DC-DC converters provide various DC voltage supplies required by electronic devices. Among all converters, the switching-mode DC-DC converter is most popular due to its high power density with high efficiency. It typically consists of switching MOSFETs, control circuits, power capacitors and power inductors. Modern portable electronics devices are becoming more and more compact while having more features integrated which demand more power. In order for semiconductor industry to meet the market need, it is imperative to reduce the size and increase the power density of DC-DC converters. Switching MOSFETs and control circuits are integrated, but power passives, especially inductors, are still bulky and off-chip components, which become the obstacle to further reduce the size of DC-DC converters.
Power Systems in Packaging (PSiP) and Power Systems on Chip (PSoC) are the two most commonly-used approaches to integrate power inductors with power ICs. PSiP uses off-shelf inductors, so they can handle large power, but their size reduction is limited. PSoC integrates power inductors directly on IC chips. This approach can minimize the size by taking advantages of the microfabrication, but it still needs to overcome the small inductance and/or low quality ( Q) of thin-film inductors to be practically useful.
In this dissertation, a new type of microfabricated power inductor, called Power Inductor in Silicon (PIiS), is proposed and experimentally demonstrated. Instead of fabricating the power inductor on the top of silicon substrate, the PIiS is fabricated into the silicon substrate to fully use the space of the inside and both sides of the substrate. Thick copper windings are embedded into the silicon substrate and have the same thickness as the substrate ranging from 200∼500 µm, which results in very low DC resistance. Two magnetic plates are fabricated on both sides of the substrate and are connected by magnetic vias, which form the magnetic core to boost the inductance. Therefore, PIiS has large inductance and high Q. PIiS can be batch fabricated at wafer level and can be integrated with power ICs by die-to-die, die-to-wafer or wafer-to-wafer bonding. Additionally, through-silicon vias (TSV) are fabricated simultaneously with the copper windings in PIiS, so the PIiS is surface mount ready and can be used as a packaging substrate, eliminating the need of bonding wires or lead frames.
In this dissertation, a silicon molding technique for embedding thick copper winding into the silicon substrate is developed. A PIiS with a permalloy core and a PIiS with a magnetic powder core are designed, fabricated and characterized. A compact DC-DC converter based on a PIiS is also demonstrated and tested to verify the feasibility of the technique.
Such a compact DC-DC converter has great potential application in the portable electrics that demand small and high performance DC-DC converter.