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This paper presents the fabrication and characterization of air-core power inductors that leverage vertically-stacked, thick copper spiral windings to deliver high inductance densities >; 100 nH/mm2 and quality factors >; 10 on silicon substrates. The inductors are designed for on-chip or in-package integration with high-frequency power converter circuits to enable chip-scale power management in size-constrained applications. A process for realizing the inductors through multilevel, three-dimensional molding of electroplated copper structures has been enhanced with capabilities for four independent copper layers in thicknesses of up to 30 μm each. The impact of increasing the thickness of the stacked copper traces is specifically explored and compared to similar inductors with 10-μm-thick stacked windings. Impedance characterization revealed a drastic decrease in the series resistance in the thicker inductors as desired but also revealed undesired eddy current and magnetic coupling effects.
Date of Publication: Nov. 2012