Byung-Su Joo

Three electrodes gas sensor based on ITO thin film

A three electrode type sensor was fabricated. This sensor consists of two parts. One is a tin doped indium oxide (or indium tin oxide: denoted ITO hereafter) film. It was fabricated by the metal thermal oxidation method. This produces a large surface area and low resistivity. So, this film has two functions, a gas sensitive film and an effective heater. The other part is a platinum resistor line covered with oxidized aluminum film. This part is located in the backside of the sensor. It plays three roles. The first is a load resistor for a measuring system. Secondly, it also acts as a heater. Most of the consumed power in the sensor is used as the thermal energy of gas sensing. The last role is catalytic combustible gas sensing in high gas concentrations. In this paper, the fabrication method of gas sensitive ITO film is researched. A complex sensor structure is fabricated. It acts as a hot wire semiconductor sensor at low gas concentration and a combustible catalytic sensor at high concentration.

The fabrication of novel micro hot-wire sensor

Many works to improve the stability and sensitivity of metal oxide based gas sensors have been carried out during last several decades. As one of stability problems, we can present one example that the resistance alters to some extent when the devices are exposed to the normal air, which becomes an obstacle to its practical use. In this paper, we proposed new type of micro gas sensor with single electrode for improving stability and sensitivity. Generally, metal oxide gas sensors have two electrodes for heating and sensing. But this new type sensor has only a single electrode by forming a sensing material onto heating electrode. This micro gas sensor shows low resistance(a few hundred /spl Omega/) for parallel values of Pt resistance and SnO/sub 2/ sensing resistance. Therefore, it is appeared that the deviation of total resistance in the normal air condition is very low and stability is improved. Pt as a heating and sensing electrode is sputtered on glass (pyrex 7740) substrate and SnO/sub 2/ sensing material is thermally evaporated on Pt electrode. SnO/sub 2/ is patterned by lift-off process and then thermally oxidized in O/sub 2/ condition for 1 hr., 600/spl deg/C. The size of fabricated sensor is 2/spl times/2 mm/sup 2/. As a result of CO gas sensing characteristics, this sensor shows 100 mV voltage output for 1,000 ppm and linearity for wide range(0/spl sim/20,000 ppm) of gas concentration. And the sensor shows a good recovery characteristic of 1% deviation compared to initial resistance. The deviation of sensor resistance is about 5% for 6 months and a little influence for humidity of 90 R.H.%.

A field effect transistor type gas sensor based on polyaniline

The researched sensor at this paper is based on gas sensitive conducting polymer. Some types of conducting polymer have gas sensitivity at room temperature (Matsuguchi et al., 2002). These conducting polymers have several shortages such as low sensitivity. The transistor type gas sensor has been studied for several decades. These sensors are based on metal oxide ceramics such as tin dioxide (Wollenstein et al., 2000 and Popova et al., 1991). We researched a polymer FET gas sensor. The sensor structure is similar to thin film transistor (TFT) one. This sensor can detect very low concentration of ammonia gas. The synthesized polyaniline has the resistance change for ammonia gas. Normally IDT electrode resistor sensor can detect several decade ppm or more ammonia gas. The FET type sensor which has the same polyaniline film can detect 5 ppm of ammonia.

Thin Film Gas Sensors Based on Tin Oxide for Acetonitrile

Thin film gas sensor based on tin oxide was fabricated and its characteristics were examined. Target gas is acetonitrile () that is simulant gas of blood agent gas. Sensing materials are , /Pt, and (Sn/Pt)oxidation with thickness from to . Sensor was consisted of sensing electrode with interdigit (IDT) type in front side and a heater in back side. Its dimension was . Fabricated sensor was measured as flow type and monitored real time using PC. The optimal sensing material for was {Sn()/Pt()}oxidation and its sensitivity and operating temperature were 30%, in 3 ppm.

Room temperature operating nitrogen dioxide sensor based tellurium thin films

The characteristic of tellurium thin films was studied for detecting nitrogen dioxide gas at room temperature. The film was deposited on substrate by using thermal evaporator. The subsequent process was heat treatment by several conditions. (temperature, flowed gases) Surface and grain boundary was investigated using SEM. The results showed that resistance of the tellurium film decreases reversibly in the presence of nitrogen dioxide. The sensitivity of this device depends on the gas concentration and detect lower concentrations less than 10 ppm.

Electrical modelling for thermal behavior and gas response of combustible catalytic sensor

This study provides the electrical model of combustible catalytic gas sensor. Physical characteristics such as thermal behavior, resistance change were included in this model. The finite element method analysis for sensor device structure showed that the thermal behavior of sensor is expressed in a simple electrical equivalent circuit that consists of a resistor, a capacitor and a current source. This thermal equivalent circuit interfaces with real electrical circuit using two parts. One is `power to heat` converter. The other is temperature dependent variable resistor. These parts realized with the analog behavior devices of the SPICE library. The gas response tendency was represented from the mass transferring limitation theory and the combustion theory. In this model, Gas concentration that is expressed in voltage at the model, is converted to heat and is flowed to the thermal equivalent circuit. This model is tested in several circuit simulations. The resistance change of device, the delay time due to thermal capacity, the gas responses output voltage that are calculated from SPICE simulations correspond well to real results from measuring in electrical circuits. Also good simulation result can be produced in the more complicated circuit that includes amplifier, bios circiut, buffer part.

Thermally stable micro hot wire sensor with Borofloat glass membrane

We report on the fabrication and characteristics of micro hot wire sensor with Borofloat glass membrane on diaphragm by using anodic bonding. Borofloat membrane was thermally stable and strong for mechanical stress. The four types of sensing materials SnO/sub 2/, In/sub 2/O/sub 3/ and mixed oxide of adequate ratio with these two type metal oxide semiconductor were prepared by thermal evaporation method and then oxidized at 600 /spl deg/C, 3 hrs. Various sensing materials for NO/sub x/, CO, etc. were prepared. And then the response characteristics were investigated. For mixed oxide semiconductor with Sn and In oxide, these showed the high sensitivity for NO/sub x/ and CO gas. And by using the Borofloat glass substrate, the power consumption was much reduced about 60 mW in condition of the best sensitivity.

Fabrication of Optical Fiber Gas Sensor with Polyaniline Clad

Optical fiber sensors have been used to detect small amounts of chemical species. In this work, a new thin polymer-clad fiber sensor is developed. Polyaniline is chemically synthesized and thin clad layers of the polymer are easily deposited on optical fiber by dip-coating technique. The optical property of polyaniline as a sensing material is analyzed by UV-Vis-NIR. The light source is stabilized He-Ne laser at 635 nm wavelength with 1 mW power. The light power transmitted through the optical fiber is measured with a spectrophotometer. By selecting a fixed incident angle, variation of transmitted light intensity through the optical fiber can be detected as gas molecules absorbed in the polyaniline clad layer. Among the various gases, the fabricated optical fiber sensor shows good sensitivity to gas. The optical fiber sensors was shown more improved properties than polymer based sensors which measure conductivity changes.

Volatile Organic Gas Recognition Using Conducting Polymer Sensor array

We fabricated gas recognition system using conducting polymer sensor array for recognizing and analyzing VOCs(Volatile Organic Compounds) gases. The polypyrrole and polyaniline thin film sensors which were made by chemical polymerization were employed to detect VOCs. The multi-dimensional sensor signals obtained from the sensor array were analyzed using PCA(principal component analysis) technique and RBF(radial basis function) Network. Throughout the experimental trails, we confirmed that RBF Network is effective than PCA technique in identifying VOCs.

Classification of Indoor Environmental Gases Using Temperature Modulation

Two based sensing films(pure and /Pt) and a Pt thin film for temperature sensor on an alumina substrate were designed and fabricated for classifying the indoor environmental gases. By controlling the heating power in the shape of trapezoid, unique four sensing response curves created from both film and /Pt film. Then, various parameters were extracted from sensing response curves and carried out principal component analysis(PCA). The results confirm that a sensor array with the proposed operating mode was extremely effective in classifying indoor environmental gases such as , , .