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 solutions1,2. 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.

Determination of Free Acid
Figure 1. Determination of 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.

Determination of Tin
Figure 2. Determination of Tin

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.

Determination of Total Acid and Lead
Figure 3. Determination of Total Acid and Lead

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 described3,5. Figure 4 shows curves obtained for 100mL/L of the Primary Additive with different levels of the Secondary Additive component.

Determination of Primary
Figure 4. Determination of Primary

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.

Spectrometric determination of Secondary
Figure 5. Spectrometric determination of Secondary

Determination of Secondary
Figure 6. Determination of Secondary

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.

Determination of Active Secondary
Figure 7. Determination of Active Secondary

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.

Determination of Antioxidant
Figure 8. Determination of Antioxidant

Linearity of Antioxidant Response
Figure 9. Linearity of Antioxidant 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.

On-line Analyzer for Sn/Pb Electrodeposition Process
Figure 10. On-line Analyzer for Sn/Pb Electrodeposition Process

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