Sub-100nm interconnects using multistep plating
10/01/2003
Overview
Electroplating sub-100nm Cu interconnects using large electrolyte baths faces limitations in the area of defect control repeatability and gap-fill consistency. A modular plating cell design with an independent electrolyte circulation loop supports a multistep plating process with different chemistries in different cells, enabling approaches to meet both gap fill and film planarization for sub-100nm Cu metallization. The small-volume plating cell with a periodic refill of plating chemicals also allows minimum buildup of organic additive breakdown products and particles in the plating bath.
Since introduction of electrochemical plating for Cu fill in the late 1990s, many improvements have been made to plating chemistry and equipment. Copper electrochemical plating (ECP) is currently used in production at the 180nm and 130nm technology nodes. In the sub-100nm node, however, increasingly stringent process requirements in Cu interconnect pose critical production challenges for Cu ECP, which only addresses basic process requirements.
For example, using an accelerator and suppressor at appropriate concentrations provides bottom-up fill in small via and trench features in Cu ECP. A third organic additive (leveler) has been introduced to reduce Cu over-deposition (mounding) in the dense area of small features. The improvement of overall film planarization benefits the subsequent Cu chemical mechanical polishing (CMP) process.
First-generation Cu ECP equipment design adopted general concepts from the centuries-old electroplating industry, including a conventional large bath (>150 liters) of electrolyte, which is typically maintained for a few weeks to months. To meet stringent process control requirements in wafer processing, these large ECP baths required the implementation of a closed-loop cyclic voltametric stripping (CVS) control scheme.
The goal of a closed-loop control is to compensate for the organic additive consumption and to maintain additive concentration levels for process results (gap fill, planarization, etc.). Though the concept has provided a valuable baseline in first-generation Cu ECP technology, it also creates significant limitations for further plating process and equipment improvement. In addition, although the CVS closed-loop control solves some problems, a few other concerns about the large baths remain to be addressed.
Plating challenges
Plating from large baths must address a buildup of bath contamination from both organic additive breakdown products and particles. Bath contamination buildup has the potential to impact process consistency (gap fill, defectivity, etc.) at different Cu interconnect levels, different technology nodes, and within different devices.
Advances in semiconductor manufacturing (every 1.5–2 years/technology node) require continuous process improvements, including new chemistry development. Large bath-plating tools severely hamper new plating chemistry developments due to enormous capital and chemical costs associated with evaluation and development of any new plating chemistry. The current reliance on CVS closed-loop control results in a further slowdown in the chemical/process development cycle due to a lengthy CVS methodology development/qualification period and the necessity of long-term bath stability studies.
Lack of multiple chemistry capabilities in large-bath ECP is another issue for advanced plating. With a single large bath, the need to compromise chemical/process selections between multiple requirements (gap fill, planarization, defectivity, etc.) cannot address the increasing complexity in the sub-100nm Cu-wiring process. This is compounded by the system's complexity when using CVS closed-loop control.
Finally, CVS closed-loop control itself can result in poor system reliability, reduced system availability, large system footprint, complex system facility and chemical management requirements, high operating cost, and concerns about day-to-day/tool-to-tool process stability due to inconsistency in CVS measurements.
A new approach
In a departure from the traditional large-bath concept for electroplating, a small-volume cell was developed. An individual modular cell plumbing design prevents cell cross-contamination and allows independent operation of different plating cells. To deliver the electrolyte mixture to the individual cells, a centralized on-board chemical delivery unit that measures volumes of inorganic and organic components was used. To address process flexibility requirements, the chemical delivery system was designed with the capability of supplying more than one electrolyte chemistry. For better long-term bath consistency, small-volume cell baths were periodically refilled instead of using CVS closed-loop control.
Typical large baths used in first-generation electroplating have a volume of 150–250 liters. In comparison, total electrolyte volume in small-volume cell/plumbing is reduced by more than one order of magnitude. The small-volume electrolyte circulation loop was set up to be emptied and recharged automatically by the number of wafers processed. It was found that this periodic bath refill minimized by-product buildup in the plating solution that resulted in stability of plating results.
Figure 1 shows high-performance-liquid-chromatography (HPLC) analysis results of electroplating baths with a conventional three-component plating chemistry. Without periodic bath refill, large-bath ECP showed significant organic additive breakdown product buildup, which may impact consistency of on-wafer results, including gap fill and defectivity. In comparison, by-product buildup was minimized using a small-volume plating cell with periodic refill, as shown by negligible HPLC peaks for both suppressor and accelerator breakdown products.
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The electrolyte stability in a small-volume cell results in stable process performance. Figure 2 demonstrates gap-fill stability in a small-volume plating cell, from the first wafer after bath refill to the last wafer before the next bath refill. Complete gap fill is achieved within wafer, throughout the bath lifetime, and on all process cells.
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Periodic bath refill yields better defect performance (Fig. 3). Not only is the overall defect number reduced by 3–10x compared with data from large-bath electroplating, but the consistency of defect performance is also improved.
 Figure 3. Defectivity improvement with small-volume plating cells. The defect analysis is conducted on an Applied Materials' Compass defect inspection tool. |
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Two-step plating
The small-volume bath cell allowed different chemistries to be used in multiple plating cells, offering the opportunity to optimize chemistries in each cell for individual requirements in copper plating, including gap fill, film planarization, defect reduction, plating on different seed thicknesses, electrical performance improvement, etc. For example, in current electroplating chemistry, two components of organic additives (accelerator, suppressor) provide gap-fill phenomena, and a third organic additive (leveler) is introduced to reduce film over-deposition (mounding) at dense, small feature areas.
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In a single large-bath-electroplating system, concentrations of three components in the electrolyte bath are selected as a compromise between gap fill and planarization. If the leveler concentration is low, gap-fill performance can be good, but mounding at the small feature area will remain poor.
On the other hand, with a high leveler concentration, good film planarization can be achieved but some center voids are visible inside small via structures due to poor gap fill (Fig. 4). With small-volume plating cells, a two-step plating scheme was implemented. The first step focused on gap-fill performance, and the second step focused on film planarization. The overall results are a combination of better gap fill and better film planarization performance than the results on current single-bath electroplating (Fig. 5).
 Figure 4. An example of benefits in two-step plating over single-step-plating processes: comparison of gap-fill and film planarization performance. |
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It is worth noting that small-volume plating offered a significant advantage in lower chemical consumption even with the periodic bath refill. Some of the cost saving originates from elimination of a time-based electrolyte partial replenishment currently implemented in large-bath electroplating.
 Figure 5. Decreased mounding is achieved through two-step plating. Trench structures shown are 0.18µm wide, 0.5µm deep, with 0.18µm spacing. |
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Conclusion
The use of small-volume plating cells vs. large-bath ECP for advanced sub-100nm interconnect manufacturing is compared in the table. In general, the small-volume plating cell design with periodic bath refill enhances system performance and process stability (gap fill, defectivity, etc.). Additionally, a small-volume plating cell system enables advanced plating process development to address new challenges in sub-100nm Cu interconnect.
Michael X. Yang, Daxin Mao, Chunman Yu, John Dukovic, Ming Xi, Applied Materials Inc., Santa Clara, California
Acknowledgments
The authors are grateful for technical input and process testing results from Nicolay Kovarsky, Hsuan-Sheng Yang, Yi-Chiau Huang, Kapila Wijekoon, Bo Zheng, You Wang, and engineering designs from Dima Lubomirsky, Saravjeet Singh, Sheshraj Tulshibagwale, Alexander Hoermann, and Andrew Chang.
For more information, contact Michael X. Yang at Applied Materials Inc., 3050 Bowers Avenue, Santa Clara, CA 95054; e-mail [email protected].