Measurements of Residual Stress in the Layers of Thin Film Micro-Gas Sensors
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Measurements of residual stress in the layers of thin film micro-gas sensors Youngman Kim and Sung-Ho Choo Department of Metallurgical Engineering, Chonnam National University, Kwangju 500-757, Korea [email protected] Abstract The mechanical properties of thin film materials are known to be different from those of bulk materials, which are generally overlooked in practice. The difference in mechanical properties can be misleading in the estimation of residual stress states in micro-gas sensors with multi-layer structures during manufacturing and in service. In this study the residual stress of each film layer in a micro-gas sensor was measured according to the five difference sets of film stacking structure used for the sensor. The Pt thin film layer was found to have the highest tensile residual stress, which may affect the reliability of the micro-gas sensor. For the Pt layer the changes in residual stress were measured as a function of processing variables and thermal cycling. Introduction A SnO2 micro-gas sensor processed in combination with thin film and micro-machining technologies has the advantages in terms of manufacturing such as mass production oriented stabilized process, low cost, low power consumption and precise temperature control. Since the relevant circuits for the sensor can be manufactured on one chip, the miniaturization of the system is achievable and the reliability can be improved simultaneously. Thus it is a general tendency that thin film type sensors gradually substitute for bulk and thick film types. The standardization for the sensor manufacturing is a challenging task and the mechanical strength of the sensors is known to be a problem [1-3]. Thin film micro-gas sensors are composed of gas-sensing material, electrodes, heaters and insulating layers. The sensing mechanism for the micro-gas sensor is detecting the difference in conductivity of sensing material before and after the adsorption of gases. Since the current passing through the gas-sensing material is about several pAs, the sensitivity of sensor highly depends on the interfaces of gas-sensing materials, electrodes and electrode materials. Micro-gas sensors essentially require heaters to increase the operation temperature of gas-sensing material to specific temperature range for the highest sensitivity of specific gases. Low power consumption and thermal stability are essential features for the heaters of the sensors in the wide temperature range. For low power consumption the micro-gas sensors have diaphragm structures with multi-layers of metals and ceramics, such as the layers of insulator, heater, electrode and sensing materials layers [2]. Any type of multi-layer structure results in the difference in thermal expansion coefficient and elastic modulus between layers due to the artificial bonding of dissimilar materials. Inevitably thermal and residual stresses between layers arise from the bonding. In turn the stresses may cause dimensional instability of parts, such as twisting and bending, which can trouble the
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