High accuracy concentration analysis of accelerator components in acidic Cu superfilling bath

We have devised a modified cyclic voltammetry stripping (CVS) method to measure the concentrations of bis-(sulfopropyl) disulfide (SPS) and 3-mercapto-1-propane sulfonate (MPS) in Cu plating solutions. Though MPS, a breakdown product of SPS, enhances the Cu deposition rate on flat electrodes, it is not a superfilling-capable accelerator for the damascene structure, unlike SPS. Therefore, accurate measurement of SPS in damascene Cu plating baths is important. However, enhancement of the Cu deposition rate by MPS interferes with the electrochemical signal of SPS, leading to a significant error when using the modified linear approximation technique (MLAT)-CVS analysis method. To evaluate their concentrations individually, a two-step CVS analysis was performed 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 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.