Tether- and post-enabled flip-chip assembly for manufacturable RF-MEMS

Abstract

This work demonstrates a cost-effective technology for integrating microelectromechanical systems (MEMS) devices with other RF components on a substrate. We have designed, fabricated and tested a new variable capacitor with a two-dimensional (2-D) array of capacitor plates. The MEMS devices were fabricated in a surface-micromachined foundry process, multiple user MEMS processes (PolyMUMPs) and then flip-chip transferred to a RF circuit substrate. With tethers, the MEMS could be released before the flip-chip assembly. Such pre-assembly release is critical in integrating hundreds of MEMS with other components on the same substrate. With the consideration of necking configuration, mechanical stops and placement, we have achieved 100% yield for the flip-chip assembly and device transfer. The precise gap between the MEMS and the RF substrate could be achieved with posts even with large bump height variations in the flip-chip assembly. Such a precise gap is critical to assure repeatable digital capacitance performance and to allow the use of compliant couplings to reduce thermal mismatch effects. We have developed 43 different plate/post configurations for 15 different gaps with a 0.25 μm increment. The 2-D variable capacitor assembled showed excellent RF performance. Its Q-factor was above 350 at 0.97 GHz with a capacitance ratio of 4.7:1 for a tuning range of 171 MHz in a resonator circuit.