Along with the decrease in the critical dimension of semiconductor chips, there are strong demands for higher electron mobility metal interconnects to permit faster signal transmission and reduce resistor-capacitor (RC) delay of the integrated circuit (IC). In addition, the metal deposition process must be able to create void-free deposits in the chip interconnects, which are as narrow as 65 nm. Replacing the traditional aluminum interconnect with copper interconnect can reduce the wiring resistance by up to 45%. The void-free electrochemical copper deposition (ECD) deposition is often referred to as superfilling or superconformal deposition and is enabled by several additives at optimal concentration ranges and proper processing conditions. In a typical copper electroplating bath, chloride, poly(ethylene) glycol, and mercapto acids, such as MPS and SPS, are the necessary additives to achieve superconformal metal interconnects in sub-micron vias or trenches.

At negative plating potentials, Cu2+ ions in the vicinity of the cathode surface adsorb onto the surface, which is followed by two single electron transfer steps. Chloride ions are the most common additive in copper plating chemistries. Affinity for adsorption of chloride onto copper surfaces has been studied with a variety of techniques. Chloride ions, at trace (parts-per-million, ppm) level in the electrolyte, are able to catalyze the Cu2+/Cu+ reaction by changing the reaction mechanism from an outer-sphere reaction (water-water bridge) to an inner-sphere reaction (chloride bridge), and therefore the copper deposition potential is depolarized. Two related chemicals, 3-mercapto-1-propanesulfonic acid (MPS) and disodium bis(3-sulfopropyl)disulfide (SPS), are well-known accelerators or brighteners in the electrochemical deposition bath chemistry. The two brightening agents are unstable at open circuit potential and during electrolysis. Formation of cuprous thiolate intermediates is the key step which leads to the brightening of electrodeposited copper.

The individual roles of and the interactions between two additives, chloride ions (Cl-) and 3-mercapto-1-propanesulfonic acid (MPS), in copper electrochemical deposition (ECD) from an acidic copper sulfate solution are studied here with electrochemical methodologies such as linear sweeping voltammetry (LSV), chronoamperometry, and alternating-current electrochemical impedance spectroscopy (EIS). The LSV results reveal that the current density is not increased by MPS without the presence of chloride in the electrolyte. The EIS results reveal a low frequency capacitive loop associated with chloride interaction with cupric ions (Cu2+) and MPS and a high frequency capacitive loop related to double layer charging. A detailed reaction mechanism is proposed based on the experimental observations and an EIS model based on the reaction mechanism is developed. The EIS results are used to estimate parameters in a simple equivalent circuit model and the EIS model. The validity of the mechanism is established by comparing predictions of the EIS model with experimental EIS data. The EIS model predictions reflect, in a qualitative manner, the features observed in the EIS experimental data.