Suppressing ionic percolation in ultralow-loss SiO₂/glass composite low-temperature co-fired ceramics through quartz/cristobalite phase engineering for mmwave radio frequency filter applications

Low-temperature co-fired ceramics (LTCCs) are widely utilized in millimeter-wave (mmWave) communication modules, owing to their excellent dielectric properties and process compatibility. With the rapid advancement of mmWave communication, low-loss LTCCs have become increasingly important for minimizing insertion loss and signal attenuation in mmWave frequency range. Among the various candidates in LTCC compositions, SiO₂-based glass/ceramic filler composites have attracted considerable attention, owing to their low dielectric constant and glass-assisted densification behavior under low-temperature sintering process. However, despite the strong influence of glass-phase transformations on dielectric loss characteristics, systematic studies on the underlying mechanisms remain limited. Herein, we systematically investigate phase engineering in SiO₂-based glass/ceramic filler composites to achieve ultralow dielectric loss while maintaining comparable sintered density, thereby clarifying the effects of glass-phase transitions on dielectric-loss behavior. With increasing sintering temperature, the crystalline phases evolved from quartz to mixed quartz–cristobalite and finally to cristobalite structures. In particular, the quartz–cristobalite mixed-phase sample sintered at 900 °C exhibited superior dielectric properties and the most stable dielectric-loss characteristics, showing a dielectric constant of 4.5, an ultralow dielectric loss of 0.0006, and a high Q × f value of 20,400 GHz, outperforming previously reported glass-based LTCC materials. These enhanced dielectric properties were attributed to suppression of ionic percolation during the crystallization of mixed phases consisting of quartz and cristobalite structures. Finally, a multilayer-integrated mmWave (~ 28 GHz) RF filter chip was successfully fabricated using the developed LTCC material. The fabricated device exhibited stable band-pass filtering characteristics near 28 GHz, achieving a low insertion loss of 0.9 dB and an exceptional figure of merit (FoM) of 325.7, outperforming previously reported LTCC-based RF filter materials. These findings provide an insight into the design of low-permittivity LTCC systems through glass phase-engineered dielectric control.