Packaging-Compatible Micromagnetic Devices with Screen- Printed Polymer/Ferrite Composites
In this paper, an approach to realize fully integrated microinductors and microtransformers is investigated. These integrated i ductors and transformers are composed of electroplated fine copper coils and polymer filled magnetically soft ferrites (NiZn and MnZn), in which fine ferrite particles are added to a polyimide matrix to form a composite. The electrical resistivity of the fabricated NiZn and Mn Zn ferrite composites are approximately 1 Mohm-cm and 0.01 Mohm-cm, respectively. The MnZn ferrite composite has higher saturation flux de ns ty (B s = 0.43 T) and relative permeability than the NiZn ferrite composite (B s = 0.28 T). Using these magnetic composite materials and screenprinting techniques, four different integrated inductor and transformer geometries are designed, fabricated, tested, and compar e . A two layer vertically stacked spiral microinductor with 50 μm height has a specific inductance of 15 μH/cm , and a quality factor of 15 at 10 MHz, and a dc resistance of 2.1 ohms. Sandwich type spiral microinductors have specific inductance of 6.5 μH/cm , quality factor of 17 at 10 MHz, and dc resistance of 1.3 ohms. Sandwich type spiral microtransformers have the best gain characteristics of the four geometries investigat d (1.5dB loss for a nominal 1:1 turns ratio device at 25 MHz). The magnetic shielding performance of the ferrite composite materi al is also investigated. Spiral coils shielded by screen printed ferrite composite had stray magnetic field emissions reduced by up to 75% when compared to their unshielded counterparts of identical geometry.