Copper Electrodeposition From Non-Cyanide Alkaline Baths Containing Methylene Diphosphonic Acid As A Complexing Agent

In this paper, methylene diphosphonic acid (MDPA, H₄L) was used as the base complexing agents in non-cyanide copper plating system. Firstly intermediate product (Tetraisopropyl methylenediphosphonate) was synthesized from Triisopropyl phosphite and dibromomethane by Condensation reaction of substituted. Finally MDPA was synthesized from Tetraisopropyl methylenediphosphonate by hydrolysis reaction.The plating bath was investigated by potentiometric titration, and the result showed that three different complex species present in the bath with the growing pH: Cu(H₃L)₂﹑[Cu(H₃L) (H₂L)]^- and [Cu(H₂L)₂]^2-. MDPA is more likely to form complex species with Cu²⁺ than 1-hydroxyethylene-1,1-diphosphonic acid (HEDPA) in the rang of pH=7?10. The result also indicated that the dissociation constants of MDPA were: pK₁=1.86, pK₂=2.65, pK₃=6.81,pK₄=9.04, and the stability constants were: pK_ML=10.65, pK_ML2=5.59,pK_ML3=2.5.Comparing MDPA-Cu system with HEDPA-Cu by electrochemical measurements indicated that the [Cu(H₃L) (H₂L)]^- and [Cu(H₂L)₂]^2- are reduce to Cu at cathode,both in MDPA-Cu and HEDPA-Cu system, and the reduction reaction is not reversible. However, the coordination compound formed by MDPA and Cu²⁺ is more stable than HEDPA, obtained the kinetic parameters was 0.33, diffusion coefficient D =1.82×10^-8cm²·s^-1.The effect of of temperature,pH,complexation ratio and CO₃^2- on copper electrodeposits from MDPA baths was investigated by electrochemical measurements, XRD,IR,EDS spectra of the complex and SEM analysis .The result showed that:Increased temperature could reduce the cathodic polarization,and the negative electrode potential is benefit for discharge both the low coordination complex ion and high coordination complex ion.Changes the pH of the bath can change the form of coordination compounds, which led to reduction and oxidation of copper and product changes.Increases MDPA could not only enhance the cathodic polarization but also promote anodic dissolution, however, the Ip in the MDPA: Cu = 3:1 reached maximum.,therefore MDPA: Cu = 3:1 is the optimal ratio of MDPA copper system.CO₃^2- could enter the complex and stabilize Cu²⁺ in the structure as the second ligand, and it can made the the changing of preferred orientation of the copper deposits. CO₃^2- can make a significant slowdown of the high current density region on hull cell sample of hydrogen, the cathode current density was also expansion.