Experimental and Mechanistic Study of Copper Electrodeposition in the Absence and Presence of Chloride Ions and Polyethylene Glycol

Abstract

The trend to miniaturize electronic devices has led to the development of new fabrication processes. Copper electrodeposition has been used extensively in the fabrication of microelectronic circuits due to the excellent conducting properties of this metal. Control of the operating conditions and understanding of the mechanism of metal deposition is necessary in order to successfully produce the micron–scale features required in these new devices. The implementation of new processes and operating conditions in the fabrication of microelectronic devices has spurred a considerable amount of research into their understanding and improvement. An approach to achieve the desired electrodeposits is the incorporation of mixtures of chemical additives into the electroplating solutions. Many modeling and experimental studies have been devoted to exploring the mechanisms by which additives operate. However, details of these mechanisms are not completely understood. A part of this study focuses on the investigation of the conditions and dynamics of the adsorption and desorption of the additives chloride ions and polyethylene glycol (PEG) on copper substrates in voltammetry and multi–step voltammetry–chronoamperometry experiments. Voltammetry scans are classified into three categories according to the range of potentials where the inhibition of Cu2+ reduction in the presence of various concentrations of Cl− and PEG is observed. Each type is explained based on the results of this study and the ideas presented in the literature on how the conformation of adsorbed PEG on the substrate can change during the course of deposition. One of the techniques that is widely used to study electrochemical processes is electrochemical impedance spectroscopy (EIS). Insight into these processes gained from measured EIS data is better when it is combined with the use of a physicochemical model. However, the models typically used involve a number of simplifying assumptions, partly due to mathematical complications. One of the purposes of this study is to relax some of these assumptions such as the neglect of convection, migration and homogeneous reactions and investigate their effect by comparing the model results to experimental data. This approach is applied to Cu2+ reduction onto a rotating disk electrode in acidic additive–free solutions. Estimates of the kinetic parameters are obtained with the non-linear least squares method. A statistical analysis reveals that the model is further improved by accounting for the correlation between consecutive residuals. The experimental data are found to be poorly predicted when the parameters estimated from the full model are used in simpler models that do not include convection and/or homogeneous reactions. The model of Cu2+ reduction in additive–free solutions is extended to account for the presence of Cl− and PEG under transient conditions. The model accounts for the formation of the inhibiting film, blockage of adsorption sites on the electrode surface and displacement of the inhibiting film by depositing copper. A distinction is made between the condition when the electrode is completely covered with the inhibiting film and when it is only partially covered. Estimates of the kinetic parameters are obtained from fitting the model to electrode responses of linear potential scans obtained at various Cl− and PEG concentrations. The model is able to predict both the sudden loss of inhibition that occurs at intermediate Cl− and PEG concentrations and the more gradual increases in current at low and high additive levels. EIS spectra are also predicted and compared to measured ones.