Testing of aluminium and its alloys as structural materials for a MEMS switch

Abstract

An important step in the MEMS switch fabrication is the formation of a suspended micron-sized beam. Typically, the beam is made of gold due to its high electrical conductivity and chemical inertness. However, the fabrication process is complicated by poor suitability of Au for chemical etching and deformation of the beam under residual mechanical stress. An additional disadvantage is a high price of gold. Aluminium and its alloys are considered as a promising alternative. In this work, the magnetron-sputtered Al 99.99%, Al–1% Si and Al–1.5% Ti are tested as structural materials for a MEMS switch. We fabricate 1 μm thick beams and investigate surface morphology, electrical resistivity and mechanical properties. Single-layer films of Al 99.99% and Al–1% Si have coarse-grained microstructure with the root-mean-square roughness higher than 50 nm. The multi-step deposition reduces this value to 15 nm without significant deterioration of the resistivity and Young’s modulus. However, multilayer films of Al 99.99% have interlayer voids, which may degrade the switch reliability. Al–1.5% Ti provides much smoother and fine-grained film with plain sidewalls, which results in a higher quality factor of the beams. But this alloy has three times higher electrical resistivity than pure Al. Therefore, fabrication of the beam and transmission line from Al–1.5% Ti will increase insertion loss of the switch. A four-layer film of Al–1% Si is more preferable, since it also has fine microstructure and does not contain interlayer voids, but is close to pure aluminium in terms of resistivity.