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Automation Comes to Litho Inspection by Alexander E. Braun, Associate Editor, Semiconductor International
Automated defect inspection and control strategies are commonplace across fabs, except for the litho cell. Because many litho process defects are relatively large, applicable solutions are limited. For example, the cost-of-ownership of using highly sensitive inspection systems utilized in other process steps often cannot be justified. Anyway, litho error detection requires a system that can be tuned to detect defect types with a wide range of characteristics.
Since over 75 percent of yield-relevant defect types are fairly large and visible to the eye, macro after-develop inspection (MADI) traditionally has been a manual, operator-intensive process. Fabs rely mostly on manual visual inspection to determine whether a wafer passes (to further processing), is reworked (stripped and rerun through the litho cell) or scrapped. Current MADI systems are manual or semi-automated, and illumination may range from common green lights or spotlights to special point sources and flat, monochromatic panels. Always, the detector is human. This is limited; defect detection and classification are inconsistent and unreliable, with results varying due to wafer complexity, background patterning noise and human boredom and fatigue. Up to 80 percent of all MADI defects may go undetected until after etch or final test, when it is too late, resulting in higher scrap rates and lower sort yields. Depending on fab size, approximately $3.6 million a year are wasted due to defects undetected by human operators. Increases in wafer complexity are adding to problems likely to go undetected by manual inspection. While other process steps are improved by baseline defect density reduction through automated detection and control of defects, the litho cell — limited 36
Spring 1999
Yield Management Solutions
to manual inspection techniques that provide little visibility or control and collect very little usable data — is a source of yield loss and scrap issues. KLA-Tencor [has introduced] an inspection suite designed to overcome these limitations, the 2401 Automated Macro Defect Inspection System. It is expected to replace the bright-light MADI performed by human operators. It provides automated detection, classification and reporting of all yield-critical MADI defect types, including hot spots, scratches, large particles, extra and missing resist, unexposed fields, striations and developer spots and splash-back (figure 1). Hot Spots Contamination • particles • foreign materials
Scratches • handling errors
Coating • missing/extra • splashback • striations • comets
Edge-Bead Removal • missing • wrong width • miscentering Develop • wrong program • developer spots Exposure • missing • focus error • gross misalign
Figure 1. Macro inspection is relatively inexpensive (approximately $0.35 per wafer). If done properly, it ensures against expensive problems.