Research Progress on Coating of Sensitive Materials for Micro-Hotplate Gas Sensor

Micro-hotplate gas sensors are widely used in air quality monitoring, identification of hazardous chemicals, human health monitoring, and other fields due to their advantages of small size, low power consumption, excellent consistency, and fast response speed. The micro-hotplate gas sensor comprises a micro-hotplate and a gas-sensitive material layer. The micro-hotplate is responsible for providing temperature conditions for the sensor to work. The gas-sensitive material layer is responsible for the redox reaction with the gas molecules to be measured, causing the resistance value to change. The gas-sensitive material film with high stability, fantastic adhesion, and amazing uniformity is prepared on the surface of the micro-hotplate to realize the reliable assembly of the gas-sensitive material and the micro-hotplate, which can improve the response speed, response value, and selectivity. This paper first introduces the classification and structural characteristics of micro-hotplates. Then the assembly process and characteristics of various gas-sensing materials and micro-hotplates are summarized. Finally, the assembly method of the gas-sensing material and the micro-hotplate prospects.


Introduction
Gas sensors play an essential role in modern life, and the demand for gas sensors is increasing day by day [1]. Traditional gas detection relies on chromatography spectroscopy [2]. The instruments used in these methods are large in size, high in price, and complex in operation, limiting field detection application [3]. Micro-nano gas sensors have attracted attention due to their small size [4], low price [5], and easy use. Micro-nano gas sensors are widely used in hazardous chemical detection [6], air quality monitoring [7], human health monitoring [8], chemical production process [9], intelligent systems for agricultural applications [10].
Among micro-nano sensors, MEMS micro-hot plate sensors are widely used due to their low power consumption [11], small size, and superior consistency [12]. The microhotplate gas sensor consists of a micro-hotplate and a gas-sensitive material layer. The micro-hotplate is responsible for providing temperature conditions for the sensor to work. The gas-sensing material layer is responsible for redox reactions with the gas molecules to be measured, causing changes in the resistance value and realizing the detection of gas types and concentrations. At present, there are many research papers on the synthesis of gas-sensing materials, but few reviews about the film-forming coating of gas-sensing materials are involved. The effective formation of patterned nano-scale gas-sensing material

Gas Sensitive Material Coating
The efficient formation of patterned sensor nanomaterials at specific positions on the surface of the micro-hotplate gas sensor and the rational integration of the patterning method with MEMS technology are the leading technical obstacles to be overcome in the fabrication of the micro-hotplate gas sensor [13]. The first part of this section introduces the classification and structural characteristics of micro-hotplates, and the second part introduces the assembly methods of different gas-sensitive materials and micro-hotplates, such as drop coating [14], liquid deposition [15], atomic layer deposition [16], chemical vapor deposition [17], dielectrophoretic deposition [18], spray pyrolysis [19], sputtering [20], in situ growth [21], inkjet printing [22], screen printing [23], self-assembly [24].

Micro-Hotplate Classification and Structure
Micro-nano fabrication technology promotes the development of sensor miniaturization. Ceramic tube gas sensor is made by coating-sensitive material on the outside of the ceramic tube, which has some shortcomings in the production process, such as low efficiency and consistency cannot be guaranteed. To overcome the shortcomings of ceramic tube gas sensors, researchers apply microelectronic manufacturing technology to the sensormanufacturing process and design a micro-hotplate gas sensor. The micro-hotplate gas sensor has the advantages of low power consumption, small size, excellent compatibility with electronic equipment, and so on [25].
Micro-hotplate gas sensors can be divided into silicon-based [26], ceramic-based [27], glass-based [28], etc. According to the different positions of the heating electrode and the test electrode of the micro-hotplate, it can be divided into a co-planar micro-hotplate [29] and a non-planar micro-hotplate [30]. Whether the support membrane is closed can be divided into closed membrane [31] type micro-hotplate and suspended membrane [32] type micro-hotplate.
Silicon-based micro-hotplates are common, and silicon-based micro-hotplates have fantastic compatibility with integrated circuits [33]. L. Xu et al. fabricated a micro-hotplate on an n-type silicon wafer, as shown in Figure 1. The thickness of the silicon wafer substrate is 350 µm. First, a silicon oxide layer is grown on the surface of the silicon wafer. Then, 1 µm low-stress SiN 0.8 was deposited on the silicon substrate under the condition of 800 • C by using low-pressure chemical vapor deposition technology. Using the atomic layer deposition method, an aluminum oxide bonding layer is deposited, and a platinum electrode is deposited on the aluminum oxide bonding layer. Finally, the release window of the micro-hotplate is etched by an ion beam, and the micro-hotplate is released by TMAH solution [34].
During the processing of the silicon-based micro-hotplate, the heating electrode, the test electrode, the support layer, and the insulating layer materials have various choices, as shown in Table 1. The heating electrode material can be selected from Ni, Cr, Ti, Pt, Mo, etc. Test electrode materials can choose Ni, Al, Cr, Au, Ti, Pt, Pd, Ag, etc. The supporting layer and insulating layer are mainly made of SiO 2 and Si 3 N 4 .
During the processing of the silicon-based micro-hotplate, the heating electr test electrode, the support layer, and the insulating layer materials have various as shown in Table 1. The heating electrode material can be selected from Ni, Cr, T etc. Test electrode materials can choose Ni, Al, Cr, Au, Ti, Pt, Pd, Ag, etc. The su layer and insulating layer are mainly made of SiO2 and Si3N4. Ni [3 SiO2 Ni SiO2 Al [3 SiO2 Cr/Pt SiO2 Cr/Au [3 SiO2 Ti/Pt SiO2 Ti/Pt [3 SiO2 Pt SiO2/Si3N4 Pt [3 SiO2 Ni SiO2 Pd/Ag [4 SiO2/Si3N4 Mo Si3N4 - [4 Compared to silicon, ceramics have a smaller coefficient of thermal expansion fore, the thermal stress of the ceramic-based micro-hotplate is smaller than that o icon-based micro-hotplate. W. J. Zhao et al. designed a ceramic micro-hotplate b AlN, as shown in Figure 2. A Pt test electrode and a Pt heating electrode are des the surface of the AlN ceramic. The heating electrode is a ring-shaped structure, th electrode is a comb-like structure, and the signal electrode is coated with a gasmaterial [42].   Compared to silicon, ceramics have a smaller coefficient of thermal expansion. Therefore, the thermal stress of the ceramic-based micro-hotplate is smaller than that of the silicon-based micro-hotplate. W. J. Zhao et al. designed a ceramic micro-hotplate based on AlN, as shown in Figure 2. A Pt test electrode and a Pt heating electrode are designed on the surface of the AlN ceramic. The heating electrode is a ring-shaped structure, the signal electrode is a comb-like structure, and the signal electrode is coated with a gas-sensitive material [42]. During the processing of the silicon-based micro-hotplate, the heating electr test electrode, the support layer, and the insulating layer materials have various as shown in Table 1. The heating electrode material can be selected from Ni, Cr, Ti etc. Test electrode materials can choose Ni, Al, Cr, Au, Ti, Pt, Pd, Ag, etc. The sup layer and insulating layer are mainly made of SiO2 and Si3N4.  Figure 2. A Pt test electrode and a Pt heating electrode are desig the surface of the AlN ceramic. The heating electrode is a ring-shaped structure, th electrode is a comb-like structure, and the signal electrode is coated with a gas-s material [42]. Low temperature co-fired ceramic (LTCC) micro-hotplates have a wide rang plications in gas sensors and microfluidic devices. It is easy to construct three-dime structures. D. K. Kharbanda et al. used laser irradiation technology to sinter con paste on a low temperature co-fired ceramic (LTCC) substrate to generate heati trodes and test electrodes to complete the fabrication of micro-hotplates [43]. L. Ku Low temperature co-fired ceramic (LTCC) micro-hotplates have a wide range of applications in gas sensors and microfluidic devices. It is easy to construct three-dimensional structures. D. K. Kharbanda et al. used laser irradiation technology to sinter conductive paste on a low temperature co-fired ceramic (LTCC) substrate to generate heating electrodes and test electrodes to complete the fabrication of micro-hotplates [43]. L. Kulhari et al. designed two LTCC micro-hotplates with different structures, and the area of the micro-hotplate is 3.35 mm × 3.35 mm, as shown in Figure 3. In the first micro-hotplate structure, the PdAg temperature measuring electrode is installed in the middle of the double-layer micro-hotplate. The upper and lower surfaces are platinum heating and gold test electrodes. In the second type of micro-hotplate structure, the platinum temperature measuring electrode is installed on the bottom surface of the micro-hotplate. The middle of the double-layer micro-hotplate is a platinum heating electrode, and the surface of the micro-hotplate is a gold test electrode [27].
al. designed two LTCC micro-hotplates with different structures, and the area of cro-hotplate is 3.35 mm × 3.35 mm, as shown in Figure 3. In the first micro-hotpla ture, the PdAg temperature measuring electrode is installed in the middle of the layer micro-hotplate. The upper and lower surfaces are platinum heating and g electrodes. In the second type of micro-hotplate structure, the platinum temp measuring electrode is installed on the bottom surface of the micro-hotplate. The of the double-layer micro-hotplate is a platinum heating electrode, and the surfa micro-hotplate is a gold test electrode [27]. Compared with alumina ceramics, zirconia ceramics have extremely low conductivity (2.5 W/m•K). Compared with alumina ceramic micro-hotplates, th consumption of micro-hotplates made of zirconia ceramics is lower. A. A. Vasili fabricated a yttrium-stabilized zirconia ceramic micro-hotplate by sintering at a t ture of 1150 °C for 12 h using a sintering process. The film thickness is 10 μm, surface roughness is 0.1-0.2 μm. Platinum heater electrodes can be easily deposite film surface using a magnetron sputtering process. After depositing the platinum electrodes, the micro-hotplate was encapsulated using the TO-8 process [44]. S. A et al. fabricated a zirconia micro-hot plate oxygen sensor, and the test results show the sensor power consumption was greatly reduced to 80 mW, and the sensitivity gen was 0.6 μA/% [45].
The thermal conductivity of glass substrates is lower than silicon substrates ramic substrates [25]. A typical micro-hotplate structure is shown in Figure 4 MEMS technology, W. Y. Chang et al. fabricated a micro-hotplate on a glass substr the sensor structure is shown in Figure 4. The sensor is designed as a multilayer st and the heating electrode is Pt. The test results show that the power consumptio W when the sensor is heated to 498 °C. In the stability test, the sensor can work ously for 72 h at the heating temperature of 496.5 °C without damage [46]. Compared with alumina ceramics, zirconia ceramics have extremely low thermal conductivity (2.5 W/m·K). Compared with alumina ceramic micro-hotplates, the power consumption of micro-hotplates made of zirconia ceramics is lower. A. A. Vasiliev et al. fabricated a yttrium-stabilized zirconia ceramic micro-hotplate by sintering at a temperature of 1150 • C for 12 h using a sintering process. The film thickness is 10 µm, and the surface roughness is 0.1-0.2 µm. Platinum heater electrodes can be easily deposited on the film surface using a magnetron sputtering process. After depositing the platinum heater electrodes, the micro-hotplate was encapsulated using the TO-8 process [44]. S. Akasaka et al. fabricated a zirconia micro-hot plate oxygen sensor, and the test results showed that the sensor power consumption was greatly reduced to 80 mW, and the sensitivity to oxygen was 0.6 µA/% [45].
The thermal conductivity of glass substrates is lower than silicon substrates and ceramic substrates [25]. A typical micro-hotplate structure is shown in Figure 4. Using MEMS technology, W. Y. Chang et al. fabricated a micro-hotplate on a glass substrate, and the sensor structure is shown in Figure 4. The sensor is designed as a multilayer structure, and the heating electrode is Pt. The test results show that the power consumption is 2.35 W when the sensor is heated to 498 • C. In the stability test, the sensor can work continuously for 72 h at the heating temperature of 496.5 • C without damage [46]. al. designed two LTCC micro-hotplates with different structures, and the area of the micro-hotplate is 3.35 mm × 3.35 mm, as shown in Figure 3. In the first micro-hotplate struc ture, the PdAg temperature measuring electrode is installed in the middle of the doublelayer micro-hotplate. The upper and lower surfaces are platinum heating and gold tes electrodes. In the second type of micro-hotplate structure, the platinum temperature measuring electrode is installed on the bottom surface of the micro-hotplate. The middle of the double-layer micro-hotplate is a platinum heating electrode, and the surface of the micro-hotplate is a gold test electrode [27]. Compared with alumina ceramics, zirconia ceramics have extremely low therma conductivity (2.5 W/m•K). Compared with alumina ceramic micro-hotplates, the power consumption of micro-hotplates made of zirconia ceramics is lower. A. A. Vasiliev et al fabricated a yttrium-stabilized zirconia ceramic micro-hotplate by sintering at a tempera ture of 1150 °C for 12 h using a sintering process. The film thickness is 10 μm, and the surface roughness is 0.1-0.2 μm. Platinum heater electrodes can be easily deposited on the film surface using a magnetron sputtering process. After depositing the platinum heater electrodes, the micro-hotplate was encapsulated using the TO-8 process [44]. S. Akasaka et al. fabricated a zirconia micro-hot plate oxygen sensor, and the test results showed tha the sensor power consumption was greatly reduced to 80 mW, and the sensitivity to oxygen was 0.6 μA/% [45].
The thermal conductivity of glass substrates is lower than silicon substrates and ceramic substrates [25]. A typical micro-hotplate structure is shown in Figure 4. Using MEMS technology, W. Y. Chang et al. fabricated a micro-hotplate on a glass substrate, and the sensor structure is shown in Figure 4. The sensor is designed as a multilayer structure and the heating electrode is Pt. The test results show that the power consumption is 2.35 W when the sensor is heated to 498 °C. In the stability test, the sensor can work continuously for 72 h at the heating temperature of 496.5 °C without damage [46].  Silicon-based micro-hotplates and ceramic-based micro-hotplates are mainly manufactured by MEMS technology. Hotplate has small volume and low power consumption, but its processing technology is complex [26]. Ceramic-based micro-hotplate is mainly processed by high temperature sintered ceramics, which has large volume and high power consumption, but simple processing technology.
According to whether the support layer is closed, the micro-hotplate can be divided into closed membrane and suspended membrane. Taking the silicon-based micro-hotplate as an example, the closed-film micro-hotplate has high power consumption, but the thermal plate has excellent stability. The power consumption of the suspended micro-hotplate is lower than that of the closed micro-hotplate, and the stability of the hotplate is poor. The number of suspension beams of the suspended micro-hotplate will also affect the power consumption of the micro-hotplate, as shown in Table 2. In the design process of the micro-hotplate, it is necessary to balance the relationship between the type of the support layer of the micro-hotplate, the number of cantilever beams, and the power consumption. In addition to the conventional rectangular cantilever beam, the researchers proposed a special-shaped cantilever beam design idea to reduce the power consumption of the micro-hotplate and improve the uniformity of the temperature distribution on the surface of the micro-hotplate. Q. Liu et al. designed four kinds of special-shaped cantilever beams, as shown in Figure 5. The test results show that Design 1 has a slender cantilever beam, which can significantly reduce the power consumption of the micro-hotplate. Design 2 mesh cantilever structure has higher power consumption in Design 2 due to more shorter cantilever beams. Design 3 and Design 4 have the same cantilever beam design as Design 2, but with different heating electrodes. Design 4 has a more uniform temperature distribution on the surface of the micro-hotplate with variable width heating electrodes [32].
Silicon-based micro-hotplates and ceramic-based micro-hotplates are mainly manufactured by MEMS technology. Hotplate has small volume and low power consumption, but its processing technology is complex [26]. Ceramic-based micro-hotplate is mainly processed by high temperature sintered ceramics, which has large volume and high power consumption, but simple processing technology.
According to whether the support layer is closed, the micro-hotplate can be divided into closed membrane and suspended membrane. Taking the silicon-based micro-hotplate as an example, the closed-film micro-hotplate has high power consumption, but the thermal plate has excellent stability. The power consumption of the suspended micro-hotplate is lower than that of the closed micro-hotplate, and the stability of the hotplate is poor. The number of suspension beams of the suspended micro-hotplate will also affect the power consumption of the micro-hotplate, as shown in Table 2. In the design process of the micro-hotplate, it is necessary to balance the relationship between the type of the support layer of the micro-hotplate, the number of cantilever beams, and the power consumption. In addition to the conventional rectangular cantilever beam, the researchers proposed a special-shaped cantilever beam design idea to reduce the power consumption of the micro-hotplate and improve the uniformity of the temperature distribution on the surface of the micro-hotplate. Q. Liu et al. designed four kinds of special-shaped cantilever beams, as shown in Figure 5. The test results show that Design 1 has a slender cantilever beam, which can significantly reduce the power consumption of the micro-hotplate. Design 2 mesh cantilever structure has higher power consumption in Design 2 due to more shorter cantilever beams. Design 3 and Design 4 have the same cantilever beam design as Design 2, but with different heating electrodes. Design 4 has a more uniform temperature distribution on the surface of the micro-hotplate with variable width heating electrodes [32].  The temperature control methods of the micro-hotplate can be mainly divided into isothermal heating and dynamic heating. Continuous heating refers to applying a fixed voltage across the micro-hotplate to make the micro-hotplate work at a constant operating temperature. By comparing the response values of the micro-hotplate at different operating temperatures, the optimal working temperature of the micro-hotplate is determined. Q. Zhou [53]. Dynamic heating refers to applying dynamic voltage at both ends of the micro-hotplate, and the voltage waveform includes rectangular wave, trapezoidal wave, triangular wave, pulse [54], etc. The dynamic response curve of the sensor is obtained by modulating the temperature of the micro-hotplate by dynamic voltage. In dynamic heating, pulse heating is typical. F. Shao et al. compared the response of ZnO nanowires to NH3 under the conditions of isothermal heating and pulse heating, and found that the response value of the sensor was higher than that of the isothermal heating condition under the condition of pulse heating. They think that the reason for the increase in the sensor response value under the condition of pulse heating is that in the low temperature range of pulse heating, there are high temperature surface oxygen species on the surface of the gas-sensing material, which enhances the sensor response. At the same time, the high temperature range has a cleaning effect on the material, which is also the reason for the enhanced sensor response [55].

Drop Coating
By mixing the gas-sensing material with chloroplatinic acid and terpineol to form a gas-sensing slurry, a gas-sensing layer can be formed on the surface of the micro-hotplate sensor by direct drop coating or DDG droplet technology. Z. H. Tao et al. mixed the synthesized In 2 O 3 hollow spheres with ethanol to form a slurry and coated the slurry on the ceramic micro-hotplate by a drop coating method as shown in Figure 6 [56]. W.-J. Zhao et al. mixed the synthesized In/Nb powder with chloroplatinic acid and terpineol to form a slurry, which was coated on the test electrode of the ceramic micro-hotplate. Moreover heating in an electric furnace at 500 • C for 2 h to complete the device fabrication [42]. W. Yan et al. synthesized ZnO@Co 3 O 4 porous hollow cage, using terpineol and porous hollow cage mixed grinding, drop coating to the micro-hotplate electrode area. The microhotplate was heated at 250 • C for 2 h [57]. N. Luo et al. first synthesize PdRh bimetallic hollow nanocubes and mix them with ethanol to make a paste. The paste was then transferred to a Pt interdigitated electrode using a micropipette and dried at room temperature. Finally, the sensor was annealed at 150 • C for 5 h to improve sensor stability [58]. Y. Chen et al. mixed tin dioxide nanoparticles, deionized water, and triethanolamine grind using ball milling technique and drop coated the sensitive material onto a micro-hotplate using drop coating method [59]. K. Yuan et al. dispersed the prepared CeO 2 modified WO 3 nanowires into deionized water. Next, the dispersion liquid was dropped onto the MEMS heating sensor and dried in air at room temperature [60].
M. Li et al. developed a DDG droplet technique for small area application. First, as shown in Figure 7a, a PicoTipTM emitter having an inner diameter of 8 µm is connected to the injector through a bidirectional peek pipe, as shown in Figure 7b. The injector is filled with deionized water, driven slowly by the pump, with water dripping out of the needle tip. With the assistance of a microscope, deionized water was dropped on the micro-hotplate. Then, the sensitive material powder is sprinkled on deionized water with a spoon and left to dry. Finally, that sensor was inverted to remove redundant sensitive materials and then the sensor was placed in a muffle furnace for heating [61].  Figure 7a, a PicoTipTM emitter having an inner diameter of 8 μm is connected to the injector through a bidirectional peek pipe, as shown in Figure 7b. The injector is filled with deionized water, driven slowly by the pump, with water dripping out of the needle tip. With the assistance of a microscope, deionized water was dropped on the micro-hotplate. Then, the sensitive material powder is sprinkled on deionized water with a spoon and left to dry. Finally, that sensor was inverted to remove redundant sensitive materials and then the sensor was placed in a muffle furnace for heating [61].

Liquid Deposition
LPD is a low-temperature growth process with advantages of simplicity, easy to change film composition, and easy mass production [62]. J.-C. Chiou et al. deposited porous SnO2-sensing film on the sensing area of silicon-based micro-hotplate by liquid phase deposition (LPD) [63]. I. Cho et al. achieved a local liquid-phase synthesis of porous SnO2 nanotubes on a micro-hotplate by liquid deposition, and the process is shown in Figure 8. First, ZnO nanowires were grown in situ using a hydrothermal method. Then, a precursor solution for liquid deposition was prepared by mixing SnF2, HF, H2O2, H3BO3 and deionization. The PH of the precursor solution was controlled by adding NaOH. When PH = 4, ZnO nanowires were formed during a liquid deposition for 15 min. When PH = 6, a SnO2/ZnO composite structure was formed during liquid deposition for 15 min. Finally, the micro-hotplate was rinsed with ethanol and dried [64].   Figure 7a, a PicoTipTM emitter having an inner diameter of 8 μm is connected to the injector through a bidirectional peek pipe, as shown in Figure 7b. The injector is filled with deionized water, driven slowly by the pump, with water dripping out of the needle tip. With the assistance of a microscope, deionized water was dropped on the micro-hotplate. Then, the sensitive material powder is sprinkled on deionized water with a spoon and left to dry. Finally, that sensor was inverted to remove redundant sensitive materials and then the sensor was placed in a muffle furnace for heating [61].

Liquid Deposition
LPD is a low-temperature growth process with advantages of simplicity, easy to change film composition, and easy mass production [62]. J.-C. Chiou et al. deposited porous SnO2-sensing film on the sensing area of silicon-based micro-hotplate by liquid phase deposition (LPD) [63]. I. Cho et al. achieved a local liquid-phase synthesis of porous SnO2 nanotubes on a micro-hotplate by liquid deposition, and the process is shown in Figure 8. First, ZnO nanowires were grown in situ using a hydrothermal method. Then, a precursor solution for liquid deposition was prepared by mixing SnF2, HF, H2O2, H3BO3 and deionization. The PH of the precursor solution was controlled by adding NaOH. When PH = 4, ZnO nanowires were formed during a liquid deposition for 15 min. When PH = 6, a SnO2/ZnO composite structure was formed during liquid deposition for 15 min. Finally, the micro-hotplate was rinsed with ethanol and dried [64].

Liquid Deposition
LPD is a low-temperature growth process with advantages of simplicity, easy to change film composition, and easy mass production [62]. J.-C. Chiou et al. deposited porous SnO 2 -sensing film on the sensing area of silicon-based micro-hotplate by liquid phase deposition (LPD) [63]. I. Cho et al. achieved a local liquid-phase synthesis of porous SnO 2 nanotubes on a micro-hotplate by liquid deposition, and the process is shown in Figure 8. First, ZnO nanowires were grown in situ using a hydrothermal method. Then, a precursor solution for liquid deposition was prepared by mixing SnF 2 , HF, H 2 O 2 , H 3 BO 3 and deionization. The PH of the precursor solution was controlled by adding NaOH. When PH = 4, ZnO nanowires were formed during a liquid deposition for 15 min. When PH = 6, a SnO 2 /ZnO composite structure was formed during liquid deposition for 15 min. Finally, the micro-hotplate was rinsed with ethanol and dried [64].

Atomic Layer Deposition
Atomic layer deposition (ALD) has unique advantages in preparing ultrathin shell material films and precisely controlling film thickness and composition even at low temperatures. K.-P. Yuan et al. used ALD technology to prepare WO 3 @SnO 2 composite thin films, and the response to 15 ppm NH 3 could reach 1.55 at 200 • C. The ALD deposition process of the WO 3 @SnO 2 composite film is shown in Figure 9. First, WO 3 nanosheets were synthesized by the hydrothermal method. Then, SnO 2 was deposited on the surface of WO 3 nanosheets using an ALD deposition system (BENEQ TFS-200). The SnO 2 film growth temperature is 150 • C, the tin source is from TDMASn, and the oxygen source is deionized water. An ALD deposition process consisted of a 0.5-s pulse of TDMASn, a 5-s pulse of argon, a 0.2-s pulse of deionized water, and a 5-s pulse of argon. WO 3 @SnO 2 composite films with different thicknesses can be prepared by controlling the number of ALD deposition processes. Finally, the micro-hotplate with the WO 3 @SnO 2 composite film was annealed at 550 • C for 2 h in an air atmosphere to complete the fabrication of the sensor. At an operating temperature of 200 • C, the sensor has a response value of 1.5 to 15 ppm ammonia [65].

Atomic Layer Deposition
Atomic layer deposition (ALD) has unique advantages in preparing ultrathin shell material films and precisely controlling film thickness and composition even at low temperatures. K.-P. Yuan et al. used ALD technology to prepare WO3@SnO2 composite thin films, and the response to 15 ppm NH3 could reach 1.55 at 200 °C. The ALD deposition process of the WO3@SnO2 composite film is shown in Figure 9. First, WO3 nanosheets were synthesized by the hydrothermal method. Then, SnO2 was deposited on the surface of WO3 nanosheets using an ALD deposition system (BENEQ TFS-200). The SnO2 film growth temperature is 150 °C, the tin source is from TDMASn, and the oxygen source is deionized water. An ALD deposition process consisted of a 0.5-s pulse of TDMASn, a 5-s pulse of argon, a 0.2-s pulse of deionized water, and a 5-s pulse of argon. WO3@SnO2 composite films with different thicknesses can be prepared by controlling the number of ALD deposition processes. Finally, the micro-hotplate with the WO3@SnO2 composite film was annealed at 550 °C for 2 h in an air atmosphere to complete the fabrication of the sensor. At an operating temperature of 200 °C, the sensor has a response value of 1.5 to 15 ppm ammonia [65].

Atomic Layer Deposition
Atomic layer deposition (ALD) has unique advantages in preparing ultrath material films and precisely controlling film thickness and composition even at lo peratures. K.-P. Yuan et al. used ALD technology to prepare WO3@SnO2 compos films, and the response to 15 ppm NH3 could reach 1.55 at 200 °C. The ALD dep process of the WO3@SnO2 composite film is shown in Figure 9. First, WO3 nanoshee synthesized by the hydrothermal method. Then, SnO2 was deposited on the su WO3 nanosheets using an ALD deposition system (BENEQ TFS-200). The Sn growth temperature is 150 °C, the tin source is from TDMASn, and the oxygen s deionized water. An ALD deposition process consisted of a 0.5-s pulse of TDMAS pulse of argon, a 0.2-s pulse of deionized water, and a 5-s pulse of argon. WO3@Sn posite films with different thicknesses can be prepared by controlling the number deposition processes. Finally, the micro-hotplate with the WO3@SnO2 composite f annealed at 550 °C for 2 h in an air atmosphere to complete the fabrication of the At an operating temperature of 200 °C, the sensor has a response value of 1.5 to ammonia [65].  At the working temperature of 250 • C, the detection limit of the sensor for triethylamine reaches 22 ppb. The deposition process follows: First, diethylzinc was selected as the zinc source and water as the oxidant precursor. Diethylzinc and water were added to the atomic layer deposition vessel in sequence. The diethylzinc pulse time was 0.02 s, and the water pulse was 0.1 s. Then, a high-purity nitrogen purge was performed between successive pulses for 25 s. Finally, the ZnO thin films were annealed at 700 • C for 2 h at a heating rate of 10 • C/min in an air atmosphere. The response value of ZnO film to 10 ppm triethylamine was 7.3 under the working condition of 700 • C [66].

Chemical Vapor Deposition
Chemical vapor deposition technology can obtain gas-sensitive thin-film materials with good crystallinity, large area and fantastic uniformity by controlling the deposition parameters [67]. Using a chemical vapor deposition method, R. L. Wilson et al. deposited NiO with different thicknesses on micro-hotplate. Under the optimal working temperature of 125 • C, NO 2 detection is realized. The test results show that the response signal of the sensor to NO 2 increases with the decrease of the thickness of NiO, which proves that the dependence of sensor sensitivity on Debye length applies to NiO [68].
Graphene has been used in gas sensors due to its large surface area, high sensitivity, and low electronic noise. For the large-area fabrication of graphene gas-sensing materials, chemical vapor deposition proved to be an effective method. S. Vollebregt et al. achieved wafer-level fabrication of graphene devices using chemical vapor deposition. First, a 4-inch silicon wafer was used as the substrate. Mo thin films were sputtered on top of silicon oxide using a 99.95% Mo target. Then graphene was deposited on the Mo layer using an AIXTRON BlackMagic Pro system at 1000 • C with argon, hydrogen, methane as feedstocks at a pressure of 25 mbar. After the deposition is complete, Mo is etched using phosphoric acid, and the wafer is rinsed and dried. The distance between graphene and silicon oxide is small, and the graphene will stick to the silicon oxide. Finally, Cr/Au electrical contacts are deposited using a lift-off process [69].

Dielectrophoretic Deposition
Dielectrophoretic deposition uses a non-uniform AC electric field to generate a dipole moment to induce a dipole moment on the gas-sensitive material. The gas-sensitive material is arranged along the electric field gradient direction on the surface of the micro-hotplate. Compared with chemical vapor deposition, dielectrophoretic deposition enables roomtemperature operation without the transfer process after material synthesis. The DEP deposition method is simple and has excellent repeatability and low cost [70].
A dielectrophoretic deposition method, F. Shao et al. deposited ZnO nanowires on micro-hotplate. The deposition process is shown in Figure 10. First, ZnO nanowires were sonicated in isopropanol. Then the AC potential of the function generator was connected to the micro-hotplate electrode, and 5 µL ZnO nanowire solution was dripped onto the micro-hotplate by microtube. Apply a voltage of 5 MHZ, 15 V, and turn off the AC signal when the solvent evaporates. Observing the surface of the micro-hotplate by using a scanning electron microscope, the neat ZnO nanowires on the micro-hotplate surface can be observed. Finally, the sensor was annealed by heating at 400 • C for two hours [55]. X. Li et al. successfully arranged indium oxide nanowires on interdigital microelec trodes using dielectrophoresis, and the process is shown in Figure 11. First, the nanowire were added to ethanol and sonicated for 3 min. Then, transfer the nanowire suspension to a micro-hotplate using a pipette. The FG502 function generator was connected to th electrode of the micro-hotplate to generate an electric field, and a 6 V, 5 MHz sine wav was applied to realize the volatilization of ethanol. In a nitrogen atmosphere, they wer heated at 110-210 °C to complete the bonding of the nanowires to the micro-hotplate. Fi nally, in the air atmosphere, heating at 450-500 °C completes the oxidation of the senso [71]. X. Li et al. successfully arranged indium oxide nanowires on interdigital microelectrodes using dielectrophoresis, and the process is shown in Figure 11. First, the nanowires were added to ethanol and sonicated for 3 min. Then, transfer the nanowire suspension to a micro-hotplate using a pipette. The FG502 function generator was connected to the electrode of the micro-hotplate to generate an electric field, and a 6 V, 5 MHz sine wave was applied to realize the volatilization of ethanol. In a nitrogen atmosphere, they were heated at 110-210 • C to complete the bonding of the nanowires to the micro-hotplate. Finally, in the air atmosphere, heating at 450-500 • C completes the oxidation of the sensor [71]. was applied to realize the volatilization of ethanol. In a nitroge heated at 110-210 °C to complete the bonding of the nanowires nally, in the air atmosphere, heating at 450-500 °C completes th [71].

Spray Pyrolysis
Compared with traditional deposition, spray pyrolysis method. Spray pyrolysis is mainly used for thick film, porous fi tion does not need vacuum conditions and is more flexible. The pyrolysis deposition is shown in Figure 12, which is divided into atomization; (2) aerosol transport; (3) precursor decomposition f

Spray Pyrolysis
Compared with traditional deposition, spray pyrolysis is a simple deposition method. Spray pyrolysis is mainly used for thick film, porous film, and powder production does not need vacuum conditions and is more flexible. The general scheme of spray pyrolysis deposition is shown in Figure 12, which is divided into three steps: (1) solution atomization; (2) aerosol transport; (3) precursor decomposition film growth [72].
Spray pyrolysis provides a simple technique for preparing thin films of vario positions. A typical spray pyrolysis apparatus is shown in Figure 13. The no placed on the right side of the hotplate at a distance of 20 cm from the sample base micro-hotplate simulation is carried out on a hotplate, and spray pyrolysis st heating for 5 min. A mechanical shutter is used between the nozzle and the microto control the duration of the spray [73]. However, the gas-sensitive films obt spray pyrolysis have poor adhesion. Spray pyrolysis provides a simple technique for preparing thin films of various compositions. A typical spray pyrolysis apparatus is shown in Figure 13. The nozzle was placed on the right side of the hotplate at a distance of 20 cm from the sample. Silicon-base micro-hotplate simulation is carried out on a hotplate, and spray pyrolysis starts after heating for 5 min. A mechanical shutter is used between the nozzle and the micro-hotplate to control the duration of the spray [73]. However, the gas-sensitive films obtained by spray pyrolysis have poor adhesion.
Spray pyrolysis provides a simple technique for preparing thin films of various compositions. A typical spray pyrolysis apparatus is shown in Figure 13. The nozzle was placed on the right side of the hotplate at a distance of 20 cm from the sample. Siliconbase micro-hotplate simulation is carried out on a hotplate, and spray pyrolysis starts after heating for 5 min. A mechanical shutter is used between the nozzle and the micro-hotplate to control the duration of the spray [73]. However, the gas-sensitive films obtained by spray pyrolysis have poor adhesion.

Sputtering
The sputter deposition of sensor-sensitive material thin films is a mature semiconductor process suitable for manufacturing mass-produced, highly repeatable gas-sensing devices. Sputter deposition allows precise control of deposition conditions, substrate temperature, and sputter power. The gas-sensitive film produced by the sputtering deposition method has the advantages of high quality and marvelous adhesion [74]. J. Zeng et al. prepared WO3 sensors by magnetron sputtering. The specific process is as follows: first, deionized water, acetone, and ethanol are used for cleaning the Al2O3 substrate to remove pollutants from the surface. Then, a 100 nm platinum electrode was deposited on the substrate using a radio frequency sputtering method. Finally, a tungsten film was deposited on the platinum electrode using a 2-inch round tungsten target and DPS-II ultra-high vacuum system [75]. J. G. Kang et al. deposited 120 nm Pt-modified SnO2 thin film on microhotplate by RF sputtering method and realized the detection of toluene gas [76]. L. Y.

Sputtering
The sputter deposition of sensor-sensitive material thin films is a mature semiconductor process suitable for manufacturing mass-produced, highly repeatable gas-sensing devices. Sputter deposition allows precise control of deposition conditions, substrate temperature, and sputter power. The gas-sensitive film produced by the sputtering deposition method has the advantages of high quality and marvelous adhesion [74]. J. Zeng et al. prepared WO 3 sensors by magnetron sputtering. The specific process is as follows: first, deionized water, acetone, and ethanol are used for cleaning the Al 2 O 3 substrate to remove pollutants from the surface. Then, a 100 nm platinum electrode was deposited on the substrate using a radio frequency sputtering method. Finally, a tungsten film was deposited on the platinum electrode using a 2-inch round tungsten target and DPS-II ultra-high vacuum system [75]. J. G. Kang et al. deposited 120 nm Pt-modified SnO 2 thin film on micro-hotplate by RF sputtering method and realized the detection of toluene gas [76]. L. Y. Sheng et al. prepared SnO 2 thin films by sputtering using a Denton SJ/24LL multi-target system with a 99.99% SnO 2 target [77]. B. Behera et al. realized the assembly of ZnO-CuO thin film and MEMS sensor by RF sputtering method, and the sensor structure is shown in Figure 14 [35]. Sheng et al. prepared SnO2 thin films by sputtering using a Denton SJ/24LL multi-target system with a 99.99% SnO2 target [77]. B. Behera et al. realized the assembly of ZnO-CuO thin film and MEMS sensor by RF sputtering method, and the sensor structure is shown in Figure 14 [35]. K. Lee et al. deposited a MoS2 thin film on a silicon substrate using a Gatan-PECS sputtering method, as shown in Figure 15. A lay of Mo is deposited on a Si substrate. Then, the Si substrate was placed in an adaptive tube furnace and heated to 750 °C at a temperature rise rate of 20 °C/min. After annealing for 30 min, the sulfur powder was heated to 113 °C and sulfur was introduced in the upstream hot zone, and the sulfur vapor completely reacted with Mo to form a MoS2 film [78].  Figure 15. A lay of Mo is deposited on a Si substrate. Then, the Si substrate was placed in an adaptive tube furnace and heated to 750 • C at a temperature rise rate of 20 • C/min. After annealing for 30 min, the sulfur powder was heated to 113 • C and sulfur was introduced in the upstream hot zone, and the sulfur vapor completely reacted with Mo to form a MoS 2 film [78]. K. Lee et al. deposited a MoS2 thin film on a silicon substrate using a Gatan-PECS sputtering method, as shown in Figure 15. A lay of Mo is deposited on a Si substrate. Then the Si substrate was placed in an adaptive tube furnace and heated to 750 °C at a temper ature rise rate of 20 °C/min. After annealing for 30 min, the sulfur powder was heated to 113 °C and sulfur was introduced in the upstream hot zone, and the sulfur vapor com pletely reacted with Mo to form a MoS2 film [78].

In Situ Growth
Using the vapor deposition method to deposit gas-sensitive materials on the micro hotplate has disadvantages such as high growth temperature and expensive deposition system. Compared with vapor deposition, using the method of in situ growth on the sur face of the micro-hotplate to realize the assembly of the gas-sensitive material and th micro-hotplate has the advantages of the cheap process and easy to mass produce [79].
J. Xuan et al. achieved the in situ growth of ZnO on the FTO gas-sensing electrode as shown in Figure 16. Zinc acetate and diethanolamine were added to ethanol to obtain ZnO seed solution. After stirring at 30 °C for 30 min, the ZnO seed solution was added dropwise to the FTO gas-sensing electrode; the ZnO seed layer was deposited and

In Situ Growth
Using the vapor deposition method to deposit gas-sensitive materials on the microhotplate has disadvantages such as high growth temperature and expensive deposition system. Compared with vapor deposition, using the method of in situ growth on the surface of the micro-hotplate to realize the assembly of the gas-sensitive material and the micro-hotplate has the advantages of the cheap process and easy to mass produce [79].
J. Xuan et al. achieved the in situ growth of ZnO on the FTO gas-sensing electrode, as shown in Figure 16. Zinc acetate and diethanolamine were added to ethanol to obtain ZnO seed solution. After stirring at 30 • C for 30 min, the ZnO seed solution was added dropwise to the FTO gas-sensing electrode; the ZnO seed layer was deposited and annealed at 400 • C for 20 min. Zinc nitrate, HMTA, and PEI were added and hydrothermally treated in an autoclave at 95 • C for 6 h. Finally, the samples were rinsed with deionized water and ethanol to obtain ZnO grown in situ on the FTO gas-sensing electrode [80].

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annealed at 400 °C for 20 min. Zinc nitrate, HMTA, and PEI were added and hydrothermally treated in an autoclave at 95 °C for 6 h. Finally, the samples were rinsed with deionized water and ethanol to obtain ZnO grown in situ on the FTO gas-sensing electrode [80].  I. Cho et al. synthesized ZnO nanowires directly on micro-hotplate by hydrothermal reaction and realized the detection of NO 2 . The gas sensitivity of the sensor is improved by ultraviolet light activation [81]. T. J. Hsueh et al. deposited ZnO seed layer on the surface of micro-hotplate by RF sputtering method. The micro-hotplate structure for growing ZnO nanowires is shown in Figure 17 [82]. Figure 16. Diagram of the preparation process of in-situ growth of ZnO on FTO gas-sensing electrode [80].
I. Cho et al. synthesized ZnO nanowires directly on micro-hotplate by hydrothermal reaction and realized the detection of NO2. The gas sensitivity of the sensor is improved by ultraviolet light activation [81]. T. J. Hsueh et al. deposited ZnO seed layer on the surface of micro-hotplate by RF sputtering method. The micro-hotplate structure for growing ZnO nanowires is shown in Figure 17 [82]. Y. Chen et al. realized the in situ growth of ZnO nanowires on the surface of the micro-hotplate, and the specific process is shown in Figure 18. First, ZnO nanowires with diameters of 50-70 nm were synthesized in an aqueous solution. Then, with the help of a microscope, a micromanipulator is used to drop zinc acetate ethanol solution on the sensing area of the micro-hotplate. They covered a zinc acetate seed film on the surface, heating to 350 °C in the air for 20 min to generate a nano zinc oxide seed layer. Finally, ZnO nanowires are grown on the surface of the WAF in situ by adopting a hydrothermal method [83]. Y. Chen et al. realized the in situ growth of ZnO nanowires on the surface of the micro-hotplate, and the specific process is shown in Figure 18. First, ZnO nanowires with diameters of 50-70 nm were synthesized in an aqueous solution. Then, with the help of a microscope, a micromanipulator is used to drop zinc acetate ethanol solution on the sensing area of the micro-hotplate. They covered a zinc acetate seed film on the surface, heating to 350 • C in the air for 20 min to generate a nano zinc oxide seed layer. Finally, ZnO nanowires are grown on the surface of the WAF in situ by adopting a hydrothermal method [83].

Inkjet Printing
The assembly of gas-sensitive material and micro-hotplate can be realized using inkjet printing technology. EHD inkjet printing is a printing technology capable of producing droplets more minor than the inner diameter of the nozzle. But inkjet printing technology requires expensive, sophisticated equipment [84]. H. Wu et al. achieved the assembly of Pd-modified SnO2 nanofibers on a micro-hotplate using EHD inkjet printing. The EHD inkjet printing device is shown in Figure 19a, including metal needles, microinjection pumps, relays, high-speed cameras, high-voltage power supplies, light sources, glass plates, and stainless steel electrodes. During the EHD inkjet printing process, the microsyringe pump replenished the Pd-modified SnO2 nanofiber ink into the metal needle

Inkjet Printing
The assembly of gas-sensitive material and micro-hotplate can be realized using inkjet printing technology. EHD inkjet printing is a printing technology capable of producing droplets more minor than the inner diameter of the nozzle. But inkjet printing technology requires expensive, sophisticated equipment [84]. H. Wu et al. achieved the assembly of Pdmodified SnO 2 nanofibers on a micro-hotplate using EHD inkjet printing. The EHD inkjet printing device is shown in Figure 19a, including metal needles, micro-injection pumps, relays, high-speed cameras, high-voltage power supplies, light sources, glass plates, and stainless steel electrodes. During the EHD inkjet printing process, the microsyringe pump replenished the Pd-modified SnO 2 nanofiber ink into the metal needle at a rate of 2 µL/min. The stainless steel electrode is ground, the relay is turned on, and the metal needle receives a high voltage of 2 KV. The spherical ink droplet first hangs on the top of the metal needle. When the relay is disconnected, the high-voltage electric field generates electrostatic force, pulling the ink droplet into a cone shape. The ink droplet is separated from the top of the Taylor cone and falls on the surface of the micro-hotplate. Repeat the above steps until the ink completely covers the micro-hotplate test electrode area, as shown in Figure 19b [85].
The assembly of gas-sensitive material and micro-hotplate can be realized using inkjet printing technology. EHD inkjet printing is a printing technology capable of producing droplets more minor than the inner diameter of the nozzle. But inkjet printing technology requires expensive, sophisticated equipment [84]. H. Wu et al. achieved the assembly of Pd-modified SnO2 nanofibers on a micro-hotplate using EHD inkjet printing. The EHD inkjet printing device is shown in Figure 19a, including metal needles, microinjection pumps, relays, high-speed cameras, high-voltage power supplies, light sources, glass plates, and stainless steel electrodes. During the EHD inkjet printing process, the microsyringe pump replenished the Pd-modified SnO2 nanofiber ink into the metal needle at a rate of 2 μL/min. The stainless steel electrode is ground, the relay is turned on, and the metal needle receives a high voltage of 2 KV. The spherical ink droplet first hangs on the top of the metal needle. When the relay is disconnected, the high-voltage electric field generates electrostatic force, pulling the ink droplet into a cone shape. The ink droplet is separated from the top of the Taylor cone and falls on the surface of the micro-hotplate. Repeat the above steps until the ink completely covers the micro-hotplate test electrode area, as shown in Figure 19b [85]. M. A. Audio et al. added glycerol to SnO 2 particles and then subjected the mixture to ultrasonic treatment to prepare the ink required for printing. SnO 2 nanoparticle ink was printed onto a micro-hotplate using a JetLab II inkjet print system. After printing, the micro-hotplate was dried at 300 • C for 3 h. The sensor surface obtained after inkjet printing is shown in Figure 20 [86].

Screen Printing
The sensitive material and the sensor array can be directly assembled using the screen printing method for the sensor array with a large area. Using screen-printing process, A Ibrahim et al. fabricated a Pd/g-C3N4 gas sensor for hydrogen detection. The screen-print ing process is shown in Figure 21. First, the interdigitated electrode pattern was printed on a glass substrate and then heated to 120 °C for curing. Then, 20 mg of Pd/g-C3N4 mate rial was added to terpineol to make a screen-printing paste. To ensure the effect of screen printing, a 135-micron screen was chosen with a 2-inch rubber squeegee to spread the gas sensitive material evenly on the electrode area of the micro-hotplate. Finally, the senso

Screen Printing
The sensitive material and the sensor array can be directly assembled using the screen printing method for the sensor array with a large area. Using screen-printing process, A. Ibrahim et al. fabricated a Pd/g-C 3 N 4 gas sensor for hydrogen detection. The screen-printing process is shown in Figure 21. First, the interdigitated electrode pattern was printed on a glass substrate and then heated to 120 • C for curing. Then, 20 mg of Pd/g-C 3 N 4 material was added to terpineol to make a screen-printing paste. To ensure the effect of screen printing, a 135-micron screen was chosen with a 2-inch rubber squeegee to spread the gas-sensitive material evenly on the electrode area of the micro-hotplate. Finally, the sensor was annealed for 30 min [87].

Screen Printing
The sensitive material and the sensor array can be directly assembled using the screen printing method for the sensor array with a large area. Using screen-printing process, A. Ibrahim et al. fabricated a Pd/g-C3N4 gas sensor for hydrogen detection. The screen-printing process is shown in Figure 21. First, the interdigitated electrode pattern was printed on a glass substrate and then heated to 120 °C for curing. Then, 20 mg of Pd/g-C3N4 material was added to terpineol to make a screen-printing paste. To ensure the effect of screen printing, a 135-micron screen was chosen with a 2-inch rubber squeegee to spread the gassensitive material evenly on the electrode area of the micro-hotplate. Finally, the sensor was annealed for 30 min [87].  Figure 22. The sensor array consists of four sensors, the front side of the sensor array is a test electrode, and the backside is a heating electrode [88]. W. Lu et al. used a high-resolution screen printer (MT650) to print a platinum-decorated alumina gas-sensitive film on the surface of a quartz-based micro-hotplate to realize the detection of methane [89].  Figure 22. The sensor array consists of four sensors, the front side of the sensor array is a test electrode, and the backside is a heating electrode [88]. W. Lu et al. used a high-resolution screen printer (MT650) to print a platinum-decorated alumina gas-sensitive film on the surface of a quartz-based micro-hotplate to realize the detection of methane [89].

Self-Assembly
The self-assembly technique proposed by Decher to fabricate ultrathin films by elec trostatic interaction. The most significant difference between the ultrathin films prepared by self-assembly technology and those prepared by vapor deposition and spin coating i that the films prepared by self-assembly technology are highly ordered. F. Q. Sun et a have realized the assembly of SnO2 thin films on curved surfaces by using the method o self-assembly. The self-assembly process is shown in Figure 23. First, the glass plate cov ered with PS colloid was immersed in the precursor solution, and the monolayer film would float on the surface of the solution, and the film was picked up by the curved glas rod. The glass rod covered with the film is then dried. Finally, the PS microspheres wer removed by calcination. This method allows porous ordered thin films to form on an curved surface [90].

Self-Assembly
The self-assembly technique proposed by Decher to fabricate ultrathin films by electrostatic interaction. The most significant difference between the ultrathin films prepared by self-assembly technology and those prepared by vapor deposition and spin coating is that the films prepared by self-assembly technology are highly ordered. F. Q. Sun et al. have realized the assembly of SnO 2 thin films on curved surfaces by using the method of self-assembly. The self-assembly process is shown in Figure 23. First, the glass plate covered with PS colloid was immersed in the precursor solution, and the monolayer film would float on the surface of the solution, and the film was picked up by the curved glass rod. The glass rod covered with the film is then dried. Finally, the PS microspheres were removed by calcination. This method allows porous ordered thin films to form on any curved surface [90].
have realized the assembly of SnO2 thin films on curved surfaces by self-assembly. The self-assembly process is shown in Figure 23. First, ered with PS colloid was immersed in the precursor solution, and would float on the surface of the solution, and the film was picked up rod. The glass rod covered with the film is then dried. Finally, the PS removed by calcination. This method allows porous ordered thin fi curved surface [90].   Figure 24. First, the single-layer polystyrene microspheres are put into the SnCl 4 solution. Due to surface tension, polystyrene microspheres were suspended on the surface of SnCl 4 . Then, an interdigital electrode was used for fishing out the polystyrene microspheres. Finally, the polystyrene microspheres were removed by heating the interdigital electrode to 400 • C in air and calcining for 2 h. Both double-layer and triple-layer SnO 2 films can be manufactured using polystyrene microsphere templates [91].
The self-assembly method can assemble sensitive materials and sensor array for the gas sensor array with a small area and large quantity. Using the in situ assembly method, L. Xu et al. made the nanopore array film on the wafer-level micro-hotplate collection. They achieved the goal of producing thousands of gas-sensing units simultaneously, as shown in Figure 25. The PS microsphere template is floated in the precursor solution. The sensitive material, the microsphere template, and the micro-hotplate are assembled by fishing wafers covered with the micro-hotplate units. By annealing at 400 • C for 2 h, the microsphere template was removed, and an ordered porous gas-sensitive film was formed on the wafer surface. After the wafer-level assembly of the gas-sensitive thin film is realized, the wafer is cut according to the number of units required by the sensor array [49].
in Figure 24. First, the single-layer polystyrene microspheres are put into the SnCl4 solution. Due to surface tension, polystyrene microspheres were suspended on the surface of SnCl4. Then, an interdigital electrode was used for fishing out the polystyrene microspheres. Finally, the polystyrene microspheres were removed by heating the interdigital electrode to 400 °C in air and calcining for 2 h. Both double-layer and triple-layer SnO2 films can be manufactured using polystyrene microsphere templates [91]. The self-assembly method can assemble sensitive materials and sensor array for the gas sensor array with a small area and large quantity. Using the in situ assembly method, L. Xu et al. made the nanopore array film on the wafer-level micro-hotplate collection. They achieved the goal of producing thousands of gas-sensing units simultaneously, as shown in Figure 25. The PS microsphere template is floated in the precursor solution. The sensitive material, the microsphere template, and the micro-hotplate are assembled by fishing wafers covered with the micro-hotplate units. By annealing at 400 °C for 2 h, the microsphere template was removed, and an ordered porous gas-sensitive film was formed on the wafer surface. After the wafer-level assembly of the gas-sensitive thin film is realized, the wafer is cut according to the number of units required by the sensor array [49].

Comparison of Coating Methods for Gas-Sensitive Materials
Among the many coating methods for gas-sensitive materials, the drop coating method is easy to operate and has a high gas response value, but the coating thickness is difficult to control. Liquid deposition is a low-temperature growth process that does not

Comparison of Coating Methods for Gas-Sensitive Materials
Among the many coating methods for gas-sensitive materials, the drop coating method is easy to operate and has a high gas response value, but the coating thickness is difficult to control. Liquid deposition is a low-temperature growth process that does not require complex equipment and the substrate is not limited. The thin film prepared by liquid deposition method has a dense structure, but the sensor response is low. Atomic layer deposition (ALD) is a method of depositing material layer by layer on the surface of a substrate in the form of a single atomic film. Atomic layer deposition can control the film thickness by controlling the number of reaction cycles, but atomic layer deposition (ALD) film deposition is slow and inefficient. Chemical vapor deposition technology can obtain gas-sensitive thin-film materials with marvelous crystallinity, large area, and fantastic uniformity by controlling the deposition parameters. Dielectrophoretic deposition can operate at room temperature without the transfer process after material synthesis, and the deposition method has low cost and excellent repeatability. However, it is difficult to set the voltage amplitude, frequency, and period of DEP deposition.
Spray pyrolysis is mainly used for the assembly of thick film materials and does not require vacuum conditions. The film raw materials are mixed in a solution state and are not prone to agglomeration. However, the films formed by spray pyrolysis have poor adhesion. Sputtering deposition is a mature semiconductor process. By controlling deposition conditions, substrate temperature, deposition power, and other conditions, high-quality, high-adhesion films can be prepared. However, sputtering deposition of thin films requires higher deposition conditions, generally requiring vacuum conditions.
In situ growth is accomplished by arranging a seed layer on the surface of the microhotplate, and using the induction method to complete the growth of the seed layer. The films prepared by in situ growth have excellent adhesion, but the preparation process is complicated.
The inkjet printing technology uses the ink prepared from the gas-sensitive material to form a Taylor cone at the nozzle using electrostatic force, producing material droplets smaller than the nozzle, and realizes the assembly of the gas-sensitive material. The inkjet printing technology is easy to operate, but the formed film material is a thick film type, the film thickness is difficult to control, and the inkjet printing equipment is expensive. Screen printing technology uses screen and rubber squeegee to spread the gas-sensitive material evenly on the surface of the micro-hotplate in a large area. The film-forming method is simple and efficient. However, the film thickness is difficult to control. The films prepared by the self-assembly method are highly ordered, easy to operate, and have fast sensor response. Table 3 shows the comparison of coating methods for gas-sensitive materials. Table 3. Comparison of thin film coating methods.

Coating Method
Operating Temperature Professional Setting Film Features

Drop Coating Normal No
The method is simple and the sensor response value is high, but the film thickness is difficult to control.

Liquid Deposition Low No
The thin film is dense and the sensor response is low.

Atomic Layer Deposition Low Yes
The film uniformity is excellent, but the film deposition rate is slow.

Chemical Vapor Deposition High Yes
The film has marvelous crystallinity and large area, but the deposition temperature is high, and impurities are easily introduced during the deposition process.

Dielectrophoretic Deposition Normal No
The thin film deposition method is simple and has excellent repeatability, but it is difficult to set the deposition voltage amplitude, frequency, and period.

Spray Pyrolysis High Yes
The film is a thick film material, and the film raw material is not easy to agglomerate, but the film adhesion is poor.

Sputtering
Low Yes Film adhesion is high, however, vacuum conditions are required for film deposition.

In Situ Growth Low No
The film has excellent adhesion, but the preparation process is complicated.

Inkjet Printing Normal Yes
The film material is thick film type, the film thickness is difficult to control, and the equipment price is high.

Screen Printing Normal Yes
The film formation efficiency is high, but the film thickness is difficult to control.

Self-assembly Normal No
The film is highly ordered and has a fast response time

Conclusions
In this paper, we introduce the classification standard and structure of micro-hotplates and summarize the assembly process and characteristics of different gas-sensitive materials and micro-hotplates, such as: drop coating, liquid deposition, atomic layer deposition, chemical vapor deposition, electrophoretic deposition, spray pyrolysis, sputtering, in situ growth, inkjet printing, screen printing, self-assembly, etc.
The effective formation of the patterned nano-gas-sensitive material layer at a specific position on the surface of the micro-hotplate gas sensor is the leading technical obstacle to be overcome in the manufacturing of the micro-hotplate gas sensor. Gas-sensitive material film with high stability, excellent adhesion, and marvelous uniformity is prepared on the surface of the micro-hotplate to realize the reliable assembly of the gas-sensitive material and the micro-hotplate, which can improve the stability and response value of the micro-hotplate gas sensor.
Compared with other coating methods, the in situ growth and self-assembly method can form a porous gas-sensitive material film with marvelous adhesion, does not need expensive equipment, has the advantages of the cheap process, easy to expand, and so on. Therefore, it can be predicted that in situ growth and self-assembly will play an essential role in developing gas-sensitive materials and micro-hotplate assembly technology.

Conflicts of Interest:
The authors declare no conflict of interest.