On-chip out-of-plane high-Q inductors

Integrating high-Q inductors on semiconductor circuits has been an elusive goal for years due primarily to the eddy current losses and skin effect resistance associated with in-plane spiral inductors. Three-dimensional out-of-plane coils reduce eddy current and skin effect losses by virtue of their geometry and magnetic field orientation. However, out-of-plane coils were not deemed producible by standard semiconductor fabrication methods. This paper reports on a novel use of conventional semiconductor processing techniques to batch-fabricate three-dimensional high-Q inductors on a wide range of insulating or active semiconductor substrates. Thin molybdenum-chromium films are sputter deposited with an engineered built-in stress gradient so that, when patterned and released from their substrate, they curl into circular springs. These springs self-assemble into three-dimensional scaffolds that form highly conductive windings after being copper plated. Quality factors up to 85 are observed at 1 GHz on standard CMOS silicon. The in-circuit microcoil performance is also compared in BiCMOS silicon L-C oscillators to that of state-of-the-art planar spirals with slotted grounds.