Low-profile small-size ferrite cores for powerSiP integrated inductors

Ferrite-based micro-inductors integrated on silicon are proposed as potential hybrid integration in low-power medium frequency DC-DC power converters. The inductors are fabricated at wafer level using micromachining and flip chip assembling techniques. The proposed process is based on a sintered ferrite core placed in between thick electroplated copper windings on two silicon substrates. The low profile (110 μm thick) ferrite cores are produced by thermal bonding of two green tape-casted films of 1.2 × 2.6 × 0.2 mm³, then micro-milled to the final dimensions before sintering. This paper presents the magnetic characterization of the sintered ferrite films cut in rectangular shape and sintered at different temperatures. It is concluded that these small dimension cores could be used in the frequency range applications of 5-10 MHz and would potentially allow a high inductance density in a 3D integration process.