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Reduce Lithography Tool Qualification Time with High-Throughput CD SEMs by Richard Elliott, Strategic Marketing Manager
Lithography tool and process characterization are at the core of lithography process engineering. Critical Dimension (CD) measurement and CD uniformity performance are primary elements used in the selection of equipment and process parameters. Bosung plots, swing curves, E-D trees and other forms of CD data analysis are familiar icons to lithography engineers. Assessment of system performance requires very large amounts of data, and the time required to collect this data can impede a thorough characterization experiment. Process characterization and optimization become increasingly difficult as minimum dimensions are reduced. CD tolerance limits must tighten to meet the requirements of smaller design rules and advanced device designs. The lithography process advances made to achieve these smaller CDs add new variables to the lithography equation. Advances include: • the use of different photoresists for line/space layers and contact layers, • the availability of variable illumination partial coherence (σ) and numerical aperture (NA) lithography systems which require optimization, and • production use of Optical Proximity Correction (OPC) and Phase-Shift Masks (PSM). In addition, the introduction of step and scan technology creates new requirements for equipment stability monitoring. For steppers, the CD variation across the exposure field is generally stable. For step and scan systems, reticle and wafer motion during exposure can vary over time. This “rubber lens” effect produces a new source of across chip linewidth variation (ACLV) which must be monitored periodically. Fast measurement speeds characterization
Consider a typical lens characterization designed to determine the focal plane deviation across a stepper field. Metrics of interest include the total range of focal plane variation across the field and the identification of the two points between which the largest variation is seen. To determine these values, focus curves from multiple points within the field are generated and compared. The 18
Summer 1998
Yield Management Solutions
best focus at any position is determined by measuring features printed through the range of useable focus, then plotting the CDs vs. focus value. 25 focus steps usually provide a clean curve to determine the point of best focus repeatably. To identify any performance variation due to feature type or orientation, multiple features are often measured at each site. Typical features used are isolated and dense lines in the horizontal and vertical direction, which yields four measurements at each site. Sagital and tangential orientations can be added, but this doubles the number of measurements required. With 4 measurements per site at 25 focus steps, 100 measurements are required per position in the exposure field. To compare the focal plane position between 9 points in a single field, 900 measurements are required. This volume of measurement seems immense, but fast automated move-acquire-measure (MAM) times can bring this experiment down to a manageable time. With a six second MAM time, this experiment takes about an hour and a half for one wafer. Automatic macros for analysis make this practical to do. Tests which might take hours longer on slower CD SEMs, or even days on manual metrology tools, can now be completed quickly. Fast MAM time opens the door for larger, more extensive experiments. Recent improvements in productivity parallel fundamental advancements made in CD SEM electron optics and metrology. State-of-the-art systems are capable of 3 nm measurement precision and imaging of features with aspect ratios as high as 10:1. Coupled with this advanced metrology capability, fast MAM time enables more extensive characterization in less time and ensures that metrology tools can support lithography tool characterization as process parameters increase.
Productivity features parallel increasing metrology requirements
Fast MAM time is not the only productivity capability needed to meet increasing metrology requirements. The development of the following productivity features on advanced CD SEMs has paralleled the development of the lithography tools:
• Data files with all relevant measurement conditions and that can associate specific measurement locations directly with exact conditions such as focus and exposure enable simpler data analysis.
• Advanced measurement algorithms provide data such as left vs. right line edge comparison, line edge roughness, corner rounding, or contact area information.
• Improved user interfaces enable direct labeling of image artifacts for documentation and reporting.
These productivity features available on tools such as the KLA-Tencor 8100XP can combine to make characterization a simple turnkey operation.
• High throughput enables massive measurement data collection.
• Library-based recipe programming allows for the reuse of information pertaining to common structures and enables the waferless creation of recipes offline.
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• Full automation and the subsequent removal of human errors in the association of measurement data to experimental conditions improves data integrity.
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Advanced Characterization Features. Analysis of CD measurement data to determine focal plane deviation across field is shown in 1a. Figure 1b shows the line scan overlaid on the live image, algorithms to quantify left vs. right edges, and the capability to label images with comments or CD measurements — valuable tools for process characterization.
The First Production-Ready 300 mm Resistivity Measurement System The new OMNIMAP® RS-100 delivers the features customers have been asking for. • Accurate edge measurement to 1 mm from the conductive film • Powerful and easy to use Windows NT® platform • Full GEM/SECS HSMS compatibility and more • Expanded sensitivity for highly conductive interconnects such as copper and thick aluminum • Automated probe conditioner • 3D uniformity maps • 200 mm and 300 mm configurations • Meets SEMI safety and ergonomic requirements
Visit our website at www.kla-tencor.com For more information call 800.450.5308
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