화학테러가스 검지를 위한 마이크로 가스 센서 어레이의 제작 및 그 특성
Fabrication and its characteristics of micro gas sensor array for detection of chemical warfare agents
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Semiconductor thick film gas sensors with high sensitivity and low power consumption were fabricated for detection of chemical warfare agents(CWAs). The sensing materials were prepared in three different sets. 1) The In₂O₃(1, 2, 3 wt.%), TiO₂(3, 5, 10 wt.%), SiO₂(3, 5, 10 wt.%), WO₃(1, 2, 3 wt.%), and Al₂O₃(0, 4, 12, 20 wt.%) were added to SnO₂ by physical ball milling process. 2) ZrO₂((1, 3, 5 wt.%) and ZnO(1, 2, 3, 4, 5 wt.%) were added to SnO₂ by co-precipitation method. 3) Pt and Pd(1, 3, 5 wt.%) were added to SnO₂ by impregnation method. Single sensor using the alumina substrate was fabricated to examine gas responses of sensing materials. Pt thin film for sensing electrode was deposited using a DC sputter on the front side of alumina substrate and an electrode for the heater was screen printed using a Pt paste on the back side of it. Total dimension of device was 7 ㎜ × 10 ㎜ × 0.6 ㎜ and the size of sensing film was 6 ㎜ × 4 ㎜. The temperature of the heater with power consumption of 2 W was about 300 ℃. Sensing materials were formed on the sensing electrodes using screen printing technique. Prepared materials were characterized by TGA-DTA for selection of without gravimetric temperature, SEM for surface morphology and thickness, XRD for crystallite size and BET for specific surface area. Surface morphologies of the materials were comparatively uniform. Crystallite sizes of materials fabricated by ball-milling method (SnO₂-Al₂O₃, SnO₂-In₂O₃) were between 25 to 46 ㎚ and their specific surface areas were between 6 to 9 ㎡/g. But crystallite sizes and specific surface areas of materials prepared by co-precipitation method (SnO₂-ZnO, SnO₂-ZrO₂) were between 4 to 17 ㎚ and between 20 to 70 ㎡/g, respectively. Crystallite sizes of fabricated materials by co-precipitation method were smaller than those of fabricated materials by ball-milling method. The gas sensing properties of prepared sensors were examined for four different simulants of CWAs. They are dimethyl methyl phosphonate (DMMP) as a simulant of Tabun (nerve agent), dipropylene glycol methyl ether (DPGME) as a simulant of Mustard (blister agent), acetonitrile as a simulant of HCN (blood agent), and dichloromethane as a simulant of Phosgene (choking agent). The sensing properties were examined for the sensors with fixed amounts of additive (3 wt.%) at 300 ℃ in 0.5 ppm simulant ambient. In the responses to DMMP and DPGME, four materials such as Al₂O₃, In₂O₃, ZnO, and ZrO₂ showed higher sensitivity than other materials. In the responses to CH3CN and CH2Cl2, ZnO and ZrO₂ added devices showed better sensitivity. But, Al₂O₃ and In₂O₃ added devices did not show good sensitivity. Response properties for four simulant gases were examined for sensing films with different operating temperatures and gas concentrations. After added amounts and operating temperature were fixed from above results, sensors were carried out at different concentrations, repetition test, response for other gases, and response time. Most samples showed high sensitivity over 50 % for the test gases. Repetition measurements were also very good, with 3 % on a full scale. From the above results, four materials were selected for a sensor array. Four materials were same as 4wt.%-Al₂O₃, 2wt.%-In₂O₃, 2wt.%-ZnO and 1wt.%-ZrO₂. A micro sensor array with 6 sensors was designed and fabricated to reduce the power consumption and to provide highly selective device. Sensor array was fabricated on a 4-inch 100 p-type wafer and SiNx and SiO₂ were used as a membrane and passivation layer, respectively. The heater resistance is 50 Ω and sensing electrode is designed as interdigitated structure. Overall dimension of micro sensor array was 12 ㎜ × 8 ㎜ × 0.5 ㎜ and the size of sensing area fabricated was 1 ㎜ × 1 ㎜. The temperature of the heater linearly increased with the applied power. The power consumption for the heater to elevate the temperature to 300 ℃, was just about 80 mW. Sensor array was examined to check the complex gases, at different operating temperatures, and at different concentrations. PCA method was adapted to classify the chemical agent gases because of disadvantage that metal oxide sensors have comparatively high sensitivity to most gases. Consequently, a sensor array fabricated by using the thick film and MEMS technology showed high sensitivity to simulant gases, operated with low power consumption and could discriminate between gases by using the PCA method.