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Microstructure Control of Low-Loss Ni–Zn Ferrite by Low-Temperature Sputtering for On-Chip Magnetic Film

Low-loss oxide magnetic film of Ni–Zn ferrite (Ni0.5Zn0.5Fe2O4) is deposited by RF magnetron sputtering at a low temperature (300 °C), applicable for integration in advanced LSIs with Cu/low-k interconnects. To control the film microstructure, or its essential magnetic and electrical properties, (1) selection of a buffer layer as a diffusion barrier under the ferrite film and (2) control of the O2/Ar gas ratio in the sputtering chamber are key factors. A thin TaN buffer layer provides the Ni–Zn ferrite film with a highly preferential (311) orientation, resulting in high saturation magnetization. In addition, the thin TaN buffer has a sufficiently good barrier property to be practical for integration of the magnetic film into Cu/low-k interconnects. Oxygen addition to Ar sputtering gas realizes both high (311) crystallization and high resistivity (ρ=10 MΩ cm), which are essential for low-loss properties. Simulation of the on-chip inductor with the magnetic film suggests that the high-ρ magnetic film is suitable for high-efficiency on-chip inductors with GHz ranges.