In-Situ Stress Measurements during Cobalt Electrodeposition

Abstract

Electrodeposited Co has attracted much attention over the past several decades owing in part to its interesting magnetic properties. These properties are largely dependent upon the structure and morphology of the electrodeposit, which can vary dramatically with overpotential, electrolyte pH, and the presence of strongly adsorbing anions. For example, solutions with a pH < 2.5 will generally result in a mixture of both face-centered cubic (fcc) and hexagonal close-packed (hcp) Co on Au, while more alkaline solutions yield nearly pure hcp Co. The evolution of residual stress during thin film growth also depends on deposit microstructure and growth conditions, making in situ cantilever curvature measurements a nice complement to traditional electrochemical measurements during electrodeposition. This talk will examine the surface and growth stress associated with the electrodeposition of Co films onto (111)-textured Au cantilever electrodes from dilute Co2+ aqueous electrolyte. We have measured the stress response as a function of overpotential in films measuring less than 25 nm in thickness, from 0.1 mol/L NaClO4 + 0.001 mol/L Co(ClO4)2 (pH = 4.8) and at Co current efficiencies ranging from 65% to 90%. XRD analysis indicates that under these deposition conditions the Co is face-centered cubic and maintains the (111) texture of the Au substrate, suggesting epitaxial but not pseudomorphic growth on the Au. The stress-thickness product is compressive in the first monolayer followed by increasing tensile stress as the film thickens. The initial compressive stress is most likely due to surface and interface stresses that dominate at submonolayer coverage. Steady state tensile stress, ranging from 0 to +450 MPa, developed in continuous Co films and showed a strong dependence on electrode potential. However, over this potential range there is no change in grain size or growth rate, factors typically associated with tensile stress. We attribute the tensile stress and its potential dependence to the formation of hydrogen stabilized vacancies, defects found to be quite stable in fcc Fe-group metals and alloys deposited under conditions of H+ discharge.