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This article explores the use of Cyclic Voltammetric Stripping (CVS) analysis to monitor the concentration of organic additives in copper electroplating baths. Additives like suppressors, brighteners, and levelers are essential for achieving uniformity, smoothness, and optimal copper thickness. Therefore, the focus will be on various measurement techniques used to quantify these additives, including Dilution Titration (DT), the Modified Linear Approximation Technique (MLAT), and the Response Curve (RC).
Plating and printed circuit boards
Plating is the process of coating a thin layer of metal onto the surface of an object. It is commonly used across various sectors, including automotive, aerospace, jewelry, medical devices, industrial equipment, and electronics. Electrochemical deposition of copper is commonly used in various processes like semiconductor chip production (e.g., through-silicon vias or TSVs), advanced chip packaging (microbumps), or printed circuit board (PCB) manufacturing.
Printed circuit boards are the backbone of modern electronics. They serve as a physical platform where electrical connections and components are integrated. A PCB structure typically consists of multiple layers made from a combination of conductive and insulating materials. In these layers, structures like traces (copper pathways for conducting signals and current), pads (copper areas for soldering components), and holes are incorporated (Figure 1).
Electrochemical copper deposition plays a critical role in forming, filling, and reinforcing the copper layer across different PCB structures, particularly within the holes. These holes, known as «through-holes» and «vias» enable electrical connections between the different layers of the PCB (Figure 2). Without these holes, the inner copper layers would remain isolated and could not be integrated into the current flow. There are specific copper plating requirements for the different types of holes.
Through-holes extend through the entire thickness of the PCB, connecting the top and bottom layers. In this case, the copper coating must be uniform to ensure a stable connection between these layers.
Vias connect only specific layers within the PCB, for example, from the top layer to an inner layer. These holes require a particularly dense and even copper fill to reduce mechanical stress and ensure a stable connection. Vias can be categorized into «blind vias» (starting from the surface and leading into an inner layer) and «buried vias» (completely hidden within the inner layers).
A crucial factor in meeting PCB quality demands is the precise control of organic additives in the copper plating bath. These additives ensure that the copper layer is deposited evenly and without defects.
The three main organic additive types in the copper bath
Suppressors, brighteners, and levelers are essential additives in the copper electroplating process (Figure 3), each with a specific function [1].
Suppressors, such as polyethylene glycol (PEG), slow down copper deposition by forming a bond with chloride ions, making a complex that attaches to the surface and acts as a barrier. This increases the energy needed for copper ions to settle, resulting in a smooth, even layer. By preventing quick accumulation, suppressors help create a consistent, defect-free copper coating.
Brighteners, usually sulfur-based organic compounds such as bis-(sodium-3-sulfopropyl)-disulfide (SPS), speed up copper growth in specific areas. They work directly on the copper surface to refine the grain structure, creating a denser and smoother finish. Brighteners compete with suppressors for active sites, resulting in a polished-like, high-quality surface.
Levelers, made from cationic surfactants for example, ensure an even copper thickness across the board. They reduce deposition in areas with higher current density, like edges, which prevents peaks and promotes uniform coverage. This precision is important to avoid weaknesses in the final product, where a consistent copper thickness is critical.
Together, suppressors, brighteners, and levelers create a balanced electroplating system. However, maintaining this balance requires precise control of their concentrations, and this is where Cyclic Voltammetric Stripping (CVS) analysis plays a crucial role.
How do we measure and quantify the concentration of organic additives?
Cyclic Voltammetric Stripping (CVS) and Cyclic Pulse Voltammetric Stripping (CPVS) are common analytical methods used to analyze organic additives in electroplating baths. Different techniques are used to quantify these additives: Dilution Titration (DT), Modified Linear Approximation Technique (MLAT), and Response Curve (RC).
To determine suppressor content, DT is the proper choice, while MLAT is used for brightener determination and RC measures the leveler concentration.
?ts=1733486004175&dpr=off&bfc=on "Example voltammogram for a 2 mm Pt working electrode during conditioning in virgin make-up solution (VMS).")
Cyclic Voltammetric Stripping (CVS)
CVS works by depositing and then removing copper from an electrode surface while repeatedly varying the electrical potential between a negative potential (ca. -0.3 V) and a positive potential (ca. +1.6 V). This produces voltammograms—curves that show the current response to the applied potential.
During the anodic sweep, as the potential shifts from negative to positive values, a distinct stripping peak for copper appears at a specific oxidation potential (Figure 4). The height of this peak is influenced by several factors, including the concentration of copper, the presence of organic additives, and various electrochemical parameters. This peak is used as a marker to analyze how additives affect the copper plating rate.
Cyclic Pulse Voltammetric Stripping (CPVS)
CPVS is another electrochemical technique based on chronoamperometry. This technique is useful specifically for measuring copper plating additives in samples that contain iron.
The dilution titration (DT) technique measures the concentration of suppressor additives in copper plating baths.
The process begins with a basic bath solution called the virgin make-up solution (VMS). VMS contains essential bath chemicals like CuSO₄, H₂SO₄, and NaCl—but no additives. This solution is used to prepare the working electrode.
Once a stable copper signal is established, a small amount of the suppressor is added. After each addition, the voltammetric system measures the amount of copper plated and then stripped from a rotating platinum electrode (Figure 5, center).
Since the suppressor slows down copper plating, adding more of it makes the copper stripping peak smaller (Figure 6). This change is used to create a calibration curve.
Once the calibration is complete, an unknown sample can be analyzed by following the same process. Instead of adding a suppressor standard solution, the bath sample is used. By comparing the results from the calibration curve with the sample, the unknown suppressor concentration can be determined (Figure 7).
Precision is essential in DT—each small addition must be accurately controlled. Automated equipment, like dosing devices (e.g., 800 Dosino), is strongly recommended to ensure precise measurements and avoid manual errors.
Modified Linear Approximation Technique (MLAT)
The modified linear approximation technique (MLAT) is used to measure the concentration of the brightener additive. MLAT tracks the influence in the copper plating reaction by adding the brightener to the intercept solution and measuring how the copper signal changes.
The process starts with an intercept solution, which is a mixture of basic bath chemicals—CuSO₄, H₂SO₄, NaCl, and suppressor in excess—without any brightener. First, the voltammetric system measures the copper stripping peak area in this solution to create a reference point, the so-called «intercept value». Next, a sample containing brightener is added, and the system records the increase of the copper stripping peak area caused by the brightener in the sample. Then a standard solution is added to this solution. Since the brightener speeds up copper deposition, adding more of it causes the copper peak area to increase (Figure 8).
The goal of MLAT is to create a calibration curve by plotting the copper peak area changes against the brightener concentration. Once this calibration is done, the brightener concentration in a sample can be determined.
Response Curve (RC) Technique
The response curve (RC) determination begins by preparing an electrolyte solution, usually made of the virgin make-up solution (VMS), suppressor, and brightener. This solution is added to the measuring vessel where the voltammetric system records the initial electrolyte value. This value serves as a reference point to assess the effect of the leveler.
A calibration curve is recorded in the next step. For that purpose, a series of additions of the leveler standard solution is made. After each addition, the system measures the copper stripping peak area.
As more leveler is added, it selectively slows copper plating in certain areas, leading to changes in the peak area observed during the voltammetric sweep (Figure 9). A response curve is then created by plotting the ratio of the copper stripping peak area to the electrolyte value against the concentration of the leveler.
Once the calibration is complete, an unknown sample can be analyzed by comparing the sample response to the established response curve. For this purpose, first the electrolyte value is recorded. Next, the sample response is measured. The ratio between the sample response and the electrolyte value is then used to determine the leveler concentration (Figure 10).
These three methods show that monitoring of different organic additives in the plating bath is possible. Cyclic Voltammetric Stripping (CVS) is essential here, as it provides a reliable way to measure these additives by monitoring the changes in copper deposition rate. Find out more by watching our video!
Summary
As electronic devices continue to evolve, ensuring optimal copper deposition for microelectronics and 3D chip integration remains crucial. Cyclic Voltammetric Stripping analysis offers a powerful solution for monitoring the concentration of key organic additives in copper plating baths.
Metrohm’s CVS systems make analysis more accurate and more efficient. Resources from Metrohm including webinars, daily routines, and dedicated blog posts provide an excellent foundation for those looking to enhance their understanding and performance in additive management. We encourage you to explore our resources below for further insights into advanced CVS.
Reference
[1] Huang, T. B.; Sharma, H.; Manepalli, R.; et al. Electroanalytical Study of Organic Additive Interactions in Copper Plating and Their Correlation with Via Fill Behavior. Journal of Elec Materi 2018, 47 (12), 7401–7408. DOI:10.1007/s11664-018-6680-0
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