The Fundamentals of Cyclic Voltammetric Stripping

 

Cyclic Voltammetric Stripping (CVS)1 is a patented analytical technique for the quantitative determination of organic additives, and their contaminants in
electroplating solutions. The additive concentration in the plating bath has a strong effect on the ductility, tensile strength, and solderability of the deposit.
Routine CVS analysis of the organic additives results in continuous, trouble-free operation.
 
CVS provides more objective information on the plating bath than a Hull Cell. A Hull Cell provides a qualitative indication of the bath condition. Using CVS, the plater can precisely replenish the additives to keep the bath operating properly.
 
CVS ANALYSIS
Cyclic Voltammetric Stripping is an electrochemical technique used for the measurement of organic additives in plating baths. It is based on the effect that
the additives have on the rate of electroplating. Regardless of the specific type of organic additive (brightener, leveler, grain refiner, etc.), its activity is reflected in a change in the plating rate.
 
The analysis is performed in an electrochemical cell using a three-electrode system, one of which is a platinum rotating disk electrode. During measurement, the potential of the platinum electrode is controlled by the instrument. The potential is scanned at a constant rate back and forth between negative and positive voltage limits. A small amount of metal from the plating bath is alternatively plated onto and stripped off the working electrode as the potential is changed. During the scan, the current at the working electrode is measured as a function of potential.
 
The activity of the additive will affect the plating rate of the metal onto the electrode. The plating rate is determined by calculating the charge required to strip the metal off the working electrode. The relationship between the stripping charge and the activity of the additives is used to quantitatively measure the additives and their components. Fresh additive as supplied by the manufacturer is used as a standard. The activity of the additive in the production bath is expressed as concentration (mL/L) of the fresh additive.
 
An instrument such as the QUALILAB QL-5® Plating Bath Analyzer from ECI Technology performs the patented CVS analysis automatically. The QL-5 is software-controlled and designed especially for the analysis of plating baths. The user is prompted through the analytical procedures while the QL-5 makes all of the calculations and reports the active concentrations of additives in the bath.
 
USING CVS FOR PLATING BATH ANALYSIS
An example of a voltammogram (plot of the measured current vs. applied potential) for an acid copper plating bath is shown in Figure 1.

 

a typical CVS voltammogram for an Acid Copper plating bath

Figure 1 is an actual display from the QUALILAB Plating Bath Analyzer. The potential determines the electrochemical reaction that occurs. The potential is displayed on the X-axis with potentials becoming more positive from left to right. A positive-going potential is becoming more strongly oxidizing, while a negative going potential generates a more strongly reducing condition. Current is shown on the Y-axis. A positive current corresponds to an oxidation while a negative current is due to a reduction. The voltammogram has several regions of interest:

 
Plating region: Points 1 to 2 and 2 to 3. The current is negative in this region as the platinum electrode is plated with metal. Since the plating time is well-defined by the scan rate, the plating rate can be accurately estimated as the amount of metal that is plated in the plating region.
 
Stripping Region: Points 3 to 4. As the current of the electrode becomes positive, the metal is stripped off the electrode. The area of the stripping peak represents the charge required to strip the deposited metal. The instrument integrates the current to measure the area beneath the stripping peak. The area is reported in milli-Coulombs (mC) and is shown in the box at the lower right of Figure 1. Since the quantity of stripped metal is the same as quantity of plated metal, the stripping peak area is proportional to the amount of metal plated. It is more accurate to measure the amount of metal plated by integrating the current in the Stripping Region rather that in the Plating Region. Electrochemical processes other than metal deposition, such as hydrogen reduction, may also be taking place in the Plating Region.
 
Cleaning Region: Points 4 to 5 and back from 5 to 6. A positive potential on the electrode is used to desorb organics and clean the surface of the platinum electrode. This cleaning segment is critical for accurate and reproducible results with CVS. This region is also used to monitor oxidizable organic contaminants and chloride levels in acid copper baths.
 
Adsorption Region: Points 5 to 6. In this region the organic additives are adsorbed onto the electrode surface, even though no metal is plated. The adsorption that takes place in this region enhances the sensitivity of CVS.
 
The influence of several organic additives on the plating rate is shown in Figure 2.

 

effect of some organic additives on the plating rate of metal

In general, additives such as levelers or wetters act as suppressors and decrease the plating rate while brighteners increase the rate of deposition. Use caution, however, since there is no standard terminology for additives within the industry.

 
During electroplating, the organic additives undergo various chemical transformations such as oxidation, reduction, fragmentation, and polymerization. Most of the products of such transformations also affect the plating process. CVS provides a measure of the effective concentration of additive (the activity of the additive in terms of the fresh additive) which is more important than the concentration of a specific substance. Most analytical techniques (spectroscopy, HPLC) measure absolute concentration. The transformed additive will still be electroactive, but not responsive to spectroscopy, or vice versa. Because CVS is based on the plating process, it is the only technique that provides a true measure of the activity of the organic additives.
 
DETERMINATION OF ADDITIVES AND THEIR COMPONENTS
The quantitative determination of organic additives is based on the CVS response of the production bath before and after the addition of a known amount of a standard. The fresh additive as supplied by the manufacturer, which is used for replenishment of the bath, is used as the standard. Since the fresh additive is used as the standard, the concentration of additive in the production bath is expressed as the equivalent activity of the fresh additive. In this way, replenishment of the bath to the proper additive level is very convenient.
 
It is usually possible to quantify all of the additives in the plating bath. An example of an acid copper bath is shown in Figure 3.

 

CVS response of carrier and brightener in an Acid Copper bath

The carrier acts as a suppressor and inhibits the rate of the plating process. This can be seen from the reduction in peak area as the concentration of the carrier is increased. Notice that the activity of the carrier is very high, i.e., a small increase in concentration has a large effect on the rate of plating. The fresh carrier as supplied by the manufacturer was used to spike the solution.

 
The plating rate changes dramatically as the carrier is increased from 0-1 mL/L. As the carrier concentration is increased beyond 1 mL/L, there is little additional effect on the plating rate.
 
The brightener has a different effect. In the presence of carrier, higher brightener concentration causes an increase in plating rate. The activity of the brightener is lower than the carrier. The difference in activity between the carrier and the brightener can be exploited to quantify both components. 
 
Some manufacturers offer a pre-mixed solution containing both the carrier and the brightener. These components can be clearly distinguished in Figure 3. CVS can be employed to insure that the carrier and the brightener are in balance in the production bath.
 
The analysis of plating baths using CVS benefits from experience. ECI Technology has invested over 15 years in developing CVS methods for a wide variety of plating baths. In addition to manufacturing CVS instrumentation, ECI Technology is the leading source of CVS applications information. CVS consultation is available to every user of ECI Technology products.
 
OUTGOING AND INCOMING INSPECTION OF ADDITIVES
CVS is a powerful tool for the outgoing inspection of additives by suppliers as well as incoming inspection at users’ sites. Figure 4 illustrates the variations between several lots of the same brightener.

 

Variation in response of different lots of the same additive from the same manufacturer

By performing a fast, easy assay, the user avoided under dosing or overdosing the brightener and losing control of the bath.

 
THE INDUSTRY STANDARD
Since its initial development in the late 70's, CVS has experienced tremendous growth. The contributions of CVS to both suppliers and users of plating chemistries has been significant. This patented technology is currently used by all major chemistry suppliers for quality control or routine analysis of customers’ baths. ECI Technology is the major driving force behind CVS. The development of both CVS instrumentation and analytical procedures by ECI Technology has been a critical contribution to the growth of CVS.
 
Users of CVS instrumentation from ECI Technology include the most trusted manufacturers of semiconductors and printed wiring boards such as Motorola, IBM, Intel, Texas Instruments, Northrop, Tyco, Photocircuits, Sanmina, and others. CVS may be found in plating laboratories in the Americas, Europe, and Asia. In addition the innovative engineering of ECI Technology was recognized by an R&D 100 Award.
 
APPLICATIONS OF CVS
Prevention of plating problems by quantitative measurement of the organic additives in plating solutions keeps the chemistry in balance.
 
• Quantitative determination of additives and their components
• Individual fingerprints of plating solutions
• Incoming/outgoing inspection of plating additives
• Monitoring the level of plating solution contamination
• Scheduling baths for carbon treatment and optimizing of the treatment
 
Study, develop, and optimize new plating technology
 
• Study the effect of various parameters of the plating process (concentration of additives, current density, mass transport, temperature, etc.) on the performance of plating solutions
• Study the consumption and transformation of additives during the plating process
• Study the effect of various additives on the plating process
 
1Cyclic Volatmmetric Stripping (CVS) is a patented technology. QUALILAB QL-5 is a trademark of ECI
Technology, Inc. No part of this material may be copied, used or distributed in whole or in part without
permission from ECI Technology, Inc.