Asymmetrical pore membrane optimizes copper plating filtration
05/01/2007
To achieve uniform copper plating on the surface of a wafer consistently in a production lot, a particle-free plating chemical solution must be presented to the wafer under uniform pressure, uniform flow, and in the proper chemical formulation. This is accomplished by re-circulating large volumes of the plating solution past the surface of the wafer. To maintain low particle levels, the fluid is filtered at a high flow rate. The particle filter must have low pressure drop, good particle retention, and high throughput to result in low wafer defects and high system uptime. In addition, the particle filter must be selected so it does not adsorb or remove some desirable component as it removes contamination from the bath.
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One of the major challenges in electrochemical copper deposition is the formation of particles as a result of the plating process. These particles can be gelatinous in nature and must be removed from the chemical solution to prevent the contamination of the wafer surface that leads to defects. Unfortunately, the gel-like nature of these particles significantly increases their ability to plug the filters. Under heavy-use conditions, it is not uncommon for the filter to become plugged after ~6 weeks. As the filter becomes plugged, the solution flow decreases and particle retention performance suffers, requiring the tool to be taken down for maintenance.
To increase filter service lifetime and reduce the cost of ownership, a comparative study was completed on the efficiency and performance of symmetric versus asymmetric filter media in the removal of gel-like particles generated during a copper plating process. A unique method was developed to non-intrusively and simultaneously analyze the lifetime performance of several filter media using a production tool rather than waiting two to three months to evaluate a filter on a production tool at a customer site without having to repeat the same process for each filter. The final media selection was further evaluated for additive absorption and on-wafer defect performance.
Filtration experiments
The first step in the process of increasing the filter lifetime was to analyze filters that had been plugged after six weeks in the process equipment. The pressure drop of the filter was measured using water in a controlled test system. The pressure drop of the filter after six weeks in the process was 60% higher than that of a new filter. Soaking the filter in a 10% solution of sulfuric acid reduced the pressure drop by 9%, but not enough to consider sulfuric acid soaking as a filter regeneration process.
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Used filters were cut apart and samples of the membrane were tested with more aggressive chemical cleaning solutions. Combinations of 25% sulfuric acid or sulfuric acid/oxidizer were effective in restoring a large percentage of the flow of the membrane, which supports the hypothesis that the plugging agents were organic in nature. Figure 1 shows scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) images of the filter surface for used filters. In addition to the large gel particle, a comparison of new and used filters clearly shows gelatinous residue masking the pore structure of the membrane. The EDS identifies the elemental constituents of the particulate material as carbon, oxygen, and sulfur. A nearly invisible platinum (Pt) peak is an artifact of the sample preparation. It is believed that the source of the organic particles is an electrochemical reaction with one of the plating bath additives.
 Figure 1. SEM and EDS of a filter used in a plating tool, showing a large gelatinous particle composed primarily of carbon, oxygen, and sulfur. |
Since the plugging material is a soft gel, there are two approaches to increasing filter lifetime [3]. The brute force approach is to increase filter area and spread the plugging agent over a larger surface area. However, this technique generally requires a larger footprint for the filter housing and a system with more hold-up volume. The more elegant approach is to modify the structure of the filter so there are a greater number of sites on the surface of the filter for particles to be trapped without blocking the pores. This technique uses an asymmetric membrane structure, where the upstream surface of the filter has larger pores than on the downstream surface. Large particles are retained in such a way that they don’t completely block the pores. Figure 2 shows a schematic representation of the way particles are retained on symmetric and asymmetric structures, as well as SEM cross-sections of the two different filter structures.
Since the plugging agent is formed in the plating process and is not amenable to modeling with a stable test particle, the best method for evaluating potential membrane materials for plating bath filtration is to place a sample of the filter in a production bath. While there would be a risk in placing a developmental filter on a production tool, a novel approach was developed, whereby a small sample of membrane is placed on a slipstream from the production tool. In this manner, several membrane samples could be easily tested simultaneously for direct comparisons. Since the pressure drop across the small membrane samples is greater than across the process tool filter, the evaluation of the test filters can be done in an accelerated mode. The membrane samples tested were 47mm disks in holders with plastic wetted surfaces. The flow through each disk could be measured separately to determine any change in filter resistance. In addition, the effluent from each filter sample could be plumbed to an optical particle counter to measure the effectiveness of each membrane sample in removing particles.
 Figure 3. Feed pressure and flow of 47mm dia. asymmetric and symmetric filter samples. |
Figure 3 is a summary of the evaluation of the symmetric and asymmetric filter samples, in terms of both flow rate and the pressure upstream of the membrane samples. During the first week of the test, the pressure increased slightly because the main circulation loop filter was plugging slightly. The main circulation loop filter was replaced between 9/26 and 10/4, which lowered the feed pressure to the filters and explains the sudden drop in pressure for the asymmetric filter being tested in the loop. The symmetric filter lost approximately half of its flow during the first week. The asymmetric filter had constant flow over the two week duration of the test.
Optical particle counters capable of detecting particles larger than 0.2μm were installed on the feed to the filter samples, and on the effluent from the coupon samples. Both the symmetric and asymmetric membrane coupons reduced the particle levels by a factor of >200. Though an asymmetric filter has significantly higher throughput than a symmetric filter, they demonstrate equivalent particle retention.
 Figure 4. Flow performance of asymmetric (Cell 2) and symmetric (Cell 1) filters are nearly identical, shown at installation (TO) and after 4 hours of operation (T4). |
It is important that the level of plating solution additives (accelerator, leveler, and suppressor) remain constant in the bath over time. Some membrane filter designs may remove these components due to absorption. This effect was evaluated by filtering 15 liters of low acid copper plating solution through a 10-in. asymmetric filter cartridge over four hours. The concentration of each of the additives was measured using the cyclic voltammetric stripping (CVS) method. The CVS instrument has an error rating of ±5 percent. Since the filter is new and the ratio of membrane area to solution volume is high, the sensitivity of this test is high. There were no additives lost during the testing.
Filtration in a plating tool
The table compares the attributes of commercial 10-inch filters with the membrane samples tested. Both filters have essentially the same pore size and particle retention. The symmetric filter was designed to be used in recirculated etch baths where the multiple pass mode of the operation results in more effective particle retention and is rated based upon this configuration.
 Figure 4. Flow performance of asymmetric (Cell 2) and symmetric (Cell 1) filters are nearly identical, shown at installation (TO) and after 4 hours of operation (T4). |
Based on the flow through the 47mm dia. coupons, the resistance to flow of both samples is equivalent. The rated pressure drop of the symmetric filter is about 70% that of the asymmetric filter, due in part to the larger membrane area of the symmetric filter cartridge. The increased pressure drop of the asymmetric filter is small enough that it will not have a significant influence in the application. Both filters demonstrate low enough pressure drop performance that the flow losses associated with piping and fittings will dominate, thus the symmetric and asymmetric filters will have the same flow in the tool (Fig. 4).
After the results of laboratory testing demonstrated that the asymmetric filter was promising, a marathon test was run in the laboratory of a major capital equipment manufacturer to further evaluate the initial conclusion. A new asymmetric filter was installed in Cell 2 of the process tool while new symmetric filters were installed in Cells 1 and 3. Monitor tests were conducted at wafer 0 (T0), 200 (T1), 1700 (T2), 2750 (T3), and 3500 (T4).
The additive level was measured over a period of four hours using CVS analysis with ViaForm NExT additive analysis protocols; there were no changes in the additive levels indicating no binding or leaching. Defects on the wafers were analyzed, resulting in no difference between the test filter and the product of record (POR). The fill rate and additive levels were consistent throughout the test. There was no difference in via-fill, copper thickness, or uniformity profile for the cell with the test filter and the cell with the POR.
Conclusion
A systematic approach was used to identify the type of particles plugging the filters in copper plating tools. A unique test methodology was developed to evaluate potential solutions to this filter lifetime issue that provided faster results than standard testing protocol. This testing proved the benefits of an asymmetric membrane configuration when confronted with solutions containing gelatinous particles. Testing indicates that the new asymmetric filter has twice the throughput of the established symmetric filter; the filter lifetime can be increased from six weeks to three months, increasing plating tool uptime. Further testing was performed to refine the filter selection, including both absorption and flow studies. A final asymmetric filter configuration was obtained that provides optimal results for this process.
Acknowledgments
The authors wish to acknowledge the significant contributions of the Entegris Taiwan team and the Novellus Taiwan team in collecting field data. In particular, the authors would like to acknowledge H.J. Yang, Tom Wei, Steven Hsiao, and Kamper Cheng for their assistance during this study as well as Joseph Zahka for his significant contributions to the development of this paper and Katie Wang for her assistance with the wafer defect testing. ViaForm is a registered trademark of Enthone Inc. Chemlock is a registered trademark of Entegris Inc.
References
- J. Zahka, D. Grant, C. Myhaver, “Modeling of Particle Removal from a Circulating Etch Bath,” Particles and Gases in Liquids 2, Plenum Press, NY, US, 1990.
- J. Zahka, V. Anantharaman, M. Carroll, K. Vakhshoori, “Characterization of Filters Used in Recirculated Buffered Oxide Etch Baths,” Solid State Technology, Vol. 36, No. 6, pp. 63-71, 1993.
- M. Clarke, J. Zahka, Understanding Membrane Plugging Mechanisms, (Entegris Applications Note MAL 116).
Aiwen Wu received his PhD in chemical engineering from U. of New Hampshire, Durham, NH, United States. He is a senior applications engineer in the Liquid Microcontamination Control Division of Entegris, 129 Concord Road, Billerica, MA 01821 United States; ph 978/436-6820; e-mail [email protected].
Gregg Conner received his BS in chemical engineering and his MS in environmental engineering from Oregon State U, Corvallis, OR, United States. He is the applications engineering manager in the Liquid Microcontamination Control Division of Entegris.
Vinay Prabhakar received his masters in mechanical engineering from MIT, Cambridge, MA, United States. He is a mechanical engineer at Novellus Systems Inc., Tualatin, Oregon, United States.