The present study is focused on the development of a gas sensor for application in a high temperature environment. The sensor has been realised using thin ®lms prepared on silicon substrates including a high temperature stable heating and wiring system. TiO 2 acts as sensitive layer. Measurements have been carried out in synthetic gas mixtures as well as in gases in a given application. Neural networks and multivariate data analysis have been used for determining the gas concentrations. The capability to detect CO, NO x , and toluene is shown.
IntroductionMicromachined gas sensors using semiconductive metal oxides have been established for some years in various applications [1±3]. The sensitivity of these devices is based on a change in the resistivity of the metal oxide ®lm depending on the surrounding atmosphere. The sensor temperature is one important parameter for the interactions between the several gases and the metal oxide ®lm. Based on experimental studies [4±7] metal oxides require temperatures of 200°C and more to be applicable as conductive sensor materials. The exact temperature depends on the used metal oxide as well as on the kind of gases to be detected. For that reason the sensors have to be heated by suited elements. Such sensors can be operated in an environment with temperatures far below up to the sensor working temperature. Conceivable applications are the monitoring and controlling of combustion processes. Especially for the application at high temperatures the major problem of these sensors is the long term stability. Respective device concepts have to be developed to ensure this stability during the whole device lifetime. These concepts include the selection of appropriate materials for the substrate, sensitive layer, and metallization, as well as the selection of a suitable housing and wiring technology.In this article we employ titania thin ®lms as material for gas sensing at different temperatures and show their features. We illustrate one way to prepare sensor devices as well as the results of their characterisation in laboratory gases and in gases in a given application.
Sensor design and fabrication
Sensor designThe sensor chip consists of a gas sensitive titania layer with platinum electrodes above and underneath this layer. For signal pattern analysis each sensor chip contains 3 sensor cells. To generate temperature dependent gas speci®c signal pattern the cell temperatures can be tuned independently by underlying heaters. For the temperature control platinum resistors are integrated in the electrode level underneath the sensitive layer. The gas sensors are prepared using standard silicon wafers. The sensitive elements are realised on a thin silicon membrane (50 lm) in order to reduce the required heating power. Furthermore, this leads to an optimised temperature pro®le and reduces the heat¯ow from the sensor area to the chip mounting. On the membrane 5 platinum heaters are located, one heater for each sensor cell and two supporting heaters to realise the necessary temperature grad...
Technologies for the 3D integration are described within this paper with respect to devices that have to retain a specific minimum wafer thickness for handling purposes (CMOS) and integrity of mechanical elements (MEMS). This implies Through-Silicon Vias (TSVs) with large dimensions and high aspect ratios (HAR). Moreover, as a main objective, the aspired TSV technology had to be universal and scalable with the designated utilization in a MEMS/CMOS foundry. Two TSV approaches are investigated and discussed, in which the TSVs were fabricated either before or after wafer thinning. One distinctive feature is an incomplete TSV Cu-filling, which avoids long processing and complex process control, while minimizing the thermomechanical stress between Cu and Si and related adverse effects in the device. However, the incomplete filling also includes various challenges regarding process integration. A method based on pattern plating is described, in which TSVs are metalized at the same time as the redistribution layer and which eliminates the need for additional planarization and patterning steps. For MEMS, the realization of a protective hermetically sealed capping is crucial, which is addressed in this paper by glass frit wafer level bonding and is discussed for hermetic sealing of MEMS inertial sensors. The TSV based 3D integration technologies are demonstrated on CMOS like test vehicle and on a MEMS device fabricated in Air Gap Insulated Microstructure (AIM) technology
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