A comparison of micromachined inductors with different magnetic core materials

Two integrated inductors with different electroplated cores, permalloy (Ni(80%)-Fe(20%)) and orthonol (Ni(50%)-Fe(50%)) cores, are designed, fabricated, tested, and compared in order to reach optimum designs for integrated inductors and transformers. These integrated inductors were realized on a silicon wafer by using micromachining techniques to fabricate wrapped coils wound around a `bar´ of high permeability core material. These devices have high current capability (up to 2A steady DC current) and are suitable for application in DC/DC converters. The total inductor size is 4 mm×1.0 mm×0.13 mm having 30 turns of multilevel copper coils (40 μm thick). Each inductor has nominally identical geometry and core thickness (15 μm), implying that differences in performance would be due entirely to differences in core behavior. The permalloy core inductor has a slightly higher inductance than the orthonol core inductor. However, the DC saturation current of the orthonol core inductor is much higher than the permalloy core inductor. In many high power applications, high saturation flux density is more important than permeability, because DC saturation current is proportional to saturation flux density. The measured behavior confirms on in-situ electroplated samples and fabricated components the trends expected from bulk properties of these nickel-iron alloys