Cyclic Voltammetry Stripping Analysis to Determine Iodide Ion Concentration in Cu Plating Bath

Abstract

Accurate monitoring of an electroplating bath's chemical balance is a key factor for maintaining its performance during a long-term plating operation. In this paper, a method is suggested to measure the concentration of iodide ion (I−), an inorganic leveler for through-silicon via (TSV) filling, based on its electrochemical response. During the operation of plating, I− was consumed via the reaction with Cu+ (Cu+ + I → CuI), an oxidation reaction (2I− → I2 + 2e), as well as a physical incorporation. The I− concentration decrease resulted in a degradation of the bath, while the major byproducts (CuI and I2) rarely influenced on bath performance. In order to monitor the I− concentration by cyclic voltammetry stripping (CVS) analysis, the electrochemical response of I− was examined at various conditions. I− suppressed the Cu electrodeposition rate; this response was dependent on the mass transport of I− and the applied potential of cathode. A subsequent effective coverage analysis revealed that not only I− but also Cu(I) iodide (CuI) was a key inhibitor, demonstrating that the inhibition of I− becomes weaker at a negative potential. With a responsive curve (RC)-CVS analysis conducted at an optimized condition, a linear relationship between the real and measured concentrations could be found, irrespective of other additives’ concentrations. The method suggested in this paper enabled the direct monitoring of the I− concentration in a Cu plating bath.