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A novel silicon-based inductor, power inductor in silicon (PIiS) has been proposed and experimentally demonstrated. The PIiS is fabricated at wafer level using a silicon-molding micromachining technique in which 200-μm-thick copper windings are embedded into a silicon substrate and both sides of the substrate are capped with a polymer-magnetic power composite. Through-silicon vias (TSVs) and copper routings are also added so that a PIiS can be directly used as a surface-mountable packaging substrate. A 3 ×3 ×0.6 mm3 PIiS with a measured inductance of 390 nH has been fabricated. The Q factor of this PIiS is 10 at 6 MHz. An ultracompact buck converter has been made by surface mounting off-shelf power ICs and capacitors on a PIiS. The buck converter is 3 ×3 ×1.2 mm3, which has successfully delivered 500 mA at 1.8 V with an 80% efficiency at 6 MHz.