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 . 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.
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.
Michael Pavlov, Eugene Shalyt, Peter Bratin
ECI Technology, Totowa, New Jersey
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, firstname.lastname@example.org.
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.