2014 Theses Doctoral

Investigation on Copper Electrodeposition: Impact of Inorganics and Lab-Scale Tool Design

Qiao, Feng

In the recent years, copper has been replacing aluminum to be widely used as the interconnect material in the production of integrated circuit (IC) chips and other components used in microelectronic semiconductor devices. The copper interconnects are usually fabricated using a damascene electrochemical deposition process from an acidic electrolyte (termed plating bath) containing cupric sulfate (CuSO4) as well as several organic and inorganic constituents. A copper electrodeposition process suitable for routine integrated circuit (IC) manufacturing must deliver copper films that can reproducibly fill deep and narrow features (vias and trenches) without any voids or seams. This can be realized by adding to the plating bath small quantities of selected inorganic and organic additives which lead to the copper electrodeposition preferentially occurring at the bottom of the feature, known as "bottom-up fill" or "superfill". With the continuing trend towards the miniaturization of microelectronic devices, additives are becoming more and more critical to the successful application of copper electrodeposition in producing interconnects. Therefore, the current research focuses on the copper
electrodeposition additives to provide better insight in improving the copper electrodeposition technique.
The integration of an iron redox couple (Fe(II)/Fe(III)) to traditional copper plating baths has been shown by previous studies as well as industrial practice to have several benefits in terms of its impact on anodic reactions and the behavior of several additives. However, the possible impact of the iron redox couple on direct copper electrodeposition onto foreign substrate has not received much attention before. Since direct copper electrodeposition onto non-copper substrates rather than onto a pre-deposited copper seed layer is emerging as an alternative for future copper interconnect fabrication, the role of the iron redox couple in the initial process of direct copper electrodeposition, especially copper nucleation was studied under various experimental conditions. It was found that the presence of iron redox couple lead to as much as a 5-fold increase in copper nucleus density, Additional experiments were conducted on emerging novel substrates, and similar results were achieved in most cases.
In addition to the iron redox couple, we have also investigated how other inorganics may impact copper nucleus density during direct electrodeposition. The inorganic constituents we considered included potassium sulfate (K2SO4), magnesium sulfate (MgSO4), and sodium sulfate (Na2SO4). Such inorganics are usually added to plating baths primarily to increase electrolyte conductivity. However, our galvanostatic electrodeposition results showed that K2SO4 and Na2SO4 also increased copper nucleus density by noticeable amounts. And the impact on the Cu nucleus density from adding K2SO4, MgSO4, and Na2SO4 could be predicted by the overpotential change during the electrodeposition.
During the investigation on novel inorganic additives to improve direct Cu electrodeposition technique, we also tested several novel barrier materials as substrates for direct Cu electrodeposition. The barrier material is an important factor to ensure high Cu nucleus density and hence the formation of continuous Cu thin films.
In lab-scale research, it is often desirable to deposit a thin uniform copper film during the process of studying copper plating additives. However, non-uniformity in film thickness even across a small length scale often arises. We thus designed and optimized a simple shielded rotating disk plating setup with the aim of electroplating more uniform copper films. The uniformity of the electrodeposited copper film is directly related to the current distribution on the substrate during the electrodeposition process. In our study, a simulation model was utilized to find the optimum values of the design. Then several insulating shields were fabricated, and experiments were carried out to compare with simulation. Results showed that the introduction of an insulating shield was able to improve the current distribution on the substrate during copper electrodeposition, although some hydrodynamic issues that were not included in the theoretical simulation might occur in experiments.
The concentration of certain organic additives in copper plating baths might change with time during copper electrodeposition, a phenomenon called bath aging. Bath aging may severely affect the stability of plating performance. A simple and efficient cell that allows for analyzing aging behavior of certain plating bath compositions is necessary to provide insight into aging performance of organic additives. Also, the anolyte and catholyte should be separated during the analysis since anodic and cathodic reactions might have different impact on the aging of plating additives. We thus designed and
fabricated three novel aging-analysis cells, which might fulfill the above requirements. The aging behavior of Bis(3-sulfopropyl)-disulfide (SPS), a popular plating additive that is known to age during copper electrodeposition, was studied with the three cells. The advantages and disadvantages of each cell were discussed with the preliminary aging results. The cell that led to the best aging results was selected to be used for future analysis.