Soft ferrite cores characterization for integrated micro- inductors
Résumé
Low profile soft ferrite films constitute a competitive solution for the integration of micro-inductors on silicon in low-power medium frequency DC-DC conversion applications. High resistivity of soft ferrites is indeed a major advantage for operating frequencies in the range of 5-10 MHz. We have studied several soft ferrites including commercial ferrite films and in-house made ferrites. Test inductors were fabricated at wafer level using micromachining and assembling techniques. The proposed process is based on a sintered ferrite core placed in between thick electroplated copper windings. The low profile ferrite cores of 1.2 × 2.6 × 0.1 mm3 were produced by two methods issued from green tape-casted films or ferrite powders. This article presents the magnetic characterization of the fabricated ferrite cores cut and printed in rectangular shape and sintered at different temperatures. The comparison is made in order to find out the best material for the core that can offer the micro-inductor a high inductance in the range of 200-1000 nH at 6 MHz and that generate the smallest losses. With commercial ferrite core, it is demonstrated, thanks to a test inductor, that an inductance density of 215 nH.mm-2 up to 6 MHz could be reached. Extracted losses at 6 MHz, under 10 mT are in the range of 0.7 to 2.5 W.cm-3.
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