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Integration of screen-printed tunable BST thick films in LTCC modules for microwave applications
(2017)
Reconfigurable microwave components play an important role in modern communication systems to meet the increasing demands for functionality and flexibility of the systems. Various technologies, such as semiconductor technology, microelectromechanical systems (MEMS) or ferroelectric thin and thick films, are suitable for the realization of tunable microwave components. Ferroelectric materials are characterized by high tuning speeds, negligible power consumption and low manufacturing costs. In particular, ferroelectric thick films based on barium strontium titanate (BaxSr1-xTiO3, BST) are promising systems for use in the frequency range up to approx. 12 GHz. However, due to the high sintering temperatures above 1100°C for pure BST thick films, the fabrication of tunable components based on silver or gold electrodes is limited to planar structures, which are applied after the sintering process.
In this presentation, it will be shown the development of BST-ZnO-B2O3 composite materials for reducing the sintering temperature to 850-900°C. Furthermore, the material and component properties of these composites or rather corresponding MIM (metal-insulator-metal) varactors are presented and compared to planar-structured varactors. The LTCC integration potential of such fully screen-printed MIM varactors based on the developed low sintering BST composites are investigated by the fabrication of phase shifters embedded in LTCC modules.
Thermoelectric generators can be used as energy harvesters for sensor applications. Multilayer thermoelectric generators (ML-TEGs) are a promising alternative to conventional π-type generators due to their high filling factor, high capability of automated production and the texturing potential during the production process. Calcium cobaltite is a promising thermoelectric oxide (p-type) with highly anisotropic properties. The following study shows the development of a textured unileg ML-TEG using ceramic multilayer technology.
Tape-casting and pressure assisted sintering are applied to fabricate textured calcium cobaltite. Compared to conventional sintering, pressure assisted sintering increases the strength by the factor 10. Thermoelectric properties can be tuned either towards maximum power factor or towards maximum figure of merit depending on the pressure level.
As electrical insulation material, a screen-printable glass-ceramic with high resistivity and adapted coefficient of thermal expansion is developed. From various commercial pastes a metallization with low contact resistance is chosen. The unileg ML-TEG is co-fired in one single step. The demonstrators reach 80% of the simulated output power and the power output is highly reproducible between the different demonstrators (99%). These results provide the first proof-of-concept for fabricating co-fired multilayer generators based on textured calcium cobaltite with high power factor, high density, and high strength.