E. Brunet

Kinetic parameter estimation and fluctuation analysis of CO at SnO2 single nanowires

In this work, we present calculated numerical values for the kinetic parameters governing adsorption/desorption processes of carbon monoxide at tin dioxide single-nanowire gas sensors. The response of such sensors to pulses of 50 ppm carbon monoxide in nitrogen is investigated at different temperatures to extract the desired information. A rate-equation approach is used to model the reaction kinetics, which results in the problem of determining coefficients in a coupled system of nonlinear ordinary differential equations. The numerical values are computed by inverse-modeling techniques and are then used to simulate the sensor response. With our model, the dynamic response of the sensor due to the gas-surface interaction can be studied in order to find the optimal setup for detection, which is an important step towards selectivity of these devices. We additionally investigate the noise in the current through the nanowire and its changes due to the presence of carbon monoxide in the sensor environment. Here, we propose the use of a wavelet transform to decompose the signal and analyze the noise in the experimental data. This method indicates that some fluctuations are specific for the gas species investigated here.

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.

Tin oxide nanowire sensors for highly sensitive detection of the toxic gas H2S

We have realized gas sensor devices, which are based on a single SnO2-nanowire or a multiple SnO2-nanowire network as gas sensing components and are very sensitive to the toxic gas H2S. The nanowires are fabricated in a two-step atmospheric pressure synthesis process directly on the Si-chip by spray pyrolysis and subsequent annealing. Exposure of the single SnO2-nanowire sensor H2S with a concentration of only 1.4 ppm decreases the resistance by ~ 30%, while the multiple SnO2-nanowire network sensor exhibits a resistance decrease by ~ 90%. The nanowire sensors have extraordinary sensitivity with resolution limit in the ppb range and are able to measure concentrations well below the threshold limit value of 10 ppm. Due to their high performance the nanowire based sensors are basically suited for the realization of smart gas sensing devices for personal safety issues as well as industrial applications.

Bimetallic nanoparticles for optimizing CMOS integrated SnO 2 gas sensor devices

We present gas sensor devices based on ultrathin SnO2 films, which are integrated on CMOS fabricated micro-hotplate (μhp) chips. Bimetallic nanoparticles (NPs) such as PdAu, PtAu, and PdPt have been synthesized for optimizing the sensing performance of these sensors. We demonstrate that functionalization of nanocrystalline SnO2 gas sensing films with PdAu-NPs leads to a strongly improved sensitivity to the toxic gas carbon monoxide (CO) while the cross sensitivity to humidity is almost completely suppressed. We conclude that specific functionalization of CMOS integrated SnO2 thin film gas sensors with different types of NPs is a powerful strategy towards sensor arrays capable for distinguishing several target gases. Such CMOS integrated arrays are highly promising candidates for realizing smart multi-parameter sensing devices for the consumer market.

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.

On-chip synthesis of CuO nanowires for direct gas sensor integration

We report on the direct integration of CuO nanowires into a functional gas sensing device using a simple, cheap and scalable fabrication technology. Our approach relies on thermal oxidation of evaporated copper microstructures for the synthesis of suspended CuO nanowires with very small diameters between 10nm and 30nm at well-defined locations. We operate the devices as conductometric gas sensors and show their sensitivity during exposure to small concentrations of the toxic gases CO (down to 5ppm) and H2S (down to 10ppb). Due to the promising sensing results and the CMOS backend compatibility of the fabrication process steps, the presented technology may be used for the realization of fully integrated gas sensing devices.

Chip-to-chip SnO 2 nanowire network sensors for room temperatureH 2 detection

The employment of nanowires is a very powerful strategy to improve gas sensor performance. We demonstrate a gas sensor device, which is based on silicon chip-to-chip synthesis of ultralong tin oxide (SnO2) nanowires. The sensor device employs an interconnected SnO2 nanowire network configuration, which exhibits a huge surface-to-volume ratio and provides full access of the target gas to the nanowires. The chip-to-chip SnO2 nanowire device is able to detect a H2 concentration of only 20 ppm in synthetic air with ~ 60% relative humidity at room temperature. At an operating temperature of 300°C a concentration of 50 ppm H2 results in a sensitivity of 5%. At this elevated temperature the sensor shows a linear response in a concentration range between 10 ppm and 100 ppm H2. The SnO2-nanowire fabrication procedure based on spray pyrolysis and subsequent annealing is performed at atmospheric pressure, requires no vacuum and allows upscale of the substrate to a wafer size. 3D-integration with CMOS chips is proposed as viable way for practical realization of smart nanowire based gas sensor devices for the consumer market.