Stress and microstructure in electrodeposited copper nanofilms by substrate curvature and in-situ electrochemical AFM measurements

Both AFM imaging and stress measurements were carried out insitu during potentiostatic electrodeposition of copper on gold in 0.05 mol dm^-3 CuSO₄ in 0.1 mol dm^-3 H₂SO₄. In the absence of additives, compressive stress generally developed initially and films subsequently underwent a compressive-to-tensile (C-T) transition. The nucleation density measured by AFM increased from 2.7×10⁷ cm^-2 at -75 mV to 2.5×10⁹ cm^-2 at -300 mV. Very little coalescence of nuclei was observed at -75 mV but the rate of coalescence increased rapidly with increasing negative potential. The time for the C-T transition correspondingly decreased rapidly until, at -75 mV, none was observed. This is consistent with models that attribute the C-T transition to increasing tensile stress due to coalescence of nuclei. With a combination of Cl^-, PEG and MPSA, compressive stress generally developed initially and was greater than in additive-free electrolyte. At less negative potentials, the stress continued to evolve in the compressive direction, even though the rate of coalescence of nuclei was rapid. At intermediate potentials (-90 mV to -150 mV), classical C-T-C behavior was observed; at more negative potentials the stress continued to evolve in the tensile direction. This enhancement of a compressive component of stress is attributed to incorporation of additive-derived impurities.