Partner und Internationale Organisationen
(Englisch)
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University of Warwick (UK), Eberhard-Karls-Universität
Tübingen (D), Linkoping University (S), Universita degli Studi di Roma (I), Centro Ricerche Fiat SCpA (I), VDO Adolf Schindler AG (D)
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Abstract
(Englisch)
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The automobile is an environment where the energy available is limited. The automotive systems, especially to improve the comfort in a car, require a power consumption as low as possible. In the aim of developing a gas sensing unit to regulate the air circulation in a car cabin, an important task is to reduce the power consumption of the sensors composing the whole system. The MOSFET (LIU) and Metal-Oxide (IPC) type gas sensors operate at high temperatures, 175°C for the first type and up to 350°C for the second type. Therefore, the use of micromachined-hotplates as substrates for these sensors reduces significantly their power consumption. The power consumption can drop from 1.0 W for a standard bulk device to a value of less than 100 mW for micromachined devices on silicon. Morever, the silicon technology allows to fabricate low-cost sensor substrates.
Since stable and reliable low-power gas sensors are not commercially available, they have to be custom made by depositing sensitive layers on microfabricated substrates. IMT was concerned by the fabrication of these micromachined structures on silicon for the other partners : UW, IPC and LIU, dealing with the gas sentive materials. There are two main types of structures: the resistive / capacitive type (UW, IPC) and the MOSFET (LIU) type. Concerning the resistive / capacitive type sensors, their substrates were fabricated at IMT and delivered to UW and IPC, which added their own optimised gas sensitive materials. In the case of MOSFET gas sensors, they were entirely fabricated at IMT.
Resistive gas sensors
To improve the robustness of the micro-hotplates to higher post-processing and operating temperature, we proceeded to a 3rd run (CIA 97) in which different materials (nitride and oxide) were used to make the membrane, the insulation and the passivation films. Films deposited by PECVD were avoided since they are unstable at high temperature. The micro-hotplates made only of low-stress LPCVD silicon nitride films are more robust (less breakdown during operation) than the ones with an oxide film as insulation layer. Moreover, their fabrication yield is higher. Both types can stand annealing temperature up to 700°C, necessary for the drop coating of the gas sensitive element (IPC), and also a pulse temperature mode of operation.
A 4th run (masks set CIA 98) with the optimum materials was also performed to improve the temperature homogeneity over the sensing area and the density of devices made on a silicon wafer. To improve the temperature uniformity, different heater geometry: simple and double meander, and the adding of a silicon plug underneath the membrane were investigated. FEM simulations were performed on the devices with a silicon plug and showed that it ensures an uniform distribution of the temperature all over the sensing area. On the other hand, the silicon plug increases the power consumption of the device. To increase the number of micro-hotplates fabricated on a wafer, Deep Reactive Ion Etching (DRIE) of silicon was used to release the membranes. This way, vertical walls can be achieved in the silicon and then the density of devices increased
As the conducting polymer sensors best react at room temperature, two runs of micro-hotplates, which had no membranes and therefore no heaters to ensure a good robustness, were processed. In the 1st run, the electrodes were made of gold (design used by UW before the CIA project) and the passivation layer of resist or polyimide. In the 2nd run, one innovation in the design (CIA 97) was the use of platinum as working electrodes for the conducting polymer instead of gold. This makes nitride passivation layer permissible which is superior to the previous polymer resistance because it can be used to grow polymers in organic solvents, e.g. acetonitrile. These two runs were performed to provide UW a number of devices to test new polymers.
All the wafers processed were diced at IMT and sensor chips delivered to UW or IPC depending on the design and the needs.
MOSFET gas sensors
IMT proceeded to the lay-out design of a MOSFET array gas sensor including 4 GasFETs, a heater and a diode as temperature sensor (MOSARV1). The electronics components and the heating resistor are located in a silicon plug underneath a dielectric membrane which thermally isolates the sensors from the chip frame.
Using this set of masks, IMT fabricated standard MOSFETs and low-power consumption MOSFET gas sensing devices. Bulk standard MOSFET gas sensors were characterised as a function of temperature at LIU. They are suitable for gas sensing at temperatures up to 225°C and they show a good gas sensitivity to H2 and NH3. Alternatively, the MOSFETs can be fabricated on silicon-on-insulator (SOI) wafers to achieve even higher operating temperatures. During the last months of the project, preliminary work has been done to adapt the fabrication process of MOSFETs on SOI with the MOSARV1 design and the IMT fabrication technology. Working transistors were made on SOI by using CVD oxide as doping source. These investigations permitted to show the great potential of the SOI technology in the field of MOSFET sensing devices.
Finally, a novel low-power consumption MOSFET array gas sensor has been designed jointly by IMT and LIU and entirely processed at IMT. 4 GasFETs, a heater and a diode temperature sensor are included on the sensor chip. A low power consumption of 80 mW is achieved for an operating temperature of 175°C for the array of 4 GasFETs compared to 0.5-1.0 W for one standard GasFET sensor.
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