Characteristics of a CaO–B2O3–SiO2–Al2O3–ZnO-based sandwich substrate and its dielectric properties in the 5G millimeter-wave band

In this study, a 34.5CaO–23B ₂O ₃–33.5SiO ₂–6Al ₂O ₃–3ZnO (CBSAZ) glass-ceramic substrate was prepared using a sol–gel method. A porous CaO–B ₂O ₃–SiO ₂–Al ₂O ₃–ZnO layer (CBSAZ0.5C) with a porosity of 72.6 % and thickness of 365 μm was inserted between two ∼120 μm thick dense CBSAZ layers to form a tri-layer substrate (CBSAZ/CBSAZ0.5C/CBSAZ). The dielectric constants ( ε_r) and dielectric losses (tan δ) of the dense CBSAZ substrate at 20 and 60 GHz were 6.90 and 0.029, respectively, and 6.96 and 0.017, respectively. These values were almost independent of frequency. In contrast, the ε_r and tan δ values of the CBSAZ/CBSAZ0.5C/CBSAZ sandwich substrate at 20 and 60 GHz were 3.59 and 0.022, respectively, and 3.11 and 0.020, respectively. Thus, the ε_r value of the CBSAZ/CBSAZ0.5C/CBSAZ sandwich substrate was significantly lower than that of the CBSAZ substrate, whereas the tan δ and κ values of the CBSAZ/CBSAZ0.5C/CBSAZ and CBSAZ substrates were similar.