New Ferrimagnetic Garnets for LTCC-Technology Circulators

Yttrium iron garnet-based ferrites are used in nonreciprocal devices like microwave circulators and isolators. The low dielectric and magnetic losses of those materials provide the required properties. The main drawbacks of circulators are their size and cost, due to complex mechanical assembling of the different materials. To simplify the complex manufacturing process, a possible solution would be to adapt the different materials to a common low temperature cofired ceramic (LTCC) process: the circulators would be produced with an additive multilayer process. We showed that cationic substitutions (bismuth and copper) enable a considerable decrease of the sintering temperature of garnets, from ~1,450°C to down to ~950°C. Furthermore, due to bismuth cations, a high permittivity is achieved, allowing the reduction of the circulator core size. Our most recent results show that it is possible to decrease this temperature down to 880°C, thanks to vanadium substitutions. This significant decrease of the sintering temperature leads to a compatible material for cofiring with gold and particularly with silver (melting points 1,064°C and 962°C, respectively). Different assemblies of tapes were studied: ferrite with silver or gold, ferrite with dielectric and ferrite with dielectric and metallization. Physical analyses (dilatometry, coefficient of thermal expansion, etc.) are exposed and magnetic and dielectric properties (permittivity and saturation magnetization) are discussed. Moreover, the first results of circulators in LTCC technology with gold and silver screen printing are presented (transmission, isolation, and return loss) and the compatibility of the different elements is analyzed.