Electrodeposition of Au for self-assembling 3D microstructures

Abstract

Rotation of structures fabricated by planar processing into out-of-plane orientations can be used to greatly increase the 3-dimensionality of microstructures. Previously this has been achieved by a self-assembly process based on surface tension in meltable hinges. An important application is in fabricating vertical inductors on silicon, to reduce the substrate coupling and thus increase quality factor and self-resonance frequency. Previous processes have used copper tracks, and Pb-Sn hinges. However, the use of Cu limits potential applications because of oxidation, since the final structure is not embedded. Moreover, a substitute hinge material is also required, as a result of legislative restrictions on Pb use. In this paper, Au is used as an alternative to Cu for the fabrication of self-assembled 3D inductors. A process has been developed to overcome photoresist deterioration problems due to the alkaline nature of Au electro-deposition solutions. Furthermore, pure Sn is used instead of Pb-Sn as the hinge material. A Ni metal layer is introduced between the Au coils and the Sn hinge to prevent inter-diffusion and formation of eutectic Au-Sn compounds. Finally a gold capping technique is proposed to protect the Sn hinge from oxidation during hinge reflow. The fabrication techniques developed here are compatible with post-processing on active CMOS circuits, and can be adopted for other MEMS applications.