(Invited) Development of Pulse-Heated Diaphragm Type MEMS Gas Sensors for Indoor Air Quality Measurement

Abstract

Introduction Amid growing concern about indoor air quality (IAQ), MOS type gas sensors for the detection of volatile organic compounds (VOC) are widely used in household appliances such as air purifiers for on-demand control of the appliances depending on VOC concentration in surrounding air. There are also potential demands for gas sensors to be used for mobile applications, with which people can monitor environmental conditions in dwelling or workplaces. However, detection accuracy, power consumption and cost of conventional MOS type gas sensors have not yet reached the level generally required for VOC concentration monitoring by mobile applications. To solve these issues, we have developed a new diaphragm type MEMS gas sensor, and studied their appropriate pulsed heating conditions. This optimized MEMS sensor chip was integrated with an ASIC with built-in temperature and humidity sensors in a SMD package, and its practical performance and mechanical durability were evaluated. Sensor structures Two types of MEMS structures were used for this study as shown in Fig.1. Air-bridge type MEMS structure [1] [2] which is being produced on a commercial basis has a square-shaped micro hot plate (MHP) with dimensions of 0.1 x 0.1 mm. Materials of the sensor electrodes and heater are Platinum, and the gas sensing material is Palladium doped Tin dioxide, which is turned into a paste using terpineol as a solvent. The gas sensing material was applied onto the MHP by micro dispenser, and then was calcined at 450°C for 3 hours in an electric furnace. The newly developed diaphragm type MEMS structure has a round shape MHP with 270mm diameter which can avoid stress concentration of SiN/SiO2 membrane. Thickness of each layer stacked on the MHP was also optimized to achieve uniform dispersion of internal stress. The same sensing material as above mentioned was applied onto the MHP of the diaphragm type MEMS by screen printing method so that the gas sensing layers are formed in one printing pass on a wafer that produces 74K chips of MHP. Method In order to determine the appropriate pulsed heating conditions and thickness of the sensing material, sensor resistances in air (Ra) and in various concentrations of gases (Rg) were measured by using the air-bridge type MEMS sensor. The sensing material was intermittently heated for 100 ms at several different temperature conditions in a range from 300 ∼ 450 °C, and at different heating intervals ranging from 1 ∼ 60 seconds. The thickness of gas sensing layer was varied in a range from 20 ∼ 50 μm. For the diaphragm type MEMS sensor, the sensing material was heated at 430 °C for 100 ms at 1 second intervals. Results and Conclusions Sensing characteristics of the air-bridge type MEMS sensor to toluene and ethanol were investigated at different working temperature as shown in Fig. 2. Response to toluene increased as the working temperature decreased within the range between 350 °C and 450 °C, suggesting the capability for detecting low concentration toluene below 1 ppm. On the other hand, response to ethanol within the same temperature range remained largely the same. Toluene and ethanol sensitivities both drastically dropped at 300 °C. There were significant differences in toluene response depending on the heating intervals as shown in Fig.3. Response to toluene became smaller as the heating interval was longer. This result implies that a reaction of the gas sensing material with gas was inhibited by water vapor which had adsorbed on the surface of the sensing element during the heater OFF period. As for the effect of sensing material thickness on sensitivity, ethanol sensitivity was relatively more affected by the thickness than toluene. The sensor with the thinnest sensing layer showed the highest ethanol response, especially at a short heating interval. Taking these results into consideration, driving conditions of the diaphragm type MEMS sensor were determined as described above. Its sensing characteristics to various gases were examined as shown in Fig. 4. This sensor showed a particularly large response to m-xylene and hydrogen sulfide which are both among the major indoor air pollutants, and very small response to hydrogen and methane, both of which would work as an interference gas in IAQ measurement if present in ambient air. It was confirmed in parallel that the diaphragm type MEMS sensor with the determined driving conditions has a sufficient mechanical durability under the heater ON/OFF cycle test equivalent to 10 years operation, and the drop test which is required for mobile applications. The diaphragm type MEMS sensor produced by high-efficiency screen printing method was expected to have a capability as a practical solution for mobile IAQ monitoring. In addition, user-friendliness of the sensor was much improved by integrating with ASIC in comparison with conventional type single sensor.