Control Strategy for Wafer-Edge Defects
Identifying and resolving systematic process issues on edge die before they migrate to interior die prevents production yield loss and enables faster yield ramp.
M.F. Hsu and J.H. Yang, Taiwan Semiconductor Manufacturing Co. Ltd. (TSMC), Hsinchu, Taiwan; E. Yang, H. Chen, M. Ng, M. Li and C. Perry-Sullivan, KLA-Tencor Corp., Milpitas, Calif. -- Semiconductor International, 6/1/2009
Traditionally, defect inspection and monitoring are only performed on complete die. Partially printed die at the perimeter of a wafer include only a portion of a full chip, and thus are unusable as product. However, we have determined that inspecting partial die can reveal defect signatures that point to process issues relevant to the full, yielding die. Inspections that include complete wafer information — both full and partial die — provide engineers with a comprehensive picture of a particular process and the associated defectivity.
It is important to inspect and monitor these partial die to detect and identify systematic process issues. For example, the introduction of immersion lithography for the 4× and below design node has increased yield loss on the perimeter of wafers. In the case of serious edge die defectivity issues, such as particles or pattern peeling, defects can migrate from the edge die to interior active die.
Additionally, during the R&D stage, test die are usually very large, resulting in fewer die being inspected for yield monitoring. Partial die inspection can substantially increase the wafer area inspected, providing the information required to accelerate edge die yield learning during R&D and the production ramp.
Full vs. partial die excursions
For three months, we performed in-line monitoring for 4× and 3× products using partial die inspection on a broadband brightfield inspector. We captured several unique defect excursions on the edge die, enabling the resolution of critical process issues. We present results from five different process layers.
Figure 1 shows front-end etch layer inspection data with and without partial die inspection capability enabled. The wafer maps that do not include partial die show a few clusters of defects on die near the perimeter of the wafer. In contrast, the inspections of partial die clearly show an extensive edge die defect excursion. Through defect review, we generated charts that reveal the density of different defect types for the different inspections (Fig. 2). On the wafer edge, an active-area damage defect dominates. We determined the damage was related to a dry etching process issue.
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| 1. A prominent active-area damage defect signature, missed by the full wafer inspection (left) was detected by partial die inspection (right). |
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| 2. Partial die inspection results for a front-end etch layer detected an active-area damage defect in both wafer lots, which was attributed to a dry etching process issue. |
Similar results were generated for three 4× and below etch layers. Figure 3 shows the wafer maps using partial die inspection, and defect density charts comparing partial die and no partial die inspection results. For all three layers, partial die inspection identified unique defect signatures: a surface particle excursion (4×); a film hump excursion induced by particles (4×); and new type of defect excursion related to fall-on particles (3×).
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| 3. Partial die inspection results for three etch process layers demonstrating unique edge die defect issues. |
Finally, partial die inspection results were collected for an etch layer for two different products. The partial die inspection (Fig. 4) successfully flagged a pattern failure. We determined this pattern failure defect was caused by improper etch conditions.
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| 4. Partial die inspection results from an etch layer reveal a unique edge die defect signature related to pattern failure. |
Overall, these inspection results demonstrate the value of partial die inspection in capturing unique defect excursions that can help engineers identify process issues early, reducing yield risk and accelerating yield learning.
Conclusion
Unique defect excursions at the perimeter of wafers can be detected using partial die inspection on a broadband brightfield inspector, enabling engineers to quickly resolve systematic process issues. During the R&D stage, the edge die defectivity information provided by partial die inspection allows for early excursion detection and early yield learning, significantly reducing the time to market and preventing potential yield loss of production die. In production, partial die inspection expedites the identification and resolution of process issues, preventing production yield loss.
| Acknowledgement | ||
| The authors would like to thank Andy Lee of TSMC Fab 12 for his assistance in writing this paper. | ||
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