Jürgen Wöllenstein

Cobalt oxide based gas sensors on silicon substrate for operation at low temperatures

The gas sensing characteristics and the morphology of cobalt oxide thin films have been investigated. The thin films were prepared by reactive electron beam evaporation of cobalt on “pure” and surface-oxidised silicon wafers respectively followed by an additional thermal treatment. Structural and morphological analyses of the thin Co3O4 films were performed by X-ray diffraction analysis (XRD), scanning electron microscope (SEM) and Rutherford backscattering (RBS). Two gas-sensitive parameters of the Co3O4 films were investigated: the shift of the work function and change of conductivity during gas exposure. The gas measurements were carried out with ammonia, methane, carbon monoxide, hydrogen, chlorine (only GasFET) and nitric dioxide as test gases in synthetic air at different humidities. The work function measurements were carried out with suspended gate GasFETs as transducer, the resistive measurements with single chip thin-film sensor arrays.

A novel single chip thin film metal oxide array

Abstract Two silicon-based sensor arrays are described and tested, one based on films deposited onto bulk silicon wafers and the second, on a novel micromachined device in which the heated membranes are suspended from glass posts to insure low power dissipation. The sensing elements examined in the arrays included p-type Cr1.8Ti0.2O3+z, n-type ZnO, n-type SnO2, n-type WO3 and n-type V2O5. The sensor arrays were investigated under trace gas exposure (H2, CO, NO2 and NH3) and, in all cases, at least one of the semiconducting metal oxide films reacted with sufficient sensitivity to the investigated gases. The measured response provided different patterns for each of the metal oxides and is thus appropriate for evaluation by pattern recognition techniques. A technological concept for an integrated device comprising the gas sensor array and circuitry for sensor control and data readout is discussed.

Material properties and the influence of metallic catalysts at the surface of highly dense SnO2 films

Abstract We present an approach to optimize the specific response to gases by using specially prepared nanosized platinum on highly dense sputtered polycrystalline SnO 2 . Structural and morphological analyses of the SnO 2 and platinum thin films were performed. Gas measurements were carried out with single chip thin-film SnO 2 sensor arrays on silicon substrates. Pt nanoclusters covering the sensitive layer significantly affect the O 3 , CO and NO 2 sensitivities and the corresponding dynamic response.

Micromechanical fabrication of robust low-power metal oxide gas sensors

A fabrication process for miniaturized low-power metal oxide gas sensor arrays is presented. This process, which is based on silicon-on-insulator (SOI) wafers, leads to thermally insulated silicon hot plate structures, which successfully combine low-power consumption with mechanical ruggedness and a high process yield. A second important feature of our process is that it consists of a standard-silicon front-end and a sensor-specific back-end module. This separation into front-end and back-end modules is a prerequisite for an efficient workshare between silicon foundries and SME sensor manufacturers in the industrialization of low-power gas sensor arrays. In the first part of the paper we present an outline of the fabrication flow consisting of the front-end fabrication of micromachined low-power heater platforms and the back-end fabrication of gas-sensitive layers. In the second part we report on the results of extensive mechanical, thermal and gas sensing measurements on such miniaturized devices.

MICROMACHINED THIN FILM SNO2 GAS SENSORS IN TEMPERATURE-PULSED OPERATION MODE

Abstract Gas detection measurements based on a micromachined SnO 2 gas sensor with periodically pulsed heater voltage are presented. Additionally, the field-effect-induced changes in resistivity of the sensitive layer caused by the heater voltage were investigated. The combination of both results leads to an improved design for low power SnO 2 gas sensors. In temperature-pulsed mode, the sensor resistances were measured at constant delays after the pulse edges. The measurements were carried out with the common test gases carbon monoxide and nitric dioxide in synthetic air with 50% humidity. In the cold pulse phase, the CO sensor response is higher and shows only a slow decrease with increasing pulse duration. The sensor sensitivity is related to the pulsed heated mode, on the one hand, and the continuously heated, on the other. The comparison of the measurement results reveals that the temperature-pulsed operation mode (TPOM) caused a significant reduction of power consumption and higher sensitivity.

Micromachined IR-source with excellent blackbody like behaviour (Invited Paper)

There are several micro sized thermal emitters commercially available, but compared with an ideal black body radiator, their emissivity and thus the emitted radiation is moderate. This was the motivation to develop a novel type of micromachined thermal IR-emitters. The main difference compared with common thermal micro emitters is the use of 2D structured bulk silicon. The regular ordered macropores of the emitters are obtained by electrochemical etching of prepatterend silicon substrates. Typical pore diameter of the fabricated photonic-crystal-like structures are in the range of 2.5 μm to 30 μm. The macroporous silicon shows a black-body-like emission profile for a wide wavelength range.

Conduction model of SnO2 thin films based on conductance and Hall effect measurements

Thin and porous SnO2 films (70nm thick with grain size between 10 and 30nm) have been prepared by e-beam evaporation onto alumina substrate provided with platinum electrodes. The Ohmic character of the contacts was preserved in all measurement conditions utilized for investigations. The dependence of electrical conduction on the composition of the ambient atmosphere has been studied by means of Hall and four point conductance measurements. The experiments were performed in different gas atmospheres containing N2, O2, and CO and at different operation temperatures (between room temperature and 420°C). A relatively low effective mobility (5–30cm2V−1s−1) and a high charge carrier effective concentration (1018–1019cm−3) were deduced when using the single crystals recipe, as required by the established models for granular materials. The analysis of these experimental data showed the inadequacy of the geometrical models and effective medium theories to correctly extract the electrokinetic parameters from conduc...

Ethylene detection in fruit supply chains

Ethylene is a gaseous ripening phytohormone of fruits and plants. Presently, ethylene is primarily measured with stationary equipment in laboratories. Applying in situ measurement at the point of natural ethylene generation has been hampered by the lack of portable units designed to detect ethylene at necessary resolutions of a few parts per billion. Moreover, high humidity inside controlled atmosphere stores or containers complicates the realization of gas sensing systems that are sufficiently sensitive, reliable, robust and cost efficient. In particular, three measurement principles have shown promising potential for fruit supply chains and were used to develop independent mobile devices: non-dispersive infrared spectroscopy, miniaturized gas chromatography and electrochemical measurement. In this paper, the measurement systems for ethylene are compared with regard to the needs in fruit logistics; i.e. sensitivity, selectivity, long-term stability, facilitation of automated measurement and suitability for mobile application. Resolutions of 20–10 ppb can be achieved in mobile applications with state-of-the-art equipment, operating with the three methods described in the following. The prices of these systems are in a range below €10 000.

Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems

A micromachined thermal emitter for fast transient temperature operation with a novel hot-plate concept is presented. This concept is based on a nonaxis-symmetric design with excellent mechanical properties during temperature modulation combined with high thermal decoupling. Especially, the mechanical stress induced by the thermal expansion of the hot-plate and their suspension was improved. This results in a reduced sensitivity for buckling of the hot-plate. The thermal emitter is fabricated using silicon on insulator (SOI) technology and KOH-etching. Different suspension structures were realized and mechanical and thermal characterizations were performed. Besides the realization of the new hot-plate suspension design, a high thermal emission at wavelengths > 5 ¿m has been achieved using ceramic coatings for emissivity enhancement. This kind of emission tuning owns-in contrast to the typical surface and bulk structuring methods-the possibility to act simultaneously as a heater passivation.

SENEKA - sensor network with mobile robots for disaster management

Developed societies have a high level of preparedness for natural or man-made disasters. But such incidents cannot be completely prevented, and when an incident like an earthquake or an accident in a chemical or nuclear plant hits a populated area, rescue teams need to be employed. In such situations it is a necessity for rescue teams to get a quick overview of the situation in order to identify possible locations of victims that need to be rescued and dangerous locations that need to be secured. Rescue forces must operate quickly in order to save lives, and they often need to operate in dangerous environments. Hence, robot-supported systems are increasingly used to support and accelerate search operations. The objective of the SENEKA concept is to network the various robots and sensor systems used by first responders in order to make the search for victims and survivors more quick and efficient. SENEKA targets the integration of the robot-sensor network into the operation procedures of the rescue teams. The aim of this paper is to inform on the goals and first research results of the ongoing joint research project SENEKA.