New ferrimagnetic garnets for LTCC-technology circulators

Abstract

Abstract Yttrium Iron Garnet based ferrites are used in non-reciprocal 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. In order to simplify this complex manufacturing process, a possible solution would be to adapt the different materials to a common LTCC (Low Temperature Co-fired Ceramics) 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 about 1450°C to down to 950°C. Due to bismuth cations, a high permittivity is achieved allowing the reduction of the circulator 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 co-firing with gold and in particular with silver (melting point of 1064°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 are exposed (dilatometry, coefficient of thermal expansion…) and magnetic and dielectric properties are discussed (permittivity and saturation magnetization). 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.