Study of Cu Electrochemical Polishing for Copper Interconnect Process

Surface planarization of Cu interconnects, including overburden removal in processes such as redistribution layer (RDL) and through-silicon via (TSV), is essential for achieving reliable multilayer stacking and high-resolution patterning in advanced semiconductor packaging. Generally, the removal of Cu overburden has been carried out using chemical mechanical planarization (CMP). However, the conventional CMP process for bulk copper overburden removal has problems including high manufacturing cost and mechanical defects resulting from abrasive slurry components. To address these limitations, electrochemical polishing (ECP) has emerged as a promising alternative, offering a particle free and cost effective approach to surface planarization. Cu ECP is an electrochemical process for achieving smooth Cu surfaces that are anodically polarized in ECP electrolytes. Copper electropolishing enables non-contact copper removal, preventing defects caused by mechanical polishing. Due to its ability to reduce surface roughness with no surface defects caused by abrasives, ECP is used in various metal finishing applications like medical devices, food and beverage, the aerospace industry. ECP effectively removes excess Cu overburden with the advantages of lower cost, lower mechanical damage compared to CMP. Copper electropolishing (ECP) is a mass transfer-limited process, typically performed in high concentrations of phosphoric acid electrolytes. To serve as a potential replacement for bulk copper CMP, ECP must achieve both high polishing rate and surface uniformity. Increasing the water content in the electrolyte is known to enhance the polishing rate; however, in conventional phosphoric acid (PA)-water mixtures, varying one component inherently alters the other, making it difficult to isolate the effect of water. In this study, ethylene glycol (EG) was introduced as a co-solvent to decouple water concentration from PA content. By fixing the water content and varying the PA-to-EG ratio, we observed that the polishing rate remained primarily dependent on water concentration, regardless of PA or EG levels. This indicates that water plays a dominant role in governing the polishing rate in ECP systems. In this study, Cu ECP was performed in a mixed electrolyte of phosphoric acid and ethylene glycol. The concentration ratio of phosphoric acid, water and ethylene glycol was controlled variously. Electrochemical behavior of Cu ECP was studied by linear sweep voltammetry (LSV), and the capacitance and resistance characteristics of Cu ECP process were evaluated by electrochemical impedance spectroscopy. The electropolished Cu surface was observed by scanning electron microscopy and optical microscopy, and the surface roughness and surface morphology were measured by atomic force microscopy.