Li2WO4 and LiBO2 as the basic components and additives in ULTCC/LTCC substrates for 5G/6G applications

The intense growth of modern information and communication technologies including 5G and 6G systems strengthens the demand for substrate materials whose dielectric properties ensure very high signal transmission speed and are simultaneously feasible as low- or ultra-low-temperature cofired ceramics (LTCC/ULTCC) to attain progress in miniaturization and the reduction of fabrication costs. In this study, lithium tungstate Li₂WO₄ and lithium metaborate LiBO₂ were investigated as multifunctional materials capable of performing a dual role, both as ULTCC single-phase substrates and as sintering aids for the LTCC substrates based on the CuB₂O₄ ceramic. The materials were characterized using a heating microscope and differential thermal analysis to examine sintering behavior, THz time-domain spectroscopy to explore dielectric properties, and XRD and SEM methods to reveal the phase composition and microstructure. Both Li₂WO₄ and LiBO₂ exhibited ultra-low sintering temperatures of 590–650 °C and advantageous dielectric properties, a low relative permittivity of 4.5–5.5 and a low loss tangent of 0.008–0.01 at 1 THz. As additives, both compounds significantly reduced the sintering temperature of the CuB₂O₄ ceramic by 80–90 °C, simultaneously showing a small impact on its good dielectric properties at THz frequencies.