Decomposition of Bis(3-Sulfopropyl) Disulfide and Poly(Ethylene Glycol-Propylene Glycol) during Cu Electrodeposition and Its Monitoring via Electrochemical Method

Abstract

Cu electrodeposition has been widely utilized in various industrial fields because of its high productivity, low process cost, and excellent products qualities. Cu plating bath usually contains small amount of organic additives that control the morphologies and the properties of Cu deposits. However, the organic additives are unstable under the electrolytic condition and gradually decompose through various chemical and electrochemical reactions. Because of the decomposition of the additives and the consequent degradation of the solution performance, the additives concentrations of the bath are tightly monitored with the electrochemical methods such as cyclic voltammetry stripping (CVS), and maintained by daily bleed-and-feed system. Despite those efforts, the bath is eventually unusable after long time operation of plating bath. It is because the breakdown products, which affect the properties of deposited Cu films and disturb the signal of monitoring tool, are uncontrollably accumulated in the bath. However, the present monitoring system cannot provide the information of breakdown products, and it is necessary to develop the advanced monitoring methods that enable to analyze not only the parent additives but also their breakdown products. Therefore, this study describes the influences of byproducts on Cu electrodeposition process in detail and suggests the advanced methodologies enabling the analysis of both the parent organic additives and their breakdown products. The aging of bis(3-sulfopropyl) disulfide (SPS) over 120 min led to the deterioration in bath performance. It was associated with the breakdown of SPS into 3-mercapto-1-propane sulfonate (MPS) and 1,3-propane disulfonic acid (PDS) via the electro-oxidation reaction and the sequential chemical oxidation by dissolved oxygen gases. The subsequent experiments revealed that the effects of PDS on Cu2+ reduction kinetics, Cu film properties and filling capabilities are negligible since it could not form chemical linkage to the Cu surface. However, MPS significantly affected the electrochemical response of plating bath, leading to the inaccurate results in CVS analysis of SPS concentration. Moreover, the presence of MPS deteriorated the filling capability of Cu plating bath. Both aspects implied the need for the development of advanced monitoring methods that provide both SPS and MPS concentrations. To evaluate their concentrations individually, a two-step CVS analysis was suggested in which the total accelerator concentration ([SPS] + 1/2[MPS]) and conversion ratio were separately determined. All MPS species in the bath were oxidized to SPS by controlling the plating solution pH. Subsequent modified linear approximation technique (MLAT)-CVS analysis successfully revealed the total accelerator concentration in the Cu plating solution. Individual SPS and MPS concentrations were thereby calculated using the conversion ratio evaluated from the difference in their relative accelerating abilities. This modified method enabled determination of the SPS concentration with <10% error, suggesting a reliable and high accuracy tool to predict pattern filling capabilities of plating solutions. During electrodeposition, poly(ethylene glycol-propylene glycol) (PEG-PPG), a suppressor-type additive, fragmented into lower molecular weight (MW) units by the oxidative scissoring reaction on ether bond. This reaction accompanied the change in terminal groups from hydroxyl to aldehyde, formic ester, and ketone and reduces average MW. Consequently, the aged solution contained high population of low MW PEG-PPG units, which affects both the electrochemical responses of bath and the properties of Cu deposits. This result indicated that the monitoring of MW of PEG-PPG was necessary for accurate prediction of solution performance. Considering those factors, the breakdown products from decomposition of additives significantly affect the filling capabilities and Cu film properties. Concentration analyses of both parent additives and its breakdown product are necessary for accurate diagnosis of Cu plating bath. This comprehensive study provides the general guideline for the development of advanced monitoring methods that are more accurate than conventional CVS.