Low-temperature sintering and microwave dielectric properties of B₂O₃-added ZnO-deficient Zn₂GeO₄ ceramics for advanced substrate application

ZnO-deficient Zn_{2- x}GeO_{4- x} ceramics with 0.05 ≤ x ≤ 0.15 were synthesized because a ZnO secondary phase is formed in the stoichiometric Zn₂GeO₄ ceramics synthesized using micrometer-sized ZnO and GeO₂ powders. The Zn_1.9GeO_3.9 ceramic sintered at 1000 °C showed a homogeneous Zn₂GeO₄ phase with good microwave dielectric properties: ε_r of 6.8, Q × f of 49,000 GHz, and τ_f of −16.7 ppm/°C. However, its sintering temperature was still too high for it to be used as an advanced substrate for low-temperature co-fired ceramic devices. Therefore, various amounts of B₂O₃ were added to the Zn_1.9GeO_3.9 ceramics to reduce their sintering temperature. Owing to the formation of a B₂O₃-GeO₂ liquid phase, these ceramics were well sintered at low temperatures between 925 °C and 950 °C. In particular, 15 mol% B₂O₃-added Zn_1.9GeO_3.9 ceramic sintered at 950 °C showed promising microwave dielectric properties for advanced substrates without the reaction with an Ag electrode: ε_r = 6.9, Q × f = 79,000 GHz, and τ_f = −15 ppm/°C.