Development of a MEMS-based H₂S Sensor with a High Detection Performance and Fast Response Time

1. INTRODUCTION

The quick and safe detection of hydrogen sulfide (H₂S) gas, which is a toxic, harmful, corrosive, and colorless gas, is an essential concern for the health and safety of industrial workers and the general population. H₂S gas with a rotten-egg smell is generated from natural sources, such as petroleum, natural gas, and volcanic gas, and human sources, such as petroleum and natural gas extraction and purification, paper and pulp manufacturing, textile production, chemical manufacturing, and wastewater treatment [1-5]. According to the Occupational Safety and Health Administration (OSHA), H₂S is considered a dangerous gas comparable to carbon monoxide (CO) in the workplace. Danger to H₂S exposure is determined by its concentration and is categorized into three groups: (1) acute exposure (>300 ppm), which causes collapsing, unconsciousness, and death; (2) post-acute exposure (>100 ppm) over 30 min, which causes difficulty in breathing and comas; (3) chronic exposure (<1 ppm) over several days, which causes nausea, headache, and skin/eye irritation.

Regulations for workers and the general population susceptible to H₂S have been developed and tightened; thus, the development of an H₂S sensor with excellent performance, such as high response and good selectivity, is becoming increasingly important. Various methods for detecting H₂S are available, including semiconducting metal oxide, electrochemical, and optical methods. Among these methods, H₂S sensors utilizing semiconducting metal oxides (SnO₂, ZnO, WO₃, and CuO) as sensing materials have been steadily developed over a long time [6-16]. Despite the advantages of semiconducting metal oxides for H₂S detection, their H₂S detection performance must be improved by tightening regulations for workers and the general population susceptible to H₂S [17-24].

In this study, a H₂S sensor with a high and fast response was developed using tin dioxide (SnO₂) and an optimized micro-heater to respond to tightened regulations. SnO₂ is the most widely used sensing material in semiconducting metal oxide-based gas sensors because of its excellent gas detection ability (a good compromise between price, stability, and reliability of the material, fast response, and recovery time) and many advantages for fabrication (low-cost, simple fabrication, and good compatibility with micro-electro-mechanical system (MEMS) processes) [9, 14].

A SnO₂-based sensor detects H₂S through a resistance-change mechanism, which is primarily an induced variation of depletion region owing to the adsorption of ionized oxygen species (O₂⁻, O⁻, and O²⁻) on the SnO₂ surface. The oxygen-related gas-sensing mechanism involves the absorption of oxygen molecules on the SnO₂ surface to generate chemisorbed oxygen species (O₂⁻, O⁻, and O²⁻) by capturing electrons from the conductance band, which causes the SnO₂ surface to be highly resistive. SnO₂ is exposed to traces of reductive gases. When reacting with the oxygen species in SnO₂, the reductive gas reduces the concentration of oxygen species on the surface, thereby increasing the electron concentration.

In this study, a gas sensor for detecting H₂S, consisting of a micro-heater, sensing material, and electrode, was fabricated using MEMS technology. To improve the H₂S detection ability governed by the principle mentioned above, we embedded a micro-heater in the H₂S sensor to increase the temperature. The micro-heater offers proper thermal energy for the reaction between the target gas (H₂S) and the sensing material (SnO₂). Thus, the performance of the gas sensor can be dramatically improved. To satisfy these requirements, we designed and characterized an MEMS-based H₂S sensor with micro-heaters (line and circular types). Microheaters with different designs were fabricated on the proposed H₂S sensor platform and characterized To investigate the relationship between the H₂S detection performance and heating performance influenced by the design of the micro-heater. Finally, H₂S was detected using an optimized micro-heater installed on the fabricated H₂S sensor.

2. DESIGN AND FABRICATION

The MEMS-based H₂S sensor with micro-heaters (line and circular types) was designed as shown in Fig. 1. The proposed MEMS-based H₂S sensor consisted of line and circular micro-heaters, a temperature sensor, an interdigitated electrode (IDE), and a sensing material (SnO₂). In particular, the different types of micro-heaters improve the performance of the MEMS-based H₂S sensor compared with previous types (meander, rectangular, and rectangular mesh types) because the continual improvement of the H₂S detection ability of the sensor by considering the dangers of H₂S gas is important. The sizes of the entire sensor and sensing area were 3 mm ⅹ 3 mm and 100 μm ⅹ100 μm, respectively. The width and thickness of two types of micro-heaters, temperature sensor and IDE were 20 μm and 200 nm, respectively. To minimize the loss of thermal energy generated by the micro-heater, we used a quartz wafer was used as the sensor substrate, which can also minimize the fabrication cost and difficulty level. Pt, which has a linear relationship between temperature and resistance, was used to fabricate the micro-heater and temperature sensor. Au and tin dioxide (SnO₂) were used as the IDE and sensing materials, respectively. Fig. 2(a) and (b) show the fabrication process and a photograph of the proposed MEMS-based H₂S, respectively.

Fig. 1.

Schematic of the proposed MEMS-based H2S sensor with micro-heaters.

Fig. 2.

(a) Fabrication process and (b) a photograph of fabricated MEMS-based H2S sensor.

The MEMS-based H₂S sensor was fabricated as follows. First, the quartz wafer used as the substrate was cleaned with acetone and methanol solution for 10 min. Subsequently, the proposed micro-heaters (line and circular) and temperature sensors were fabricated through a photolithography process to pattern the desired design and an e-beam evaporation process for Pt deposition. Silicon nitride (Si₃N₄), which was used as an electrical insulating and passivation layer, was deposited using plasma-enhanced chemical vapor deposition (PECVD). The IDE was fabricated using photolithography and e-beam evaporation processes for Au deposition. Finally, SnO₂, as the H₂S sensing material, was deposited via sputtering, and Si₃N₄ was etched to fabricate the electrical pads of the micro-heaters and temperature sensor.

3. RESULTS AND DISCUSSIONS

The performance of the fabricated temperature sensor and line and circular micro-heaters was characterized before estimating the H₂S detection ability of the proposed MEMS-based H₂S sensor. First, the resistance of the fabricated temperature sensor and the temperature change were measured using a commercial ceramic heater installed in the gas chamber.

The measured resistance of the temperature sensor increased linearly, as shown by the blue line in Fig. 3. Thus, the heating performance of the line and circular micro-heaters could be estimated from the measured resistance of the temperature sensor in real time. Different input voltage values were applied to the line and circular micro-heaters, and their heating performances were characterized by measuring the resistance of the temperature sensor, as shown by the red line in Fig. 3.

Fig. 3.

Measured resistance of the temperature sensor as functions of the controlled temperature and input voltage of the micro-heater.

The excellent H₂S detection ability (good response and selectivity, fast response and recovery times) of SnO₂ is ensured at operating temperature in the range 120–200 ^oC. Therefore, an input voltage was applied to the micro-heater to increase the optimum H₂S detection temperature of SnO₂. By measuring resistance of temperature sensor, the expected temperatures of SnO₂ were approximately 120, 145, and 165 ^oC when input voltages in the range of 3–4 V were applied to the circular micro-heater and approximately 110, 140, and 160 ^oC when input voltages in the range of 5–7 V are applied to line micro-heater. This implied that the circular micro-heater produced more thermal energy than the line micro-heater. Increasing the current through the micro-heater is important because its heating performance is affected by Joule heating, which is closely related to the current traveling through the micro-heater (Joule heating [cal]=I²Rt). The measured resistance values of the line and circular micro-heaters were 87.37 and 30.27 Ω, respectively. The circular micro-heater had a superior heating performance to a line micro-heater because more current flowed when an equal or lower input voltage was applied to it. Thermal energy in the line and circular micro-heaters was generated nonlinearly along with the applied input voltage, and the circular micro-heater exhibited a stronger nonlinearity, as shown in Fig. 3. This was because the generated thermal energy was proportional to the square of the current flowing through the micro-heater (Joule heating [cal]=I²Rt). Based on these experimental results, we expect that the circular micro-heater can supply proper thermal energy; thus, the MEMS-based H₂S sensor with the circular micro-heater can detect H₂S effectively.

Next, the H₂S detection ability of the MEMS-based H₂S sensor with the line and circular micro-heaters was characterized. The fabricated MEMS-based H₂S sensor was placed in the prepared chamber, as shown in Fig. 4, and H₂S gas in the range of 0–10 ppm was injected into the chamber by measuring the output currents of the MEMS-based H₂S sensor. The output current of the MEMS-based H₂S sensor increased when H₂S was injected into the chamber, as shown in Fig. 5(a). This was because the oxygen species adsorbed on the sensing material surface (SnO₂) was consumed by the chemical reaction, and electrons were donated back to the SnO₂ surface, resulting in a decrease in the electrical resistance when the MEMS-based H₂S sensor was exposed to H₂S. Therefore, the measured output current of the MEMS-based H₂S sensor increased. The response of a MEMS-based H₂S sensor is frequently defined as

Fig. 4.

Experimental set-up to test the fabricated MEMS-based H2S sensor.

Fig. 5.

(a) Measured output current of fabricated MEMS-based H2S with line and circular micro-and H2S concentration; (b) measured response/recovery times of fabricated MEMS-based H2S with line and circular micro-heaters.

where R_air and R_gas are the resistances, and I_air and I_gas are the conductances of the sensor in air and the reducing gas (H₂S), respectively.

The response time is defined as the time required to decrease the resistance or increase the conductance of the H₂S sensor by 90% of the total decrease (R_air-R_gas) or total increase (I_gas-I_air). The recovery time is defined as the time required to recover the resistance or increase the conductance of the H₂S sensor by 90% of the total decrease (R_air-R_gas) or total increase (I_gas-I_air) when H₂S injection is stopped and air is injected into the chamber. Fig. 5(a) shows the measured responses of the fabricated MEMS-based H₂S sensor at different H₂S concentrations (0–10 ppm), and Fig. 5(b) shows the measured response/recovery time of the fabricated MEMS-based H₂S sensor.

The response of the sensor to H₂S dramatically improved when the operating temperature was increased by increasing the input voltage of the line and circular micro-heaters. The response values of the MEMS-based H₂S sensor with the line micro-heater (I_gas/I_air) for H₂S concentration of 0–10 ppm were 5.03, 5.89, 6.25, 6.84, 7.43 at an input voltage of 7 V; 4.37, 6.28, 7.30, 8.31, 8.41 at 6 V; and 4.75, 6.46, 7.51, 8.54, 9.40 at 5 V. With the circular micro-heater, the response values (I_gas/I_air) for a H₂S concentration 0–10 ppm were 4.75, 6.15, 6.99, 7.73, 8.25 at an input voltage 4 V; 4.01, 5.26, 6.09, 6.65, 7.06 at 3.5 V ; and 4.22, 6.16, 7.56, 9.02, 9.16 at 3 V . As mentioned earlier, the both MEMS-based H₂S sensors with line and circular micro-heaters can detect H₂S gas in temperature range 120–200 ^oC. In particular, the fabricated MEMS-based H₂S sensor with a circular micro-heater has superior H₂S detection ability at a lower micro-heater input voltage compared with the sensor with a line micro-heater. This is because the circular micro-heater has a remarkable heating performance, as shown in Fig. 3. The response and recovery times of the MEMS-based H₂S sensor were reduced by more than twofold when the circular micro-heater was utilized. These results demonstrate that the proposed and fabricated MEMS-based H₂S sensors can detect H₂S gas more quickly and accurately.

Finally, the fabricated MEMS-based H₂S sensor with the circular micro-heater not only has a strong response but also good selectivity toward the target gas (H₂S in this study) for practical applications. To estimate the selectivity of the MEMS-based H₂S sensor with a circular micro-heater, the sensor was exposed to different gases, including ammonia (NH₃), hydrogen (H₂), and carbon monoxide (CO) because they react well with SnO₂ under various conditions. Generally, these gases react well with SnO₂ at different temperatures and H₂S reacts well with SnO₂ between 120 and 200 ^oC. As shown in Fig. 6, the fabricated MEMS-based H₂S sensor with the circular micro-heater had a high selectivity toward H₂S. This was because the fabricated MEMS-based H₂S sensor was effective under temperature conditions (120–200 ^oC) in which H₂S reacts well with SnO₂. If the sensor operated outside the optimum temperature range, its selectivity toward H₂S would have been very low. Based on the experimental results, we confirmed the importance of supplying proper thermal energy for the reaction between H₂S and oxygen species (O₂⁻, O⁻, and O²⁻) on the SnO₂ surface via a micro-heater with superior heating performance. For a reaction to occur between molecules, they must exist as close to each other as possible, and each molecule must have an energy greater than the energy required for the reaction (activation energy, E_a). According to the Maxwell–Boltzmann distribution, as the temperature increases, the number of molecules with energies above the activation energy increases:

Fig. 6.

Measured responses of the MEMS-based H2S sensor with circular micro-heater for different gases.

where k is Boltzmann’s constant, E is the energy, and T is the absolute temperature.

In summary, the H₂S detection ability of the proposed MEMS-based H₂S sensor was significantly improved by supplying thermal energy through micro-heaters embedded in the H₂S sensor, which can be achieved by fabricating a well-made micro-heater with an optimized design.

4. CONCLUSIONS

In this study, a MEMS-based H₂S sensor with a micro-heater was fabricated. The sensor uses a semiconducting metal oxide (SnO₂) as the sensing material and consists of a substrate, sensing material, IDE, and micro-heater. As micro-heaters embedded in the proposed sensor increase the temperature, they have an important role to play because the reaction between H₂S and oxygen species (O₂⁻, O⁻, and O²⁻) on the SnO₂ surface is affected by the operating temperature of the sensor. The development of a micro-heater that produces more thermal energy by minimizing power consumption or operating voltage is essential for real-time monitoring applications. To satisfy this requirement, we developed line and circular micro-heaters and characterized their heating performance by estimating the H₂S detection ability of the fabricated sensor. This was accomplished by applying an input voltage and measuring the resistance of the temperature sensor. Based on the experimental results, we confirmed that the circular micro-heater produces thermal energy more effectively. Therefore, the H₂S sensor fabricated with the circular micro-heater exhibited superior H₂S detection ability. Its responses (I_gas/I_air) were 4.75 (2 ppm), 6.15 (4 ppm), 6.99 (6 ppm), 7.73 (8 ppm), and 8.25 (10 ppm) at an applied input voltage of 4 V. Furthermore, it had a shorter response time (<16 s) and recovery time (<65 s) than the H₂S sensors with a line micro-heater. H₂S, which is a toxic and harmful gas, even at concentrations as low as hundreds of parts per million, is primarily produced in various fields, such as oil deposits, biogas, and natural gas. Developing an H₂S sensor with good responsivity and a fast response time is crucial for the health and safety of industrial workers and the general population. Therefore, we expect that the developed and optimized H₂S sensor in this study will be a good candidate for practical real-time applications.

Acknowledgments

This work was supported by the Technology Development Program (S3177927), funded by the Ministry of SMEs Startups (MSS), Korea.

This study has been conducted with the support of the Korea Institute of Industrial Technology as “Technical support project for research and development of manufacture to improve core components of eco-mobility” (Kitech JF-23-0007).

This work was supported by the Korea Innovation Foundation(INNOPOLIS) grant funded by the Korea government(MSIT) (2020-DD-UP-0348).

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