MEMS based fabrication of high-frequency integrated inductors on Ni-Cu-Zn ferrite substrates

A surface micro-machining process is described to realize planar inductors on ferrite (Ni_0.49Zn_0.33Cu_0.18 Fe₂O₄) for high-frequency applications (<30 MHz). The highly resistive nature (∼10⁸ Ω m) of the Ni-Cu-Zn substrate allows direct conductor patterning by electroplating of Cu windings through a photoresist mold on a sputtered seed layer and eliminates the need for a dielectric layer to isolate the windings from the bottom magnetic core. Measured inductances∼367 nH (DC resistance∼1.16 Ω and Q-value>14 at 30 MHz) and ∼244 nH (DC resistance∼0.86 Ω and Q-value∼18 at 30 MHz) at 1 MHz for elongated racetrack (10.75 nH/mm²) and racetrack inductors (12.5 nH/mm²), respectively show good agreement with simulated finite element method analysis. This device can be integrated with power management ICs PMICs for cost-effective, high-performance realization of power-supply in package (PSiP) or on-chip (PSoC). This simple process lays the foundation for fabricating closed core ferrite nano-crystalline core micro-inductors.