In this paper, an inductor on interposer with a ferrite film is investigated for power supply circuits in order to improve the efficiency of the power supply circuits by introducing a high permeability material to the inductor. A DC-DC buck converter is used as a power supply circuit in this work. The dependence of the inductance on the location of ferrite film is discussed with field electromagnetic simulations. Simulation results indicate that the ferrite film should be located on both upper and lower sides of the inductor and between metals. By using the inductor with the 10 µm-thick ferrite film, the efficiency of buck converter is improved by 13% when the efficiency of buck converter with the air core inductor is 60%.