Inorganic Functional Materials

High Frequency Magnetic Microinductors

The objective of the DFG funded project "Magnetic nanocomposites for rf applications in mobile communication" (MrfC, QU  146/2-1) is the development of toroidal microinductors with magnetic core material consisting of nanostructured composites of magnetic alloy and polymer material. These materials fulfill the demands of high cut-off frequencies up to the GHz range , high permeabilities, and sufficient quality factors required in modern mobile communication electronics. A toroidal design for the microinductors was chosen to combine a high inductivity and a high quality factor of the inductors with a minimization of stray fields inside the structure. The operating frequency of the magnetic core material is limited by two main loss mechanisms: the ferromagnetic resonance (FMR) and the eddy currents. The ferromagnetic resonance frequency depends on the saturation magnetization M s and the anisotropy field H k . Because the ratio of those parameters determines the permeability µ, one has to maximize M s but keep a reasonable value for H k , otherwise the permeability would be too low. Eddy currents can be minimized by using a high resistive material. M ultilayer systems of alternating layers of magnetic alloys like Fe 54 Ni 27 Co 19 and the polymer polytetrafluoroethylene (PTFE) with a thickness of several nanometers are used as core material and are deposited at the Chair for Multicomponent Materials of Prof. Faupel. The dielectric layer of polymer increases the resistivity of the core and thereby minimizes the eddy currents. The composite material shows high FMR frequencies in combination with a high quality factor Q. The fabrication of the microinductors is performed using different thin film techniques including photolithography, electroplating, and etching processes, which require a clean room facility. The magnetic core of alternating layers of polymer and magnetic alloy is separated from the gold wiring by spin-coated dielectric layers of the insulator material benzocyclobutene (BCB). Figure 1 shows a cross-section through the microinductor with the gold windings and the magnetic core. The microinductors are analyzed measuring the s 11 -parameter. The inductance L of completed inductors was calculated and the high-frequency behavior was simulated at the Microwave Group of Prof. Knöchel and shows a principal agreement between measured and simulated data. Future goals are to optimize the microinductor geometry in terms of inductivity, quality factor, and cut-off-frequency, to use optimized magnetic polymer-nanocomposites, and to realize other rf-components like transmission line transformers, DC-DC converters, and power amplifiers.