This paper proposes a 3D stacked buck converter with a 15μm thick spiral inductor on a silicon Interposer, which is suitable for fine-grain power-supply voltage control in SiP's. Our newly developed silicon interposer technology realizes a fine-pitch design rule (Line/Space=20μm/20μm, via hole diameter =30um) at the metal thickness of 15μm as well as conventional interposers with 5μm metal thickness. The measurement result shows that the 15μm thick inductor improves the power efficiency of the buck converter by 12% at the output current of 100mA compared with that by the conventional 5-μm thick inductor.