High frequency soft magnetic properties of nanocrystalline Fe-(Co)-Hf-O films with high electrical resistivity and their application to micro DC-DC converter

We have developed nanocrystalline Fe-(Co)-Hf-O films which exhibit high electrical resistivity (ρ) more than 4 μΩm as well as high magnetization (Is) more than 1T and also good soft magnetic properties even in as-deposited states. Furthermore, real part of complex permeability μ′ of these films exhibit more than 1000 up to 100 MHz by annealing in an uniaxial magnetic field. The layered inductor using nanocrystalline Fe₆₁-Hf₁₃-O₂₆ film and Co₄₄-Fe₁₉-Hf₁₅-O₂₂ film show the Q of 8.4 and 12.3 at 5 MHz. The micro DC-DC converter using the nanocrystalline Fe₆₁-Hf₁₃-O₂₆ film and Co₄₄-Fe₁₉-Hf₁₅-O₂₂ film inductor show the maximum efficiency of 83.5% and 86% in the boost converter driving at 5 MHz. So, it was evident that high resistive nanocrystalline Fe-(Co)-Hf-O films were effective for the highfrequency micro DC-DC converter.