, J.H. Lee, D.D. Lee, J.H. Lee, Lee., Silicon
bridge grill micro-gas sensor array, Sensors Actuators B Chem. 108 (2005)
271–277, https://doi.org/10.1016/2Fj.snb.2005.01.042.
[45] J. Kathirvelan, R. Vijayaraghavan, Design, simulation and analysis of platinum
microheaters on Al2O3 substrate for sensor applications, J. Eng. Appl. Sci. 9 (2014)
2307–23014.
[46] G. Saxena, R. Paily, Performance improvement of square microhotplate with
insulation layer and heater geometry, Microsyst. Technol. (2015) 21, https://doi.
org/10.1007/s00542-014-2337-y.
[47] N. Kumar, N. Mehta, Applications of different numerical methods in heat transfer-a
review, Int. J. Eng. Sci. Res. Technol. 4 (2013) 7–12, doi:10.1.1.413.5465.
[48] Z.S. Ali, F. Udrea, W.I. Milne, J.W. Gardner, Tungsten-based SOI microhotplates for
smart gas sensors, J. Microelectromech. Syst. (2008) 17–24, https://doi.org/
10.1109/JMEMS.2008.2007228.
[49] A.Z. Sadek, K. Kalantar-Zadeh, W. Wlodarski, W.L. Hughes, B. Buchine, Z.L. Wang,
H2 and NO2 gas sensors with ZnO nanobelts layer on 36 LiTaO3 and LiNbO3 SAW
transducers, Proc. IEEE Sens. (2005) 1343–1346, https://doi.org/10.1109/
ICSENS.2005.1597956.
[50] D.V. Guruviah, Design of microheaters with better thermal management for sensor
applications, Int. J. Mech. Eng. Technol. 8 (2017) 8–10.
[51] G. Sureshkannan, G.M. Kumar, Experimental investigation and 2- D thermal
analysis of high temperature microtubular heater, Eur. J. Sci. Res. 74 (2012)
403–411.
[52] H. Kumar, K.K. Singh, N. Sood, A. Kumar, Design and simulation of a micro
hotplate for MEMS based integrated gas sensing system, IEEE Sens. Appl. Symp.
(2014) 800–978, https://doi.org/10.1109/SAS.2014.6798942.
[53] A. Giberti, M.C. Carotta, V. Guidi, C. Malagu, L. Milano, Influence of ambient
temperature on electronic conduction in thick-film gas sensors, Sensors Actuators B
Chem. 137 (2009) 111–118, https://doi.org/10.1016/j.snb.2008.10.017.
[54] M. Murata, K. Wakino, X-ray photoelectron spectroscopic study of perovskite
titanates and related compunds: an example of the effect of polarization on
chemical shifts, J. Electron Spectrosc. Relat. Phenom. 6 (1975) 459–464.
[55] P. Pertosa, F.M. Michel-calendini, X-ray photoelectron spectra, theoretical band
structures, and densities of states for BaTiO3 and KNbO3, Phys. Rev. B 17 (1978)
2011–2020.
[56] C. Proust, C. Miot, E. Husson, Characterization of BaTiO3 powder obtained by a
chemical route, J. Eur. Ceram. Soc. 15 (1995) 631–635.
[57] J.A. Anderson, J.L.G. Fierro, Bulk and surface properties of copper-containing
oxides of the general formula LaZr1-XCuXO3, J. Solid State Chem. 108 (1994)
305–313.
[58] J. Morales, L. Sanchez, F. Martın, J.R. Ramos-Barrado, Use of low temperature
nanostructured CuO thin films deposited by spray-pyrolysis in lithium cells, Thin
Solid Films 474 (2005) 133–140.
[59] B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, Surface spectroscopic
characterization of Cu/A1203 catalysts, J. Catal. 94 (1985) 514–530.
[60] M. Romand, M. Roubin, J.P. Deloume, X-ray photoelectron emission studies of
mixed selenides AgGaSe2 and Ag9GaSe6, J. Solid State Chem. 25 (1978) 59–64.
[61] C. Prajapati, Shekhar, N. Bhat, Detection of NO2 in PPB range: material
optimization, fabrication and packaging of WO3 film based sensor device, RSC
Adv. 8 (2018) 6590–6599, https://doi.org/10.1039/C7RA13659E.
[62] C.S. Prajapati, P.P. Sahay, Alcohol-sensing characteristics of spray deposited ZnO
nano-particle thin films, Sensors Actuators B Chem. 160 (2011) 1043–1049,
https://doi.org/10.1016/j.snb.2011.09.023.
[63] I. T., K. K., Mizuhara, Takita, A new type of CO2 gas sensor based on capacitance
changes, Sensors Actuators B Chem. 13 (1991) 97–102, https://doi.org/10.1016/
0925-4005(91)80227-B.
[64] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and
influencing factors, Sensors 10 (2010) 2088–2106.
[65] Mostafa Shooshtari, Alireza Salehi, Sten Vollebregt, Effect of temperature and
humidity on the sensing performance of TiO2 nanowirebased ethanol vapor
sensors, IoP Puublishing, Nanotechnology 32 (2021) 325501 (13 pp.), https://doi.
org/10.1088/1361-6528/abfd54.
[66] Liu Yuliang, Ni, Wu Xiang, Xi Yuan, Wei Zhang, Study on effect of temperature and
humidity on the CO2 concentration measurement, IOP Conf. Ser. Earth Environ.
Sci. 81 (1) (2017), 012083, https://doi.org/10.1088/1755-1315/81/1/012083.
11