We present gas sensor devices based on nanocrystalline 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 proper functionalization with PdAu-NPs leads to a strongly improved sensitivity to the toxic gas carbon monoxide while the cross sensitivity to humidity and carbon dioxide is almost completely suppressed, which is of high importance for real life environmental conditions. We also present μhp chips employing Through-Silicon-Via (TSV) technology, which are capable for flexible 3D-integration of different types of gas sensors to a multi-parameter nanosensor system. Such CMOS integrated systems are promising candidates for realizing smart sensor devices for consumer market applications.

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 AμHP=100×100μm2 and the switched capacitor temperature controller, low power consumption Pel=24mW at high temperatures T=400°C and short rise time of inîe=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 Achip=3.4×2.4mm2, a power consumption Pchip=180mW and is well suited for various low power gas sensing applications.

The ppb level detection of the toxic gas H2S is of great importance for industrial and safety applications. We report on CuO nanowire gas sensors, which are capable to detect H2S concentrations as low as 10ppb in dry as well as humid atmosphere. In particular, measurements with different humidity levels up to 65% have been performed, which is of high practical relevance regarding H2S detection in ambient atmosphere. Three different types of conductometric gas sensors have been investigated: a single CuO nanowire configuration and two different multiple CuO nanowire configurations that have been developed in order to optimize sensor performance and to enable CMOS integration of CuO nanowire gas sensors in the future. All sensor devices employ suspended CuO nanowires as gas sensitive components. This is a highly favorable configuration because the nanowires are entirely surrounded by the gas atmosphere. The devices based on multiple CuO nanowires show enhanced H2S response in humid air compared to dry synthetic air. Furthermore, we have found that the sensor design, which employs CuO nanowires with the smallest average diameters around 20nm, has the highest gas response. The estimated detection limit approaches the ppt range, which shows the excellent sensor performance even in humid atmosphere. These results and the CMOS backend compatibility of the optimized CuO nanowire sensor design are of high importance for the realization of low power silicon integrated gas sensors for daily life applications.

We report on novel gas sensing devices based on cupric oxide (CuO) nanowires which are synthesized on-chip by thermal oxidation of electroplated copper microstructures. This technique enables the direct integration of a multitude of CuO nanowires, which bridge the electrical contacts of a conductometric gas sensor. The CuO nanowire bridges exhibit a huge surface-to-volume ratio and are entirely surrounded by the gas atmosphere, which is a highly favorable gas sensor configuration. As a result, the CuO nanowire gas sensor devices are able to detect carbon monoxide (CO) down to a concentration of 10ppm and exhibit extraordinary sensitivity to hydrogen sulfide (H2S) where concentrations down to 10ppb have been detected, even in the presence of humidity. For characterization of the CuO nanowires, X-ray diffraction measurements, transmission electron microscopy and electron energy loss spectroscopy are employed. As no process temperatures higher than 400°C are required for the fabrication of the CuO nanowire devices, our approach can be employed in a CMOS backend process enabling the realization of a fully silicon integrated CuO nanowire gas sensing device.

In this paper we report on conductometric gas sensors based on suspended CuO nanowire arrays for CO detection in humid atmosphere. CuO nanowires with very small diameters (10nm-30nm) are synthesized by a thermal oxidation process of Cu microstructures. Our approach allows efficient CuO nanowire integration using a CMOS backend compatible technology. We perform CO measurements in a concentration range between 25ppm and 150ppm and investigate humidity interference effects by comparing CO measurements in dry and humid atmosphere.

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.

In order to optimize the performance of gas sensor devices, nanocrystalline SnO2 thin film and single crystalline SnO2 nanowire sensors have been characterized for the target gases CO, CH4, H2, CO2, SO2 and H2S. At optimum operating temperature – varying from 250°C to 400°C – the SnO2 thin film sensor detects CO, CH4, CO2 and SO2 with responses in the range of 23–34%, H2 and H2S with responses above 80%. The SnO2 nanowire sensor shows responses in the range of 1–8% for CO, H2 and SO2, 29% for H2S, while CH4 and CO2 are not detected. Taking into account that the exposed surface area of the thin film sensor is 800 times larger than that of the single nanowire, we have correlated the number of CO gas molecules impinging the sensors’ surface with the number of electrons, which are actually detected as sensors’ response for the target gas CO. In case of the thin film sensor a single detected electron requires ∼2760 gas molecules impinging the sensor's surface. For the nanowire sensor only ∼86 gas molecules are required for a single detected electron. The SnO2 nanowire sensor thus has a detection efficiency more than 30 times higher than the SnO2 thin film sensor, which we attribute to a lack of grain boundaries. From our measurements we conclude that single crystalline SnO2 nanowire sensors provide a higher sensitivity and an improved cross-sensitivity than their nanocrystalline counterpart.

The development of smart gas sensor devices for daily life application requires considerable technological efforts related to CMOS integration of gassensitive materials. We are focusing on the heterogeneous integration of ultrathin (50nm) SnO2 layers deposited by spray pyrolysis with CMOS devices. With respect to miniaturization and implementation of the sensing layers on micro-hotplates (μhps) we compared the H2 responses for 100x100μm2 and 5x100μm2 sized sensing films and found a very high response of 42% and 28%, respectively, for 10ppm H2. We performed 2-point and 4-point electrical measurements of SnO2 layers on typical CMOS Al contacts and achieved linear V-I characteristic in the full operating temperature range up to 350¡C in 4-point configuration. We also demonstrate the excellent step coverage of the deposited SnO2 layers on passivated CMOS chips, which is highly important for post-CMOS processing of the sensor films.

Nanowire gas sensors show high sensitivity towards various gases and offer great potential to improve present gas sensing. In this work, we investigate experimental results achieved with an undoped single SnO2 nanowire sensor device for CO pulses in N2 atmosphere at different operating temperatures. We calculated the reaction parameters according to the mass action law including frequency factors, activation energies, and numbers of intrinsic as well as extrinsic surface sites. With the values obtained, we then calculated the surface charge of the nanowire sensor by solving the corresponding differential equations. The simulated results agree very well with the experimental values at an operating temperature of 200¡C and hence provide good understanding of the chemical reaction. This can be used to simulate the current through the transducer and consequently the sensitivity of the device, and the parameters provided here are useful for computational procedures to provide selectivity.

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 H-2 concentration of only 20 ppm in synthetic air with similar to 60% relative humidity at room temperature. At an operating temperature of 300 degrees C a concentration of 50 ppm H-2 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 H-2. 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.

We present a novel method for integrating single CuO nanowires as gas sensing components in a suspended configuration. In these devices, the nanowire surface area that is actually exposed to the surrounding gas atmosphere is increased, which is favourable when operating the nanowire as conductometric gas sensor. CuO nanowires bridging two adjacent Cu structures were contacted using a thermal oxidation process leading to growth of copper oxide around the nanowire. By this method, linear IV characteristics could be achieved showing the Ohmic contact properties. The sensitivity of the suspended CuO nanowire devices towards humidity and CO was investigated. High signal changes up to 60% were measured in the presence of humidity while small concentrations of CO down to 1ppm could be detected validating the excellent sensing performance of the presented devices.

Cupric oxide (CuO) nanowires were synthesized by thermal oxidation of resistively heated copper wires in ambient air conditions. Aspect ratios up to 1000 have been achieved with this easy growth method that requires only standard laboratory equipment. The as-synthesized nanowires were subsequently transferred to silicon substrates and contacted by a structured metallization layer. These devices were used for conductometric gas sensing as nanowire conductivity is strongly dependent on the surrounding gas atmosphere. The sensitivity of a single CuO nanowire towards water vapour and hydrogen sulfide (H2S) was investigated regarding a possible application in hydrogen fuel cells.

We present nanosensors based on ultrathin SnO2 films, which are very sensitive to the highly toxic gases SO2 and H2S. The SnO2-sensing films are fabricated by a spray pyrolysis process on Si substrates with a thickness of 50 nm. The sensor resistance is decreased in the presence of the toxic gases. Exposure to 50 ppm SO2 leads to a sensor resistance drop of ∼40% whereas a H2S gas concentration of only 2.5 ppm decreases the resistance by ∼85%, which demonstrates the extraordinary sensitivity of the nanosensors. With respect to further system integration a CMOS technology based micro-hotplate containing heating element and sensing layer has been simulated. Preliminary results show that the micro-hotplates can provide operating temperatures of 400 °C with a power consumption of less than 5 mW. A concept for 3D system integration of the nanosensor chip and a CMOS chip based on Through-Silicon-Via (TSV) technology is proposed as potential roadmap towards smart nanosensor systems for daily life applications.

We demonstrate a novel gas sensor device, which is based on silicon chip-to-chip synthesis of ultralong tin oxide (SnO2) nanowires. The sensor device employs an interconnected 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 has an extraordinary sensitivity to the toxic gas H2S. A concentration of only 1.4 ppm decreases the resistance of the sensor by ~ 85%, which demonstrates a detection limit far in the ppb range. 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.

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.