High aspect ratio air core solenoid inductors using an improved UV-LIGA process with contrast enhancement material

Abstract

In this paper, we investigate the application of a contrast enhancement material (CEM388SS®, Shin-Etsu MicroSi, Inc., AZ, USA) on thick SU-8 negative photoresist (Microchem Corp., MA, USA) and its use in the construction of a high aspect ratio on-chip solenoid inductor. Various SU8 test microstructures (with aspect ratio of 10:1 and 20:1) were first constructed by utilizing this improved UV-LIGA process to demonstrate the clear difference before and after the application of CEM. It was found that this material dramatically reduced the “T-topping” effect and improved the structural profile angle; and higher aspect ratio microstructures could be achieved without modification/upgrades of any kind on the existing lithography tool. Furthermore, we implemented this process in the fabrication work of high aspect ratio air core solenoid inductor and obtained a very high quality (Q) factor of 72.8 at 9.7 GHz from a 3-turn inductors and a high inductance of 28 nH from a 20-turn inductor. Experimental data showed that inductors with taller via structures (higher aspect ratio) had better radio frequency (RF) characteristics than those of lower via structures, which are highly desirable in many communication applications.