A method for probing the effects of conformal nanoscale coatings on fatigue crack initiation in electroplated Ni films

This paper describes an experimental technique to identify robust nanoscale coatings for improving the long-term reliability of metallic microelectromechanical systems. More specifically, the influence of nanoscale alumina coatings on the fatigue crack initiation process in 20μm thick electrodeposited Ni films was investigated in a mild (30°C, 50% RH) and harsh (80°C, 90% RH) environment. Atomic-layer-deposited alumina layers, with thicknesses of 5 and 25nm, were coated on Ni fatigue micro-resonators, and the fatigue degradation behavior in the very high cycle fatigue regime was compared to that of uncoated structures. Evidence based on post-test scanning electron microscopy and resonant frequency evolution plots shows that the coatings do not prevent the formation of fatigue extrusions and micro-cracks. However, their formation is likely delayed for the 25nm thick alumina-coated Ni films. ► Effect of alumina coatings (5 and 25nm thick) on fatigue initiation in nickel films ► Fatigue tests were performed at 30°C, 50% relative humidity (RH) and 80°C, 90% RH. ► Coatings did not prevent fatigue extrusions and micro-cracks. ► 25nm coatings likely delayed the formation of fatigue extrusions and micro-cracks. ► The technique can be used to identify reliable nanoscale coatings.