Novel copper borate ceramics with lithium-based sintering aids for LTCC terahertz applications

Abstract

Novel CuB₂O₄–Cu₃B₂O₆ substrates doped with three Li-based sintering aids, Li₂WO₄, LiBO₂ and Li₂CO₃, were prepared using the LTCC (low temperature cofired ceramic) technology. The main goals of the work were to confirm the feasibility of LTCC substrates based on the developed materials and to prove their good dielectric properties in a broad terahertz frequency range. The comprehensive characterization of the thermal, compositional, microstructural, and dielectric properties of the fabricated ceramics was carried out by hot-stage microscopy, thermal analysis, dilatometry, X-ray diffractometry, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and time domain spectroscopy. The ceramic powders were used for the preparation of slurries and tape casting of proprietary green tapes. Test LTCC multilayer structures with embedded conductors were fabricated. The developed ceramics doped with Li₂WO₄ and LiBO₂ seem to be very good candidates for LTCC substrates in THz applications. These substrates demonstrate a low sintering temperature 860–880 °C, a low dielectric permittivity 5.4–5.7 and a low loss tangent 0.008–0.01 at 1 THz, as well as a low thermal expansion coefficient of 5.7 ppm °C⁻¹, quite well fitted to silicon, and a high flexural strength.