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. 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