Integration of microfabricated low resistance and thousand-turn coils for vibration energy harvesting

This paper presents two microfabrication approaches for multi-layer coils for vibration-energy harvesters. A magnet array is arranged with alternating north- and south-orientation to provide a rapidly changing magnetic field for high electromagnetic energy conversion. Multi-turn spiral coils on silicon wafer are aligned to the magnet array for maximum magnetic flux change. One type of coil is made out of 300 μ m-thick copper that is electroplated with silicon mold, and the other is built on 25 μ m-thick copper electroplated with photoresist mold. The low resistive coils fabricated by the first approach are integrated in a microfabricated energy harvester of 17 × 7 × 1.7 mm 3 (=0.2 cm 3 ) weighing 0.8 g, which generates 14.3 μ W power output (into 0.7 Ω load) from vibration amplitude of 6 μ m at 250 Hz. The latter approach is used to make a 1080-turn coil for a microfabricated electromagnetic energy harvester with magnet array and plastic spring. Though the size and weight of the harvester are only 44 × 20 × 6 mm 3 (=5.3 cm 3 ) and 12 g, respectively, it generates 1.04 mW power output (into 190 Ω load) when it is vibrated at 75 Hz with vibration amplitude of 220 μ m.