Control of Accumulation of Cu(I) in Copper Sulfate Electroplating Plating Solution
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 through-silicon via (TSV) plating technology, it becomes more important to fine management of the plating solution during the manufacturing process in order to keep high quality of product. In copper sulfate electroplating, monovalent copper ions (cuprous ion: Cu(I)) has been implicitly pointed out as one of the main cause of low quality such as high roughness and dull metallic on the Cu plated surface, therefore accurate measurement of Cu(I) concentrations has been strongly required in the plating process. In spite of the needs in industry, it has been remaining as challenge to measure and monitor the Cu(I) concentration in the process because of the unstableness of Cu(I) in solution, conceitedly rapid oxidation. In fact, although a large amount of Cu(I) can be considered to be contained in the plating solution in the production lines, there is not enough knowledge about the holding structure of Cu(I) in the plating solutions. We have succeeded in optical analyzing the Cu(I) containing in the plating solutions conducted by absorption of a chelate of Cu(I) with bathocuproinedisulfonic acid, disodium salt (BCS). Here, we will report the formation and accumulation of Cu(I) in copper sulfate electroplating solutions and discuss the relationship Cu(I) activity to electrolysis conditions such as electro density, time and dissolving gas, based on the experiments in the laboratory. These results also will be important knowledge to control a process factor such as concentrations and holding structures of Cu(I) in the plating solutions even in manufactural plating line. As experimental, a time course of Cu(I) concentrations were determined from change in absorbance originated from formation of Cu(I) and BCS chelate accompanied by applying electrolysis the copper sulfate plating solutions. The electrolysis was carried out using two electrode system; anode: copper plate, cathode: platinum plate (electrochemically active area: 20×40 mm). The plating solution (150mL) was degassed with nitrogen gas through the energization. Cu(I) does not include most in the new liquid of copper sulfate, however about 1mM Cu(I) was accumulated by electrolysis for 10 minutes at a current density of 62.5mA / cm2. Cu(I) concentration in the solutions were firstly increased depending on the electrolysis time, the decreases as longer than a certain period of time, for example, about 15 minutes in 62.5mA/ cm2. Accumulation of Cu(I) tends to increases in accordance with the current density but there was no change in the following 15.5 mA/ cm2. On the contrary, the amount of Cu(I) to be accumulated even more than 78.1mA/ cm2 was less than the amount of 62.5mA/ cm2. Accumulation of Cu(I) is considered to be proportional to the current density and time within a certain range. For optimal current value in the formation of the plating film is less than this lower limit, usually the accumulation of monovalent copper would be less likely to occur. On the other hand, if it exceeds the upper limit, the precipitation of a large amount of particles in the solution was observed. In previous discussion, Cu(I) that is formed in a solution concluded that are stabilized incorporated into PEG. Excessive Cu(I) might form the copper particles by a disproportionation reaction. Through the optical analysis of Cu(I), it found that the formation and accumulation of Cu(I) depends on other conditions in electrolysis. For example, the accumulation of Cu(I) is hardly observed using phosphorus-containing copper plate as the anode instead of Cu plate. Further, when bubbling with air instead of nitrogen gas, accumulation of Cu (I) was reduced to approximately half. It can be explained that Cu (I) was oxidized by dissolved oxygen come from the bubbled air. The oxidation is a major factor that inhibits the accumulation of Cu (I). In fact by the air bubbling, Cu (I) concentration in the solution is reduced in accordance with the bubbling time. Cu (I) is more stably maintained in the degassed solution. As conclusion, these results clarified that accumulation of Cu (I) of the copper sulfate plating solution is governed by various factors, such as current-density, operation time electrode and kind of dissolved gas in electrolysis. It also shows the possibility of controlling the density and the holding structure of Cu (I) in solution. The Cu (I) is a cause of roughness of the film, it is also considered to have a positive effect on the filling in TSV plating. By controlling the formation and accumulation of Cu (I) of the copper sulfate plating solution, thereby enabling more detailed consideration of these issues.