Influence of Additive Coadsorption on Copper Superfill Behavior

The interaction between the additive components chloride, accelerator, and suppressor is the focus of the present paper. Based on Frumkin and Damaskin’s adsorption theory, an advanced concept for the superfilling mechanism is introduced. Cyclovoltammetry measurements show the additive impact on the current–potential behavior and their synergetic effect on the charge transfer across the electrode–electrolyte interface. The measurements are supported by partial fill experiments. Cross-section scanning electron microscopy micrographs reveal that the synergetic impact of an accelerator and a suppressor is very different from their individual contributions. Chloride ions support the adsorption of suppressor molecules, while the accelerator enhances the desorption of the suppressor molecules from the copper surface. The higher the local accelerator concentration in an electrolyte, the more suppressor molecules desorb from the surface. The synergetic behavior between the chloride, suppressor, and accelerator can be explained by coadsorption, which is an important key in the process of copper superfilling. For the application of interconnect structures, manifold investigations have been carried out regarding the role of plating bath additives in the electrochemical copper deposition process. To achieve void-free copper interconnects, superfilling is crucial. Superfilling with an increased copper growth rate at the bottom structure results from the impact of plating bath additives. At least two additives, called suppressor and accelerator, are required for the superfilling process. The active component of the suppressor often consists of polyethylene glycol, a long chained organic molecule with a molar mass of up to 10,000 g/mol. Due to its functional end groups, it is soluble in water and acids, while the ether groups possess free electron pairs that interact with copper ions. 1,2 Small amounts of chloride ions are necessary for the suppressor to function as a surfactant. 3,4 The active component of the accelerator bis3-sulfopropyldisulfide or mercaptopropylsulfonic acid adsorbs on Au and Ag surfaces by forming thiolate bonds. Controversies exist about the surface activity of the accelerator on copper especially in acidic solutions. 5 Spectroscopic measurements did not identify a specific coordination of the accelerator to a copper surface. 6