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

Abstract

Metallic structural components for micro-electro-mechanical/nano-electro-mechanical systems (MEMS/NEMS) are promising alternatives to silicon-based materials since they are electrically conductive, optically reflective and ductile. Polycrystalline mono-metallic films typically exhibit low strength and hardness, high surface roughness, and significant residual stress, making them unusable for NEMS. In this study we demonstrate how to overcome these limitations by co-sputtering Ni-Mo. Detailed investigation of the Ni-Mo system using transmission electron microscopy and high-resolution transmission electron microscopy (TEM/HRTEM), x-ray diffraction (XRD), nanoindentation, and atomic force microscopy (AFM) reveals the presence of an amorphous-nanocrystalline microstructure which exhibits enhanced hardness, metallic conductivity, and sub-nanometer root mean square (RMS) roughness. Uncurled NEMS cantilevers with MHz resonant frequencies and quality factors ranging from 200-900 are fabricated from amorphous Ni-Mo. Using a sub-regular solution model it is shown that the electrical conductivity of Ni-Mo is in excellent agreement with Bhatia's structural model of electrical resistivity in binary alloys. Using a Langevin-type stochastic rate equation the structural evolution of amorphous Ni-Mo is modeled; it is shown that the growth instability due to the competing processes of surface diffusion and self-shadowing is heavily damped out due to the high thermal energies of sputtering, resulting in extremely smooth films.

Low Temperature Reactive Sputtering of Thin Aluminum Nitride Films on Metallic Nanocomposites

“…Contact metals used as electrodes in piezoelectric devices should thus possess high enough tensile strength to withstand these stresses. To such effect, novel co-sputtered metal nanocomposites optimized for the fabrication of micromechanical devices have been reported [ 47 – 49 ]. These nanocomposites feature grain size as small as a few nanometers and residual stresses substantially lower than those of pure metals.…”

Nanocrystalline Electroplated Cu–Ni: Metallic Thin Films with Enhanced Mechanical Properties and Tunable Magnetic Behavior

“…However, fabrication of MEMS/NEMS from diamond requires rather complex lithographic procedures. For these reasons, pure metals and metallic alloys are emerging as potential candidates to replace Si compounds in certain MEMS/NEMS applications, particularly in magnetic MEMS/NEMS, which have the added value in that they can be remotely actuated 8, 9. In fact, metallic films exhibit more ductility and fracture toughness than silicon.…”

“…The main drawbacks are that their strength is usually lower than for silicon and surface roughness is often exceedingly large. This precludes conventional metallic films from being used in advanced structural components for NEMS 8. From a magnetic point of view, pure Ni, Ni 19 Fe 81 (permalloy) and CoFe‐based alloys have received attention due to their soft magnetic character 9.…”

Resonance properties and microstructure of ultracompliant metallic nanoelectromechanical systems resonators synthesized from Al–32at.%Mo amorphous-nanocrystalline metallic composites

“…For example, a similar microstructure composed of the base element less prone to significant oxidation than Al should offer improved detection sensitivity in parallel with all the advantages of an electrically conductive platform. To this end we have recently developed Au and Ni-based devices 16,17 with improved Q values over Al-Mo.…”