With miniaturization of semiconductor devices, the size of electroplated features is continuously reduced in the damascene process. Most recently, copper electroplating technology application has been extended in 14 nm, 10 nm, and 7 nm nodes. This advancement has caused significant changes in bath compositions. While new organic additive packages were introduced, the inorganic component concentrations were significantly altered as well. The composition of plating baths used in sub 14 nm nodes was recently described [1, 2]. These publications specified relatively low concentrations of copper between 1 and 10 grams per liter, sulfuric acid between 2 and 15 grams per liter, and 30-150 ppm chloride ions. Such electrolyte also includes organic additives traditionally used in copper electroplating baths (Suppressor, Accelerator, and Leveler). Suppressors are polymeric in nature (e.g. polyethylene glycol, polypropylene glycol, etc.). Accelerators are typically sulfur-containing compounds such as dimercaptopropane sulfonic acid, and bis-(3-sulfopropyl) disulfide (SPS). Levelers are designed to work on protruding features, and typically nitrogen contained compounds, e.g. Janus Green B.

Before a new analytical electrochemical method is developed, the interactions between bath components at low copper concentrations should be investigated and understood. For this purpose, we utilized an electrochemical cell with three electrodes connected to a potentiostat/galvanostat. We studied responses of organic components at different electrochemical and hydrodynamic conditions as well as at different concentrations of copper. Fig. 1 shows electrochemical responses of organic additives in solutions with different copper concentrations. All tests were performed with the working platinum electrode set at a constant cathodic current of -10 mA and 10-2500 RPM. As this graph indicates, the copper concentration plays a key role in the electrodeposition process. Reduction in copper concentration causes significant changes of plating potential, which is consistent with observations made in another publication [2].

Injection of Suppressor (PEG) into the solution changes plating overpotential in a similar way to reduction of copper concentration. It is critical to note that addition of Suppressor into solutions with different copper concentration causes almost the same shift in plating potential (Fig. 2). In Fig. 2, the voltages for 1 g/l and 5 g/l copper solutions were adjusted to match the initial potential for a solution with 10 g/l of copper.

Unlike the Suppressor effect, Accelerator (SPS) and Leveler (JGB) effects are clearly dependant on the changes in the copper conentrations. Accelerator component transient response shows slower and weaker depolarizatrion at the lowest concentration of copper, while the Leveler polarization effect is stronger at lower copper concentration.

Understanding the interactions between bath components at low copper concentrations enables the design of accurate electrochemical methods of analysis for each organic component. This presentation will reveal more information related to the electrochemical behavior of organic additives and inorganic components. Results of EIS investigations will also be represented. The presentation will be concluded with a discussion of analytical results for each organic additive at different concentration levels.

References:

1. J. Zhou, J. Reid, “Low copper electroplating solutions for fill and defect control”, US Patent Application, US 13/753,33
2. J. Zhou, E. Opocensky, J. Reid, “Low Cu electrolyte for advanced damascene plating”, Semiconductor Technology International Conference (CSTIC), 2015