Direct casting indialite/cordierite glass ceramic substrates for millimeter-wave dielectrics

Conventional and well-studied techniques such as gel casting and ceramic route to fabricate dielectric substrates have ruled in the last few decades. All of these have the bottom-up approach, which has its limitations when the... [ view full abstract ]

Conventional and well-studied techniques such as gel casting and ceramic route to fabricate dielectric substrates have ruled in the last few decades. All of these have the bottom-up approach, which has its limitations when the highest densification and the best of dielectric properties are required. In this paper, an alternative top-down approach is introduced, which is direct casting of glass with post-crystallization. Substrate fabrication process is demonstrated by indialite/cordierite glass ceramics with addition of TiO₂ preventing clacks due to anisotropic surface crystal growth [1-3].

The indialite/cordierite with 10 wt. % addition of TiO₂ was prepared by mixing high purity raw materials and melting at 1550 ^oC. The glass was cast on a graphite plate. The cast and shaped glass was annealed to relieve the thermal stress at 760 ^oC/4 h, being below the glass transition temperature of 780 ^oC. The annealed glass substrate is crystallized at 1200-1400 ^oC/10-20 h, and polished 1mm thickness. The developed substrate showed a high densification (> 99 %) with good mechanical/thermal characteristics. The dielectric properties of the substrate were measured in the microwave frequency range, which are permittivity of 5.6 and 5.6 and dielectric loss of 0.00025 and 0.00022 at 2.4 and 5.1 GHz, respectively. Moreover, the developed substrate shows the temperature variation of relative permittivity of -58 ppm/^oC. Glass casting and crystallization method for making substrate is a cost-effective way to produce high-quality micro/millimeter wave substrates for the next-generation telecommunication devices.

Acknowledgements: Apart of this study was supported following grants: JSPS KAKENHI Grant No. JP16K06735, Nokia Visiting Professor Project 201700003, and European Research Council Project No. 24001893.

References: [1] H. Ohsato et al., Jpn. J. Appl. Phys., 50, 09NF01-5 (2011).[2] H. Ohsato et al., J. Ceram. Soc. Jpn, 121, 649-654 (2013). [3] H. Ohsato et al., Ceramics International, 41, 588-595 (2015).