Localized Temperature Stability in Low-Temperature Cofired Ceramics

Low-temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. For LTCC modules targeting radio and microwave frequency (RF and MW) applications, a low or near 0ppm/degrees C temperature coefficient of resonant frequency ((f)) ensures temperature stability of embedded resonator and filter functions. The base dielectrics of most commercial LTCC systems have a (f) in the range -50 to -80ppm/degrees C. This study explored a method to achieve a zero (f) on stripline (SL) resonators by locally cofiring, in a multilayer LTCC structure, compensating dielectrics (CD) with an opposite (f) to that of the host dielectric. The formulation, synthesis, dielectric properties, and microstructure of SrTiO3 (STO)-based low-fire (f) CD are presented. Chemical interactions and physical compatibility between the compensating and the host LTCC dielectrics are investigated for cofireability. The dependence of (f) compensation on the wt% of STO, the printed thickness, and the location of the CD in multilayer LTCC are discussed. The most effective (f) compensation is achieved by integrating CD next to the resonator lines, and can be explained by the concentration of electromagnetic energy via total internal reflection of electromagnetic waves inside the CD layer.