Surface Acoustic Wave Devices: Materials Stability in Harsh Environments
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Surface Acoustic Wave Devices: Materials Stability in Harsh Environments Denny Richter1, Michal Schulz1, Sergey Sakharov2, Zachary J. Davis3, Holger Fritze1 1 Institute for Energy Research and Physical Technologies, Clausthal University of Technology, Goslar, Germany 2 FOMOS Materials Co., Moscow, Russian Federation 3 Centre for Microtechnology and Surface Analysis, The Danish Technological Institute, Taastrup, Denmark ABSTRACT The availability of high-temperature stable surface acoustic wave (SAW) devices would enable realization of wireless sensors for monitoring high-temperature processes. One of the most promising substrate materials for SAW based high-temperature sensors is langasite (LGS, La3Ga5SiO14). It can be excited piezoelectrically up to its melting point at 1470 °C. However, gallium evaporation and degradation of the electrodes limit the application of LGS in SAW sensors for harsh environments to some extent. The objectives of this work include the investigation of the gallium loss in the vicinity of the langasite surface in oxidizing, reducing and vacuum conditions at temperatures up to 900 °C. The gallium content in the vicinity of the LGS surface is not decreased after annealing the samples in air, while a significant gallium loss occurs in vacuum and reducing atmospheres (0.5 % H2/Ar). The latter results in a gallium oxide deficient region of 1.5 μm below the surface after annealing for 12 hours at 900 °C. The gallium loss is virtually completely suppressed after protecting the surface with a thin alumina film. Further, thin-film electrodes based on platinum and platinum/rhodium are tested. While conventional platinum based electrodes are completely destroyed at 900 °C within hours due to agglomeration, alumina protected electrodes can be operated at least for several days at this temperature. After 400 hours at 700 °C, the alumina protected platinum electrodes show insignificant degradation. The influence of alumina passivation layers on the stability of the SAW devices is examined. Different electrode configurations are tested with respect to their long-term frequency stability at 650 °C. INTRODUCTION High-temperature processes can be found in many manufacturing industries, in power plants and in transportation. In order to improve the product quality, to reduce energy consumption and to fulfill emission regulations, sensors for monitoring these processes are highly demanded. In numerous applications, wired sensors are hardly usable, because they need to be mounted on rotating parts or have to be placed in closed process chambers that are not easily accessible with conventional electrical feedthroughs. For these kinds of applications, wireless surface acoustic wave (SAW) sensors are a preferable option. In order to operate SAW devices at temperatures of 600 °C and above, the stability of all sensor components, including the piezoelectric substrate, thin film structures and packaging has to be ensured. One of the most promising candidates for high-temperature substrate is langasite (LGS, La3Ga5SiO1
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