An inlaid electroplated copper coil for on-chip powering of microelectromechanical systems devices

Inductive powering is promising for freeing microsystems incorporating microelectromechanical systems devices from wire tethering. Planar integrated coils used to intercept electromagnetic energy typically have large internal resistance, and hence low power transfer efficiency. We report a fabrication procedure developed to electroplate metal microstructures of large dimensions inlaid into silicon substrates. With a modified Bosch process, silicon substrates can be used to form electroplating molds of high lateral aspect ratio, so metal microstructures with thicknesses comparable to that from the LIGA process can be realized. In this work, copper coils of centimeter side length and several tens of micrometers thick are fabricated, and inlaid coils can be potentially integrated with circuits. The silicon molds can also be selectively removed to realize stand-alone metal structures. Three types of integrated coils are fabricated to compare their performance as a part of inductive link for power delivery. Power transfer efficiency is greatly improved by increased copper thickness, and is not noticeably affected by coil parasitic capacitance from silicon sidewalls. Also, higher voltage amplification at resonance was obtained with an electroplated coil.