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Voltammetric Detection of Low Copper Concentrations in Nickel Plating Baths

Abstract

Nickel electroplating is widely used in semiconductor manufacturing, primarily during the packaging stage. It is not used as a final coating, but instead as a barrier layer to prevent formation of copper–tin intermetallic compounds that affect the reliability of solder joints. The nickel is deposited from baths containing nickel salt (in relatively high concentrations), boric acid, and other ions.
The quality of the deposited nickel is highly dependent on the composition of the plating bath. Metallic contaminants are acceptable when their concentrations are below approximately 30 ppm. Copper, lead, zinc, and cadmium, even in relatively small quantities (higher than 30 ppm) produce a dull, black, or skip plate condition in the low-current-density areas. These metals may be removed from the plating solution by low-current-density dummyplating, but a sensitive and accurate analytical method must be used to determine when to treat the bath. Copper is considered a main contaminant due to its higher concentrations in the bath and its most detrimental effect on the nickel deposit. To prevent plating defects, the bath contaminants must be monitored.

Key Words

Nickel, electroplating, copper, contamination

I. Introduction

Traditionally, low concentrations of metals in plating solutions can be monitored using highly sensitive polarography methods or spectral methods such as AAS or ICP [1]. These methods can detect parts per trillion of copper and other metals. However, utilization of mercury electrodes makes this method less desirable due to safety and environmental concerns. In nickel-plating solutions, concentrations of copper are high enough that it would be more appropriate to measure copper with a solid electrode. Mercury electrodes are more sensitive than solid electrodes, but in the case of nickel-plating baths, such sensitivity is not required.
Prior publications describe methods for the determination of low copper concentrations using solid electrodes [2]. These methods work well when copper is present in low (ppm) concentrations and other compounds are present in low amounts or absent.In the nickel-plating bath, the main challenge is that concentrations of  nickel and other components (boric acid, chloride, or bromide ions) are very high, while the concentrations of copper are much lower [3,4]. For that reason, a new analytical method was carefully verified for possible interference with other bath components. In addition, nickel plating baths are utilized at high temperatures (40-50°C) to prevent precipitation of boric acid. This required additional precautions during analysis as boric acid can precipitate and distort the results. Results showed that if highly concentrated and hot nickel solutions are pre-treated (diluted and cooled) prior to analyses, the results of copper analysis can be accurate and reproducible.

II. Experimental Details

Chemicals and Materials- Solutions were prepared with boric acid (Fisher), nickel (II) chloride hexahydrate (Sigma-Aldrich), sodium chloride (Fisher), 50% aqueous nickel (II) sulfamate (Palm), Nikal BP wetting agent (Dow), and sodium bromide (Spectrum). For dilution tests pH 4.00 reference standard buffer from Ricca was used.

Instrumentation- Analyses were performed using an ECI QualiLab QL-10 bench top plating bath analyzer. A 4 mm Platinum Rotating Disk Electrode, an Ag/AgCL electrode with 0.1 M KCL junction solution, and a stainless steel rod counter electrode comprised the three-electrode system.

Procedures- Samples were prepared by dissolving properly measured quantities in de-ionized water. The analyzer performed modified pulse voltammetric stripping analysis (MPVS) on the samples. During the electrochemical scan, the platinum electrode surface was polarized with negative voltage to accumulate copper as per equation (1) and then subsequently polarized with positive voltage to dissolve accumulated metal.

Cu² + 2e <=> Cu⁰

The reaction of copper deposition and dissolution is quite reversible, allowing collection of multiple electrochemical cycles. The dissolution peak area was selected as a main analytical signal. Electrochemical parameters such as scan rate and deposition potential were tested and optimized. The ranges are 10 to 50 mV/sec and -0.25 to -0.35 V (vs. Ag/AgCl reference electrode) respectively. The rotation speed of the platinum disk electrode was also validated in the range between 100 and 6000 rpm. Data was then processed using a proprietary algorithm where Cu peak areas were determined and compared.

II. Results and Discussions

A.      Parameter optimization

To establish a suitable electrochemical signal, initial parameter screening was performed. It was observed that the electrochemical outputs (voltammogram peaks) were sensitive to the changes in deposition voltage, scan rate, and rotation speed of the platinum disc electrode. The shape of the copper dissolution peak affects the subsequent data processing. The analysis parameters were optimized to provide the highest and most reproducible dissolution peak.

Fig. 1 shows optimization results of platinum electrode rotation rate.

As this graph shows, the most suitable peak shape is achieved at the highest possible rotation rate. This is because the copper concentration in solution is quite low and can be easily depleted in the layer near the electrode. Further increase in rotation rate was not beneficial due to high turbulence in the electrochemical cell that caused solution disturbance and splashing. 6000 rpm was found to be the optimum rotation rate and provides reproducible and suitable data for peak processing. In all subsequent electrochemical experiments, the rotation rate of the platinum electrodes was set at 6000 rpm.
The concentration of copper can potentially be as high as 30 ppm. However, as previously mentioned, the nickel plating baths contain high concentrations of boric acid and are kept hot to allow boric acid to remain in solution. When the bath is taken for analysis, the temperature can drop, causing precipitation of boric acid. This makes further analysis complicated. The solubility of boric acid at room temperature is about 47 g/l, but the presence of other ions in the solution will reduce its solubility [5]. We determined that for analytical purposes, boric acid concentration should be maintained below 30 g/l.
When rotation rate and dilution factor were optimized, copper calibration experiments were conducted (Fig. 2). These tests were performed in a copper concentration range from 0 to 10 ppm. The responses are linear through a wide range of rotation rates. However, at the highest rpm, the response is strongest, but not as linear as at lower rpm.
The increase in currents at higher rotation rates is expected due to an increase in the supply of reactants to the electrode surface. This should agree with the Levich equation (2), where mass transport limited currents are proportional to the square root of the rotation rate.

IL = (0.620)*n*F*A*D2/3*w1/2*v-1/6* C,

where ILis the Levich current (A), n is the number of moles of electrons transferred in a half reaction, F is the Faraday constant (C/mol), A is the electrode area (cm2), D is the diffusion coefficient (cm2/s), w is the angular rotation rate of the electrode (rad/s), v is the kinematic viscosity (cm2/s), and C is the concentration (mol/cm3).

Fig. 3 shows a linear relationship between the square root of the rotation rate and currents obtained during copper deposition experiments

Similar results were observed when scan rate was varied. Increased scan rate caused shorter deposition time and reduction of copper deposited on the electrode. This was undesirable as the decrease in the copper deposition reduces the sensitivity of the analysis. A linear relationship was observed for the entire range of scan rates tested.

B.Verification of method accuracy

The effects of all components in the bath were validated prior to the final testing of the analytical procedure.

The effects of all components in the bath were validated prior to the final testing of the analytical procedure.

Concentration changes in boric acid and nickel sulfamate have been shown to slightly distort the copper peak, affecting final analytical results. Table I summarizes the data collected in this study. It must be noted that pH changes could negatively affect the final result as well. This effect was observed when the plating solution was diluted by 50% with DI water. The result did not improve when higher dilutions were used.

This interference could not be reduced by altering electrochemical parameters. Generally, the interference significantly increased when any of the main electrochemical settings were altered.
Because the bath must be diluted to avoid crystallization of boric acid, several different diluents were tested. We observed that dilution with pH 4 buffer (close to pH of the plating bath) aids in the elimination of the possible effects of boric acid and sulfamate. Essentially, both of these materials can affect pH values and buffer use or dilution simply cancels those effects.
During our testing, pH 4 buffer was used for bath dilution. The previously noted commercial buffer solution has enough buffering capacity to maintain the pH within ±0.1 units after dilution. Changes in the concentrations of multiple components in the bath had no effect (or only a negligible effect that was within analysis accuracy) on the analytical signal. A desirable dilution of the bath was achieved and boric acid did not precipitate.

Fig. 4 shows voltammograms obtained from the same nickel target bath with boric acid concentrations that are varied by dilution with DI water and with pH 4 buffer. Dilution with pH 4 buffer shows a clear advantage. The data are shown in Table II.

IV. Conclusion

A newly developed electrochemical method provides reproducible results of analysis for copper in nickel plating baths. This method has advantages in comparison to polarographic methods as it does not use a mercury working electrode. During this study, it was demonstrated that the matrix effects of other bath components can be eliminated by dilution with commercial buffer solution. This new analytical method can be easily automated to provide fast online (within 10 minutes) analysis results for copper in nickel electroplating baths.

Michael Pavlov, Mitchell Coffin, Danni Lin, and Eugene Shalyt
ECI Technology
60 Gordon Drive
Totowa, NJ 07512 USA
Ph: 973-773-8686; Fax: 973-773-8797
Email: mpavlov@ecitechnology.com

V. References