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 mu m-thick copper that is electroplated with silicon mold, and the other is built on 25 mu 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 x 7 x 1.7 mm(3) (=0.2 cm(3)) weighing 0.8 g, which generates 14.3 mu W power output (into 0.7 Omega load) from vibration amplitude of 6 mu 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 x 20 x 6 mm(3) (=5.3 cm(3)) and 12 g, respectively, it generates 1.04 mW power output (into 190 Omega load) when it is vibrated at 75 Hz with vibration amplitude of 220 mu m.