Wire bonded MEMS-scale on-chip transformers

We present a novel, wafer-level fabrication method of 3D solenoidal microtransformers using an automatic wire bonder. Automatic wire bonders allow to precisely shape 25 μm diameter wire around prefabricated yokes or magnetic cores within seconds. Former reports of wire bonded microcoils treated individual solenoids showing low mutual inductances, whereas in this study transformers with strongly coupled microsolenoids are presented. The process is fully compatible with standard microelectronic manufacturing, therefore, enables the direct integration of transformers into a given electronic circuit. Two different prototypes are presented here. A non-magnetic core transformer with a footprint of 1 mm² and 20 windings in both the primary and the secondary coil, yields a power efficiency of 67% and a coupling factor of 94%. A magnetic core transformer with a footprint of 0.64 mm² and 18 windings in both the primary and the secondary coil, yields more than 1 μH inductance and 74% power efficiency with a coupling factor of 98%. The feasibility of both prototypes with respect to the power conversion in miniaturized circuits is evaluated. Finally, the microtransformers are benchmarked to underline the potential of the wire bonded microtransformers compared to other state-of-the-art publications.