Characterization of a Wafer-Level Packaged Au−Ru/AlCu Contact for Micro-Switches

Packaging radio-frequency microelectromechanical systems (RF-MEMS) is mandatory for their integration in electronic systems. This paper discusses the integration of a Au−Ru/AlCu contact in a MEMS fabrication process with a wafer-level packaging (WLP) step through Al−Ge eutectic bonding. The Au contact is embedded in a movable fixed−fixed beam made of <tex-math notation="LaTeX">30 ~ μ m </tex-math> thick Si while Ru is used as a coating metal on top of the AlCu static electrode. The measured return loss and isolation of the contact, up to 40 GHz, are <tex-math notation="LaTeX">≥ -1.7 </tex-math> and <tex-math notation="LaTeX">≤ -12 </tex-math> dB respectively. The WLP Au−Ru/AlCu contact has an electrical resistance <tex-math notation="LaTeX">≤ 1.2 ~ Ω </tex-math> up to 3 million cycles in hot switching at 0.1 W before failing due to high electrical resistance. Au hillocking is identified as the only intrinsic limiting mechanism that can be mitigated by an appropriate design rule. It is demonstrated that Au−Ru/AlCu is a promising contact configuration, compatible with Al−Ge eutectic bonding, and can be integrated in a standard MEMS fabrication process flow for the development of a robust generation of WLP RF-MEMS.