Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Nobakhti, Mohammadali
Master of Applied Science - MASc
University of British Columbia
Owing to the significant emission of Volatile Organic Compound gases (VOCs) by the oil and natural gas industry and considering the adverse health and environmental effects associated with VOCs exposure, the need for implementing accurate and real-time VOC sensors in air quality monitoring systems is crucial. In the last few decades, various methods have been developed for detecting VOC gases. Out of all of the developed methods, Metal Oxide Semiconductor (MOS) sensors are the most known for their usage as cheap, accurate, and disposable sensors for sensing VOCs in many industrial applications. Due to its high bandgap energy, tin oxide (SnO₂) is considered as one of the best materials to manufacture a MOS sensor. Up to now, extensive research has been done on optimizing the SnO₂-based gas sensor performance; however, to the best of our knowledge, less attention has been paid to optimizing an ALD-deposited SnO₂ thin film as a sensing layer. Moreover, the relationship between the target gas polarity and the mentioned optimization has not been investigated before. In this study, we present an extensive characterization of an atomic-layer-deposited SnO₂-based thin film gas sensor to detect selected polar and non-polar VOCs, namely ethanol, acetone, and toluene, for a concentration range of 20- 227 ppm. The deposited SnO₂ thin film with an approximate thickness of 40 nm showed the highest response to target VOCs, irrespective of the operating temperature or the target VOC polarity. Importantly, our results suggested that the operating temperature of the sensor was dependent on the polarity of the target analytes. The developed SnO₂-based MOS sensors can therefore be tuned to selectively differentiate between target gases based on their polarity. This can be advantageous for detecting mixtures of polar and non-polar VOCs. A good example of a practical application for selective detection of mixture of VOCs can be in Natural Gas-related industries to analyze the gas components and optimizing the odorization system. Keywords: Atomic Layer Deposition, Tin Oxide thin film, Gas sensor, Volatile Organic Compounds
OPTIMIZATION AND CHARACTERIZATION OF ATOMIC-LAYER-DEPOSITED TIN OXIDE THIN FILMS METAL OXIDE SEMICONDUCTOR (MOS) SENSORS TO DETECT POLAR AND NON-POLAR VOLATILE ORGANIC COMPOUNDS by Mohammadali Nobakhti B.Sc., University of Tehran, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in The College of Graduate Studies (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) December 2021 © Mohammadali Nobakhti, 2021 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis/dissertation entitled: OPTIMIZATION AND CHARACTERIZATION OF ATOMIC-LAYER-DEPOSITED TIN OXIDE THIN FILMS METAL OXIDE SEMICONDUCTOR (MOS) SENSORS TO DETECT POLAR AND NON-POLAR VOLATILE ORGANIC COMPOUNDS submitted by Mohammadali Nobakhti in partial fulfillment of the requirements of the degree of Master of Applied Science. Dr. Mina Hoorfar, School of Engineering Supervisor Dr. Jian Liu, School of Engineering Co-Supervisor Dr. Homayoun Najjaran, School of Engineering Supervisory Committee Member Dr. Alexander Uhl, School of Engineering Supervisory Committee Member Dr. Mahmudur Fatmi, School of Engineering University Examiner iii Abstract Owing to the significant emission of Volatile Organic Compound gases (VOCs) by the oil and natural gas industry and considering the adverse health and environmental effects associated with VOCs exposure, the need for implementing accurate and real-time VOC sensors in air quality monitoring systems is crucial. In the last few decades, various methods have been developed for detecting VOC gases. Out of all of the developed methods, Metal Oxide Semiconductor (MOS) sensors are the most known for their usage as cheap, accurate, and disposable sensors for sensing VOCs in many industrial applications. Due to its high bandgap energy, tin oxide (SnO2) is considered as one of the best materials to manufacture a MOS sensor. Up to now, extensive research has been done on optimizing the SnO2-based gas sensor performance; however, to the best of our knowledge, less attention has been paid to optimizing an ALD-deposited SnO2 thin film as a sensing layer. Moreover, the relationship between the target gas polarity and the mentioned optimization has not been investigated before. In this study, we present an extensive characterization of an atomic-layer-deposited SnO2 -based thin film gas sensor to detect selected polar and non-polar VOCs, namely ethanol, acetone, and toluene, for a concentration range of 20- 227 ppm. The deposited SnO2 thin film with an approximate thickness of 40 nm showed the highest response to target VOCs, irrespective of the operating temperature or the target VOC polarity. Importantly, our results suggested that the operating temperature of the sensor was dependent on the polarity of the target analytes. The developed SnO2-based MOS sensors can therefore be tuned to selectively differentiate between target gases based on their polarity. This can be advantageous for detecting mixtures of polar and non-polar VOCs. A good example of a practical application for selective detection of mixture of VOCs can be in Natural Gas-related industries to analyze the gas components and optimizing the odorization system. Keywords: Atomic Layer Deposition, Tin Oxide thin film, Gas sensor, Volatile Organic Compounds iv Lay Summary This work involved the characterization and optimization of thin-film gas sensors for detecting ethanol, acetone, and toluene. Aside from determining the optimal value for the fabricated sensors, the results showed that the structure of the target gas molecules strongly influences these optimum values. These findings can be utilized to enhance the sensitivity and even selectivity of all the semiconductor-based gas sensors for detection of volatile organic compounds, especially in industrial applications, such as ethanol fermentation of fruits, natural gas odorization, VOCs leak detection, and diagnostic biomarkers for a range of diseases including lung cancer, prostate cancer, and diabetes. v Preface This thesis is original research performed by the author at the Advanced Thermo-Fluidic Laboratory (ATFL) and Advanced Materials for Energy Storage Lab in the School of Engineering, the University of British Columbia, under the supervision of Dr. Mina Hoorfar and co-supervision of Dr. Jian Liu. The details of the publications are explained below: ▪ Mohammadali Nobakhti, Nishat Tasnim, Jian Liu, Mina Hoorfar, "Optimization of Atomic-Layer-Deposited SnO2 Thin Films for the Detection of Polar and Non-polar Volatile Organic Compounds", 2021, Sensors and Actuators B: Chemical, Manuscript number: SNB-D-21-05900 (under review) Credit authorship statement ▪ Mohammadali Nobakhti: Conceptualization, Methodology, Design of Experiments, Data gathering, Data analysis, Resources, Writing - original draft, Project administration Nishat Tasnim: Writing - review & editing. Jian Liu: Conceptualization, Writing - review & editing, Resources, Supervision for fabrication, Atomic Layer Deposition, material characterization, Funding acquisition. Mina Hoorfar: Conceptualization, Writing - review & editing, Resources, Supervision for gas sensing and optimization, Funding acquisition. ▪ Mohammadali Nobakhti, Nishat Tasnim, Jian Liu, Mina Hoorfar, “"Fabrication processes of SnO2 thin-film gas sensor", (review paper under preparation). ▪ Mohammadali Nobakhti, Nishat Tasnim, Jian Liu, Mina Hoorfar, "Effect of Thickness and Operating Temperature on an ALD-Deposited SnO2-Based Ethanol Sensor", (Conference Abstract, ready to submit) vi Table of Contents Abstract .................................................................................................................................... iii Lay Summary ........................................................................................................................... iv Preface ....................................................................................................................................... v Table of Contents ..................................................................................................................... vi List of Tables ............................................................................................................................ ix List of Figures ........................................................................................................................... x Acknowledgements ............................................................................................................... xiv Dedication ............................................................................................................................... xv Chapter 1: Introduction ............................................................................................................ 1 1.1 Challenges and Motivations ...................................................................................... 1 1.2 Objectives and sub-objectives .................................................................................. 2 1.2 Thesis Organization .................................................................................................. 2 Chapter 2: literature review ...................................................................................................... 4 2.1 Selected VOCs and their health and environmental effects ...................................... 4 2.1.1 Ethanol ............................................................................................................... 4 2.1.1.2 Detection application ....................................................................................... 5 2.1.2 Acetone .............................................................................................................. 5 2.1.2.2 Detection application ....................................................................................... 6 2.1.3 Toluene ............................................................................................................... 6 2.1.3.2 Detection application ....................................................................................... 7 2.2 Detection methods ....................................................................................................... 7 2.2.1 Gas chromatography ............................................................................................. 8 2.2.2 Mass spectroscopy ...............................................................................................10 2.2.3 Optical Methods ....................................................................................................11 2.2.4 Piezoelectric sensors ............................................................................................12 vii 2.2.5 Metal oxide semiconductor sensors ......................................................................13 2.3 Gas sensing mechanism and importance of thickness ................................................13 2.4 Material Selection ........................................................................................................16 2.5 SnO2 thin film deposition techniques ............................................................................18 2.5.1 Physical deposition techniques .............................................................................18 2.5.1.1 Evaporation ...................................................................................................19 2.5.1.2 Sputtering ......................................................................................................21 2.5.1.3 Ion plating ......................................................................................................23 1.5.1.4 Rheotaxial Growth and Thermal Oxidation (RGTO) .......................................25 2.5.2.1 Solution-phase chemical deposition ...............................................................26 2.5.2.1.1 spin coating .............................................................................................27 2.5.2.1.2 Dip coating ..............................................................................................28 2.5.2.1.3 spray pyrolysis ........................................................................................30 2.5.2.2 Gas-phase chemical deposition .....................................................................32 2.5.2.2.1 Chemical Vapor deposition (CVD) ..............................................................32 2.5.2.2.2 Atomic Layer Deposition (ALD) ...............................................................35 2.6 Influential parameters on sensing performance of MOS sensors .................................40 2.7 Previous studies on optimizing thickness and operating temperature ..........................43 Chapter 3: Experimental Setup .............................................................................................. 45 3.1 Synthesis of SnO2 thin films by ALD ............................................................................45 3.2 Material structural characterization ..............................................................................45 3.3 Fabrication of MOS gas sensors..................................................................................46 3.4 Gas sensing data collection .........................................................................................47 3.4.1 Pre-test preparations ............................................................................................47 3.4.2 Test ......................................................................................................................48 3.4.3 Sources of error ....................................................................................................49 viii Chapter 4: Results and discussion ........................................................................................ 50 4.1 Material Characterization .............................................................................................50 4.1.1 Thickness measurement .......................................................................................50 4.1.1 XPS results ...........................................................................................................51 4.2 Operational Optimization of sensing temperature and the effect of polarity .................53 4.2.1 Experimental results .............................................................................................53 4.2.2 Mathematical modeling .........................................................................................57 4.3 Operational optimization of sensing layer thickness ....................................................59 4.4 Characterization of optimized sensor ...........................................................................60 Chapter 5: Conclusions and future works ............................................................................ 62 5.1 Summary .....................................................................................................................62 5.2 Contributions ...............................................................................................................63 5.3 Limitation and Future Works ........................................................................................63 Bibliography ............................................................................................................................ 65 ix List of Tables Table 1- Summary of SnO2-based MOS sensors for VOC detection .........................................17 Table 2- Gas sensor response of tin oxide-based combinations ...............................................31 Table 3- Summary of the influential parameters and the respective results of the reviewed paper .................................................................................................................................................41 Table 4- Element concentration of O, Sn, and C in the ALD-deposited SnO2 samples measured by XPS. .....................................................................................................................................53 x List of Figures Figure 1- 2D atoms configuration of ethanol molecules, the angles between atoms are not presented realistically................................................................................................................. 5 Figure 2- 2D atoms configuration of acetone molecules. The angles between atoms are not presented realistically. ................................................................................................................ 6 Figure 3- 2D atoms configuration of toluene molecules. The angles between atoms are not presented realistically. ................................................................................................................ 7 Figure 4- Gas sensing and separation mechanism of Gas Chromatography method. Molecules B has less interaction with the stationary phase. Consequently, they are faster [20] .................. 9 Figure 5- schematic of a complete gas chromatograph, including carrier gas supply, gas filter, flow control, sample injector, GC column, GC oven, detector, and computer to control and analyze the collected data [20] ..................................................................................................10 Figure 6- schematic of Mass spectroscopy technique to analyze and monitor a gas components based on the mass difference [20] ............................................................................................11 Figure 7- schematic of the sensing mechanism in one of the optical gas sensing methods [21] 12 Figure 8- schematic of a piezoelectric gas sensor, including sensing layer, piezoelectric substrate and metal thin film IDTs [23] ......................................................................................13 Figure 9- gas sensing mechanism of a metal oxide semiconductor sensor (n-type), (a) when it is exposed to the fresh air and ionized oxygen adsorbed to the surface. (b) when it is exposed to the target gas and the target gas will react with the adsorbed oxygen on the surface ...............15 Figure 10- Electrical circuit for a MOS gas sensor [37] ..............................................................16 Figure 11- schematic of vacuum-evaporation process, in this case electron beam used as the evaporator [56] ..........................................................................................................................20 Figure 12- (a) Sensitivity as a function of operating temperature, (b) comparing different samples with different thicknesses and deposition conditions [57] ............................................21 Figure 13- schematic of the vacuum-sputtering process [59].....................................................22 xi Figure 14- Sensitivity (%) as a function of operating temperature for pure SnO2 and Pt doped SnO2 [62] ..................................................................................................................................23 Figure 15- schematic of Ion plating process [63] .......................................................................24 Figure 16- comparing gas sensor response as a function of CH4 concentration between undoped SnO2 and Ca doped SnO2 , deposited through Ion Plating [66] ...................................25 Figure 17- categorizing the chemical deposition methods, inspired by [71] ...............................26 Figure 18- three phases of spin coating deposition process: dropping the solution, rotating, and evaporating [72] ........................................................................................................................27 Figure 19- schematic of dip coating deposition process, including three stages: dipping, withdrawing, and drying [77] ......................................................................................................28 Figure 20- Comparison between pure and NiO/SnO2 composite gas sensor [82] ......................30 Figure 21- schematic diagram of spray pyrolysis deposition process [87] .................................31 Figure 22- Sn(DMA)4 chemical formula .....................................................................................34 Figure 23- critical steps in a typical CVD process [96] ...............................................................34 Figure 24- Four steps of ALD process in one cycle ...................................................................37 Figure 25- Cross-sectional SEM of an ALD deposited high aspect ratio hole with 190 ALD cycles [107] ...............................................................................................................................37 Figure 26- Cross-sectional TEM of a nano laminated structure deposited by ALD [108] ...........38 Figure 27- Gas sensor response of a tin oxide film toward 250 ppm of CO in different sensing temperatures [109] ....................................................................................................................39 Figure 28- Schematic representative of (a) the fabricated MOS gas sensor using KMHP-100 MEMS Micro-Hotplate. 1-4 are the electrical pins connecting the heater and sensing electrodes to the electrical measurement circuits, 5 is the gas sensor chip. (b) top view of gas sensor chip before deposition, 1: sensing electrodes, 2: heater electrodes, 3: sensing layer location. (c) cross-sectional view of gas sensor chip after deposition, 1: deposited SnO2 as the sensing material 2: patterned Pt microheater 3: sensing electrodes. (d) measurement electrical circuits, xii pins 1 and 2 are connected to the sensing layer, and resistance change is measured by measuring the Vout; pins 3 and 4 are connected to the heater. (e) an example of a normalized gas response diagram with three exposures to a target gas......................................................47 Figure 29- (a-d) Cross-sectional view and (e-h) top-view SEM images of 100-SnO2, 200-SnO2, 300-SnO2, and 400-SnO2 deposited on Si/Si3N4 substrates by ALD at 100 °C using TDMASn and H2O as the precursors. .......................................................................................................50 Figure 30- (a) SnO2 film thickness as a function of ALD cycle numbers on Si/Si3N4 substrates deposited at 100 °C. The film thickness was measured by both SEM and profilometer. A linear fitting yields a growth per cycle (GPC) of ~ 0.21-0.25 nm; (b) The weight percentage of Sn and O in the ALD-SnO2 samples as a function of ALD cycle numbers. ............................................51 Figure 31- (a) XPS full survey, and deconvolution of (b) Sn 3d and (c) O 1s spectra for 100-SnO2, 200-SnO2, 300-SnO2, and 400-SnO2 on the Si/Si3N4 substrates. ....................................52 Figure 32- Deconvolution (a) Sn 3d, (b) O1s, and (c) C 1s for 400-SnO2 on Si/Si3N4 substrates as a representative example. ....................................................................................................53 Figure 33- (a-d) gas sensor response (Ra/Rg) of the deposited SnO2 thin films toward 227 ppm of ethanol, acetone, and toluene in different operating temperatures (microheater input voltages) (a) for sample 100-SnO2 (b) 200-SnO2 (c) 300-SnO2 (d) 400-SnO2. (e) the optimum operating temperatures (microheater input voltages) to achieve the highest gas sensor response, in all different thicknesses, for ethanol (Topt= 325 °C), acetone (Topt= 295 °C), and toluene (Topt= 265 °C). "rp" represent "relative polarity of the target gas to water molecules." (f) gas sensor resistance of the deposited thin films in the air in different operating temperatures (microheater input voltages)...........................................................................................................................54 Figure 34- The trend of gas sensor response (Ra/Rg) of 200 cycles deposited SnO2 thin films toward 227 ppm of Ethanol, Acetone, and Toluene as a function of the operating temperature.57 xiii Figure 35- (a-c) gas sensor response of the deposited SnO2 thin films toward 227 ppm of Ethanol, Acetone, and Toluene in different thickness (ALD cycles) (a) heater voltage as 5 V (b) heater voltage as 4.5 V (c) heater voltage as 4 V ......................................................................59 Figure 36- Gas sensor performance based on the optimum deposited sensing layer (sample 200- SnO2): (a) sensitivity values for ethanol, acetone, and toluene are 0.0277, 0.0178, and 0.0204, respectively. (b) Long-term stability performance for sample 200-SnO2. (c-e) Short-term stability and repeatability performance for sample 200-SnO2 towards (c) ethanol, (d) acetone, and (d) toluene. For each target gas, their optimum heater voltage has been used to collect the data. ..........................................................................................................................................61 xiv Acknowledgements Foremost, I am incredibly grateful to my advisors Dr. Mina Hoorfar and Dr. Jian Liu, for the continual support of my Master's study and research and their patience, motivation, enthusiasm, and profound field knowledge. Throughout the research and writing of this thesis, I was guided by them. It was an honor to work with my advisors as a Research Assistant. Moreover, my thesis committee, including Dr. Najjaran and Dr. Uhl, deserve my appreciation. Their encouragement, thoughtful comments, and difficult questions helped guide my work. Nishat Tasnim deserves my appreciation. In addition to her role as our lab manager, she has been a valuable resource to me in regard to writing and organizing my publications. I thank my fellow lab mates in the Advanced Thermo-Fluidic Laboratory (ATFL) and Advanced Materials for Energy Storage Lab for all the guidance, discussions, and fun we have had in these two years. In particular, I am grateful to Eva Gaggioli, my friend who selflessly helped me on this thesis and other publications. Lastly, but certainly not least, I wish to thank my parents, Mohammad and Zahra, for giving me life in the first place and supporting me spiritually throughout it all. xv Dedication 1 Chapter 1: Introduction Real-time monitoring of volatile organic compounds (VOCs) using low-power and miniaturized gas sensors is crucial for various health and environmental applications. VOCs are the primary causes of many environmental concerns as they contribute to stratospheric ozone depletion and, consequently, climate change, endanger the existence of susceptible wildlife, and more. Moreover, human exposure to some VOCs, even at the sub-ppm level of concentration, can cause fatal diseases, such as nasopharynx cancer (toluene exposure) [1], pneumonia (ethanol long term exposure) [2], pharyngitis (acetone exposure) [3], asthma and lung cancer. For these reasons mentioned above, VOCs detection systems must be implemented for human and environmental safety monitoring. This need for a precise VOCs sensing system is also necessary since VOCs can be utilized as diagnostic biomarkers for various diseases, including lung cancer, prostate cancer, and diabetes [4-6]. 1.1 Challenges and Motivations The current VOC gas sensors have limitations due to their complexity in sample extraction, manufacturing, and operating processes. Consequently, they need an expert operator. Moreover, colossal space-consuming and expensive costs can emphasize the importance of a miniaturized, accurate, fast-response, and cheap gas sensor. MOS sensors meet all the mentioned needs regarding size, accuracy, fast response, and cost; however, improving their sensitivity and response has always been a subject of debate in industrial and academic projects. In this project, the atomic layer deposition technique allowed us to deposit sensing layers with different thicknesses. Additionally, optimizing thickness and operating temperature simultaneously towards different target gases is a promising way to investigate the effect of the target gas polarity on the optimized values and enhance the gas sensor response. 2 1.2 Objectives and sub-objectives This research project explains the fabrication process of a SnO2-based MOS sensor through Atomic Layer Deposition (ALD) technique. Moreover, a unique approach to enhancing the fabricated sensor's sensitivity and general sensing performance is presented. In this regard, we defined three variables: 1) Thickness of the SnO2 sensing layer, 2) Operating temperature, and 3) target gases with different polarities. The first two parameters are controlled by the number of ALD deposition cycles and microheater input voltage, respectively. Furthermore, for the last parameter, the target gas polarity, we exposed the sensors to ethanol, acetone, and toluene, representing polar to non-polar volatile organic compounds (VOCs). Changing each of the mentioned variables could lead us to accomplish the following objectives: 1) Optimizing the deposited thickness to achieve the highest response towards the selected target gases, 2) Optimizing the operating temperature to achieve the highest response towards the selected target gases, and finally 3) investigating the effect of target gas polarity on the previous optimized values. Understanding the objectives mentioned can significantly increase the performance of the MOS sensors detecting VOCs. 1.2 Thesis Organization In Chapter 1, a brief overview of the project, including targeted applications and challenges, is presented. Moreover, the overall objectives and sub-objectives of this project are defined. Also, the organization of this thesis is explained. In Chapter 2, after introducing the selected target gases and their health and environmental effects, different gas detection methods and some examples from the literature are explained in detail. Regarding the needs and requirements in this project, metal oxide semiconductor gas sensing is selected as the sensing method among all the mentioned detection methods. Additionally, by presenting solid reasoning, tin oxide is chosen as the sensing material. In the following of Chapter 2, a detailed review of deposition techniques to fabricate a thin layer of tin oxide as the sensing layer is presented, including their mechanism, advantages, and 3 limitations. At the end of this chapter, in addition to choosing Atomic Layer Deposition technique as the fabrication method, influential parameters on the sensing performance of a MOS sensor are discussed. In Chapter 3, our experimental procedure to fabricate and characterize the fabricated MOS sensor is explained. Moreover, the setup and experiments plan to investigate and optimize the effect of thickness and operating temperature on the sensing performance is presented. Up to Chapter 4, the general concepts, overview and goals, literature review, and methodology of this project are explained. In Chapter 4, first, the material characterizations of the deposited thin film are shown. Then, the collected gas sensing results are categorized based on the selected target gases, different deposited thicknesses, and different operating temperatures and presented in diagrams. Finally, the optimized fabricated sensor is characterized in terms of its sensing performance, such as sensitivity and stability. To wrap it up, in Chapter 5, a summary of the achievements in this project is presented. Additionally, the contributions, limitations, and possible future works are discussed. 4 Chapter 2: literature review In this chapter, after introducing the selected target gases and the motivation to monitor them accurately, different detection methods and their mechanism is explained. By mentioning each gas detection techniques' advantages and disadvantages, a comprehensive conclusion led us to choose the SnO2-based MOS sensor method as our detection method. Then, possible fabrication processes to deposit and fabricate the mentioned sensor are well-explained by bringing some examples from the previous literature. Finally, the atomic layer deposition technique, as the selected fabrication technique, is deeply discussed, and influential parameters on the sensing performance are presented. 2.1 Selected VOCs and their health and environmental effects In this project, ethanol, acetone, and toluene were selected as the target gases, representing polar to non-polar volatile organic compounds. In the following, their chemical formula, polarity, and their application are discussed. 2.1.1 Ethanol 2.1.1.1 Chemical Formula Ethanol or ethyl alcohol is a volatile organic compound with the chemical formula of C2H6O (atomic configuration shown in Figure 1). As it can be seen in Figure 1, in this molecule, a hydroxyl group (-OH) is attached to a hydrocarbon. Due to the difference in electron configuration and electronegativity between hydrogen and oxygen in the hydroxyl group, there is no symmetric electron distribution in the -OH group. It would be a polar chemical compound. This polarity can be generalized to the whole ethanol molecules. Accordingly, ethanol is a polar molecule with a relative polarity of 0.654 in relation to the polarity of water molecules. 5 Figure 1- 2D atoms configuration of ethanol molecules, the angles between atoms are not presented realistically 2.1.1.2 Detection application Ethanol is a colorless, flammable, and volatile chemical compound and is usually applicable in the form of liquid or gas. It is crucial to have precise and reliable ethanol detection due to the following application [2], [7]–[11]: (a) ethanol sensors to breath analyzing the drivers, which can reduce the number of car accidents due to drunk drives; (b) controlling and monitoring the level of ethanol in the fermentation process in the fruits and wine industries; (c) extracting and refining ethanol as a biofuel; and (d) finally, it has been shown that prolonged exposure to ethanol can cause lung injury. Therefore, monitoring it, especially in hospital environments, is a must to protect vulnerable people. 2.1.2 Acetone 2.1.2.1 Chemical Formula Acetone or dimethyl ketone is a volatile organic compound with the chemical formula of CH3CO CH3 (atomic configuration shown in Figure 2). As it can be seen in Figure 2, in this molecule, a carboxyl group (-OC) is attached to two hydrocarbons. Due to the difference in electron configuration and electronegativity between carbon and oxygen in the carboxyl group, there is no symmetric electron distribution in the -OC group. It would be a polar chemical compound. This polarity can be generalized to the whole acetone molecules. Accordingly, acetone is a polar 6 molecule with a relative polarity of 0.355 in relation to the polarity of water molecules, which is less polar than ethanol. Figure 2- 2D atoms configuration of acetone molecules. The angles between atoms are not presented realistically. 2.1.2.2 Detection application Acetone is a colorless, flammable, and highly volatile chemical compound and is usually applicable in the form of liquid or gas. It is vital to have precise and reliable ethanol detection due to the following application: Other than controlling and monitoring a combustible gas and its environmental hazardous, there are two important reasons to detect acetone regarding human health [3], [4], [12]: (a) chronic exposure to acetone (i.e., in the work environment) can cause pharyngitis, catarrhal bronchitis, and gastritis, and (b) the more important reason is monitoring biomarkers through breath analyzing. It has been shown that detecting acetone in breath air can signify type 2 diabetes, one of the most common diabetes. 2.1.3 Toluene 2.1.3.1 Chemical Formula Toluene or methylbenzene is a volatile organic compound with the chemical formula of C7H8 (atomic configuration shown in Figure 3). As it can be seen in Figure 3, this molecule is symmetric. Therefore, there would be no difference in electron configuration and electronegativity between 7 atoms in this molecule. Accordingly, toluene is a non-polar molecule with a relative polarity of 0.099 in relation to the polarity of water molecules, which is almost equal to zero. Figure 3- 2D atoms configuration of toluene molecules. The angles between atoms are not presented realistically. 2.1.3.2 Detection application Toluene is a colorless, flammable, and highly volatile chemical compound and is usually applicable in liquid or gas. It is essential to have precise and reliable ethanol detection due to the wide usage of toluene in ink manufacturing, varnishes, adhesives, thinner, interior decorations, and automotive parts [13]–[17], which are closely interconnected to our daily lives. Toluene is one of the most harmful VOC air pollutants. It has been shown that prolonged or repeated exposure to this chemical compound can cause deadly diseases, such as asthma, lung cancer, and nasopharyngeal cancer, or it can damage the nervous system. 2.2 Detection methods In each gas detection method, the following characteristics should be considered to have a better comparison for the mentioned purposes: Selectivity 8 Selectivity is the ability to detect a particular gas or a group of gases in the presence of other gases. This means that the sensor should be significantly more sensitive to the target gas than the other gases in the mixture [18]. Sensitivity By knowing this term, one can interpret the response change of the detector (e.g., electrical signal) per unit of the target gas concentration [18]. Sample extraction Depending on the application of the gas sensor, the need for a proper sample extraction technique could vary. However, in all the mentioned purposes, a fast and easy sample extraction is desired. As an example, this feature becomes particularly important in detecting gas leaks or breath analysis [19]. Operation complexity In industrial applications, the need to have a technician to operate the device can make a huge difference compared to automated devices. Fabrication process and cost An essential factor that should be considered in each industrial decision is the cost. Therefore, considering the fabrication complexity and its price should be evaluated in each gas sensing technique. Thus, the ideal detector would have high selectivity, sensitivity towards the selected target gases. Moreover, it has the advantage of easy sample extraction, easy operation, and easy fabrication process compared to the other methods. VOCs are detected in various ways. The most commonly used methods include gas chromatography, mass spectroscopy, optical methods, and chemical methods, which are described in detail below. 2.2.1 Gas chromatography This technique works based on separating the target gas mixture into its components, and it can measure each component concentration in a complex gas mixture. The separation occurs due to 9 the interaction of two phases: the mobile and stationary phases. The mobile phase is usually an inert gas, such as argon, nitrogen, or helium. The purpose of the mobile phase is to carry the target mixture through the whole process and primarily through the chromatographic bed to separate the compounds without any chemical reaction. The stationary phase is usually a long tube with a layer of viscous liquid film on it. The gas components will be separated by passing the target gas through the long tube and considering different interaction of gas molecules with the stationary phase. The separation process is illustrated in Figure 4. Figure 4- Gas sensing and separation mechanism of Gas Chromatography method. Molecules B has less interaction with the stationary phase. Consequently, they are faster [20] At the end of this tube, there is a detector to analyze the separated gases. The most commonly used detectors are Flame Ionization Detector (FID), Flame Photometric Detector (FPD), and Thermal Conductivity Detector (TCD). A schematic of the gas chromatograph is shown in Figure 5. 10 Figure 5- schematic of a complete gas chromatograph, including carrier gas supply, gas filter, flow control, sample injector, GC column, GC oven, detector, and computer to control and analyze the collected data [20] 2.2.2 Mass spectroscopy In mass spectroscopy, after ionization of the target gas molecules, ionized molecules are accelerated through an electrical field. Then they undergo a magnetic field that bends them at different angles depending on their mass to charge ratio. The output of the bent ionized atoms is analyzed in a mass analyzer. Figure 6 depicts the schematic of this process. 11 Figure 6- schematic of Mass spectroscopy technique to analyze and monitor a gas components based on the mass difference [20] 2.2.3 Optical Methods The working principle of all the optical methods lies in measuring gas molecules' interactions and the light. This interaction could be light absorption by gas molecules, the excitement of the gas molecules, the analysis of the resulting exposed light, or the simple rules of reflection and the amount of molecule absorbed at a reflecting surface. Different chemical species absorb at different wavelengths in the electromagnetic spectrum, which provides high selectivity for the optical absorption methods. Other than high selectivity to many chemical compounds, this method can be highly sensitive (detecting in ppb level of concentration [Ref]), quickly responsive (Short response time). However, applying this technique in an industrial application requires huge and expensive facilities and a knowledgeable technician. Consequently, a few industrial gas sensors are based on optical methods, and their applications are limited to laboratories purposes. Figure 7 depicts the schematic of the optical gas sensing method. 12 Figure 7- schematic of the sensing mechanism in one of the optical gas sensing methods [21] 2.2.4 Piezoelectric sensors Piezoelectric materials are capable of transforming electrical signals to high-frequency movements and vice versa. In a piezoelectric gas sensor, a piezoelectric material, such as ZnO, LiTaO3, and GaPO4, is used to produce an acoustic wave to resonate the sensing layer. The sensing layer is coated with a material that selectively absorbs the target gas molecules. The absorption of the target molecules changes the resonator mass, and therefore, its frequency. There are two main types of piezoelectric sensors, namely: surface acoustic wave (SAW) and quartz crystal microbalance (QCM). These devices are susceptible and very small in size (millimeter size). However, their drawbacks on poor selectivity and high operating temperature (around 800 °C) cannot be neglected. [22] 13 Figure 8- schematic of a piezoelectric gas sensor, including sensing layer, piezoelectric substrate and metal thin film IDTs [23] 2.2.5 Metal oxide semiconductor sensors Metal Oxide Semiconductor (MOS) sensors are a subgroup of chemiresistors sensors. In these types of sensors, a chemical reaction between the target gas molecules and the metal oxide molecules change the resistance of the sensing layer material. The resistance change is proportional to the target gas concentration. Out of all of the mentioned methods, Metal Oxide gas sensors have gained the most popularity due to their low cost, fast response, and ease of sample extraction and calibration. Moreover, considering the new technologies' demands toward miniaturization and portability, MOS gas sensors can be an excellent alternative to other detection methods for rapid and accurate detection applications [24]. Further details on their gas sensing mechanism and influential parameters will be explained in the following sections. 2.3 Gas sensing mechanism and importance of thickness Before explaining the gas sensing mechanism of MOS sensors, the definition of gas response should be clarified. In general, the gas response is defined as the ratio of the resistance of the 14 sensitive layer in two conditions: when it is exposed to air and when it is exposed to the target gas ((Equation 1). 𝑆 =𝑅𝑎𝑅𝑔 (Equation 1) Where S is the gas response, 𝑅𝑎 is the resistance of the sensing layer in air and 𝑅𝑔 is the resistance of the sensing layer in the target gas [25]. Therefore, the larger and more observable the difference between these two resistances, the more responsive the sensor. To better understand why these two resistances are different and when this difference is more observable, one should take into the gas detection mechanism of a MOS sensor. A MOS gas sensor consists of three main components, namely the substrate, microheater, and sensing layer. The substrate is made of an insulator, such as silicon/silicon oxide, glass, sapphire, and aluminum oxide, and separates the sensing layer from the microheater. The microheater is a patterned metal deposited on the back of the substrate and is made of highly conductive metals such as platinum and gold. The most crucial component is the sensing layer, a thin layer of a semiconductor on top of the substrate. It can be made of SnO2 [26], TiO2 [27], ZnO [28], and WO3 [14] or a combination of them [18-20], with different morphologies, such as thin films, nanorods [32], core/shell [33], nanosphere [34], nanotubes [35]. When an n-type semiconductor is exposed to the air, ionized oxygen will absorb to its surface through a physical bond (Figure 9-a). In the second state, when it is exposed to the target gas, the gas molecules will oxidize on the semiconductor surface (Figure 9-b) [36]. 15 Figure 9- gas sensing mechanism of a metal oxide semiconductor sensor (n-type), (a) when it is exposed to the fresh air and ionized oxygen adsorbed to the surface. (b) when it is exposed to the target gas and the target gas will react with the adsorbed oxygen on the surface In the case of an oxidizing gas (such as CO, CH4, or TBM), there is a difference in bond energies between atoms in the gas and the adsorbed oxygen; therefore, they will react together, and an electron will be donated to the semiconductor (Equation 2). 𝐶𝑂 + 𝑂− → 𝐶𝑂2 + 𝑒− (Equation 2) Each electron will fill a hole (electron vacancy) in the sensing layer and create a charge carrier depleted region near the surface; this will cause a decrease in the conductivity of the sensing layer. Therefore, after the calibration of the sensor, by measuring the conductivity, one can measure the concentration of the target gas. In the case of depositing the sensing layer on top of another dielectric, due to the interaction of two semiconductors and difference of Fermi energy (EF) level, a heterojunction joint will be 16 created. This heterojunction structure can change the sensing characteristics of the deposited sensing layer. The band bending of a heterojunction interface due to the p-n and n-n junction and the resulting sensing performance change have been well-explained in [30]. However, in this research, the deposited thin film is electrically isolated from the substrate through Si3N4 and there would be no electrical interaction. The electrical circuit needed to measure the resistance or conductivity of the sensing layer is shown in Figure 10. Figure 10- Electrical circuit for a MOS gas sensor [37] In addition, if the whole thickness of the semiconductor is in the order of the thickness of the charge carrier depleted region, the sensor will be more responsive. In other words, by having a thinner sensing layer, the effect of gas oxidation and electron donation will be more observable, and one can get a higher response from the sensor[38]. However, It has been shown this relationship is not entirely linear and the optimal thickness of SnO2, in terms of the higher response, is in the range of 30-40 nm [39]. 2.4 Material Selection Due to its wide bandgap energy (3.6 eV), SnO2 is one of the most widely used semiconductor oxides in gas sensing applications. The dual valency of tin in the O=Sn=O structure facilitates the reduction of Sn4+ to Sn2+ on its surface composition, creating unique electrochemical properties, 17 which make SnO2 an excellent choice to detect reducing gases such as VOCs [40]. Table 1 summarizes previous studies using pure SnO2-based MOS sensors to detect the selected VOCs of ethanol, acetone, and toluene. Several factors affect the response (Ra/Rg): operating temperature, target gas concentration and type, and the sensing layer thickness, porosity, and specific area. The highest responses tabulated studies are towards sensing layers with nanostructured tin oxide (e.g., nanoparticles [41]). And as expected, the lowest one corresponds to a thick film of pure SnO2 [42]. Moreover, the majority of previous investigations on the response of pure SnO2 towards ethanol, acetone, or toluene have reported that the ratio of (Ra/Rg) for these three gases are in the order of ethanol > acetone > toluene [28-32]. Table 1- Summary of SnO2-based MOS sensors for VOC detection Structure Deposition Technique Operation Temp. (°C) Conc. (ppm) VOC gases Ra/Rg Ref. Porous SnO2 film Colloidal template method 120 10 Ethanol 7 [46] Acetone 12 SnO2 Nanoflowers SnO2 Hydrothermal/ annealing process 200 20 Ethanol 5.3 [44] SnO2 nanoparticles One-step hydrothermal method 250 100 Ethanol 18 [41] Acetone 9 Toluene 3 Undoped-SnO2 nanoparticles Chemical co-precipitation 320 100 Ethanol 7.5 [7] SnO2 sphere-like nanoparticles Photochemical method 270 50 Ethanol 10 [47] SnO2 hollow nanofibers Electrospinning technique 240 200 Acetone 12 [12] An array of SnO2 thick film Microelectronic technology 350 2317 Acetone 1.8 [42] 18 As pointed out in previous literature, focusing on MOS sensing mechanisms, especially for the two key features of sensing layer thickness and sensor operating temperature, is pivotal for optimizing sensing performance towards target gases [36-39]. To investigate the effect of thickness, first, we need to know the fabrication techniques to deposit a thin layer of SnO2. In the following, a detailed explanation of the manufacturing methods of MOS sensors is provided. Also, an analysis of the influential variables of each manufacturing technique is used to understand better enhancing gas sensor response. 2.5 SnO2 thin film deposition techniques The techniques of preparation of SnO2 thin films can be categorized into two classes: i) Physical Vapor Deposition (PVD) or Physical Deposition and ii) Chemical Deposition (CD). The first class, or PVD processes, are an atomistic deposition method in which, after the vaporization of a condensed source (solid or liquid), the vaporized material's atoms will be deposited inside a vacuum or vacuum/plasma chamber and stick to the target substrate [51]. The second class, or CD processes, is an atomistic deposition method in which one or more precursors will chemically react or dissociate on the target substrate to create the thin film. Same as PVD, CD processes also need to be carried out in high temperatures, mostly at sub-atmospheric pressure and in some cases in plasma environments [52]. Since the later methods do not need a high vacuum chamber, they developed before the PVD techniques. These two processes are similar as they present the same outcome; both are used to deposit a skinny layer; on the other hand, what differentiates them the most is how the vaporized precursors will be deposited to the substrate. 2.5.1 Physical deposition techniques PVD processes can be classified into three basic methods: 1) Evaporation, 2) Sputtering, 3) Ion plating. Each technique, their advantages and disadvantages, and application in SnO2 thin film sensor fabrication are discussed in the following sections. 19 2.5.1.1 Evaporation In the evaporation process, source materials are vaporized in vacuum chambers from powder or liquid sources. Then, a certain distance is traveled by the vaporized atoms before the latter collide and deposit onto the substrate. The materials are usually evaporated or boiled using thermal or electron beam heaters [53]. Adding a small amount of reaction gas to the vacuum chamber will allow SnO2 or any other metal oxide, nitride, or carbide to be deposited. Upon chemical reaction with this gas, the evaporated metal forms the desired metal oxide thin film. It is referred to as "Reactive Evaporation" (RE). By converting atoms into energetic atoms or ions through the plasma environment, chemical reactivity will be increased and so will the rate of deposition. As described above, this is called "Activated Reactive Evaporation" (ARE) [54]. Using this method, one is able to better control the final properties of deposited films by separating the formation and deposition processes of the metal oxide. Further, the same layer thickness can be deposited with less source material concentration than the normal evaporation [55]. 20 Figure 11- schematic of vacuum-evaporation process, in this case electron beam used as the evaporator [56] Amit Kumar Mauraya et al. [57] deposited three SnO2 thin-film sensors with the thickness of 100, 200, and 500 nm by vacuum evaporation combined with the post-oxidation process. SnO2 thin films were deposited on top of Aluminium and Sapphire substrates at the vacuum pressure of 7.5 × 10-6 Torr. Under different operating temperatures, these sensors were exposed to 915 ppm of CO. Deposition occurred at a rate of 2–5 Å/s. Their results (Figure 12) indicate that the highest response, which was S=Ra/Rg=3.41, reached for a film deposited at a minimum thickness of 100 21 nm, using direct oxidation and at a maximum operating temperature of 400 °C. This sensor detected CO between 10 and 1000 ppm [57]. Figure 12- (a) Sensitivity as a function of operating temperature, (b) comparing different samples with different thicknesses and deposition conditions [57] 2.5.1.2 Sputtering Sputtering is also a physical vacuum deposition process in which fast ions collide with the surface of the source material to release atoms and sputter them out. A thin film will be deposited on top of a substrate if the substrate is positioned correctly. In most cases, DC sputtering is used for the deposition of thin films. Ions are accelerated by applying DC voltage between anodes (substates) and cathodes (source materials). The limitation of this method is that the target thin film cannot be dielectric because a conductive cathode must be used. Radiation-Frequency (RF) sputtering is used for the deposition of dielectric materials, such as metal oxides. This method generates an electrical potential at a frequency of 13.56 MHz to prevent the effect of surface charges on the substrate and dielectric [58]. Figure 13 shows how the process works. (a) (b) 22 Figure 13- schematic of the vacuum-sputtering process [59] Using 200 nm sputtered Indium Tin Oxide with 200-nm thickness, G. Sberveglieri et al. detected 25-200 ppm of NOx gases [60]. The deposition process employed a forward-rf power of 60 W, an accelerating voltage of 600 V, a substrate temperature of 520 K, and a bias voltage of 25 V. Using these sputtering conditions, the researchers showed how zinc oxide crystallizes more efficiently to be used for sensing applications. In addition, the combination of Indium Tin Oxide makes gas sensors more stable. Also, a highly sensitive sensor for VOCs can be synthesized using DC sputtering in conjunction with other deposition methods such as photolithography. For example, the photolithography and sputtering of SnO2/Pt bilayers were used by Thai et al. to monitor two reducing gases (H2 and NH3). Their measurements were performed at 25-250 ppm concentrations and showed that 5- 10 nm of Pt over 150 nm of SnO2 was the best combination to obtain the optimum sensitivity [61]. The role of Pt in these situations would be to activate or catalyze. Studies have shown that these activators must be added to the tin oxide to attain elevated sensitivity toward reducing gases at lower temperatures. Figure 14 shows that the highest sensitivity occurred at a lower temperature 23 for Pt added to SnO2. Moreover, the activator changed the maximum sensitivity value towards a certain amount of CO from about 40% to 140% [62]. Figure 14- Sensitivity (%) as a function of operating temperature for pure SnO2 and Pt doped SnO2 [62] 2.5.1.3 Ion plating Ion plating consists of continuously or intermittently bombarding the substrate with energetic particles before and during the deposition. Electrons or reactive gas ions may be used as energetic particles. The bombardment has two functionalities. The first is to remove any contamination from the substrate surface before deposition through sputtering. Secondly, the bombardment during the film formation can be controlled, thus modifying some thin-film characteristics. Also, chemical reactivity will be increased by the high flux energy on the surface, so there would be no need for a heater [63], [64]. This method requires that the substrate is energized with a negative voltage to accelerate ions. In this way, ions will adhere to the substrate following the electric fields even inside the cavities. Furthermore, as with any reactive deposition method, the evaporated materials will collide with the gas molecules and scatter; consequently, the thin film will be relatively uniform [65]. Figure 15 shows how the process works [63]. 24 Figure 15- schematic of Ion plating process [63] Through the Ion Beam Sputtering (Ion Plating) process, Bong-KiMin et al. synthesized SnO2 methane sensors doped with Ca. Si/SiO2 wafers were utilized for their experiments as the substrate, and the ion gun source was argon gas and Torr as working pressure. The sensing layer had a thickness of 5000 Å and 1.8 response to 5000 ppm of CH4 [66]. Figure 16 compares pure SnO2 with Ca-doped SnO2. 25 Figure 16- comparing gas sensor response as a function of CH4 concentration between undoped SnO2 and Ca doped SnO2 , deposited through Ion Plating [66] 1.5.1.4 Rheotaxial Growth and Thermal Oxidation (RGTO) Aside from the above three PVD processes, another minor method for depositing metal oxides is prevalent and must be discussed here since many papers are on this topic. In RGTO (Rheotaxial Growth and Thermal Oxidation), either sputtering or evaporation is used to accomplish the PVD process. PVD must be followed by extended oxidation of the metallic film to deposit the final metal oxide. Here are some examples: G. Sberveglier described the deposition of thin layers of SnO2-Pd on silicon substrate using RGTO. For 8 hours, the thin film was maintained at 793 K [67]. As a result of this technique, they found that the tin oxide's surface area was increased, thereby increasing its sensitivity to hydrogen (200 ppm) [68]. They also observed high sensitivity in the case of a thin film with a thickness between 150-200 nm when exposed to ethanol with a concentration between 50-1000 ppm [69]. The RGTO technique was also used by G. Partridge et al. to deposit 80 nm 26 layer Sn on top of the Si3N4 membrane. The thin film was oxidized in the air by keeping it at 200 °C for 2 h, at 400 °C for 2 h, and after that at 600 °C for 2 h. The device responded well to 600 ppm of H2 [70]. 2.5.2 Chemical deposition techniques Chemical deposition methods can be divided into two groups according to the precursor carrier state during the deposition: 1) Solution-based or wet deposition, and 2) Gas-phase chemical deposition (Figure 17). Figure 17- categorizing the chemical deposition methods, inspired by [71] 2.5.2.1 Solution-phase chemical deposition Solution-phase and wet depositions methods work based on the evaporation and decomposition of chemical compounds on the surface of the target substrate. For example, one way to deposit SnO2 is that the substrate will be wetted by the solution of SnCl4·2H2O in ethanol. As a result of the evaporation and the chemical reaction of SnCl4 and H2O, as shown in the chemical equation below, the thin layer of solid tin oxide will remain on the surface, and other by-products will be removed in a gas state. 27 𝑆𝑛𝐶𝑙4 + 2𝐻20(𝑔) → 𝑆𝑛𝑂2(𝑠) + 4𝐻𝐶𝑙 (𝑔) (Equation 3) The three main solution-phase deposition processes are a) spin coating, b) dip-coating, and c) spray pyrolysis. In the following subsections, each process is described. 2.5.2.1.1 spin coating A certain amount of the source material is dissolved in a volatile solvent during the spin coating process. Then an excess amount of the solution will be dropped on top of the surface. At last, A high-velocity rotation is performed to apply the solution uniformly all around the surface and to remove the excess solution from the edges (Figure 18). Two factors influence the thickness of thin films: first, the spinning speed; as the speed increases, the thinner the final film. Second, the concentration ratio of solvent to the solution. Due to its mechanism, this process is limited to depositing thin films on flat surfaces, such as wafers. This technique allows the deposition of thin films down to 10 nm in thickness [72]. Figure 18- three phases of spin coating deposition process: dropping the solution, rotating, and evaporating [72] The spin-coating method was used by N.S. Baik et al. to deposit a 1-micrometer layer of tin oxide with an 8 nm crystal size. This layer was used to detect H2 and CO gases. By reacting tin chloride with ammonium bicarbonate, the hydrated tin oxide was precipitated. In order to prepare the right 28 solution for spinning, the gel was suspended in an ammonia-water solution (1.8 wt.% sol). It was spun at 1000 rpm to clean the excess solution from the Al substrate [73]. 2.5.2.1.2 Dip coating Dip coating involves immersing a substrate at a certain speed into a container of precursor solution without juddering. It is ideal for letting the substrate interact correctly with the solution for a sufficient length of time. The substrate will then be pulled out at a constant rate. The excess solution will drain from the substrate, leaving behind a thin film of solution at the surface. Upon forced-drying or heating, a solidified thin film will be deposited after the solvent has evaporated [74]. This process can be broken down into four steps: 1. Immersion 2. Dwelling 3. Taking it Out 4. Solidification. There is a direct correlation between the thickness of the thin film deposited on a substrate, its pulling-out velocity, and the solution viscosity. According to the literature [75], [76], the thicker the film is deposited, the faster the substrate is pulled out. Figure 19- schematic of dip coating deposition process, including three stages: dipping, withdrawing, and drying [77] 29 In addition to the difficulties in coating the cavities, the major design flaw of this process is its inability to achieve uniform thickness as an excess material would be deposited on the bottom [78]. Using dip-coating, Zhao et al. (2006) examined how grain size and multilayer deposition affected alcohol detection of nanosized SnO2 films. 0.5 mol / l and 0.1 mol / l solutions were dipped into a glass substrate at a speed of 80 mm/min. A 15-minute period at 100 °C was used to evaporate the solvent, followed by 30 minutes at 500 °C for crystallization and heat treatment. Using two different sol concentrations, they demonstrated that two different grain sizes of 87 nm and 5 nm are produced. Moreover, the above-mentioned procedure is repeated multiple times to deposit a multi-layered thin film. They found that thin films with grain sizes of about 5 nm are more sensitive to alcohols and acetone. Also, increased sensitivity can be obtained by increasing the number of deposited layers. Their conclusion about grain size is in line with other literature; however, their claim regarding the effect of thickness on sensitivity is not completely accurate. The thickness effect on sensing efficiency cannot be clearly observed by itself due to the created interface between each layer deposition [79]. It should be mentioned that the reason behind this phenomenon is that increasing the surface area by repeating this process and depositing a second layer has been cited as the reason for the increase in sensitivity. By having more surface area, the sensing layer and gas will interact more, leading to increased sensitivity [80]. Dip-coating was used by Sutichai Chaisitsak to prepare a thin film of tin oxide for detecting LPG with a thickness of 250 - 500 nm. A 2.4 mm/min speed of pulling was used to dip the substrate into the solution and pull it out, followed by 30 s of drying at 400 °C. This process could deposit pure SnO2 at a rate of 23 nm per cycle. As discussed before, changes in withdrawal speed and other conditions can affect the deposition rate[81]. Through multilayered deposition, composite thin films, heterojunctions, doped thin layers, and in general, two different materials can also be deposited [1], [82]–[85]. Tianyu Liu et al. compared the methanol sensitivity of a pure deposited SnO2 on top of a ceramic tube and a 30 SnO2/NiO composite. Dip-coating was used to deposit both materials, and they were dried for 2 h at 500 °C. There is evidence that creating a heterojunction joint in the sensing layer bends its bandgap, changes its resistance, and also creates more surface area. Thus, by selecting the right material, the sensor will be more sensitive [82]. Figure 20- Comparison between pure and NiO/SnO2 composite gas sensor [82] 2.5.2.1.3 spray pyrolysis Similar to other chemical processes, pyrolysis also relies on evaporation and chemical decomposition. As part of this method, the precursors, usually salt solubles, are dissolved in proper solvents, such as water. Then, a nebulizer is used to spray the solution onto the heated substrate. Similar to dip coating or spin coating, all of the byproducts of the chemical decomposition are volatile at substrate temperatures, except for the desired thin film. The following parameters are important in controlling the film thickness and morphology: 1. % solvent concentration, 2. nebulizer output speed, 3. spray droplet size, 4. nozzle to substrate distance, 5. spraying time, and 6. the substrate's temperature [59], [86], [87] 31 Figure 21- schematic diagram of spray pyrolysis deposition process [87] K. Nomura et al. [88] used spray pyrolysis to prepare different SnO2 thin films with a thickness of 200 nm, doped with Sb, Cu, Fe, Co, Ti, Ni, Mn. Based on their findings, the more resistant the sensor layer, the more sensitive the sensor will be. A more sensitive sensor can be produced by manufacturing them thinner or by adding p-type semiconductors, such as MnO, and creating hetero-joints to increase the sensing layer's sensitivity. A combination of Sn, Mn, and Pd will provide the best sensitivity toward 1000 ppm of ethanol, as shown in Table 3 [88]. Table 2- Gas sensor response of tin oxide-based combinations Materials Mole ratio Temperature (oC) Resistance (kΩcm-2) Gas response Rair/Rgas Sn 1 350 10 2 Sn:Pd 100:1 350 10 2 Sn:Ca:Pd 100:500:1 350 100 3 Sn:Ti:Pd 100:100:1 350 100 6 Sn:Cu:Pd 100:100:1 375 100 3.2 32 Materials Mole ratio Temperature (oC) Resistance (kΩcm-2) Gas response Rair/Rgas Sn:Fe:Pd 100:100:1 350 100 6.4 Sn:Co:Pd 100:100:1 350 10 3.4 Sn:Mn:Pd 100:1000:1 350 2 × 105 43 Sn:Ni:Pd 100:1000:1 350 105 33 Correlation between the deposition temperature and sensitivity, as well as the resistance of sensing layer and the sensitivity, is demonstrated again by G Korotcenkov in 2001. According to their findings, pyrolysis of SnO2 deposited at a thickness under 20 nm can produce discontinuous deposits, causing a significant increase in the resistance, and this will not cause the coating to react towards reducing gases as expected [89]. Using spray pyrolysis, R. H. Bari et al., have also carried out research on this topic. They deposited SnO2 thin films for 10 minutes, 20 minutes, 30 minutes, and 40 minutes, depending on the deposition duration. A thin film that had been deposited for 30 minutes had the best response toward H2S; the resistivity of the sensing layer was 3500 times less when exposed to 600 ppm of the target gas [90]. 2.5.2.2 Gas-phase chemical deposition Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) are the two main gas-phase chemical deposition methods. Even though both approaches are based on the same principles, there are some differences in their steps and sequences, which will be explained in the following paragraphs. 2.5.2.2.1 Chemical Vapor deposition (CVD) Chemical Vapor Deposition (CVD) is a process in which a non-volatile solid material will be deposited due to a chemical reaction or decomposition of one or more gas-phase volatile materials. In contrast with PVD, these techniques do not need to be in a vacuum chamber. In these processes, the chemical reactions can be occurred both on top of the substrate or in the 33 gas phase before colliding with the surface substrate. Also, the rate of the chemical reaction can be increased by high-temperature, Microwave Plasma (Microwave plasma-assisted CVD (MPCVD)), or, in general, low-frequency waves, UV light, or high-frequency waves, and plasma environment (plasma-enhanced CVD) [91], [92]. There are four influential variables on the morphology and crystallization of the final deposited material: 1) Substrate material, 2) Substrate temperature, 3) Chamber pressure, and the most critical one, 4) Precursors and chemical reactions [93]. Regarding the precursors, there are two main groups according to their nature, inorganic and metalorganic materials. For example, in the case of depositing SnO2, the most used inorganic precursors are SnCl4 and SnI2, and the chemical reaction, in this case, can be one of the followings: 𝑆𝑛𝐶𝑙4(𝑔) + 𝑂2 (𝑔) → 𝑆𝑛𝑂2(𝑠) + 2𝐶𝑙2(𝑔) (Equation 4) 𝑆𝑛𝐶𝑙4 + 2𝐻2𝑂(𝑔) → 𝑆𝑛𝑂2(𝑠) + 4𝐻𝐶𝑙 (𝑔) (Equation 5) 2𝑆𝑛𝐼2 + 𝑂2(𝑔) → 𝑆𝑛𝑂2(𝑠) + 𝑆𝑛𝐼4(𝑔) (Equation 6) As seen in the abovementioned equations, usually, by-products of the inorganic chemical reactions would be toxic. To eliminate these hazardous gases, increase the growth rate and decrease the deposition temperature, researchers started to use metalorganic reagents. In case of depositing SnO2, the most used metalorganic precursors are Tetrakis (dimethylamino) tin (TDMASn), Dibutyltin diacetate (DBTA), Tin (IV)-butoxide, N,N2-trimethyl-2-propoxy tin (Sn(dmamp)2), N2,N3-di-tert-butyl-butane-2,3- diamidotin(II) (Sn(tbba)), Acetylacetonate tin (II) (Sn(acac)2) and Tetramethyl and tetraethyl tin (SnMe4 and SnEt4)[94]. For TDMASn, the chemical reaction and its chemical formula are as below [95]: 𝑆𝑛(𝐷𝑀𝐴)4 + 2𝐻2𝑂2 → 𝑆𝑛𝑂2 + 4𝐻𝐷𝑀𝐴 + 𝑂2 (Equation 7) 34 Figure 22- Sn(DMA)4 chemical formula The CVD process can be divided into six key steps [91], [92]: (a) Transportation of gas precursors from the source tanks to the reaction chamber (b-d) The chemical reaction of gas precursors in the gas phase and before sticking to the surface to produce the new reactive materials (e) Transportation of the initial precursors and the produced intermediates to the substrate surface (e) Surface absorption of precursors and surface diffusion (After e) Nucleation and film formation (f) Desorption and purging of the gaseous by-product from the reaction chamber Figure 23- critical steps in a typical CVD process [96] 35 2.5.2.2.2 Atomic Layer Deposition (ALD) CVD and ALD processes are indeed similar in deposition mechanism and chemistry; their differences in sequences justify defining them into two distinct subsets of gas-phase chemical deposition. Atomic Layer Deposition (ALD) is a chemical thin film deposition method in which, by separating the gas-solid reaction, self-limiting surface reactions will be repeated successively to deposit the target thin film [97], [98]. In CVD processes, all of the precursors (reactants) chemically react at once within the chamber; however, in each ALD cycle, the chemical reaction will be broken into two half-reactions [99]. For example, in depositing a metal oxide thin film, the metal precursor (First reactant) will react with the substrate surface or surface-active sites in the first half-reaction. Then, in the second half-reaction, the oxygen source (Second reactant) will react with the deposited product of the first half-reaction, and this cycle will be repeated. Here is the comparison of CVD and ALD for depositing SnO2 in terms of chemical reactions [53], [94]: CVD: Chemical reaction 𝑆𝑛(𝐷𝑀𝐴)4 + 2𝐻2𝑂2 → 𝑆𝑛𝑂2 + 4𝐻𝐷𝑀𝐴 + 𝑂2 (Equation 8) ALD cycle: 1st half-reaction 𝑂𝐻𝑥∗ + 𝑆𝑛(𝐷𝑀𝐴)4 → (𝑂)𝑥𝑆𝑛(𝐷𝑀𝐴)4−𝑥∗ + 𝑥𝐻𝐷𝑀𝐴 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑔𝑟𝑜𝑢𝑝 + 𝑇𝑖𝑛 𝑃𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟→ 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑 𝑠𝑜𝑙𝑖𝑑 + 𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑏𝑦_𝑝𝑟𝑜𝑑𝑢𝑐𝑡 (Equation 9) 2nd half-reaction (𝑂)𝑥𝑆𝑛(𝐷𝑀𝐴)4−𝑥∗ + 2𝐻2𝑂2→ 𝑆𝑛𝑂2(𝑂𝐻)𝑥∗ + (4 − 𝑥)𝐻𝐷𝑀𝐴 + 𝑂2 1𝑠𝑡𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + 𝑂𝑥𝑦𝑔𝑒𝑛 𝑠𝑜𝑢𝑟𝑐𝑒 → 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑 𝑆𝑛𝑂2 +𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑏𝑦_𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 (Equation 10) 36 1st +2nd half-reactions 𝑆𝑛(𝐷𝑀𝐴)4 + 2𝐻2𝑂2 → 𝑆𝑛𝑂2 + 4𝐻𝐷𝑀𝐴 + 𝑂2 (Equation 11) The separation between the two half-reactions is provided by purging an inert gas, usually, Nitrogen or Argon, to ensure there is no reaction between reactants in the gas phase. Therefore, each ALD cycle can be divided into four time-steps: 𝑡1, Pulsing the first reactant (e.g., metal precursor), 𝑡2, purging the inert gas, 𝑡3, pulsing the second reactant (e.g., oxygen source), 𝑡4, purging the inert gas [100]. 𝑡2 and 𝑡4, or purging times should be long enough to remove all the remained unreacted precursors and by-product of the previous half-reaction, and also to avoid any probable chemical reaction in gas phase. In the other world, never two gaseous precursors will be in contact during the deposition process. In addition, 𝑡2 and 𝑡4, or pulsing times should be long enough to provide sufficient precursors to cover all the substrate surfaces. A unique characteristic about the ALD process is that the deposited thickness in each cycle is independent of the precursors pulsing time and its dose. After the reactants covered all the active surfaces, since a precursor is not chemically reactive toward itself, there will be no more chemical reaction in the ALD chamber and the system will be saturated. This characteristic also known as self-limiting or layer-by-layer deposition. Thanks to the self-limiting deposition, the rate of increase in thickness in each cycle or growth per cycle (GPC) will remain constant, and for a saturated ALD it will be limited in the order of atoms dimension (𝐺𝑃𝐶𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛~1 Å/𝑐𝑦𝑐𝑙𝑒); This means the ALD deposition can control the thickness in Angstrom scale [101]. Moreover, due to layer-by-layer deposition, one can easily deposit an alternating layer of different materials [102]. 37 Figure 24- Four steps of ALD process in one cycle Another remarkable advantage of this deposition method is its capability to deposit a conformal [103], high quality, and uniform thin film, which are all results of the self-limiting sequential deposition [104], [105]. Consequently, the ALD allows perfectly depositing complex and high aspect ratios non-planner surfaces, such as deep trenches and porous media [106]. Jaeyeong Heo et al. achieved 80% uniformity in the deposition of SnOx inside a hole with a 50:1 aspect ratio [107]. Figure 25- Cross-sectional SEM of an ALD deposited high aspect ratio hole with 190 ALD cycles [107] 38 Figure 26- Cross-sectional TEM of a nano laminated structure deposited by ALD [108] In 2006, Gwangpyo Choi et al. investigated three different tin oxide thin films in their gas sensing characteristics: 100, 200, and 400 of ALD cycles, which were deposited through PE-ALD and dibutyl tin diacetate (DBTA) as the reactant of tin. The effects of deposition temperature, purging time, operation temperature, and thickness have been discussed in this study. Based on their results, due to the surface morphology and the capability of absorbing oxygen ions, the best sensitivity toward a reducing gas such as CO, would be for 200 cycles deposited thin film at 450 oC [109]. This result was compatible with other studies in this regard [110]–[112]. 39 Figure 27- Gas sensor response of a tin oxide film toward 250 ppm of CO in different sensing temperatures [109] In 2010, Antti J. Niskanen et al. deposited a 17.5 nm tin oxide thin film through ALD using tin (IV) chloride to manufacture a gas sensor on top of a micro hot plate. The sensor was exposed to ethanol, acetone, and Acrylonitrile gas in the range of 20-500 ppm and showed an acceptable response. However, in terms of stability, the sensor lifetime was around 11-16 days, limiting its application to fast and one-time sensing usages [113]. As explained before, surface reactions and surface-to-volume ratio play a crucial role in the sensitivity of a gas sensor. Therefore, utilizing nano-scale structures, which have the maximum active surface sites, has always attracted much attention among researchers in gas sensing applications. Moreover, the right set of circumstances in the atomic layer deposition technique allows us to deposit on top of extremely fragile substrates, such as nano films [114], nano tubes [115], nanowires/rods [32], [116], or nano-scale porose media [117]. By considering the facts mentioned earlier about gas sensing and the ALD, one can understand the great opportunity to implement ALD in sensing layer manufacturing. For instance, in 2010, Won-Sik Kim et al. combined electrospinning and ALD to manufacture a SnO2 nanotube network as a sensing layer. 40 The best response toward 188-100 ppm of ethanol was for 8 nm tin oxide deposited on top of nano tubes with 100- 200 nm diameter [35]. Additionally, this method can be used to deposit two semiconductors on top of each other in nano-scale structures to create a heterostructure joint. The effect of heterostructure joints on gas sensitivity has been well explained by Catherine Marichy et al. [48]. This heterostructure can be in form of nanoporous [118], nanospheres [34], core-shell nanowires [119], quantum dots [120], and nanofilms or nanocomposite [121][112]. These kinds of the sensor are capable of detecting VOCs even in the range of ppb concentrations. Finally, through the ALD, a monolayer of a catalyst such as Pt or Pd can be deposited on the sensing layer. No need to mention that these catalysts will increase the surface reactions and consequently the sensitivity. For example, in 2020, Yongshan Xu et al. synthetized various thicknesses of a thin film of tin oxide (4-18 nm) and covered by a single-atom layer of Pt. According to this study, 9 nm deposited tin oxide showed the highest response toward triethylamine at different concentrations. Also, adding an ultrathin layer of Pt on top of the sensing layer noticeably increased the sensitivity and decreased the limit of detection (LOD) to 7 ppb [122]. 2.6 Influential parameters on sensing performance of MOS sensors The previous section discussed the physical and chemical deposition methods of a thin tin oxide layer for gas sensing purposes. Moreover, the ways to enhance SnO2-based MOS sensors' sensitivity through optimizing the deposition conditions and working parameters have been reviewed. There are five main influential parameters on gas sensor response: 1. Thickness: Supposing the thickness of the sensing layer is in the order of the electron depletion layer, the effect of surface chemical reactions will be more observable. As a result, the sensor will show a higher response [48]. Additionally, the sensing temperature will affect the number of 41 charge carriers, chemical reactivity, and adsorption rate of oxygen ions on the surface, altering the sensor response [123], [124]. 2. Operating temperature: generally speaking, high temperature increases chemical reactivity. However, some other effects, such as the rate of molecules absorption, should not be neglected. 3. Dopant materials: By adding a catalyst such as Pt or Pd as a dopant in the sensing layer, the sensor will have a higher response since the surface reaction will increase drastically. 4. Heterojunction joints: The overall bandgap energy will change by creating a heterojunction joint between two different semiconductors, such as SnO2 and ZnO. Therefore, choosing the suitable materials to combine will result in a more sensitive sensor toward reducing gases. 5. Porosity and crystallinity size: Manipulating these two factors will indirectly impact the active surface site area. Consequently, the surface reactions with the target gas and the gas response will be influenced. Moreover, there are some additional minor factors such as oxygen concentration and temperature during the deposition; these will impact the sensing layer's ability to absorb oxygen Ions in the sensing process. Here is the summary of the abovementioned parameters and the respective results of the reviewed paper: Table 3- Summary of the influential parameters and the respective results of the reviewed paper Deposition technique Dimension (nm) Additives Target Gas Rang of detection (ppm) Highest response (at max ppm) Year Ref. Evaporation 8.5 pure CO 2-20 30 2019 [125] 300-350 Pd doped ethanol 100-3000 2.9 1981 [126] r.f. Reactive sputtering 45 Pd CO 1000 35 1990 [39] 60 Pure NOx 5 5.5 2008 [127] 100 pure NOx 5-100 NR 1979 [128] 50 pure CO <36 6 1979 [129] 200 Indium polycrystalline NOx 1-1000 100 1989 [60] NR pure CO NR NR 1991 [130] 300-400 pure CO 10-100 43 1996 [131] 150 Pt NH3 25-250 75 2020 [61] NR Pt CO 25-500 4.82 1995 [62] 50 Pure CO 1-100 NR 1979 [132] 42 All these factors enhance the gas response based on either the increase of surface reactions or their observability. However, it is only possible to approximate the optimum amount of these Deposition technique Dimension (nm) Additives Target Gas Rang of detection (ppm) Highest response (at max ppm) Year Ref. Ion plating 10-15 Ca-doped CH4 1000-5000 1.8 2004 [66] 60 Pure ethanol 100-1500 100 1988 [133] 70-100 Pure H2 100-1000 9 1992 [134] RGTO 200 Pd ethanol 50-1000 151 1992 [69] 80 pure NOx 0.51-0.106 3 2009 [111] 45 pure NOx 0.500 2.25 2008 [70] 200-300 CdO NOx 10-1000 10000 1991 [135] Wet method 20 nm Pt-La2O3 ethanol 1000 1000 1991 [38] spin-coating 10* pure CO 800 600 2000 [73] 10* pure methanol 0.01-64 NR 2001 [136] 110 Ru hydrocarbon 100-1000 350 2003 [110] NR Pure ethanol 100-1500 10 2002 [137] 100 Pure NO 300-1800 - 2004 [138] 34* Pure O2 100-1000 30 2001 [139] Dip-coating 87* Pure ethanol 2-10 28 2006 [79] 913 Pure CO/NOx 50 2.2 2017 [80] 250-500/5* F LPG 5000 1.4 2011 [81] 300 NiO methanol 1-100 15.12 (10 ppm) 2019 [82] 20* NiO toluene 0.1-1000 19.2 (10 ppm) 2020 [1] spray pyrolysis 140 In doped ethanol 400-800 3.5 2014 [25] 20-100 Pure ethanol 0.1-1000 100 1979 [140] 200 Mn: Pd ethanol 1000 43 1988 [88] 20 Pure H2 2000 100 2001 [89] 67* Pure H2S 100-600 3500 2012 [90] 300 Pure H2 1000 40 2009 [141] CVD <1000 Pure ethanol 50-500 1075 2005 [142] 20* Pure CO 50-1000 31.7 2005 [32] ALD 20 Pure CO 250 1.7 2006 [109] 90 TiO2 H2 1000 380 2010 [112] 17.5 Pure ethanol 100-500 1.64 2010 [113] 10 Pure CO 1000 NR 2003 [114] NR ZnO formaldehyde 0.07-20 38.2 2021 [34] NR TiO2 formaldehyde 0.06-0.6 6 2019 [121] 9 Pt trietylamine (TEA) 10-100 25 2020 [122] ALD/ Evaporation NR TiO2 formaldehyde 0.1-10 1.3 2021 [118] 5* Ga2O3 ethanol 1000 70 2011 [119] 43 variables under certain circumstances, such as a specific target gas, detection range, and surface morphology. To thoroughly understand the enhancement of a SnO2-based MOS sensor's performance in the following chapters, we selected two influential parameters: thickness and the operating temperature for an ALD-deposited thin film, and we optimized them simultaneously towards ethanol acetone and toluene as the target gases. 2.7 Previous studies on optimizing thickness and operating temperature Up to now, extensive research has been done on optimizing the thickness and operating temperature of MOS gas sensors; however, less attention has been paid to optimizing an ALD-deposited SnO2 thin film as a sensing layer. Lack of grain boundaries and complex crystallization structures in ALD-deposited thin film makes it easier to understand the fundamentals of influential sensing parameters. A. Rosental et al. [114] deposited a SnO2 thin film on α-Al2O3 (012) substrate via ALD using SnI4-O2 as the precursors. By testing a range of thicknesses from 1 to 100 nm, the authors determined that the peak of response toward 1% carbon monoxide (CO) in fresh air occurred at the thickness of 10 nm. In another study, G. Choi et al. [109] deposited SnO2 thin films with three different thicknesses, i.e., 10 nm (100 cycles), 20 nm (200 cycles), and 40 nm (400 cycles), through plasma enhanced-ALD using DBTA and O2 precursors. They exposed these samples to 250 ppm of CO in the temperature range of 250 to 500 °C. The 200 cycles deposited sample (20 nm) showed the highest response at 450 °C. To justify this result, the authors claimed, based on their XPS and AFM results, that the optimum sample had the highest capability of adsorbing oxygen ions and the most favorable surface microstructures. Another explanation for this relationship between sensor response and thickness in the case of ALD-deposited SnO2 as the sensing layer has been proposed by Yongshan Xu et al. [122]. In this study, four different thicknesses of SnO2 thin film (4 nm, 9 nm, 13 nm, and 18 nm) were grown on a Si/SiO2 substrate by ALD using TDMASn and H2O as the precursors. The optimum operational condition to detect triethylamine (TEA) in the sensing temperature range of 100 to 300 °C was the 9 nm SnO2 sample at the sensing temperature of 260 °C. To investigate this peak performance, they examined their 44 samples through Hall experiments to measure the charge carrier concentration in each sample. Even though the sensor exhibited the highest response when its thickness was near the Debye length [143], based on the results, the resistance change was undetectable for very thin layers (4 nm) since the carrier concentration was very low. These observations of changes in sensor response based on the film thickness and sensing temperature have also been made for other semiconductors [27], [120], [123] with different morphologies [119] and via different deposition techniques [39], [57]. By comparing the results of all the mentioned studies on the optimum thickness and sensing temperature, it can be found that these values are highly dependent on the chemical nature of target gases (e.g., the polarity of target gas [144]) and some other factors such as deposition temperature. This conclusion has been drawn both theoretically [145] and experimentally [39] in previous studies. Moreover, G. Korotcenkov et al. [141] claimed that these two parameters, optimum thickness and operation temperature, are not independent variables, and one cannot be optimized without considering the other. The majority of studies in this area have been focused on the optimization of gas sensors toward one or a group of specific gases such as CO, TEA, or VOCs. However, to the best of our knowledge, the influence of the polarity of target gases on the optimal sensing layer thickness and operation temperature has not been thoroughly examined. In this study, we optimized both the thickness and sensing the temperature of an ALD-deposited SnO2 thin-film sensor to detect selected polar and non-polar VOCs. To better understand how the target gas polarity influences the optimization of thickness and operating temperature, we exposed the samples to three different VOCs, i.e., ethanol, acetone, and toluene, representing polar to non-polar VOCs for a concentration range of 20-227 ppm. Other influential factors, such as the physical and chemical properties of the deposited thin films, were also investigated. Finally, the optimized sensor was characterized in terms of sensor response and short- and long-term stability toward the mentioned target gases. 45 Chapter 3: Experimental Setup In the following, the deposited MOS sensor fabrication and characterization procedure are explained. Additionally, a setup and experiment plan to evaluate and optimize the effects of thickness and temperature on the performance of the sensor is presented. 3.1 Synthesis of SnO2 thin films by ALD The SnO2 thin film was deposited on a silicon wafer (Si/Si3N4, the microheater chip) substrate in a GEMStar XT-P PEALD system (Arradiance, USA). The ALD deposition temperature was 100 °C. TDMASn (Sn[(CH3)2N]4) and H2O were used as the precursors. TDMASn was heated at 70 °C to obtain proper vapor pressure for the deposition. Nitrogen gas (99.999%) was used to carry the precursors and purge by-products in each cycle. The chamber pressure baseline was maintained at about 10 mbar. The ALD-SnO2 procedure was set as follows: (1) a 0.5 s supply of TDMASn; (2) a 5 s extended exposure of TDMASn to the substrate; (3) a 15 s purge of oversupplied TDMASn and any by-products; (4) a 0.5 s supply of H2O vapor; (5) a 5 s extended exposure of H2O to the substrate; (6) a 15 s purge of unreacted H2O and any by-products. The thickness of SnO2 was controlled by varying the number of ALD cycles from 100 to 200, 300, and 400 cycles, which are named 100-SnO2, 200-SnO2, 300-SnO2, and 400-SnO2, respectively. 3.2 Material structural characterization Scanning Electron Microscopy (SEM, FEI Nova NanoSEM, 15 kV) and profilometer measurement (DektakXT stylus Profiler, Bruker) were applied to measure the thickness of ALD-deposited SnO2 thin films. The surface morphology and surface elemental compositions were investigated using SEM equipped with Energy Dispersive Spectrometer (EDS, Oxford Instruments X-Max EDS detector). The chemical composition and bonding of ALD deposited SnO2 thin films were examined by X-ray Photoelectron Spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical). In each measurement, the error is calculated as the standard deviation of the five successive data series that have been collected, which in the case of thickness measurement, it was about 5- 10 nm. 46 3.3 Fabrication of MOS gas sensors KMHP-100 MEMS Micro-Hotplate (Kebaili Corporation, USA) was used as the substrate for deposition of SnO2 sensing layers because it offered a temperature range of 300-600 °C. The structure of the ALD-SnO2 MOS gas sensors is shown in Figure 28a. Four pins (1-4) are connected to a 1 mm2 microchip (5 in Figures 28a and 28b). The top two pins (1 and 2 in Figures 28a and 28d) are sensing pins, and the bottom two (3 and 4) are the heater pins. Figure 28b illustrates the schematics of the microheater chip. Area 3 in this figure is the designated area for depositing SnO2 sensing layers. This space between electrodes 1 is about 250 microns. The two other electrodes are connected to a patterned Pt microheater, deposited underneath the silicon oxide layer. The patterned part of the heater is located just below the distance between the two electrodes (Figure 28b-3). According to the datasheet of this microheater, it can raise the temperature to 350 °C. In the experiment condition, temperature as a function of heater input voltage is as follow: 𝑇(𝑉)(°𝐶) = 59.5 × 𝑉𝑖 + 27 (0.4 𝑉 < 𝑉𝑖 < 6 𝑉) (Equation 12) Where T(V) is the temperature of the microheater according to its input voltage and Vi is the microheater input voltage. It has been shown that at high temperatures, e.g., more than 300 °C, the thin layer of SnO2 can act as a variable resistor in the microheater circuit (Figure 28d). Thus, when it is exposed to air/oxygen, oxygen ions adsorb to the surface, changing the thickness of the electron depletion layer, thereby changing the conductivity and resistance of the thin film. When this thin film is exposed to a reducing gas, the adsorbed oxygen will oxidize the gas molecules, resulting in an electron donation to the semiconductor. Consequently, the conductivity and resistivity will change, producing a response signal towards the target gas [48]. Since our deposited SnO2 is considered an n-type semiconductor, donating an electron is equivalent to increasing the number of charge carriers on the sensing layer surface, or in other 47 words, increasing the conductivity and decreasing the resistivity of the gas sensor (Figure 28e: valleys are the exposure time to the target gas). Figure 28- Schematic representative of (a) the fabricated MOS gas sensor using KMHP-100 MEMS Micro-Hotplate. 1-4 are the electrical pins connecting the heater and sensing electrodes to the electrical measurement circuits, 5 is the gas sensor chip. (b) top view of gas sensor chip before deposition, 1: sensing electrodes, 2: heater electrodes, 3: sensing layer location. (c) cross-sectional view of gas sensor chip after deposition, 1: deposited SnO2 as the sensing material 2: patterned Pt microheater 3: sensing electrodes. (d) measurement electrical circuits, pins 1 and 2 are connected to the sensing layer, and resistance change is measured by measuring the Vout; pins 3 and 4 are connected to the heater. (e) an example of a normalized gas response diagram with three exposures to a target gas. 3.4 Gas sensing data collection 3.4.1 Pre-test preparations Sensing layer resistance change was measured by a potentiostat (Princeton Applied Research VersaSTAT 4) with a resolution of 6 μV and 120 fA for voltage and current measurements, respectively. Depending on the experimental conditions, such as mechanical and electrical noises and the target gas, the resistance error was between 0.005 to 1 Ω, less than 0.001 % of the 48 reading resistance. The sensor input voltage was kept at a constant value of 5 V (Vin in Figure 28d), and current, and output voltage (Vout) were the variables. The microheater input voltage (V3-4 in Figure 28d) was provided by a DC power supply (Triple Output DC supply, B&K Precision) with an accuracy of 0.1 V, varied in the range of 1 to 6 V. The fabricated SnO2 MOS sensors were allowed to rest for 20 minutes after ALD deposition. Then, their pins were connected to the potentiostat and the DC power supply for 30 minutes to raise the temperature and stabilize the surface condition, consequently recording a constant resistance corresponding to the air exposure. This 30-minute recovery period was considered between each test with different microheater input voltages. To prepare a specified target gas concentration, the liquid form of the target gas was injected by a single-channel pipette into a gas testing chamber (previously described in [18]). A heater and a fan were implemented for 10 minutes to evaporate the liquid and circulate the air in the box, ensuring the sample's homogeneity. 3.4.2 Test Four different thicknesses (100, 200, 300, and 400 cycles) were deposited. Each deposited sensor was exposed to three VOCs (ethanol, acetone, and toluene) at 227 ppm. For simultaneous identification of the optimum thickness and microheater input voltage, we gathered the data at six operating temperatures (2, 3, 4, 4.5, 5, and 5.5 V as the microheater input voltages), which were 146, 205, 265, 295, 325, 355 °C. In total, gas responses were recorded in 72 different conditions regarding thickness, microheater input voltage, and the target gas. A recorded gas response was repeated five times to ensure the reliability and repeatability of the results. The average value and the standard deviation of the collected data series were reported as the final value and its uncertainty. Between each data gathering, the gas chamber was fully flashed with an air compressor. Moreover, the gas sensor was exposed to the empty chamber to ensure the baseline equality between free air and air inside the box. 49 After finding the optimized sensor, it was exposed to our target gases in four concentrations from 20 to 227 ppm, to characterize its sensitivity: 𝑆 =𝜕(𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒)𝜕 (𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛) (Equation 13) Where S is the sensitivity, the Response is (Ra/Rg), and the Concentration is the target gas concentration in ppm. Additionally, Short-term and long-term stability (7 days) were investigated in the optimized condition. 3.4.3 Sources of error The primary source of error in this experiment can be originated from the gas sample preparation process. As mentioned before, between each data gathering, the gas chamber was fully flashed with an air compressor, then depending on the data set, the target concentration was prepared through injection of a specific liquid form of gas. Regarding the size of the gas chamber (about 8 L) and precision of the micropipette (0.1 μL), the prepared concentration could be slightly varied between each experiment; moreover, the mentioned error limited the lower possible prepared concentration down to 20 ppm. Besides the above-discussed limitation, microheater performance change could be considered the second primary source of error in this experiment. According to the literature, the gas sensor response is highly dependent on the sensing temperature. With this in mind and considering the fact that not all of our commercial microheaters are exactly performing the same, it can be concluded that another source of error is coming from the microheater's stability. Additionally, electrical and mechanical noises, air and light pollutant in the laboratory environment, and human error can be other sources of error in this experiment. 50 Chapter 4: Results and discussion This chapter discusses the material characterizations of the thin film deposited. On the basis of selected target gases, different deposited thicknesses, and different operating temperatures, the collected gas sensing results are categorized and shown in diagrams. Lastly, this optimized sensor is evaluated regarding its sensing performance, such as sensitivity and stability. 4.1 Material Characterization 4.1.1 Thickness measurement Figure 29- (a-d) Cross-sectional view and (e-h) top-view SEM images of 100-SnO2, 200-SnO2, 300-SnO2, and 400-SnO2 deposited on Si/Si3N4 substrates by ALD at 100 °C using TDMASn and H2O as the precursors. Figure 29(a-h) shows the cross-sectional and top-view SEM images of SnO2 thin films deposited on Si wafers using four ALD cycles (100, 200, 300, and 400) at 100 °C. It can be seen that the SnO2 thin films exhibit excellent uniformity on the Si/Si3N4 substrates. Some small nanopillars are observed in 100-SnO2 thin film, probably due to the impurities introduced during the sample handling process. The thickness of SnO2 thin films increases proportionally with the ALD cycle numbers and is determined as 26, 50, 73, and 99 nm for 100-SnO2, 100-SnO2, 100-SnO2, and 100-SnO2, respectively under SEM. A linear fitting of the SnO2 film thickness as a function of ALD 51 cycle numbers yields a growth per cycle (GPC) of 0.25 nm (Figure 30a). Profilometer measurement on the SnO2 film thickness gives a slightly lower GPC of 0.21 nm. The different GPCs are due to measurement errors using SEM and profilometer techniques. Moreover, the atomic weight percentage of SnO2 films is measured by EDX and is presented in Figure 30b. The mass fraction of Sn and O elements increases with ALD cycles linearly. Figure 30- (a) SnO2 film thickness as a function of ALD cycle numbers on Si/Si3N4 substrates deposited at 100 °C. The film thickness was measured by both SEM and profilometer. A linear fitting yields a growth per cycle (GPC) of ~ 0.21-0.25 nm; (b) The weight percentage of Sn and O in the ALD-SnO2 samples as a function of ALD cycle numbers. 4.1.1 XPS results The ALD SnO2 films were further analyzed by XPS to confirm the elemental composition and chemical bonding, and the results are shown in Figure 31. As seen in Figure 31a, all the samples contain Sn, O, and C elements in the XPS full spectra. Deconvolution of Sn3d peaks leads to one strong peak at 487.1 eV (Sn 3d5/2) and the other one at 495.7 eV (Sn 3d3/2) with a binding energy separation of 8.6 eV, indicating the presence of Sn4+ in all SnO2 films [13]. Deconvolution of the O 1s peak (Figure 32) suggests two peaks located at 530.9 eV (Sn-O) and 531.9 eV (-OH) [7]. Moreover, the atomic ratio of O/Sn is calculated to be 2.1 for all the samples (Table 2), which confirms the chemical formula of SnO2. The existence of the C element is likely due to residual precursors, adsorbed hydroxyl groups, and adventurous carbon on the thin film surface [146]. 52 Another interesting observation from XPS data is that the maximum atomic concentration of oxygen ions in the 200-SnO2 sample and the minimum observed amount of oxygen species were in the 400-SnO2 sample. This fact will directly affect the gas sensing ability of the ALD deposited sample, which will be discussed further in detail. Nevertheless, XPS analysis proves the successful deposition of SnO2 films by ALD. Figure 31- (a) XPS full survey, and deconvolution of (b) Sn 3d and (c) O 1s spectra for 100-SnO2, 200-SnO2, 300-SnO2, and 400-SnO2 on the Si/Si3N4 substrates. 53 Figure 32- Deconvolution (a) Sn 3d, (b) O1s, and (c) C 1s for 400-SnO2 on Si/Si3N4 substrates as a representative example. Table 4- Element concentration of O, Sn, and C in the ALD-deposited SnO2 samples measured by XPS. Sample 100-SnO2 200-SnO2 300-SnO2 400-SnO2 O (at.%) 45.2 48.0 44.8 39.6 Sn (at.%) 21.7 23.0 21.4 18.6 C (at.%) 31.8 27.5 32.2 40.1 O/Sn atomic ratio 2.1 2.1 2.1 2.1 4.2 Operational Optimization of sensing temperature and the effect of polarity 4.2.1 Experimental results The 72 collected data (4 thicknesses × 3 target gases × 6 applied voltages) were categorized into four groups based on the thickness of SnO2 layers. Then, the gas sensor response vs. microheater temperature was drawn for each deposited thickness (Figure 33 (a-d)) towards the three target gases. Surprisingly, the sensors showed the highest response toward each target gas in a specific microheater temperature (with the precision of 20 °C based on the 0.5 V voltages steps) for that gas regardless of the thickness. These results have been summarized in Figure 33e. 54 Figure 33f shows the change in Ra (sensor resistance in the air) of all the deposited sensors in different operating temperatures. When the temperature is raised, the charge carriers' mobility increases, so the resistance decreases. This logarithmic behavior of semiconductors' resistance with temperature has been observed and formulated by Steinhart and Hart [147]. Moreover, it can be concluded that the thicker the thickness, the less the resistance. Figure 33- (a-d) gas sensor response (Ra/Rg) of the deposited SnO2 thin films toward 227 ppm of ethanol, acetone, and toluene in different operating temperatures (microheater input voltages) (a) 55 for sample 100-SnO2 (b) 200-SnO2 (c) 300-SnO2 (d) 400-SnO2. (e) the optimum operating temperatures (microheater input voltages) to achieve the highest gas sensor response, in all different thicknesses, for ethanol (Topt= 325 °C), acetone (Topt= 295 °C), and toluene (Topt= 265 °C). "rp" represent "relative polarity of the target gas to water molecules." (f) gas sensor resistance of the deposited thin films in the air in different operating temperatures (microheater input voltages) Based on the equations (14-16), depending on the temperature, the adsorbed oxygen can be ionized in three different oxygen species: 𝑂2− (superoxide ions), 𝑂− (singly), and 𝑂2− (doubly ions) [148]. 𝑂2 (𝑎𝑑𝑠) + 𝑒− ↔ 𝑂2 (𝑎𝑑𝑠)− 𝑇 < 𝑇1 (Equation 14) 𝑂2 (𝑎𝑑𝑠)− + 𝑒− ↔ 2𝑂 (𝑎𝑑𝑠)− 𝑇1 < 𝑇 < 𝑇2 (Equation 15) 𝑂(𝑎𝑑𝑠)− + 𝑒− ↔ 𝑂 (𝑎𝑑𝑠)2− 𝑇 > 𝑇3 (Equation 16) The exact temperature range for these equations (14-16) was reported differently in the literature; however, all studies mentioned that 𝑇1 < 𝑇2 < 𝑇3. Moreover, it can be concluded that desorption of 𝑂 (𝑎𝑏𝑠)2− results in more electron donation to the sensing layer conduction band rather than 𝑂 (𝑎𝑏𝑠)− , and same between 𝑂 (𝑎𝑏𝑠)− and 𝑂2 (𝑎𝑏𝑠)− . Therefore, the higher the temperature, the sensor tends to have a higher response. At the same time, at a higher temperature, less oxygen and target gas molecules are adsorbed onto the sensing layer surface, resulting in a lower response value [15]. Combining these two effects together justifies the increase and decrease in the response (𝑅𝑎𝑅𝑔) vs. temperature (microheater input voltage) curve, and the existence of an optimum operating temperature. After the formation of ions on the surface, the target gas oxidation is another important factor in the gas sensor response. For the selected target gases in this study, the chemical reactions are as follow [32], [63-65]: 𝐶2𝐻5𝑂𝐻 + 6𝑂−𝑎𝑑𝑠 ↔ 2𝐶𝑂2 + 3𝐻2𝑂 + 6𝑒− Ethanol (Equation 17) 𝐶2𝐻5𝑂𝐻 + 6𝑂2−𝑎𝑑𝑠 ↔ 2𝐶𝑂2 + 3𝐻2𝑂 + 12𝑒− Ethanol (Equation 18) 56 𝐶𝐻3𝐶𝑂𝐶𝐻3 + 8𝑂−𝑎𝑑𝑠 ↔ 3𝐶𝑂2 + 3𝐻2𝑂 + 8𝑒− Acetone (Equation 19) 𝐶6𝐻5𝐶𝐻3 + 18𝑂−𝑎𝑑𝑠 ↔ 7𝐶𝑂2 + 4𝐻2𝑂 + 18𝑒− Toluene (Equation 20) 𝐶7𝐻8 + 18𝑂2−𝑎𝑑𝑠 ↔ 7𝐶𝑂2 + 4𝐻2𝑂 + 36𝑒− Toluene (Equation 21) Figure 33e shows that in all the deposited sensing layer thicknesses, the order of the optimum operating temperatures (microheater input voltage) for the three different target gases is: 𝑇𝑜𝑝𝑡 (𝐸𝑡ℎ𝑎𝑛𝑜𝑙) > 𝑇𝑜𝑝𝑡 (𝐴𝑐𝑒𝑡𝑜𝑛𝑒) > 𝑇𝑜𝑝𝑡 (𝑇𝑜𝑙𝑢𝑒𝑛𝑒). Moreover, it is known that the relative polarities (rp) of these gases are as follows: 𝑟𝑝 (𝐸𝑡ℎ𝑎𝑛𝑜𝑙) > 𝑟𝑝 (𝐴𝑐𝑒𝑡𝑜𝑛𝑒) > 𝑟𝑝 (𝑇𝑜𝑙𝑢𝑒𝑛𝑒). The sensing behavior of a SnO2-based nanostructured sensor towards ethanol and acetone has been investigated by Abokifa et al. [149], who theoretically calculated the adsorption energies for ethanol and acetone on a SnO2 surface. Based on their results, the higher polarity of ethanol (as compared to that of acetone) can lead to larger adsorption energy, in other words, stronger bindings between ethanol molecules and the SnO2 surface. Therefore, it can be generalized that the higher the polarity, the more robust the adsorption bindings. As mentioned previously, the optimum operating temperature is affected by two phenomena: increasing the oxygen ions' negative charge and decreasing the number of adsorbed target gas /oxygen molecules on the sensing surface due to raising the temperature. The first effect is independent of the polarity of the target gas. However, the second phenomenon will be less effective in the case of polar target gases. Since higher polarity results in stronger adsorption, raising the temperature has a smaller effect on breaking the bonds of adsorbed molecules. Consequently, the second function would be less effective. The interaction between these two functions and their impact on the optimal operating temperature is modeled mathematically in the next section. 57 4.2.2 Mathematical modeling Figure 34- The trend of gas sensor response (Ra/Rg) of 200 cycles deposited SnO2 thin films toward 227 ppm of Ethanol, Acetone, and Toluene as a function of the operating temperature. As shown in Figure 34, the response value as a function of temperature has a maximum point for each gas. This behavior originates from increasing the oxygen negative charge and decreasing the number of adsorbed molecules due to raising the temperature. Assumptions: Since these two functions are working independently, the final function of gas sensor response, [𝑅𝑎𝑅𝑔]𝑇 = 𝑅𝑇, and the two influential functions can be simplified as: 𝑓(𝑇): The first influential function corresponding to increasing the oxygen negative charge due to raising the temperature. 𝑓′ (𝑇) > 0: It is an ascending function. 𝑔(𝑇): The second influential function corresponding to decreasing the number of adsorbed molecules due to raising the temperature 𝑔′(𝑇) < 0: It is a descending function. The final function can be written as bellow; constants a and b represent the effectiveness of the functions f and g. 58 𝑅(𝑇) = 𝑎𝑓(𝑇) + 𝑏𝑔(𝑇) (Equation 22) 𝑎, 𝑏 > 0 𝑅′(𝑇𝑜𝑝𝑡) = 0 (Equation 23) 𝑅" < 0: Since it has a maximum (Equation 24). Modeling and analyze: From (Equation 22) and (Equation 23): 𝑅′ = 𝑎𝑓′ + 𝑏𝑔′ = 0 → 𝑓′(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡= −𝑏𝑎 (Equation 25) From (Equation 24): 𝑅" = 𝑎𝑓" + 𝑏𝑔" < 0 (If 𝑔" > 0 ) 𝑓"(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡< −𝑏𝑎 (Equation 26-a) (If 𝑔" < 0 ) 𝑓"(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡> −𝑏𝑎 (Equation 26-b) From (Equation 25) and (Equation 26-a), and since 𝑓′ > 0, 𝑔′ < 0, 𝑔" > 0: 𝑓"(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡< 𝑓′(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡 → 𝑓"(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡> 𝑓′(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡 From (Equation 25) and (Equation 26-b), and since 𝑓′ > 0, 𝑔′ < 0, 𝑔" < 0 : 𝑓"(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡> 𝑓′(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡 → 𝑓"(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡> 𝑓′(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡 In both cases, we have: 𝑓"(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡- 𝑓′(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡 > 0 → 𝑓"(𝑇)𝑔′(𝑇)|𝑇𝑜𝑝𝑡- 𝑓′(𝑇)𝑔"(𝑇)|𝑇𝑜𝑝𝑡(𝑔′(T))2|𝑇𝑜𝑝𝑡> 0 Therefore: (𝑓′(T)𝑔′(T))′|Topt> 0 (Equation 27) Moreover, we have the derivative of the equation (Equation 25) of the variable b as: (𝑓′(T)𝑔′(T))′|Topt× 𝑑𝑇𝑑𝑇𝑜𝑝𝑡|𝑇𝑜𝑝𝑡× 𝑑𝑇𝑜𝑝𝑡𝑑𝑏|𝑇𝑜𝑝𝑡= −1𝑎 59 Since 𝑑𝑇𝑑𝑇𝑜𝑝𝑡|𝑇𝑜𝑝𝑡= 1 and from (Equation 24): (𝑓′(T)𝑔′(T))′|Topt> 0, and 𝑎 > 0: 𝑑𝑇𝑜𝑝𝑡𝑑𝑏|𝑇𝑜𝑝𝑡< 0 (Equation 28) This means by increasing b (effectiveness of the second function), 𝑇𝑜𝑝𝑡 will be decreased. As shown in Figure 33e and based on the mathematical model, it can be concluded that the optimal operating temperature depends on the polarity of the target gas, and secondly, the higher the polarity, the higher the optimum operating temperature. 4.3 Operational optimization of sensing layer thickness To find a correlation between the thickness of the deposited layer and the sensor response, the 72 collected data (4 thicknesses × 3 target gases × 6 applied voltages) were categorized into three groups based on the applied voltages (three optimal voltages for the selected targes gases: 4, 4.5, and 5 V). Then the gas sensor response vs. the ALD cycle (SnO2 thickness) was obtained for each optimum applied voltage (Figure 35 (a-c)) towards the three target gases. The results showed that the 200-SnO2 sensor, with a thickness of about 40 nm, has the highest response towards the selected target gases regardless of the target gas and the operating temperature. This result can be justified by considering the XPS data (Table 2) and the presence of the highest percentage of adsorbed oxygen on the 200-SnO2 surface. Figure 35- (a-c) gas sensor response of the deposited SnO2 thin films toward 227 ppm of Ethanol, Acetone, and Toluene in different thickness (ALD cycles) (a) heater voltage as 5 V (b) heater voltage as 4.5 V (c) heater voltage as 4 V 60 The independence of the optimal thickness from the operating temperature and the target gas polarity has been reported in previous research [39], [109]. As shown in Figure 27, the gas sensor response corresponding for 200 ALD cycle samples is higher than the other fabricated sensors at all operating temperatures [109]. This independence means that the optimization of the fabrication process (thickness) can be separated from the optimization of the operating condition (working temperature and the target gas). This finding will give us the opportunity to use commercial sensors and optimized them based on the target VOC, through changing the input voltage and working temperature. 4.4 Characterization of optimized sensor Since this work focuses on the gas sensor response value (Rair/Rgas), the essential sensing characteristics related to the sensor response are reported after optimizing the sensing operation conditions. Figure 36a shows the sensitivity diagram of the 200-SnO2 sample towards the selected target gases. Each target gas has been tested at its optimal microheater input voltage (operating temperature). The sensor showed a linear response in the concentration range of 20-227 ppm (point: 0 ppm and 1 as the response is added to all the lines). Below are the equations corresponding to each target gas: 𝑅𝑎/𝑅𝑔 = 0.0277 𝑋𝑔𝑐 + 1.1801 𝑉ℎ𝑒𝑎𝑡𝑒𝑟 = 5 𝑉, 𝑇𝑜𝑝𝑡 = 325 ℃ Ethanol (Equation 29) 𝑅𝑎/𝑅𝑔 = 0.0178 𝑋𝑔𝑐 + 1.3245 𝑉ℎ𝑒𝑎𝑡𝑒𝑟 = 4.5 𝑉, 𝑇𝑜𝑝𝑡 = 324 ℃ Acetone (Equation 30) 𝑅𝑎/𝑅𝑔 = 0.0204 𝑋𝑔𝑐 + 1.3245 𝑉ℎ𝑒𝑎𝑡𝑒𝑟 = 4 𝑉, 𝑇𝑜𝑝𝑡 = 295 ℃ Toluene (Equation 31) Where 𝑅𝑎/𝑅𝑔 is gas sensor response and 𝑋𝑔𝑐 is the target gas concentration (ppm). To ensure the long-term performance stability of the optimized sensor, its response towards 227 ppm of the selected target gas was monitored over seven days of continuous operation. As shown in Figure 36b, the sensor presented desirable stability over this period, with less than 10% drift which may be due to surface oxidation, changes in the surface morphology and microstructures, 61 and changes in the microheater performance due to the non-stop heating. Furthermore, the normalized short-term stability and repeatability have been illustrated in Figure 37 (c-e) for ethanol, acetone, and toluene. A linear relationship was observed between the response value and the target gas concentration. Figure 36- Gas sensor performance based on the optimum deposited sensing layer (sample 200- SnO2): (a) sensitivity values for ethanol, acetone, and toluene are 0.0277, 0.0178, and 0.0204, respectively. (b) Long-term stability performance for sample 200-SnO2. (c-e) Short-term stability and repeatability performance for sample 200-SnO2 towards (c) ethanol, (d) acetone, and (d) toluene. For each target gas, their optimum heater voltage has been used to collect the data. 62 Chapter 5: Conclusions and future works 5.1 Summary Four SnO2 thin-film sensors with four thicknesses were deposited on a microheater with silicon/silicon nitride substrate by atomic layer deposition (100, 200, 300, and 400 ALD cycles), using (TDMASn- Sn[(CH3)2N]4) and water as the precursors, at the deposition temperature of 100 °C. The deposited thin films' chemical and physical surface characteristics were investigated through SEM, EDS, Profilometer, and XPS analyses. The microheater input voltage and the deposited thickness were optimized to achieve the highest sensing response towards ethanol, acetone, and toluene, representing polar to non-polar VOCs. Moreover, a thorough explanation has been proposed on the effect of target gas polarity on the optimized operating temperature. Finally, the optimized sensor was characterized by sensitivity and long-term and short-term stability towards the selected target gases. The 200-SnO2 sample, with a thickness of ~40 nm, exhibited the highest response towards the selected target gases for a concentration range between 20-227 ppm at all operating temperatures. In our case study, the optimum operating temperatures for ethanol, acetone, and toluene were determined to be 325 °C, 325 °C, and 295 °C, respectively. Additionally, it can be concluded from our results that VOCs with higher polarity require a higher operating temperature to achieve the highest response. This observation can be attributed to the highest binding energies and the stronger adsorption in a higher polar VOC. This finding can be utilized to enhance the selectivity of SnO2-based MOS sensors to differentiate between VOCs in mixtures based on their polarities selectively. The developed ALD-deposited SnO2 thin film sensor showed a stable, reliable, and repeatable linear response in the detection range and could have promising application as a selective, sensitive, miniature, low-power VOCs sensor. 63 5.2 Contributions (a) Fabrication process of a thin film SnO2 as the sensing layer, in a sensor configuration, through Atomic Layer Deposition has been explained, including suitable precursors, required temperature, and timing. ALD deposition occurred at the growth rate of 2.2 Å per cycle. (b) a highly sensitive SnO2-based MOS sensor has been developed with the sensing layer thickness of 40 nm for the detection of Volatile Organic Compounds. (c) Sensing layer thickness and operating temperature have been optimized to achieve the highest response towards the selected VOCs in the concentration range of 20-227 ppm. (d) The effect of target gas polarity on the sensing performance and the optimization of the deposited thickness and the operating temperature have been investigated. It was found that target gas with a higher polarity required a higher sensing temperature to be effectively detected. (e) To the best of our knowledge, it is the first time that the relationship between the target gas polarity and the optimum operating temperature, and the optimum sensing layer thickness is investigated. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Hoorfar, Mina
Liu, Jian
Optimization and characterization of atomic-layer-deposited tin oxide thin films metal oxide semiconductor (MOS) sensors to detect polar and non-polar volatile organic compounds
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