Electrochemical and Microstructural Studies of the Additives’ Effects on Copper Electrodeposition
George C. Tu
In this thesis, electrochemical and microstructure effects on copper electrodeposition with two additives, thiourea and chloride ion, were studied and reported. The copper electroplating was performed at relatively high temperature and high current density, which was conducted in a conventional copper sulfate-sulfuric acid bath with various concentrations of additives. The annealing behavior of electroplated copper deposits is reported in this study. Moreover, this thesis is also to study the electrocrystallization behavior of the copper deposit on pure titanium substrate. The copper deposits were electroplated with a current density of 0.7 A/cm2 in a sulfuric acid bath containing various concentrations of thiourea. The hardness values of the copper deposits were measured before and after annealing, and the DSC diagrams of the as-electroplated copper deposits were recorded. An improvement of the softening resistance of the copper deposits was observed when the bath contained thiourea ≥3 ppm. By adding thiourea in the plating bath, smaller grain size of the copper deposits can be achieved. As thiourea content increased ≥3 ppm, the twin boundary of the copper deposits was significantly increased, and many sulfur-rich particles were deposited along the grain boundaries and a few within the grains of the deposit. These sulfur-rich particles are capable of impeding migration of the grain boundaries, and, hence, improving the softening resistance of the copper deposits during annealing. The aforementioned microstructures of the copper deposits were examined with transmission electron microscope (TEM) integrated with energy-dispersive x-ray spectrometer (EDS) for chemical composition analysis. The electroplating was conducted at 0.7 A/cm2 in cupric sulfate-sulfuric acid bath with various chloride additions. Initial growth morphology and microstructure of the deposit were examined with field-emission scanning electron microscope (FESEM) and high-resolution transmission electron microscope (HRTEM) integrated with energy dispersive X-ray spectrometer (EDS). Results of cathodic polarization and galvanostatic plating experiments show that the increasing concentration of chloride ion in the plating bath would significantly increase the cathodic potential. This cathodic potential increase was found to be induced by the presence of CuCl precipitates on the cathodic surface as well as on the copper cluster formed during electrocrystallization process. At initial plating stage both copper and hexagonal-shaped CuCl precipitate were produced simultaneously. Eventually, the pyramid-shaped CuCl precipitates were observed, which consisted of many parallel hexagonal planes stemming from a screw dislocation. The polarization effect of CuCl precipitate on the Ti-substrate was further confirmed and clarified. Through measurement of the electrode response and investigation of the deposit using FESEM, it was observed that under the identical electroplating conditions much less CuCl forms on a copper substrate in the initial stages than on a titanium substrate, leading to lower polarization.
|Appears in Collections:||Thesis|