Residual stress gradients in electroplated nickel thin films

Abstract

Residual stress gradients in electroplated nickel films of 1 mu m thickness are characterized for a wide range of current densities (1-20 mA/cm(2)) and electroplating temperatures (30-60 degrees C) in a nickel sulfamate bath. Although a variety of stress measurements is available, exploration of stress gradients remain unstudied at the scale of 1 mu m. Stress gradients - unlike uniform stresses - can cause significant bending even in monolayered released structures. Moreover, examples of misinterpretation of wafer curvature data as a measure of stress gradients exist in the literature. Based on these motivations, monolayered Ni microcantilevers are employed in this work as mechanical transducers for the characterization of stress gradients within the nickel film. Experiments are supported with finite element simulations. Residual stress gradient is found to vary in the range of about 130 to 70 MP/mu m with the sign change indicating a transition from downward to upward deflection of the microcantilever. Thus, a window of electroplating parameters is established yielding zero residual stress gradients, i.e. straight cantilevers, without the use of any additive agents.