Spectroscopic and Electrochemical Analysis of Cu(I) Complex of Copper Sulfate Electroplating Solution and Evaluation of Plated Films
Authors: Koga Toshiaki , Hirakawa Chieko , Sakata Yoshitaro , Noma Hiroaki , Nonaka Kazuhiro , Terasaki Nao 
Published: February 2020

With density growth of printed circuit board and the development of TSV plating technology, it becomes more important to management of the plating solution during the manufacturing process. In copper sulfate electroplating, accurate measurement of the monovalent copper ions (Cu(I)) concentrations in the solutions is required, because they are pointed out to cause roughness of the plated surface. We have succeeded in analyzing the Cu(I) contained in the plating solutions conducted by absorption of a chelate of Cu(I) with bathocuproinedisulfonic acid, disodium salt (BCS). Although Cu(I) in the solutions is considered to not be detected because they are oxidized immediately, in fact, a large amount of Cu(I) is contained in the plating solution of the production lines. However, there is not enough conclusions about the effect of Cu(I) in the solution is on the plated surface. We had found that formation and accumulating Cu (I) by electrolysis under suitable conditions for copper sulfate plating solutions. Here, we will discuss a mechanism of Cu(I) accumulation in CuSO4 plating solution during the electrolysis from the view point of (1) dynamics on formation of 1:2 complex between Cu(I) and BCS chelate and (2) MALDI-MS analysis of the plating solution. In addition, by considering of implicit knowledge about the role of Cu(I) for product quality in production line, we will discuss (3) the relationship of Cu(I) amount and qualities of the Cu plated surface. (1) Dynamics on formation of 1:2 complex between Cu(I) and BCS chelate Cu(I) concentration in the plating solution can be determined from the absorbance of Cu (I) and BCS chelate. Furthermore, it is possible to classify the holding structures of Cu(I) by analyzing the kinetics reaction between BCS and Cu (I). Reaction of BCS with Cu (I) can be divided into three components, the instantaneous component of the time constant is less than 1 seconds (A0), an fast component is completed within a few seconds (AS), and components having a long time constant which is slow than a few minutes (AL). Although Cu(I) concentration in the solution was increased by electrolysis, change in each of the reaction components was not uniform (AS component was not observed in this experiment). That AL component is increased in the initial (<4min) of the electrolysis, A0 component was increased with a delay to between 6 and 10 minutes. This suggests that some Cu(I) holding structures occurring at different processes in solution. (2) MALDI-MS analysis of Cu plating solution Results of MALDI-MS analysis show some intermolecular interaction between Cu(I) and BCS. It can be considered that the some of the Cu(I) ion must be incorporated into poly ethylene glycoside (PEG) and stabilized for accumulation in the Cu plating solution. In the case of deeply incorporating into the PEG chain, Cu(I) must be isolated from the solution and attack from BCS by the PEG, and formation of Cu(I)-BCS2 complex was probably inhibited to be long kinetics component (AL). On the other hand, A0 component is not be described in clear, these Cu (I) is considered to be held in a state to quickly react with BCS. (3) Relationship of Cu(I) amount and qualities of the Cu plated surface Cu(I) in these states would have played a different role each in the electroplating process. So far, the results of the plating solution analyzing of the production line, mainly A0 component is suggested to affect the roughness of the plated surface. This effect is also supported by electrochemical measurements. Changes in the A0 values for the current time are correlated well with the changes in the area values in CVS (Cyclic Voltammetry Stripping) measurement. On the other hand, no clear relationship was observed between the AL values and area values. PEG might have to reduce the impact on the film surface by incorporating excess Cu(I). In order to investigate the effect of Cu(I) to the structures of the plating, surface were electroplated onto the copper plates with electrolysis solutions (electrolysis time : 0 to 20 minutes). In the film formed in 4 min electrolysis solution (negligible A0), roughness on the plate surface did not occur. However, in the 6 to 10 min electrolysis solutions, roughness occurs on the film surface depending on the electrolysis time. It was confirmed that Cu(I) accumulation (especially A0 component) is affecting the defect occurrence of coating appearance. These results show that spectroscopic methods for the analysis of Cu(I) in solution are effective, and the performance change of the solutions can be easily evaluated.