# Large Thermal Expansion LTCC System for Cofiring with Integrated Functional Ceramics Layers
###### Abstract  
We have studied the sintering behavior of CT708 LTCC tapes with large CTE of 10.6 ppm/K. This low-k dielectric LTCC material is a quartz-based glass ceramic composite system with partial crystallization of celsian upon firing. The shrinkage, densification and dielectric properties were examined using different heating rates and a sintering temperature of 900 \({}^{\circ}\)C. The maximum shrinkage rate is at 836 \({}^{\circ}\)C (for a heating rate of 2 K/min) with a sintering density of 95% and a permittivity of \(\epsilon^{\prime}=5.9\) and tan \(\delta=0.0004\) (at 1 GHz). Due to their similar shrinkage and thermal expansion properties, CT708 tapes may be cofired with functional ceramic layers. As an example, we report on cofiring of a multilayer laminate of CT708 and a Sc-substituted hexagonal ferrite for applications as integrated microwave circulator components. This demonstrates the feasibility of cofiring of functional ceramic tapes and tailored LTCC tapes and documents the potential for the realization of complex LTCC multilayer architectures. low temperature cofired ceramics; LTCC; glass ceramic composites; hexagonal ferrites; embedded passives +
Footnote †: journal: materials  
**Citation:** [PERSON], [PERSON]. [PERSON]-T. [PERSON], [PERSON] Large Thermal Expansion LTCC System for Cofiring with Integrated Functional Ceramics Layers. **Materials 2022**, _15_, 564. [[https://doi.org/10.3390/ma15020564](https://doi.org/10.3390/ma15020564)]([https://doi.org/10.3390/ma15020564](https://doi.org/10.3390/ma15020564))  
Academic Editor [PERSON]. [PERSON]. 20 to 50 vol% of ceramic powders mixed with a majority low softening point glass phase are classified as glass-ceramic composites (GCC). In non-reactive systems, no or little reaction between the ceramic fillers and the glass phase is observed, whereas in other cases the glass partially crystallizes or reacts with the ceramic powder and forms additional phases which contribute significantly to the final properties and specifications. The sintering of these LTCC materials is governed by a liquid-phase sintering mechanism including different stages, i.e., rearrangement, dissolution, precipitation, and skeletal sintering. DuPont 951 [6] is an example of a GCC tape system. The effect of processing conditions on the properties of this material was studied in detail by [PERSON] et al. [7] and the crystallization of anorthite (CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\)), induced by dissolution of parts of the alumina filler phase in the glass matrix, was reported. [PERSON] investigated the densification and crystallization behavior of the high-frequency DuPont 9k7 tape system [8] and revealed the formation of La borate (LaPO\({}_{4}\)) and calcium aluminum borate (CaAl\({}_{2}\)B\({}_{2}\)O\({}_{7}\)) as reaction products upon firing of the alumina-filled glass tape. [PERSON] et al. studied the sintering and crystallization in a calcium-alumo-borosilicate glass/alumina GCC system and demonstrated wollastonite and anorthite formation and their effect on densification and properties [9]. Finally, glass-bonded ceramics (GBC) with low glass content and majority ceramic phases represent another category of LTCC materials. In that case, the glass acts as sintering flux agent and bonds the ceramic particles. The integration of layers of functional ceramic materials (e.g., middle-k dielectrics [10], or ferrites [11]) into LTCC multilayer modules through co-firing of a multi-material green laminate has spurred considerable interest recently as a promising option to increase the packaging density in ceramic multilayer substrates. Combination of ferrite tapes with LTCC multilayer laminates and 3D printed coil patterns enables embedding of magnetic components (inductors, LC filters, transformers). Integration of hexagonal ferrites as self-biasing circulators into LTCC microwave modules has emerged as vital field of research [12]. However, cofiring of multi-material laminates is challenging.