Cyclic Voltammetry Stripping Method for Determining Additive Concentration in Cu Electroplating Solution
Authors: Yoon Young , Kim Tae Young , Choe Seunghoe , Kim Jae Jeong 
Published: February 2020

Cu electroplating has been used in various fields. Electroplating process can form thin film of metal on a substrate and the application of Cu electroplating process expands from fabricating Cu foil to filling Cu microvia and interconnect in semiconductor chip and printed circuit boards (PCBs). [1]

The mechanical properties of the Cu deposit and the morphology of Cu films deposited by electrochemical systems are controlled by additives. [2] Various combinations of additives are used to control the film properties of the deposit and induce bottom-up filling in various scales of vias and interconnects. Conventional additives used in Cu electroplating are classified into three categories. Accelerators increase Cu electrodeposition rate where they adsorb. Suppressors physically block the access of Cu2+ ions to the surface of the electrode, retarding the Cu electrodeposition rate. Levelers, generally slowing the plating rate like suppressors, have specific behavior dependent on the mass transport and are newly introduced to fill micro-scaled vias and through-holes. [3]

As like other chemical processes, Cu electroplating accompanies degradation. Plating bath contains many kinds of components and bath conditions can be degraded during the Cu electroplating process. Therefore, monitoring the properties of the bath is necessary for various purpose. The additives, which are hard to monitor their chemical properties are decomposed to byproducts or incorporated into the deposit due to the chemical/electrochemical effects.

Therefore, the degradation of organic additives decreases the concentration of the additives and produces byproducts. [4, 5] Because the concentration of the additives have critical effects on the fabricated Cu deposit, monitoring the concentration of the additives is important. CVS (cyclic voltammetry stripping) uses voltammetric approach in determining the concentrations and it concentrates on the stripping charge, which represents the electroplating rate affected by the concentration/type of additives. CVS obtains calibration curve of a target additive using standard solution whose concentration of the target additive is known and determines the concentration of the additive by interpolating the stripping charge of a sample solution.

When monitoring additives used in Cu electroplating solution, optimized CVS methods have been devised for each type of additives. MLAT-CVS (modified linear approximation technique-CVS) is used for determining accelerator concentration, however in case of SPS (bis(3-sulfoproply) disulfide), the degradation byproduct, MPSA (3-mercapto-1-propanesulfonic acid), acts as an interference due to its different accelerating effects on Cu electroplating compared with SPS. Controlling the pH of the base and target solutions could completely convert the MPSA to SPS, which enabled the determination of total amount of accelerating agent when using MLAT-CVS method. The conversion ratio from SPS to MPSA responded to the devised function related to stripping charge, which enabled the determination of individual SPS/MPSA concentration.

DT-CVS (dilution/titration-CVS) method is generally used for determining suppressor concentration. When the conventional DT-CVS analysis was applied to accelerator/suppressor two-additive target solutions, the accelerator could act as an interference as its effect on the DT-CVS was shown in Fig. 1. Even small concentrations of accelerator (CA) in CVS bath can make a change in determined suppressor concentration (CS). We suggested new method of determining the suppressor concentration in two-additive Cu electroplating solution by introducing iodide ion (I-) as an inhibitor of the accelerator.

As mentioned above, I- has been recently developed to fill high-aspect ratio TSV (through silicon via) due to its excellent inhibition effect on Cu electrodeposition by deactivating SPS. RC-CVS (response curve-CVS) method could be applied to monitor I- concentration due to its leveling effect. From the optimization for the determination process, the mechanism of I- affecting Cu electroplating in addition to the successful monitoring of I- concentration in wide range with high resolution. This method was used to recover the performance of three-additive Cu electroplating solution for TSV filling.

Fig. 1. Q/Q0 plots with various CAs as a function of CS and deviated concentrations of PEG-PPG by the effect of SPS at 0.6 evaluation value.

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Figure 1