MTL Annual Research Report 2011

Figure 1: Schematics of buried toroidal inductors in silicon. Left: 3-D rendering. Right: cross-sectional view. (For clarity, the polymer via layer is not represented.)

Passive components, namely inductors, transformers, and capacitors, are often the largest and most expensive components in power electronic circuits, and the magnetic components (inductors and transformers) are often responsible for a large portion of the power loss. As operating frequencies are increase, the physical size of the passive components can, in theory, be correspondingly reduced while maintaining or improving efficiency. However, increases in frequency also increase the severity of several losses. Realizing the potential for miniaturization and ultra-high efficiency requires new magnetic component designs, fabrication strategies, and materials.

As the switching frequencies of the power electronics rise and the size of the magnetics fall, new fabrication strategies for the magnetics become possible. In particular, with sufficiently small volume the magnetics can be embedded in the substrate of the power circuit or within a secondary substrate and flip-bonded above the power circuit. Moreover, toroidal magnetics fabricated in this manner provide the self-shielding necessary to mitigate electromagnetic interference, and they can be fabricated using standard MEMS bulk fabrication processes. The purpose of this work is to develop the analyses, design rules and fabrication processes necessary for the implementation and demonstration of such embedded magnetics that enable high-frequency operation and realize small size and low loss.

The work carried out here focuses on toroidal inductors such as that shown schematically in Figure 1. It is anticipated that all conductors in the figure will be fabricated from electroplated copper. The labeling in the figure suggests that the inductor is buried in the power electronics chip itself. Fabrication within a separate insulating substrate is under study, too.