LTCC glass-ceramic composites for microwave application
Introduction
Low-temperature-cofired ceramics (LTCC) for microwave application represent a key position in the development of future electronic products in a high frequency application for IC packaging radar, antennas and wireless technologies. The integration of passive components in LTCC is, therefore, particularly interesting in multi-layers technology. Integration of passive devices in wireless application corresponds to the trend of mobilization and miniaturization with high electrical performance using conductive electrode materials such as gold, silver and copper. The melting temperature of silver electrodes in a multi-layer device limits the sintering temperature to 900°C. Material systems of BaO–Re2O3–TiO2 (Re=La, Sm, Nd, Eu) in 1:1:4 composite are especially suited for the development of dielectric materials1, 2 which are characterized by excellent dielectric constant ε, high quality factor Q as inverse of dielectric loss Q=(tan δ)−1 and low temperature coefficient of frequency TCf characteristics.
Exceptional dielectric characteristics of 1:1:4 composite are achieved at sintering temperature >1300°C and, therefore, contrary compared to the demanded temperatures of 900°C in the case of LTCC technology with Ag-electrodes. Development trends of LTCC materials are glass–ceramics systems, including low softening glasses and high sintering microwave ceramic. Typical glass systems are borosilicate, lead borosilicate or earth alkali CaO–B2O3 SiO2 glass which crystallize at optimized processing parameters.3 The requirement catalogue for glass–ceramic composites is: densification temperature <900°C, combined with dielectric characteristics ε 20…70, Q 1000 and TCf approximately 0 ppm/°C of composite. Promising candidates of middle ε 20…30 composite are glass–ceramic by restricting on lead-free glasses especially rare earth derived microwave glasses La–B–Ti–O (LBT).4 An alternative route for high ε materials 60–80 are low viscosity reactive glasses in a B–Bi–Zn–O (BBSZ) system. BaNd2Ti4O12 microwave ceramic filler was added with earth alkali ZnO less 1 wt.% to optimized sintering and dielectric properties in adjustment of glasses.
Section snippets
Experimental
Microwave ceramic compositions in the BaO–Nd2O3–TiO2 system were prepared using a conventional mixed-oxide method. The BaNd2Ti4O12 ceramic was derived by BaCO3, TiO2, Nd2O3 high purity powders with and without functional additives less 1 wt.%. Ball milled powders were calcinated at 1100–1270°C depending on additives and milled again for 24 h. Ground powder (average particle size D50=3 μm) were granulated by mixing with 10% polyvinylalcohol solution and pressed into a disk of 12 mm diameter and
Microstructure and dielectrics
For a glass-ceramic system with rare earth derived glass LBT the microwave ceramic filler content is limited <45 vol.% by the crystallization of glasses at temperatures <900°C. Ceramic filling material contents >80 vol.% are realized by sinter active glass BBSZ with low viscosity at a temperature of 400°C and a high solubility of rare-earth derived ceramic. Glass softening point Ts (log η=7.6 d.Pas) as significant processing parameter is 725°C for LBT and 430°C for BBSZ, respectively.
Dielectric
Conclusion
Glass–ceramic composites of defined dielectric material with TCf=0 ppm/°C, quality factor Q>1000, permittivity ε of 25–70 in the frequency range of 0.5–3 GHz for high frequency application are demonstrated. Depending on the glass–ceramic concentration evaluated by modified tridimensional modeling and significant processing parameters e.g. powder preparation techniques dielectric properties and sintering modification at temperature <900°C were controlled. Processing parameters of passive
Acknowledgements
Financial support for this cooperative project of Heraeus, Siemens, BAM and TU-Dresden by the BMBF program MaTec is gratefully acknowledged.
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