POLYMER-CORE CONDUCTOR APPROACHES FOR RF MEMS
Yong-Kyu Yoon, Jin-Woo Park and Mark Allen
In many radio frequency (RF) microelectromechanical systems (MEMS) applications, currents are confined to the outermost portions of conductors due to the skin effect. Conductors consisting of polymer cores coated with metal, the so-called polymer-core conductor, are appropriate to consider for these applications, and in many instances are easier to fabricate than their solid-metal-core counterparts. Implementation of polymer-core conductors using an SU-8 epoxy-core patterning and subsequent metal electrodeposition is reported. The SU-8 core approach allows for relatively simple formation of extremely high-aspect-ratio columns for inductor sidewalls. In addition, an SU-8 bridge fabrication technique has been realized using a double exposure and single develop scheme. The bridge thickness has been characterized as a function of the optical dose and the post bake time in an oven. Three-dimensional, high-aspect-ratio, high Q-factor, solenoid-type RF inductors are fabricated and tested to demonstrate the feasibility of the polymer-core conductor approach for RF applications. A single, vialess metallization over SU-8 back-bone structure provides the complete conducting paths of the inductor. A single turn inductor that is 900 μm in height and 600 μm in lateral extension shows a maximum Q-factor of 84 and an inductance of 1.17 nH at 2.6 GHz.
Keywords: Double-exposure-single-development, SU-8 bridge fabrication, epoxy-core conductor, high-aspect-ratio solenoid inductor, polymer-core conductor and radio frequency (RF) microelectromechanical systems (MEMS)