Mg-Cu-Zn Ferrites for Multilayer Inductors

摘要

Sinter active soft ferrite materials with adequate permeability profiles are commonly used for multilayer ferrite inductors (MLFI). A sintering temperature of T ≤ 950°C is required for LTCC compatibility. For MLFI applications in the intermediate frequency range (i.e. 1-100 MHz) Ni-Cu-Zn ferrites are so far the most prominent materials. However, there are unresolved issues that trigger the search for substitute or improved MLFI materials. Problems include the omnipresent desire for sinter active high permeability materials, better compatibility between ferrite and silver coil material and, finally, the substitution of hazardous nickel oxide raw materials. Mg-Cu-Zn ferrites may represent an alternative and their potential for multilayer applications is evaluated. The composition strongly effects the shrinkage and sintering behavior; ferrites with less than 50 mol% Fe{sub}2O{sub}3 reveal enhanced densification behavior. Magnetic properties as saturation magnetization and Curie temperature also have to be optimized by proper selection of composition. Ferrite powder morphology is a key issue for the fabrication of multi-layer devices. The effect of powder particle size on the sintering behavior of powder compacts was investigated. Sub-micron powders prepared by fine milling show enhanced sintering activity and a high density after sintering at 900°C. Nano-size ferrite powders prepared by co-precipitation or flame synthesis lead to high density; maximum shrinkage already occurs at T < 800°C. The use of Bi{sub}2O{sub}3 as sintering additive further improves the densification, but also effects the microstructure and, hence, the permeability. Maximum permeability is in the order of μ{sub}i = 600-700. The relations between powder particle size, sinter additive concentration, microstructure and permeability are discussed in the context of MgCuZn ferrite multilayer inductor fabrication.