Monolithic fabrication of electroplated solenoid inductors using three-dimensional photolithography of a thick photoresist

A novel and high-yield fabrication process has been devised for monolithic integration of solenoid inductors. In order to simplify the fabrication ste ps, we decompose the solenoid inductor into two parts, bottom conductor lin es and air bridges. The air bridge is formed as a single body during a sing le electroplating step. This single-step fabrication of the air bridges is possible by forming a three-dimensional (3D) photoresist mold using multipl e exposures with varying exposure depths, followed by a single development step, which realizes the 3D latent image of the unexposed volume in the pho toresist. We have successfully fabricated solenoid inductors with and witho ut a magnetic core using this process. This process is easy and simple, so that one can significantly improve the fabrication yield over that achieved by conventional methods. Also, this process has good compatibility with th e integrated circuit (IC) process owing to a low process temperature (< 120 degrees C) and the monolithic feature.