Abstract
Ferrite-based micro-inductors are proposed for hybrid integration on silicon for low-power medium frequency DC-DC converters. Due to their small coercive field and their high resistivity, soft ferrites are good candidates for a magnetic core working at moderate frequencies in the range of 5–10 MHz. We have studied several soft ferrites including commercial ferrite film and U70 and U200 homemade ferrites. The inductors are 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 × 0.2 mm3 are produced by two methods from green tape-casted films and ferrite powder. This paper presents the magnetic characterization of the sintered ferrite films 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 reach the required inductance (470 nH at 6 MHz) under 0.6A current DC bias and that generate the smallest losses. An inductance density of 285 nH/ mm2 up to 6 MHz was obtained for ESL 40011 cores that is much higher than the previously reported devices. The small size of our devices is also a prominent point.
Highlights
The trend towards miniaturization of mobile electronic products with the demand of improving functionality and performance raises challenges for power management systems for which higher power density with higher energy conversion efficiency is required
The induction saturation is in the range of 0.25-0.30 T, with a coercive field of 0.8-5.8 Oe. 40010 exhibits the highest coercive field which is due to the highest amount of magnetic phase (Ni and Fe) in 40010 ferrite compared to other ferrites ; a similar effect was observed when doing some copper substitution into Ni-Zn ferrites [35]
When superimposing a DC bias current, all four materials chosen for core fabrication exhibit a large decrease of permeability, which is a potential disadvantage of these soft ferrite cores for high DC current application
Summary
The trend towards miniaturization of mobile electronic products with the demand of improving functionality and performance raises challenges for power management systems for which higher power density with higher energy conversion efficiency is required. Four materials are compared: cores either cut from commercial films or screen printed from an in-house made paste We believe that this approach presents a low-cost solution due to minimum number of fabrication steps and high throughput deposition techniques: electroplating for copper windings and screen printing or milling for cores. The specifications, defined according to typical commercial inductors, are the following: an inductance in the range of 200-1000 nH, an equivalent series resistance in the range of 0.1 0.2 at bias current of 0.6 A and operating at 6 MHz. In the objective of evaluating performance prior to inductor fabrication, several NiZn ferrites were selected among which: - Commercial ferrite films by ESL ElectroScience [31,32]: ESL 40010® and ESL40011® with expected permeabilities of 60 and 200 respectively.
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