Articles

Control of Electroless Nickel Baths

This paper reports the authors’ work on developing methods to analyze in-situ key parameters, including pH, nickel concentration and reducing agent concentration of electroless nickel baths. The purpose is to enable production line automated control of the deposition process. The analytical techniques developed within the scope of this work are discussed and their inclusion into an automated chemical monitoring system described. Thousands of data points have been collected to evaluate the system’ s performance. The corresponding results are presented in the context.

Introduction

As PCB manufactures comply with the requirements of lead-free regulations, alternative finishes such as ENIG (Electroless Nickel/Immersion Gold), immersion tin, and immersion silver have become widely adapted. Among those, ENIG provides a highly solderable surface that does not tarnish nor discolor – ensuring a relatively long storage time as compared to other alternative finishes. In addition, ENIG is known as an effective barrier preventing copper diffusion and maintaining solderability of the PCB pads.

One possible drawback of the ENIG finish is the probable nickel corrosion during immersion gold deposition – a defect commonly referred to as “black nickel” or “black pad” [1]. Black pads typically cause solderability failure and therefore need be avoided. The structure and the phosphorous content of the nickel layer are among the key factors in determining the subsequent formation of black pads. Those factors are, in turn, related to the composition and pH of the bath during electroless nickel deposition. Control of the electroless nickel bath is therefore key to defect-free ENIG processes. Bath properties change as solution ages, through consumption of components as well as creation of byproducts. To be able to control the bath, one must know the properties of the bath, and then make adjustment accordingly. Being able to analyze bath properties is thus the first step towards effective control of baths.

Unlike electroplating in which an external circuit provides the electrons to reduce metal ions into metal deposits, an electroless process must use reducing agents to provide the electrons. The most commonly used reducing agent in an electroless nickel bath is sodium hypophosphite. This paper reports the authors’ approaches to analyze divalent nickel (Ni²⁺) and sodium hypophosphite concentrations as well as pH in the bath.

Analytical Methods and Results

Analyses of pH and nickel concentration were conducted by a Quali-Stream inline chemical monitoring system (ECI Technology, Inc.), Figure 1.

The system samples and analyzes solutions alternately from two production tanks based on pre-set schedules, and the solutions are automatically returned to the original production tanks after analysis. Solution inlets and outlets for multiple tanks are located on the left side panel of the system.

Analysis of nickel concentration is by spectroscopic method, based on Beer’s Law. Incoming light is partially absorbed by the solution under analysis. The higher the nickel concentration, the stronger the absorbance is, resulting in a weaker outgoing optical signal in the corresponding wavelength ranges. The outgoing light is collected by fiber optics and brought to an internal high-performance detector for analysis to ensure sensitivity and accuracy. A calibration curve is built by measuring the absorbance of solutions of known different nickel concentrations (carefully prepared ahead of time), Figure 2.

Absorbance of tank solution is measured and the corresponding nickel content is determined by mapping the absorbance to the calibration curve (an automatic process performed by software, while eliminating contribution from other species). Figure 3 shows the performance test results of the system measuring divalent nickel concentration.

More than 4,000 data points were collected over a 3-day period, with the spectrometer automatically calibrating itself periodically. As can be seen from the figure, while analyzing the same standard solution of 6.0 g/l nickel concentration, the analytical method achieved very high accuracy – with the highest reading during the test period being of 6.094 g/l and the lowest 5.922 g/l. Statistical analysis of this data set further affirmed the high reliability of the method, with relative standard deviation at 0.86%.

Measurement of pH was conducted by a pH meter that has been built into the Quali- Stream analyzer. Figure 4 shows the long-term performance test of the system on measuring pH.

More than 4,000 data points were taken at the same time as the aforementioned nickel concentration test was performed. The pH output reading had been maintained in a very narrow range throughout the 3-day period, with max at 4.727 pH unit, and min at 4.740 pH unit. Statistical analysis of this data set also showed a small relative standard deviation of only 0.06%. The accuracy of the system’s pH measurement was further affirmed by conducting an additional set of test. In this 2nd set of performance test, pH of one buffer solution was measured at several different temperatures. It’s been well documented that pH readings, even for the same solution, changes with the solution’s temperature. The pH vs. temperature results of this work, presented in figure 5 (blue data set), matched very closely with published data (pink data set), affirming the performance of the system.

Analysis of reducing agents was conducted by CVS (Cyclic Voltammetric Stripping) technique, the most widely adapted method to determine organic components concentrations in a copper electroplating bath [2]. The system used in this work to analyze sodium hypophosphite concentration was a Qualilab QL-5 plating bath analyzer (ECI Technology, Inc.). CVS technique applies a cyclic potential onto a platinum working-electrode that is immersed in the working solution (containing copper ions as well as precisely diluted bath sample from the process tank under analysis). The cyclic potential swings between pre-determined positive and negative limits. Copper is deposited onto the working electrode during the negative potential portion of the cycle and then completely stripped away during the positive potential portion of the cycle. The concentrations of additives in the solution affect the rate of copper plating onto the working electrode. When measuring reducing agents, the authors found that the deposition rate of copper in the working solution (note that Cu is the working metal used in the CVS analysis, though the reducing agent concentration in electroless nickel solution is being analyzed) increases monotonically with the increase of reducing agent concentrations.

Figure 6 illustrates the effect of hypophosphite concentration on voltammogram (I-V diagram monitoring the progress of CVS).

Four carefully as-prepared test solutions of different hypophosphite concentrations gave distinct I-V curves during voltage scan. The enclosed areas under the curves, referred to as ‘peak area’ or Ar, correspond to the integration of current against the applied voltage and are therefore proportional to how fast plating/stripping occurred. A calibration curve, figure 7, plotting peak area vs. hypophosphite concentration can thus be built to compare with the peak area of an unknown solution and accordingly determine the hypophosphite concentration of the unknown solution.

Long-term statistics showed that using CVS to measure hypophosphite concentration could achieve better than 4% relative accuracy and 3.5% repeatability.

Summary and Conclusion

Methods for analyzing pH and nickel concentration in electroless nickel baths have been developed. Engineering efforts based on instrumentation know-how’s have integrated the developed methods into one automated system, enabling PCB production environments to analyze tank solutions in real time. The corresponding long-term results demonstrated both high accuracy and repeatability of the measurements. Closed-loop dosing based on the analytical results can ensure the stability of the electroless nickel bath and give production engineers full control of their parts quality. Additionally, reducing agent in the electroless nickel solution can be measured by CVS technique using a separate lab analyzer, although in this case sampling from the tank needs be performed manually.

The authors have established similar analytical approaches to analyze palladium activation solution, electroless copper solution and electroless cobalt solution, achieving comparable accuracies. Discussions of some of those topics have been published elsewhere [3].

Authored by:

Eugene Shalyt, Semyon Aleynik, Michael Pavlov, Peter Bratin, Chenting Lin
ECI Technology, Totowa, New Jersey, USA

Reference

1. George Milad and Jim Martin, “Electroless Nickel/Immersion Gold, Solderability and Solder Joint Reliability as Functions of Process Control,” CircuiTree, October 2000.

2. D. Tench and C. Ogden, “A New Voltammetric Stripping Method Applied to the Determination of the Brightener Concentration in Copper Pyrophosphate Plating Baths,” J. Electrochem. Soc. n. 125, p. 194 (1978).

3. P. Bratin, et. Al., “Development of Chemical Metrology for Electroless Deposition Baths,” ISTC Proceedings, March 2006.

Detection of Accelerator Breakdown Products in Copper Plating Baths

The mercaptopropylsulfonic acid (MPS) breakdown product of the bis(sodiumsulfopropyl)disulfide (SPS) additive used in acid copper plating baths accelerates copper electrodeposition and can be detected by cyclic voltammetric stripping (CVS) analysis. In the presence of oxygen, MPS decomposes rapidly in acid copper sulfate baths so that the CVS stripping peak area (Ar) decreases on successive cycles. The slope of a plot of Ar vs. log of the CVS cycle number (or time) provides a measure of the initial MPS concentration.

INTRODUCTION & BACKGROUND

Acid copper sulfate baths are employed in the “Damascene” process (1) to electrodeposit Cu within fine trenches and vias in dielectric material on semiconductor chips. Two organic additives are required to provide bottom-up filling of the Damascene features. The “suppressor” additive, which is typically high-molecular-weight polyalkene glycol (e.g., PEG), adsorbs strongly on the Cu cathode surface, in the presence of chloride ion, to form a film that sharply increases the overpotential for Cu deposition. The “anti-suppressor” or “accelerator” additive counters the suppressive effect of the suppressor to provide the accelerated deposition within trenches and vias needed for bottom up filling.

Close organic additive control needed for Damascene plating is provided by Cyclic Voltammetric Stripping (CVS) analysis, which involves alternate plating and stripping of Cu at a Pt rotating disk electrode. The additives are detected from the effect that they exert on the electrodeposition rate measured via the Cu stripping peak area (Ar). The accelerator concentration is typically determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by Bratin (2). During Damascene Cu plating, however, additive species break down into breakdown products that may interfere with the electrodeposition process. These breakdown products need to be controlled to ensure that high quality Damascene deposits are obtained. A method for detecting suppressor breakdown products was described in our earlier publications (3,4).

This paper describes a CVS method for detecting breakdown products of accelerator additives that are widely used for Damascene copper plating. Results are presented for the 3-mercaptopropylsulfonic acid (MPS) species, which is a breakdown product of the bis(sodiumsulfopropyl)disulfide (SPS) additive (5).

EXPERIMENTAL DETAILS

CVS measurements were made using a Qualilab QL-10® plating bath analyzer (ECI Technology, Inc.) with a polyethylene beaker cell containing 50 mL of solution (open to the atmosphere). For some experiments to verify that oxygen plays a role in MSA decomposition, the cell was partially sealed and deaerated via nitrogen bubbling (stopped during the CVS measurements). The supporting electrolyte contained 40 g/L Cu (added as CuSO₄ . 5 H₂O), 10 g/L H₂SO₄, 50 ppm chloride ion, and 2.0 g/L of 5000 molecular weight (MW) polyethylene glycol (Aldrich). The SPS and MPS materials were purchased from Raschig Chemical (Germany). The working electrode was a Pt rotating disk (4 mm diameter, 2500 rpm). Unless otherwise noted, the potential was scanned at 100 mV/s between a positive limit of +1.575 V and a negative limit of either -0.225 and -0.325 V vs. SSCE-M (standard silver-silver chloride electrode modified by replacing the solution with a saturated AgCl solution also containing 0.1 M KCl and 10 volume% sulfuric acid). The counter electrode was usually a stainless steel rod (6 mm diameter). During CVS measurements, the solution temperature was controlled at 24°C within +0.5°C. Specimens of MPS and SPS were injected into the cell at the positive limit in the CVS cycle. The effects of the commercial Viaform™ (Enthone Inc.) and Ultrafill™ (Shipley, Inc.) additives (at normal concentrations) were also investigated.

RESULTS & DISCUSSION

MPS Analysis Method

Figure 1 shows that Ar measured on the first CVS cycle after addition of MPS to the acid copper electrolyte varies monotonically with the MPS concentration. However, a simple Ar measurement cannot be used for MPS analysis since organic additives and other species present in plating baths also affect Ar values.

Figure 2 shows that Ar remains constant for acid copper baths containing only SPS, but decreases monotonically with potential cycling in the presence of the MPS breakdown product. Both compounds tend to accelerate the copper deposition rate but the accelerating effect of MPS is stronger and much more time-dependent. After addition of the Viaform™ and Ultrafill™ accelerator additives at the normal concentrations, the Ar values were also constant (not shown) but were smaller (1.5 and 1.4, respectively) than the value of 2.2 observed for SPS at 1.0 ppm concentration. For the potential scan rate and limits used, a CVS cycle corresponded to 38 seconds and copper deposition occurred over a time frame of about 6 seconds. Since the MPS and SPS specimens were injected at the positive potential limit, the copper deposition rate measurement for cycle number 1 began after about 16 seconds and ended at about 22 seconds (onset for copper deposition is about 0.0 V vs SSCE-M). It is clear from these data that MPS decomposes rapidly when its concentration is high, and much more slowly as its concentration decreases.

Figures 3 and 4 illustrate the effects of delaying CVS cycling (after addition of the MPS sample) and interrupting CVS cycling.

Note that a relatively negative potential scan limit (-0.325 V) was used to enhance measurement sensitivity. It is evident that Ar continues to decrease unabated even when no voltage is applied to the working and counter electrodes, indicating that MPS decomposes chemically in the bath. When the beginning of the curve for a 3-minute delay was shifted to the 5th CVS cycle (corresponding to 3.2 minutes), the 3-minute delay curve practically coincided with the curve for which cycling had been interrupted for 2 cycles (Fig. 4). Potential cycling appears to actually slow MPS decomposition, possibly because of SPS formation.

Figure 5 illustrates that an exposed copper counter electrode tends to increase the rate of MPS decomposition compared to a stainless steel electrode (or a copper electrode partitioned from the electrolyte via a double-junction glass frit). This effect is relatively small and may result from adsorption of MPS on the relatively large copper counter electrode.

Figure 6 shows that the decrease in Ar after MPS additions is exponential since a plot of Ar vs. Log (CVS cycle number) is linear.

The theoretical basis for this empirical relationship would be difficult to ascertain from the present data since the measured copper deposition rate is a composite for a range of potentials, and both electrochemical and chemical processes may be involved in the decomposition process. Nonetheless, as shown in Fig. 7, the slope of such plots provides a measure of the initial MPS concentration.

Figure 10 illustrates the effect of negative potential scan limit on plots of Ar vs. Log (CVS cycle number). Linear plots are obtained in all cases, although the slopes vary. Figure 11 shows the dependence of the slope on the negative potential limit. Obviously, the negative potential limit must be held constant for the MPS analysis.

Previous work by Healy et al. (6) has shown that SPS and MPS undergo complicated chemical and electrochemical reactions in acid copper plating baths. The SPS species, also known as 4,5-dithiaoctane-1,8-disulphonic acid, is slowly oxidized chemically in the bath but only in the presence of copper metal, although the oxidation rate is accelerated in the presence of oxygen. A complex involving Cu(I) and MPS, e.g., Cu(I)SCH₂CH₂CH₂SO₃H, apparently plays a key role as an intermediate in electrochemical reduction of SPS and oxidation of both SPS and MPS in acid copper baths. Nonetheless, Healy et al. conclude that oxidation of MPS via this intermediate does not lead to regeneration of the SPS species. However, Moffat et al. (5) provide convincing evidence that decomposition of MPS eventually results in formation of SPS in acid copper baths.

Figure 12 shows that the CVS stripping peak area (Ar) for a 1.0 ppm MPS acid copper electrolyte decays (after about two days) to a constant value corresponding to that for a 1.0 ppm SPS electrolyte.

Since the concentration of the two species based on weight was the same (1.0 ppm), the molar concentration of the MPS electrolyte was initially double that of the SPS electrolyte. The equivalent Ar values observed for the aged MPS electrolyte and the fresh SPS electrolyte indicate that the MPS dimerized to SPS, resulting in the same molar concentration of SPS in both solutions. Thus, our results support the conclusion of Moffat et al. (5) that MPS dimerizes to form SPS in acid copper baths. Our results also indicate that this process is reversible (under some conditions) since the initial Ar value for the MPS solution aged for one day was somewhat greater than the final Ar value from the previous day (fresh MPS solution).

Oxygen also plays a role in the decomposition of MPS in acid copper baths since partial deaeration of the solution was found to significantly reduce the decomposition rate (slow the decrease in Ar value with time). On the other hand, stirring of the solution had little effect, indicating that mass transport is not an important factor. Likewise, removal of the Pt rotating disk electrode from the solution had no effect. Future studies will determine the effects of acidity and copper ion concentration. The goal of this work is to provide metrology that helps tool manufacturers, chemical suppliers, and users to better control acid copper plating processes.

CONCLUSIONS

The mercaptopropylsulfonic acid (MPS) breakdown product of the bis(sodiumsulfopropyl)disulfide (SPS) additive used in acid copper plating baths can be detected by cyclic voltammetric stripping (CVS) analysis. Decomposition of MPS in acid copper baths apparently involves dimerization to SPS, which is accelerated in the presence of oxygen.

Authored By

M. Pavlov, E. Shalyt, P. Bratin and D. M. Tench
ECI Technology, Inc.
60 Gordon Drive, Totowa, NJ 07512

REFERENCES

1. P. C. Andricacos, Electrochem. Soc. Interface, p. 32, Spring 1999.
2. P. Bratin, Proc. AES Analytical Methods Symp., Chicago, IL (March 1985)
3. P. Bratin, G. Chalyt, A. Kogan, M. Pavlov, J. Perpich and M. Tench, “Detection of Suppressor Breakdown Contaminants in Copper Plating Baths”, 203rd ECS Meeting, Paris, France (Apr. 27 – May 2, 2003)
4. M. Pavlov, E. Shalyt and P. Bratin, Solid State Tech. 46(3), 57 (2003)
5. T. P. Moffat, B. Baker, D. Wheeler and D. Josell, Electrochem. Solid State Lett. 6(4), C59 (2003)
6. J. P. Healy, D. Pletcher and M. Goodenough, J. Electroanal. Chem. 338, 167 (1992)

Control of Tin/Lead Solutions for Electrodeposition of Bumps

Ingredients of commercial SolderOn SC and SolderOn BP chemistry as well as samples of aged solutions were provided by Shipley Company. Reagents used were from Aldrich.

Abstract

Typical bumping process includes formation of bumps through UBM copper electrodeposition, followed by deposition of tin/lead coating. Quality control of the tin or tin/lead electroplating solutions is critical to meet demands on the properties of the plated deposit, cost, and environmental issues. Even though lead is being phased-out as enemy of environment and many replacements are being tested, it is still widely used. As all electrochemical processes, tin/lead plating is a dynamic system; unless precisely controlled, concentration of consumable components and breakdown products soon gets outside of acceptable range resulting in manufacturing problems and eventually rejects.

Copper Electrodeposition process for bumping is similar and somewhat less challenging than Damascene copper electrodeposition process used for the interconnects. The control of the Damascene copper process has been a focus of research by all major semiconductor companies in recent years. We have previously demonstrated an on-line controller for complete analysis of copper electroplating solutions^1,2. This paper will focus on automated on-line determination of tin/lead coatings, by reviewing analysis of up to 6 components in a commercial tin/lead electrodeposition bath. While the benchtop analysis of tin/lead solutions has been utilized for a number of years, many obstacles exist when transitioning such procedures into automated on-line system due to steps such as gravimetric or extraction procedures.

While determination of metals and acid is reasonably common, measurement of proprietary organic additives remains a challenging task. Organic additives are the key ingredients in the plating solution that influence the properties and quality of the deposits. Cyclic Voltammetric Stripping (CVS) method is an established analytical technique that has long been demonstrated to be applicable to analysis of tin and tin/lead additives in various plating solutions (fluoroborate, sulfate, MSA, and PSA). Detailed description was published in an earlier paper³. This paper will describe an on-line analysis of all components generally used in a tin/lead bumping process. The total automated analysis takes about 40-60 minutes with accuracy better than 10% and reproducibility better than 5%.

Experimental

Experiments were performed using Qualilab QL-10® CVS/Titration benchtop analyzer from ECI technology, Qualiline QLC-7500® on-line analyzer from ECI technology, S2000 Spectrometer from Ocean Optics. Titration was performed using pH and ORP combination electrodes from Thermo Orion. Analysis of three inorganic components (Methanesulfonic acid – H-MSA, Tin (II), and Lead) and three organic components (Primary additive, Secondary additive, and anti-oxidant) was tested and validated using both, benchtop and on-line, analyzers.

A. Analysis of Inorganic components

1. Methanesulfonic Acid (H-MSA)

As for many acids, an acid-base titration of H-MSA with NaOH is a recommended procedure. Manual analysis typically employs a color indicator; use of pH electrode to follow the titration allows for easier automation of this analysis, as well as improved accuracy and reproducibility.

Regardless whether color indicator or pH electrode is employed, titration with NaOH measures “total” acid, which is a sum of tin and H-MSA concentrations, because tin hydroxide formation is at pH values too close to the acid-base titration end-point, making it difficult to separate these two species. To measure “Free” H-MSA, one recommended procedure³ is to add masking agent, such as magnesium EDTA, which ties tin from forming hydroxide or shifts hydroxide formation away from the acid-base endpoint. This allows measurement of free H-MSA. Figure 1 shows titration curve for the free acid.

2. Determination of Sn (II)

There are two approaches that we have tested for analysis of tin (II) in the tin/lead plating solutions: (a) iodometric titration in acidic solution using starch (manual) or ORP electrode (on-line) as end-point indicators and (b) acid-base titration with NaOH with and without Mg-EDTA masking agent and subsequent extraction of tin from the two experiments.

The acid-base titration with NaOH requires two experiments; first titration of the solution under test with NaOH, where endpoint is proportional to the sum of concentrations of tin (II) and free acid (“total acid value”), and second experiment where Mg-EDTA is added prior to titration with NaOH, resulting end-point volume being proportional to concentration of free acid only. The difference of the two end volumes then yields the concentration of tin (II). This approach yields results with accuracy of 10% and reproducibility of 5%. This method, however, looses accuracy and precision with “high lead/ low tin” formulations, when tin concentration falls below 10 g/l. At these low tin levels the difference between total and free acid is small, and falls within the experimental error for determination of end-point.

The industry recommended determination of Sn (II) is based on its oxidation by iodine in acidic media with starch as color indicator for detection of equivalence point. This method is employed with most benchtop units. Our on-line approach utilizes ORP electrode to automate the equivalence point detection without need for any color indicator. Figure 2 shows curves obtained with this method.

Experimental data indicates that “iodometric” titration yields better accuracy and reproducibility than set of acid-base titrations, while the latter has an advantage of simpler analysis, less electrodes, less reagents, improved reliability, and thus lower cost and cost of ownership.

3. Determination of Pb

Two approaches are described in literature:

• Determination by AAS
• Oxidation of Sn(II) followed by titration with EDTA

Both procedures contain multiple steps, which are very difficult, if not impossible, to automate.

The technique implemented on the on-line unit is a second step in the analysis of total acid, which is the approach used for analysis of Copper in the Damascene systems. After titration of the solution under test with NaOH to determine total acid (acid + tin), excess of EDTA is added. EDTA reacts with lead releasing additional acid proportional to amount of lead in tested solution, which is then further titrated by NaOH using pH electrode as an indicator. The second equivalence point is then proportional to concentration of lead. Resulting curve is shown in Figure 3.

For higher tin concentrations, complete set of 4 analyses (free acid, total acid, tin, and lead) can be determined using set of two titrations with but single electrode (pH) and 3 reagents: Magnesium EDTA, Sodium EDTA, and NaOH. Needless to say, pH electrode is the most robust ion selective electrode, thus yielding highly reliable methods.

B. Analysis of Organic components (additives)

1. Determination of Primary Additive

Primary additive is the key component of the Tin/Lead electrodeposition solution, responsible for grain size and other properties of deposit. It is Primary suppressor and is typically monitored by CVS (Cyclic Voltammetric Stripping) technique. In this technique, activity of the primary additive is measured from its effect on the plating rate of the metal, in this case tin or tin/lead. CVS method employed for the Primary additive is Dilution Titration, where the Supporting electrolyte is “titrated” with solution under test, causing suppression (reduction of plating rate). Titration curve of unknown solution is then compared to that of calibration standard. Details of this method as well as its application to the analysis of the Primary Additive in Tin/Lead systems have been previously described^3,5. Figure 4 shows curves obtained for 100mL/L of the Primary Additive with different levels of the Secondary Additive component.

There is a wide variety of commercially available Sn/Pb formulations. All additives tested to date display different degrees of the same suppression behavior, and can therefore be monitored using the same technique. The very same technique is used for analysis of suppressors in the copper electroplating processes.

2. Determination of Secondary Additive

Secondary Additive component provides “Secondary” suppression. The suppression effect of this component is much weaker than that of Primary additive. At standard conditions, effect of Secondary component can be seen, but is not quantitatively analyzable due to variation of other components in the solution, even if they are within process spec.

Secondary additive contains dye and most chemical suppliers recommend analysis using a photometric method with extraction, in order to avoid interferences from other ingredients. Extraction technique can be tricky, and is hardly viable for automation.

We have equipped the analyzer with spectrophotometric capability and optimized experimental conditions in order to provide on-line photometric analysis for the Secondary additive component without need for extraction. Figure 5 shows spectral curves obtained for low, target, and high levels of Secondary additive, as well as effect (or better non-effect) of other solution components. Calibration curve is shown in Figure 6.

Results with standard and aged solutions indicate that on-line technique without extraction gives results that are consistently 5-10% higher than benchtop extraction technique.

3. Determination of “Active” Secondary Additive

During the electrodeposition additive undergoes multiple transformations, forming a variety of breakdown products. The photometric methods measure analytical concentration of a compound or a group of compounds that absorb in the same frequency region, thus they might or might not include effect of the breakdown products. Since Secondary additive affects plating through its suppression of the electroplating process rather than color, analysis of secondary by photometry might not be the most appropriate analytical method. CVS technique, on the other hand, is an electroanalytical procedure, which monitors concentration of “active” Secondary additive, or its activity.

As mentioned above, the suppression effect of Secondary Additive is quite weak at standard parameters. By optimization of parameters and use of Pulse (CPVS) technique, sensitivity of analysis can be significantly improved. Figure 7 shows response curve for the Secondary Additive utilizing the optimized conditions.

Results obtained for various chemistries indicate that in aged solutions, the activity of the Secondary Additive as determined by CVS is about 20% lower than total Secondary Additive concentration as obtained with photometric methods, with activity decreasing with increasing bath age.

During these study, we also compared results of Photometric and Electrochemical analysis obtained from standard solutions, which did not go through electro-plating process. The results obtained for Secondary additive were the same, which confirms the ability of electrochemical (CVS) technique to monitor its “active” concentration. This approach, when two independent techniques are used for analysis of the plating bath, allows to characterize the conditions of plating solutions and its transformations during electroplating process.

4. Determination of Anti-oxidant

Recommended analysis of Antioxidant is performed by photometric analysis after extraction and precipitation. Needless to say, such procedures, similar to the previously mentioned procedure for the Secondary Additive, are very difficult, if not impossible, to automate. We have optimized the analytical methods to allow use of spectrophotometric measurement without need for extraction and precipitation steps, thus allowing us to automate the analysis. Figure 8 shows spectral responses for low, target, and high concentrations of anti-oxidant and effect of other solution components. Figure 9 shows linearity of the response.

C. Validation of Results

All analytical procedures described above were validated by:

• repeated analysis of standard solutions with various ratios of components to establish parameters, reproducibility and accuracy
• repeated analysis of aged sample to establish reproducibility
• recovery of known dilution of aged sample to establish accuracy and reproducibility
• recovery of known spike with component of interest into aged sample to establish that we are measuring component of interest, as well as accuracy
• effect of spike with other components in the matrix into aged sample to evaluate interference from these components
• correlation of results to recommended benchtop methods

The results obtained for standard solutions and recovery of spike and dilution were within 10% of expected values, as were correlations with recommended benchtop methods.

D. On-Line Analysis

All of the above mentioned analyses were tested individually in the on-line analyzer. Analyzer employs 2 cells; one cell is used for inorganic titrations and second cell for analysis of organic additives by CVS and UV-VIS Photometry. Standard qualification included 10 data points for each of 3 compositions: Target, Low end of process window, and High end of process window. Accuracy in all experiments was within 10% and relative standard deviation within 5%. Cycle time for complete analysis is 40 min (without CVS measurement for activity of Secondary Additive) or 60 min with activity of Secondary Additive. Picture of the online analyzer is shown in Figure 10.

Authored By

P. Bratin, E. Shalyt, M. Pavlov, J. Berkmans
ECI Technology, 60 Gordon Drive, Totowa, NJ 07512

References

1. P. Bratin, E. Shalyt, M. Pavlov. “Automated On-line Control of Plating Bath Additives Increases Wafer Yield”, Semiconductor Fabtech –14th Edition, pp. 205-207

2. M. Pavlov, E. Shalyt, P. Bratin. “A New Generation of CVS Monitors Cu Damascene Plating Bath”, Solid State Technology, March 2003, vol. 46, N 3, pp. 57-60

3. P. Bratin, E. Shalyt, M. Pavlov, and R.Gluzman “Control of Tin and Tin/Lead Electroplating Solutions” Proc. IPC’96 , pp S16 1-14

4. Atotech Technical Spec Data Sheets for Sulfotech-M process.

5. P. Bratin “New developments in use of CVS for analysis of plating solutions”, Proc. AES Analytical Methods Symposium, March 1985

A New Generation of CVS Monitors Cu Damascene Plating Baths

Overview

With damascene processing gaining acceptance in high-volume IC manufacturing, reliable analysis of copper plating baths is increasingly important. Cyclic voltammetric stripping has shown that it is an accurate and precise method for the control of the organic additives. The latest generation adds reliability, simplicity, speed, and acceptable cost of ownership.

In copper electro-deposition processing, it is critical to achieve uniformity of the copper film across the wafer surface, the desired mechanical film properties, and super-filling characteristics in trenches and vias. These important properties are typically achieved using multicomponent plating solutions that include organic and inorganic components. There is a need for accurate, sensitive, easy-to-use, and cost-effective on-line monitoring metrology.

Copper electroplating baths are normally formulated using highly stable electrolytes containing copper sulfate and sulfuric acid. Copper concentration in these electrolytes is 14 to 60 g/1 and sulfuric acid 1 to 240 g/1. Other components added into the bath are present in relatively small amounts. These components are organic additives and chloride ions. The organic additives, depending on the concentration and chemical composition, affect the properties of electro-deposited copper, including uniformity, hardness, ductility, and tensile strength. The control of the additives is known to be critical in maintaining the desired properties of the deposits.

Organic additives added to copper electroplating baths fall into three major categories:

• Accelerators are compounds that contain pendant sulfur atoms that locally accelerate deposition where they are adsorbed.
• Suppressors are polymers, such as polyethylene glycols (PEGs), that have the ability to form a current-suppressing film on the entire wafer surface, especially in the presence of chloride ions. (Chloride enhances the suppression.)
• Levelers are secondary suppressors and work only on protruding surfaces where mass transfer mechanisms are most efficient.

Organic components are consumed during an electroplating process at various rates, thus requiring individual control. Accelerators, for example, are consumed faster than the other organic species. There are other interactions between the organic additives and inorganic compounds during the electroplating process that cause decomposition and modification of initial organic compounds. These by-products can affect the deposition process, sometimes significantly more than the initial organic compounds. This means that monitoring the pure analytical concentrations of the original organic additives is insufficient to control copper electroplating bath performance. The presence of multiple components of the additives and breakdown products make it necessary to monitor total effective concentration (“activity”) of individual bath components, which affect suppression, acceleration, and leveling properties. At the same time, the accumulation of breakdown products might increase to a level where they can seriously affect the plating process. Monitoring of bath aging is therefore another important aspect in the analysis and control of copper electroplating solutions.

The CVS method

Among the wide range of analytical approaches that have been used to monitor the concentrations of organic additives in electroplating solutions, only cyclic voltammetric stripping (CVS) has demonstrated the ability to reliably monitor the activity of a wide range of components in plating baths. In this method, the classic three-electrode cell is used, where the main indicator is a platinum (Pt) rotating-disc electrode [1]. CVS involves cycling the potential of the rotating-disk electrode so that metal is alternately plated and stripped at its surface. Organic additives are detected by CVS from the effect that they exert on the electro-deposition rate measured via the metal stripping peak area.

This approach has become the most popular method for controlling damascene copper deposition processes. The initial method was to adapt bench-top analyzers that have been used for more than 20 years by PC-board manufacturers. IC manufacturers soon discovered that the far more critical requirements of their process often created a need for non-stop on-line measurement of the plating solution.

On-line plating bath analysis

As electroplated copper deposition has become an integral part of the latest semiconductor manufacturing process, CVS chemical monitoring systems have established themselves as well, forming a closed-loop system with the plating tools.

The first generation of CVS-based analyzers was developed in 1997 and controlled only two components of organic additives. These systems were used in several research locations and produced valuable results for understanding and improving the plating process.

One of the lessons from these early monitoring systems was that both organic and inorganic components of plating solutions needed to be monitored, and organic additives need to be significantly modified from their original formulations. More organic components were introduced into the bath and concentrations of inorganic components were modified.

A second-generation analyzer — capable of controlling six components of a plating solution — was introduced in 1999 and became a popular system for analysis of copper electroplating solutions for high-volume manufacturing by virtually every major semiconductor facility.

Today, we have a new, third-generation CVS-based monitoring system that has been designed to handle the challenges of 300mm wafer processing. This system was developed to answer the challenge, based on experiences collected from more than 100 analyzers running worldwide at various semiconductor facilities. The major differences of this analyzer from the previous design include its dual-cell design and significant advance in its software, giving improvements in accuracy, precision, reliability, cost of ownership, and time of analysis.

Today’s CVS

With the dual-cell configuration, organic additives are analyzed in one cell and inorganic analysis is performed in the second cell. This significantly enhances analytical capabilities. The biggest benefit of dual-cell design is the reduction in analysis time from 100 min using a previous single cell configuration to <40 min/cycle for all six components in the dual-cell configuration. Faster analysis is achieved by simultaneous analyses and by use of new algorithms designed to reduce total time of analysis, chemical consumption, and waste.

The overall benefits of the shorter time to first analysis yields a decreased time delay in bringing a plating tool up after prolonged downtime, a decrease in cross-contamination between analyses, allowing for reduction in rinsing and cell preparation, and significant reduction in conversion from one chemistry to another, including method development if required.

Figure 3. CVS endpoint volume for standard and aged production solutions using two parameter sets (a and b).

Improved accuracy and precision is another important aspect of dual-cell design (see table). Single-cell analyzers have successfully passed all specifications set by users and tool manufacturers, but these specifications are continuously changing due to demands on plating processes (i.e., the need to fill finer features on larger wafer areas). Today, these demands are being achieved through decreased chemical interference and thus decreased effect on electrodes during an analysis cycle.

Since organic additives and inorganic components are analyzed simultaneously and in separate cells, pre-conditioning and cleaning of various electrodes before analysis is significantly reduced. This is very beneficial for stability of analytical results during a long process run, as well as for the reliability of the analyzer. Better stability of electrodes results in less frequent preventive maintenance procedures and thus lower cost of ownership.

Even with the increased performance of this generation of CVS, the hardware is more straightforward, with improved solution delivery and electronics. Improvements include such critical parts as syringe modules, valves, electrochemical circuitry, and electrodes. Operation of the system is also easier via new user-friendly software. Changes include new analytical procedures for accelerator and suppressor, new rinsing and conditioning procedures, new drivers, and new analysis parameters. A new combined procedure for suppressor and accelerator completely eliminates the need to replace solutions, as well as to rinse and condition electrodes between analyses, producing a drastic reduction in consumption of reagents and waste generation, as well as analysis time and maintenance frequency (Fig. 1).

Suppressor breakdown products

To detect suppressor breakdown products with this new generation of CVS, we used CVS dilution titration analyses of a copper-plating bath using two parameter sets. The volume needed to reach endpoint using a first parameter set with a -0.4V limit yields a direct measure of the active suppressor component concentration, since the suppressor breakdown contaminants are not effective at suppressing the copper deposition rate with these parameters. The volume needed to reach endpoint using a second parameter set with a -0.225V limit yields a measure of the concentration of the active suppressor plus the suppressor breakdown products. The ratio of the two endpoint volumes provides a relative measure of the suppressor degradation.

To confirm the presence and behavior of the breakdown products, we performed CVS dilution titrations with -0.400V and -0.225V limits for solutions containing 1.0 g/L of PEG polymer comprising mixtures of the 900 and 10,000 molecular weight (MW) species. Figure 2 shows plots of the CVS endpoint volumes for the two potential limits as a function of the weight fraction of the 900 MW species. For the -0.400V limit, the required endpoint volume increases (activity decreases) as the fraction of the 900 MW species increases, since the concentration of the 10,000 MW species is decreasing and the 900 MW species is relatively inactive as a suppressor for this potential limit. For the -0.225V limit, the endpoint volume exhibits a slight decrease as the weight fraction of the 900 MW species increases, since both PEG species are effective suppressors at this potential limit and the overall PEG molar concentration is increasing.

In addition, we performed CVS dilution titrations for solutions containing fresh standard and aged production solution, using both parameter sets (Fig. 3). Results show that the response with parameter set 1 was the same for both solutions, while with parameter set 2, the fresh solution showed higher activity (less volume needed to suppress VMS).

We also looked at how decreasing molecular weight of PEGs changes their CVS response and therefore their suppressing activity (Fig. 4). As a bath ages, more PEGs are cleaved into shorter (lower MW) fractions, thus affecting their performance.

Recently, possible mechanisms for functioning and breakdown of accelerators have been presented at the Electrochemical Society meetings. We plan to publish our work describing analysis of breakdown products of accelerators and levelers in the near future.

Authored by

Michael Pavlov, Eugene Shalyt, Peter Bratin
ECI Technology, Totowa, New Jersey

Reference

1. P. Bratin, et al., Semicondutor Fabtech, 14th Edition, Summer 2001.

Michael Pavlov received his MS in electrochemistry from the Moscow Institute of Steel and Alloys. He is product manager at ECI Technology, 60 Gordon Drive, Totowa, NJ 07512; ph 973/890-1114, fax 973/890-1118, mpavlov@ecitechnology.com.

Eugene Shalyt received his PhD in electrochemistry from Mendeleev University of Chemical Technology in Moscow. He is R&D manager at ECI Technology.

Peter Bratin received his PhD in electroanalytical chemistry from CUNY in New York. He is VP at ECI Technology.

Control of Damascene Copper Processes By Cyclic Voltammetric Stripping

Use of electroplated copper for onchip metallization in ultra-largescale integrated circuits (ULSI) devices is gaining momentum because of the low cost and high throughput of the process. Electroplated lines and trenches with submicron dimensions, however, are strongly affected by changes in the composition of the plating solution, thereby creating a high demand for control techniques. The most dynamic ingredients of electroplating solutions are organic additives. Even a small imbalance between components of the additive system can cause various defects in the filling of the trenches and properties of electroplated copper. On-line monitoring and control of these additive components is therefore desirable. Cyclic voltammetric stripping (CVS) analysis has long been used for just such a purpose in the manufacturing of printed circuits.

Numerous chemical and electrochemical processes (i.e. thermal decomposition, cathodic reduction, occlusion in the deposit, anodic oxidation, drag-out) contribute to additive consumption, as does timedependent chemical decomposition in the bulk solution. Additive dosing is often done based on time or amp hours. Some companies have developed proprietary algorithms that predict additive consumption. However, even the best prediction algorithm requires an analytical tool to fine-tune the algorithm’s coefficients, and it cannot predict changes due to equipment failure and human errors. There is a variety of analytical equipment for monitoring the analytical concentration of plating additives. However, only CVS can monitor activity of individual components in a whole range of proprietary organic additives. Over the years, CVS has become a standard for monitoring these additives in copper plating solutions in the printed circuit industry, and is currently being tested by the majority of companies involved with Damascene processes.

With the CVS technique, the potential of the inert electrode is cycled at a constant rate in the electroplating bath, so that a small amount of Copper is alternately deposited on the electrode surface and stripped off by anodic dissolution. The measured area under the stripping peak is proportional to the plating rate, which strongly depends on active concentration of plating additives and their components (Fig. 1).

A modification of the CVS technique, cyclic step voltammetric stripping (CSVS) employs a series of potential steps, instead of the linear sweep, to measure the effect of additives. CVS and CSVS scans, combined with proprietary analytical techniques, allow independent determination of up to three components (brightener, carrier and leveler) of the additives, even in cases of premixed additive system (Figs. 2-3).

Additional applications of the CVS technique include monitoring of plating bath ageing. While most PWB manufacturers use bench-top analyzers, the requirements of the semiconductor industry are much more demanding. The small volume of plating solution creates very dynamic changes in chemical component activity that, when combined with a narrow operating window, generates demand for continuous on-line, closed-loop analytical systems. Other requirements adding to the value of on-line units are: (1) high frequency of analysis; (2) sample size limitation; (3) clean room requirements; and (4) automation concerns.

Demand Has Caused Changes

The demands of the semiconductor industry have led to the recent redesign of the proprietary on-line acid copper bath controller* to meet the particular requirements of the semiconductor industry (Fig. 4).

The unit, supported by all major additive suppliers, samples up to four plating tanks and standard solutions. It incorporates analysis for brightener, leveler, carrier and contamination level, and it replenishes the plating solutions as needed. The accuracy and reproducibility of on-line analysis are significantly better than lab instruments because the on-line unit includes temperature control, automatic calibration, reproducible conditioning and it eliminates human variations. Table 1 summarizes results of online analysis of several Damascene plating solutions at high, middle and low levels of additive specifications.

Results indicate very good accuracy and reproducibility of analysis for all components. Figure 5 illustrates the history of one plating solution followed for 160 measurements.

Before measurements #79 and #130, the customer performed carbon treatment procedures. In both cases, treatment effectively removed brightener, but left intact most parts of carrier the component.

Early Warning

Under ideal conditions, Damascene plating is performed at steady-state conditions, and readings in production solution are almost as stable as in standard solution. However, an online analyzer provides an early indication of process malfunctioning– e.g., dosing pump failure (Fig. 6).

To test response of CVS to changes of additive levels, a series of additive spikes to a production solution was performed and measured. Figure 7 shows the results obtained for these spikes.

Because of the extremely high cost of wafers, a significant amount of work is being performed to correlate CVS results to the performance of the plating baths with different levels of additives and inorganic components. Although organic additives present the main challenge in controlling plating solution, one needs to monitor inorganic components as well. Therefore, a new generation of CVS units is capable of monitoring both organic and inorganic components (copper, acid and chloride). All . components are analyzed by potentiOmetric titration using proprietary procedures. Table 2 summarizes the results of analysis for standard solution.

At the preparation time of this paper, the analyzer was tested at 10 facilities for an overall period of seven years, including four facilities performing Damascene plating for a 12-month period. Testing included several 5,000-wafer marathons. Use of the analyzer allowed manufactures to keep the process in a very tight window, resulting in a high yield of wafers. Results of the analyzer are in excellent correlation with results of bench-top analyzers. Results of longterm testing of the analyzer at customer facilities are now available.

Authored by

Peter Bratin, Eugene Shalyt & Michael Pavlov

Evaluating Finishes Using SERA

Over the years, the hot air solder leveling (HASL) process has been established as a primary surface finish for circuit boards. It still remains the most solderable, and arguably the most durable, surface finish. Recent developments in PCB manufacturing have highlighted some problems with these finishes, creating an urgent

Solder has been used for a long time as the preferred coating to protect the solderability of circuit boards and components, and HASL has become a favorite process for the application of solder to PCBs. In general, it has been a reliable coating process for retaining the solderability of printed circuits; however, it is far from being a trouble-free process.

For years, the industry has been searching for a viable method to evaluate the solderability of solder finishes. Tench et al. has evaluated the use of electroanalytical techniques for this purpose, and proposed the technique known as SERA for the evaluation of tin and solder finishes.^1,2

The SERA technique involves isolation of a surface pad or a plated through-hole using a rubber gasket with an accurately defined area. Then the hole, or covered surface pad, is filled with borate electrolyte, and a small, but accurately defined, current is applied between the tested surface and the auxiliary electrode placed within the cell above the board. The electrode potential of the tested surface is then measured as a function of time against a reference electrode, which is placed in the electrolyte’s storage vessel. (Figure 1)

The recorded potential-time curve consists of a series of plateaus with characteristic shapes depending on the type and quality of the surface. For example, Figure 2 shows one such curve obtained for a tin-lead surface.

It shows the presence of oxidized Cu/Sn intermetallic compounds, SnO and SnO₂ on the surface of the solder coating. The final plateau on the curve indicates hydrogen evolution due to reduction of the electrolyte, thus providing the highest cathodic value achievable with the given electrolyte. Other species that need to be reduced at a more cathodic potential cannot be detected in the above manner.

A number of publications^1-5 have described the use of SERA for evaluation of surface species and solderability of solder finishes on printed circuits, as well as for process control during the various stages of the PCB manufacturing process. The objective of this article is to concentrate on the evaluation of alternate finishes, currently being proposed to replace HASL and electroplated tinlead coatings.

Despite its wide-spread usage, the HASL coating is not trouble-free or universally applicable. Presently, when circuit board features are being miniaturized and environmental pressures are mounting, the search continues for the best candidate to replace the HASL process. Features, advantages, disadvantages, and costs of alternative coatings are being considered.

In spite of the many papers presented and published highlighting or evaluating various alternative coatings, no single coating has been established as a single viable process to replace HASL. All seem to have some advantages as well as disadvantages. The SERA technique can be used as a tool for monitoring quality and consistency of alternative coatings. In addition, it is our feeling that all of these coatings will be used to a certain extent, and more will emerge in the near future as “the only viable process.”

SOLDER COATINGS

SERA is presently used for evaluating the solderability of solder finishes, as well as a process control tool during its preparation. It became clear from the initial work at Rockwell¹ that an increase in the tin oxide reduction potential of the solder surface can be correlated with an increase in the PCB defect rate during the assembly

process. Tench and others have shown that as the potential of the reduction process shifts to more cathodic (negative) values, the defect rate increases for a given flux. This provides a basis for evaluation of surfaces, fluxes, and soldering conditions during PCB manufacturing, as well as prior to assembly. Since higher cathodic potential means that the reduction of the oxide becomes more difficult, flux requirements can be selected prior to the assembly, based on the quality of the solder surface. This means that the higher the amount of SnO₂ on the solder surface, the more aggressive the flux condition must be to assure proper soldering.

In addition to the value of the reduction potential, other SERA curve features are being evaluated as potential process control indicators. These features include:

presence and amount of the intermetallic compounds on the surface, open circuit potential, and hydrogen evolution potential. All of these parameters have been observed to vary during comparison of “good” v. “bad” boards. Currently, only the presence of intermetallic compounds has been clearly correlated with solderability problems. When a significant amount of intermetallic compounds are detected on the board’s surface, solderability issues arise. Figure 3 shows one such “bad” board with the presence of intermetallic compounds.

HASL ALTERNATIVES

A number of finishes have emerged as potential alternatives to HASL. They include thin and thick OSPs, immersion silver, immersion tin, and immersion gold over nickel. As these finishes are examined, we will discuss how the SERA method can be used as a tool for process control, and for surface evaluation of alternative coatings.

ORGANIC SOLDERABILITY PRESERVATIVES

Unlike the HASL process, where soldering is done by melting the existing solder coating, soldering with OSPs requires the removal of the OSP coating with heat and acid (flux). This must be done so that the attachment can take place on the copper surface under the OSP coating. Hence, OSPs must keep the Cu surface solderable prior and during the soldering process.

During storage, unprotected Cu surfaces are oxidized, leading to the formation of Cu₂O, and later, to a mixture of Cu₂O and CuO. If the copper surface has not been protected from oxidation, it might indicate the presence of three SERA detectible species. The nature of these species and reduction voltages are given in Table 1.

Coating copper surfaces with OSP protects the surface by formation of a complex between the OSP (imidazole, benzotriazole or substituted benzimidazole) and the copper surface.8 Figure 4 shows SERA curves of protected and unprotected Cu surfaces after one year of natural storage at ambient conditions.

Clearly, the imidazole-protected surface prevented further oxidation of copper. This observation can be used as a basis for detection of OSP on the surface. Figure 5 shows SERA curves for surfaces with and without thin OSP coating after a short heating cycle.

The protected surface is clearly distinguishable. Such a procedure is currently used by some PCB manufacturers to detect problems with thin OSP coatings soon after the manufacturing process. Since thick OSPs shift the reduction potential of the Cu-OSP complex, depending on their thickness, their presence is easily detected even without application of the heat. Thus, SERA offers a simple method for detecting the presence of OSPs on the PCB surface. Also, variations in the curve can be used as a useful process control indicator.

Moreover, thin OSPs (25-100Å) are generally nearmonolayer coatings, and their application is quite uniform across the surface. However, the coating thickness is too thin to be measured electrochemically. On the other hand, thick OSPs (500-6,000Å) generally vary in thickness depending on the type of OSP, the control of the chemical composition of the applied solution, and the type and quality of the application process. By using different electrolytes, SERA curves can be used to measure the thicknesses of thick OSP coatings. As an OSP thickness increases, the amount of the reducible material increases, and the time required to “drill” through the OSP coating is then correlated to the thickness of the OSP. Because SERA can be used on very small surfaces (1.6 mm diameter or less), it can be used to measure the thickness distribution of OSPs directly on the PCB, thus becoming a very powerful process control tool. Measurements of the relative
distribution of the OSP on the surface allows one to improve and control the process itself. A number of papers describing SERA testing of OSPs have been published.6-8

The ideal goal is to correlate SERA measurements of OSP coatings to their solderability. Work performed at NPL9 revealed that SERA curves, obtained with yet another electrolyte, showed an increase in copper oxide formation and a decrease in OSP thickness, as the boards undergo consecutive simulated reflow cycles.

The NPL study involves a number of U.K.-based PCB shops, using different types of OSP coatings. Another, more thorough study has been focused on a single OSP coating with different thicknesses and different storage conditions.10 During this work, the results of SERA measurements were correlated to the results of wave soldering, which was performed by the counting of filled holes in specially prepared solderability test vehicles. It was found that the SERA technique can be used for monitoring boards coated with a “proprietary” OSP coating.

Summarizing the work with OSPs, the SERA method can be used as a process control tool to evaluate the presence, quality and thickness of the OSPs, as well as to show some correlations to solderability after simulated reflow cycles.

IMMERSION SILVER

Silver is considered to be a “precious” metal, but its chemical properties distinguish it from other precious metals such as gold and palladium.

The silver surface is quite reactive, and atmospheric corrosion of the silver surfaces occurs almost immediately upon contact with air. Corrosion, as expected, proceeds even faster in the production environment than in clean air. Presence of free sulphur or sulphur compounds in the environment causes rapid formation of the silver sulfide (Ag2S). Higher humidity increases the rate of silver corrosion in the presence of sulphur compounds.

The main components of tarnish films on the silver surface are silver oxide (Ag2O), silver sulfide (Ag2S), silver sulphate (Ag2SO4) and silver chloride (AgCl). The
presence of sulphur compounds on the surface causes solderability problems.11, 12

Since the immersion silver coating is relatively thin (0.1 μm) and sensitive to tarnishing, the surface of silver must be protected to maintain its solderability
characteristics. The corrosion of silver can be inhibited by placement of thin organic or inorganic films on the surface.12 Commercially available immersion silver coatings are coupled with thin organic films which are supposed to provide the required protection from tarnishing; however, this compound is fragile, especially at elevated temperatures and under humid conditions. The solder wettability of fresh silver coatings is good, but degrades rapidly with the formation of a tarnish film.

Therefore, it follows that the presence of a protective film on the surface of silver is extremely important for the protection of silver from tarnishing. Also of importance is a sufficient thickness of the silver layer itself. The presence of proper analytical techniques to monitor conditions and thicknesses of protective layers, tarnishing layers, and the thickness of silver itself is of utmost importance.

When a constant current is applied to a silver surface (SERA test), the resulting potential-time curve reveals a number of plateaus that correspond to the sequential reduction of compounds in the tarnishing film. SERA curves obtained from silver surfaces are similar in shape to those obtained from copper surfaces.

Fundamental work has recently been performed to identify and quantify the species of tarnishing films.13 It was found that components of tarnishing film reveal the following reduction potentials: Ag2O +0.28 V, AgCl +0.12 V, Ag2S -0.62 and Ag2SO4 -0.92 V.

Since SERA has detected the presence of both organic inhibitor and tarnish iflm, SERA can consequently be used as a process control tool for immersion silver coating. In addition, using an anodic current, the silver coating can be stripped coulometrically. This allows for an accurate (but destructive) measurement of the thickness of the silver coating.

IMMERSION TIN

Immersion tin is another of the emerging alternative coatings that can be evaluated using the SERA technique. For this process about a 1-μm-(40μ inches)-thick
immersion tin layer is placed on top of a copper substrate. Some of the coatings might contain organic or inorganic materials that act as diffusion barriers between copper and tin, or protect the tin surface from excessive oxidation.

We have studied several immersion tin processes. Figure 7 shows SERA curves for a thin tin coating (0.26 μm, 10μ inch), before and after heat treatment (reflow).

As can be seen from the curves, before the treatment, only SnO is visible on the surface. This does not pose any solderability problems. After the heat treatment, only oxidized Cu-Sn intermetallics are present on the surface. This indicates that all of the tin has diffused into the copper substrate. Hence, this surface would present serious solderability problems.

The above observation was confirmed by the destructive oxidation of the surface. Before the heat treatment, tin is clearly present on the surface, followed by a thicker layer of Cu5Sn6 intermetallic, and a very thin layer of Cu3Sn intermetallic. After heat treatment, no pure tin can be observed on the surface, the Cu5Sn6 intermetallic layer is slightly thinner, while the thickness of the Cu3Sn material has increased.

The presence of SnO2 and oxidized intermetallic compounds can be correlated to solderability problems on tin coatings. Thus, SERA can be used for QA process
control of this coating. The destructive oxidation of the coating can be used for precise thickness determination of the tin coating, as well as for determining intermetallic thicknesses.14 The solderability of heated immersion tin was shown to diminish as the thickness of the Cu3Sn layer increased.

ELECTROLESS PALLADIUM

The promise of electroless palladium coating lies in its capability to be applied directly over copper. Previously published results10 showed that between 6 to 9
microinches (0.15 to 0.23 μm) thick palladium coating are needed to completely cover a copper surface. Even thicker coatings are required to prevent copper migration through the palladium layer during heating. Thus, the presence of copper on the surface or in the pores of a palladium coating is easily detected, and it can be used for evaluation of electroless palladium coatings on a copper substrate.

IMMERSION GOLD

The last coating to be discussed is immersion gold over a nickel barrier. This coating is being tested extensively as a promising alternative coating that can yield solderable and wirebondable surfaces. Some preliminary results have been presented in an earlier paper.10

One possible cause for solderability problems on immersion gold coatings is the porosity of the gold, which allows oxidation of the exposed nickel barrier through the gold layer pores. As shown in Figure 8, the reduction of nickel oxides takes place at potentials more negative (cathodic) than hydrogen evolution on the gold surface. This makes it impossible to detect the reduction of nickel oxides in the presence of an exposed gold surface.

Tench1 has described a technique similar to SERA, named potentiomentric evaluation of substrate oxidation (PESO). The PESO technique uses an acidic electrolyte such as NH4Cl (pH=4.0), and the corrosion potential of the gold surface is measured as a function of time. The presence of plateaus can be related to the presence (and thicknesses) of the nickel oxides, and the corrosion potential can be related to the porosity of the gold surface.

A second solderability problem arises due to the presence of organic materials on the gold surface. These materials can come from various sources, such as solder mask, oils, or contaminated ovens. Their presence on the surface of gold can be observed by a shift in the rest potential of the gold surface in the borate buffer (pH=8.4) electrolyte. Both the potential shift, as well as the signal “noise” disappear when the surface is cleaned with a strong acid and the solderability is restored.

The combination of SERA and PESO techniques can be used to evaluate immersion gold coating surfaces for organic contamination, porosity, presence of oxidized nickel in the pores of the gold, and presence of copper oxides on the gold surface.

These can lead to a loss of both solderability and wirebondability for this alternative coating. Further tests on this surface are being conducted.

CONCLUSION

In conclusion, it has been shown that the SERA technique can be a powerful tool for evaluating the surfaces of alternative coatings. More work needs to be done and additional studies are being performed which will be reported in the near future. Despite the work that still remains, SERA has already been established as a valuable process control tool for PCB manufacturing.

AUTHORED BY

Peter Bratin, Michael Pavlov and Eugene Shalyt

REFERENCES

1. D.M. Tench, “Solderability Assessment via SERA,”Journal of Applied Electrochemistry, No. 24, pp. 46-50, December, 1994.

2. D.M. Tench, D.P. Anderson, Method of Assessing Solderability, U.S. Patent 5,262,022, Nov. 19, 1993.

3. J. Reed, J. Cheng, “Sequential Electrochemical Reduction Analysis (SERA) Application on Process Characterization and Troubleshooting for Printed CircuitBoard Fabrication,” Proceedings of IPC Technical Conference, pp. 12-22, April 1995.

4. D.M. Tench, M.W. Kendig, D.P. Anderson, D.D. Hillman, G.K. Lucey, and T.J. Gher, “Production Validation of SERA Solderability Test Method,”Soldering & Surface MountTechnology,No. 13, 46, pp. 18-29, February, 1993.

5. M. Pavlov, P. Bratin, E. Shalyt and R. Gluzman, “Solderability Assessment by SERA,” Proceedings of AESF SUR/FIN ’94 Technical Conference, June 1994.

6. P. Bratin, M. Pavlov and E. Shalyt, “New Applications of SERA Method – assessment of the Protective Effectiveness of Organic Solderability Preservatives,” Proceedings of AESF SUR/FIN ’95 Technical Conference, pp. 538-589, Indianapolis, June 1995.

7. S. Gutierrez, P. Tune, “Organic Coatings and The Challenge No Clean Presents,” Proceedings of IEEC/ECTC ’95 Technical Conference, May 1995.

8. G. Wenger, U. Ray, “Four OSPs: Quality Assessment,” SMT magazine, May, 1996.

9. C. Hunt, “A Model of Solderability Degradation,” Proceedings of the SMI Technical Conference, pp. 650-660, September, 1996.

10.P. Bratin, M. Pavlov and E. Shalyt, K. Wengenroth, J. Fudala, A. DelGobbo, “Studying the Solderability of Organic Solderability Preservatives Using SERA,” Proceedings of National Conference on Solderability and Alternative Finishes, Vol. 2, pp. 86-91, September, 1998.

11. D. Hillman, P. Bratin, M. Pavlov, “Demonstrating the Relationship Between Wirebondability and Solderability of Various Metallic Finishes for Use in Printed Circuit Assembly,” Proceedings of SMI Technical Conference, pp. 687-693, September, 1996.

12. K. Wlassink, Soldering in Electronics, 2d ed; 1989, Electrochemical Publications, Scotland.

13.P. Bratin, M. Pavlov and E. Shalyt, “Surface Evaluation of the Silver Finishes via SERA,” Circuit World, Vol. 25, October, 1998.

14. R. Edgar, “Immersion White Tin,” Printed Circuit Fabrication, December, 1998.