Cyclic Stabilization of Electrodeposited Nickel Structural Films

Tensile, room-temperature creep, and fatigue tests were conducted to determine the stability of the mechanical properties of electrodeposited films and to establish a unique strategy to reduce variation in micromachined devices made from them. Microcrystalline nickel films with columnar grains with a typical diameter of less than 1 μm and nanocrystalline films with 20-nm equiaxed grains were evaluated. While the tensile strengths of the films were higher than bulk forms of the material, the strength, strain to failure, and apparent elastic modulus were highly variable. Creep tests revealed that the films accumulated plastic strain rapidly at room temperature and that the apparent elastic modulus increased after exposure to stress. However, the response of the film was sensitive to the local deposition conditions. Fortunately, incremental-step fatigue tests demonstrated that the films cyclically hardened and that further changes in the elastic and plastic deformation responses do not occur after cyclic stabilization. As a result, cyclic stress strain curves and the transient stabilization behavior can be used to define a mechanical “burn-in” sequence for electrodeposited nickel films that will improve the stability and reproducibility of micromachined devices.