Hybrid flex foil-ferrite technology for miniaturized power and RF applications

A hybrid technology for the realization of threedimensional miniaturized power inductors, transformers and RF inductors is presented. This technology consists of combining planar Cu coils on polyimide substrates with high permeability and high resistivity ferrite cores. For miniaturized power inductors and transformers, high permeability (µr ~2000 Mn-Zn ferrite cores are used. These cores operate up to 10 MHz and are obtained by threedimensional micro-patterning of ferrite wafers using the powder-blasting technique. High resistivity (107 Ohm.cm) Ni-Zn ferrite plates cores are used for the realization of RF inductors. The plates are deposited on top of the coils to enhance the magnetic properties of the inductor at GHz frequencies. The planar Cu coils are realized using an inhouse developed high-resolution polyimide spinning and Cu electroplating process on a flexible support of Kapton. Inductive and resistive properties of the power inductors are characterized as a function of frequency; inductance values in the 1 mH range have been obtained. RF inductors show a 40 % enhancement of the inductance and a 25 % enhancement of the quality factor for frequencies up to 0.2 GHz. Our results indicate the potential of using bulk ferrites in a hybrid assembly process for power and RF applications.