Tailoring the microstructure and surface morphology of metal thin films fornano-electro-mechanical systems applications

Abstract

Metallic structural components for micro-electro-mechanical/nano-electro-mechanicalsystems (MEMS/NEMS) are promising alternatives to silicon-based materials sincethey are electrically conductive, optically reflective and ductile. Polycrystallinemono-metallic films typically exhibit low strength and hardness, high surfaceroughness, and significant residual stress, making them unusable for NEMS. In thisstudy we demonstrate how to overcome these limitations by co-sputtering Ni–Mo.Detailed investigation of the Ni–Mo system using transmission electron microscopyand high-resolution transmission electron microscopy (TEM/HRTEM), x-raydiffraction (XRD), nanoindentation, and atomic force microscopy (AFM) reveals thepresence of an amorphous–nanocrystalline microstructure which exhibits enhancedhardness, metallic conductivity, and sub-nanometer root mean square (RMS)roughness. Uncurled NEMS cantilevers with MHz resonant frequencies and qualityfactors ranging from 200–900 are fabricated from amorphous Ni–Mo. Using asub-regular solution model it is shown that the electrical conductivity of Ni–Mo is inexcellent agreement with Bhatia’s structural model of electrical resistivity in binaryalloys. Using a Langevin-type stochastic rate equation the structural evolution ofamorphous Ni–Mo is modeled; it is shown that the growth instability due to thecompeting processes of surface diffusion and self-shadowing is heavily damped outdue to the high thermal energies of sputtering, resulting in extremely smoothfilms.