Hochtemperaturstabile Mikroheizer für miniaturisierte Sensorsysteme

During the past decade the development of microheater devices has been an active field of investigation. Driver applications for such devices have been micromechanically fabricated metal oxide gas sensors. In the majority of these investigations standard silicon micro-machining technologies have been employed to produce thermally insulated membrane structures, which feature low levels of heating power consumption (~50 mW at 400°C) and small thermal response times (~10 ms). Up to the present time the heating of such structures is generally accomplished by passing currents through evaporated or sputtered films of noble metals, like platinum (Pt). Using such material combinations long-term stable device operation has been found feasible up to about 600°C. Over the years, an increasing demand of microheaters with higher operating temperatures for gas sensing systems as well as for thermal infrared (IR) sources in optical sensor systems arose. The currently employed metal heaters, however, have limited stability above 600°C and fail rapidly when operating at higher temperatures. The objective of the present thesis therefore was finding alternative heater materials and metallisation techniques to enable microsystems with prolonged operating temperatures of about 800°C and beyond. As a representative high-temperature application, micromachined thermal infrared (IR) emitters have been realised and their performance characterised. The electric heating of the emitter hotplate is achieved with electrically conducting heater structures employing different kinds of metallic and semiconductor materials. In order to assess the long-term performance of these heater elements, accelerated degradation tests had been performed and the results compared to each other. The results clearly revealed that doped semiconductors provide a high-temperature performance that is superior to metallic ones. The best results so far have been obtained with antimony-doped tin oxide (SnO2:Sb) heaters. This kind of heater metallisation combines the moderate electrical conductivity of doped semiconductors with the advantage of complete oxidation stability of stoichiometric oxides.In the second part of this thesis detailed investigations of this new kind of heater metallisation are presented to reveal the high-temperature degradation mechanisms that limit the lifetime of SnO2:Sb heater elements. The results show that - unlike most other heater materials - the high-temperature stability of SnO2:Sb is not limited by electromigration but rather by outdiffusion and evaporation of Sb impurities from the SnO2 bulk. Continuing such heating experiments towards higher temperature revealed that SnO2:Sb coated hotplates can be heated up to the melting temperature of silicon (~1410°C) before they get destroyed. Aiming at even higher temperatures microheaters based on silicon carbide (SiC) have been realised. Apart from a much higher mechanical strength, SiC has a substantially higher temperature stability (up to ~2300°C). The acquired knowledge leads to microheater devices with an estimated lifetime of 10 years at operation temperatures up to 950°C. Heater elements based on SnO2:Sb films therefore form a very useful complement to the existing silicon micromachining technology that opens up interesting new applications in the field of high-temperature MEMS devices.

In den vergangenen Jahren wurden von Mikroheizern für die Gassensorik, aber auch für thermische Infrarotquellen zunehmend höhere Betriebstemperaturen gefordert. Bei Temperaturen von über 600°C stoßen die bisher verwendeten Heizermaterialien jedoch an ihre Leistungsgrenzen. Ziel dieser Doktorarbeit war es daher, alternative Materialien und Technologien zu erforschen, welche einen langzeitstabilen Betrieb dieser Mikro systeme auch bei Temperaturen von 800°C und mehr ermöglichen. Als repräsentative Hochtemperaturanwendung wurden thermische IR-Emitter mit unterschiedlichen Heizermetallisierungen mikromechanisch aus Silizium und Siliziumkarbid hergestellt und auf ihre Eigenschaften hin untersucht. Die gewonnenen Erkenntnisse führten schließlich zur Entwicklung einer hochtemperaturfähigen Heizermetallisierung aus Antimon-dotiertem Zinnoxid (SnO2:Sb), welche einen permanenten Betrieb bei Temperaturen von bis zu 950°C über einen Zeitraum von mindestens 10 Jahren ermöglicht. Diese Tatsache eröffnet der Mikrosystemtechnik ein weites Feld neuer Anwendungsgebiete im Hochtemperaturbereich.


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