Parylene-Insulated Ultradense Microfabricated Coils

This paper details the microfabrication and characterization of electrodeposited coils with high packing density. The process consists of electroplating a first sequence of metal microstructures, followed by conformal insulation of these conductors by a thin vapor-deposited layer of parylene, and subsequent electrodeposited metal filling between the first-layer conductors. Using this approach, the packing density limitation due to photoresist aspect ratio is overcome. The microcoils, which are fabricated onto a dummy substrate, are released and embedded into a parylene layer to reduce parasitic substrate losses at high frequencies, as well as to facilitate the device integration. Comblike test structures were designed and fabricated in order to validate the approach and to explore the electrical properties of such microconductors. Furthermore, ultradense parylene-insulated spiral windings were fabricated and electrically characterized. A large number of turns per volume can be fabricated because of this fabrication approach, which is a requirement for highly efficient small-scale magnetic actuators. Finally, an array of substrateless parylene-coated 2-D coils were built, then folded on top of each other, and electrically connected to form 3-D coil devices. A 14.6-mm-diameter 96-turn three-layer copper winding was fabricated and characterized. The packing density of the 3-D fabricated coil was 81%.