Jörg Siegert

Modeling the Growth of Tin Dioxide Using Spray Pyrolysis Deposition for Gas Sensor Applications

In order for the gas sensor devices to enjoy the miniaturization trend that has consumed much of the electronic device industry, major research in the field is undertaken. The bulky sensor devices of previous generations can not easily be incorporated into a CMOS processing sequence, because of their bulky nature and potential higher cost of production. More recently, materials such as zinc oxide and tin dioxide have shown powerful gas sensing capabilities. Among many potential deposition methods, spray pyrolysis has become a popular approach because of its ease of use and cost effectiveness. A model for spray pyrolysis deposition is developed and implemented within the level set framework. The implementation allows for a smooth integration of multiple processing steps for the manufacture of smart gas sensor devices. From the observations, it was noted that spray pyrolysis deposition, when performed with a gas pressure nozzle, results in good step coverage, analogous to a CVD process. This is mainly due to the atomizing nozzle being placed at a reasonable distance away from the wafer surface and reducing the droplets volume and mass in order to ensure they fully evaporate prior to contact with the substrate surface. A topography simulator for this deposition methodology is presented.

Fully integrated System-On Chip gas sensor in CMOS technology

Within this work the development of an integrated gas sensor as System-On Chip (SOC) in a 0.35μm standard CMOS process plus CMOS compatible SnO₂-deposition and Si-release steps is presented. The SnO₂ layer provides high gas sensitivity to 10ppm for CO in humid air. An optimized Micro-Hotplate (μHP) consisting of a fully released membrane with a poly-Si heater in the oxide stack is designed. Due to the small area of A_μHP=100×100μm² and the switched capacitor temperature controller, low power consumption P_el=24mW at high temperatures T=400°C and short rise time of i_nîe=11.8ms are achieved. The differential setup contains sense and dummy sensors in order to compensate drift and tolerances. The readout stage consists of a gain adjustable amplifier with digital offset compensation and shows a relative error e<;±1%. The complete multichannel chip carries six sensors at a size A_chip=3.4×2.4mm², a power consumption P_chip=180mW and is well suited for various low power gas sensing applications.

Modeling and Analysis of Spray Pyrolysis Deposited SnO2 Films for Gas Sensors

Metal oxide materials such as tin oxide (SnO2) show powerful gas sensing capabilities. Recently, the deposition of a thin tin oxide film at the backend of a CMOS processing sequence has enabled the manufacture of modern gas sensors. Among several potential deposition methods for SnO2, spray pyrolysis deposition has proven itself to be relatively easy to use and cost effective while providing excellent surface coverage on step structures and etched holes. A model for spray pyrolysis deposition using a pressure atomizer is presented and implemented in a Level Set framework. A simulation of tin oxide deposition is performed on a typical gas sensor geometry and the resulting structure is imported into a finite element tool in order to analyze the electrical characteristics and thermo-mechanical stress present in the grown layer after processing. The deposition is performed at 400 °C and the subsequent cooling to room temperatures causes a stress to develop at the material interfaces due to variations in the coefficient of thermal expansion between the different materials.

Modeling the growth of thin SnO2 films using spray pyrolysis deposition

The deposition of a thin tin oxide film allows for the manufacture of modern gas sensors to replace the bulky sensors of previous generations. Spray pyrolysis deposition is used to grow the required sensing thin films, as it can be seamlessly integrated into a standard CMOS processing sequence. A model for spray pyrolysis deposition is developed and implemented within the Level Set framework. The implementation allows for a seamless integration of multiple processing steps for the manufacture of smart gas sensor devices. From observations it was noted that spray pyrolysis deposition, when performed with a gas pressure nozzle, results in good step coverage, analogous to a CVD process. This is due to the liquid droplets evaporating prior to contact with the heated wafer surface and subsequently depositing on top of the exposed silicon in vapor form.

Metal oxide nanowire gas sensors for indoor and outdoor environmental monitoring

We present performance results of SnO2 and CuO nanowire gas sensor devices, where single and multi-nanowire device configurations have been employed in order to optimize sensor design. In particular the response to the target gases CO, H2, and H2S has been measured in dry and humid air; both the SnO2 and CuO nanowire sensors are able to detect CO in the low ppm concentration range, which is important for environmental monitoring. The CuO multi-nanowire devices show an extraordinary high response to H2S with sensitivity in the low ppb concentration. We present our developments of CMOS technology based micro-hotplates, which are employed as platform for gas sensitive thin films and nanowires. Potential heterogeneous integration of nanowires on the micro-hotplate chips as well as an approach towards gas sensor arrays is discussed. We conclude that CMOS integrated multi-nanowire gas sensors are highly promising candidates for the practical realization of multi-parameter sensor devices for indoor and outdoor environmental monitoring.