Roughness development in electrodeposited ultrathin cobalt and nickel layers

For both Co and Ni, a series of electrodeposited films of varying thickness (2–10 nm) was grown under otherwise identical conditions using potentiostatic control. The substrates were pieces of Si wafer onto which a Cu basal layer had been thermally evaporated. Contact mode atomic force microscopy was used to measure both the root-mean-square peak height (nm) and the areal peak density (μm−2) of each film. Root-mean-square (rms) peak heights for Co initially increase with film thickness and then plateau at a layer thickness of 3 nm. For Ni, the rms peak heights increase almost linearly for layer thicknesses less than 11 nm, reaching a value of 6 nm. Peak density shows the opposite trend, decreasing with layer thickness before reaching an approximately constant value for both metals at a film thickness of 4 nm. The atomic force microscopy data indicate that Ni and Co have different deposition mechanisms. A Co film initially nucleates rapidly; then the nucleation phase is followed by multinuclear, multilayer growth. Ni deposits also have initial rapid nucleation, but the dominant growth mode is primarily vertical, with increasing peak heights but no change in peak density. Increased peak density is linearly correlated with decreased peak height for the thinnest films in both systems.