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The More and Less of Effective Overlay Control Ming Yeon Hung, Taiwan Semiconductor Manufacturing Company Xuemei Chen, Kelly Kuo, Steven Fu, Viral Hazari, Kevin Monahan, Mike Slessor, and Amir Lev, KLA-Tencor Corporation George Shanthikumar, Zhoujie Mao, and Shiming Deng, Universtity of California at Berkeley
With shrinking design rules and the transition to 300 mm wafers, the risk and cost associated with process excursions become more severe. With the increased number and value of transistors per wafer, any process or product excursion that goes undetected or is not forestalled, implies significant material at risk and unnecessary production cost. Therefore, a systematic approach to excursion management that ensures effective detection, identification, and reduction of process excursions is essential for realizing the productivity and cost benefits of the technology shifts. In this article, we describe excursion management as applied to overlay in lithography, in the context of a total lithography metrology ROI analysis framework for 300 mm high volume production.
Introduction
In addition to capital investment such as tool and footprint cost, the potential revenue gained through effective excursion control and process control must be accounted for in a total lithography metrology ROI cost model. Effective excursion control reduces the potential yield loss due to excursions by detecting and resolving process excursions efficiently. Effective process control improves the revenue through the variance reduction achieved with efficient feedback. Both are affected by the chosen sampling plans and time to response afforded by the fab metrology configurations. Effective excursion management needs to achieve two objectives: 1) Timely detection of yield-impacting excursions, which is dependent on the following criteria: • Optimized sampling based on proper separation and characterization of excursion and baseline statistics
• Shorter excursion detection delay by improving metrology queue and measurement time 2) Excursion risk reduction, which involves the following: • Characterizing excursion types, and eliminating sources for excursion effectively • Reducing lots exposed to excursion through faster time to results • Reducing excursion frequency by improving baseline variance distribution It becomes apparent that sufficient sampling and shorter cycle time are two major factors that contribute to the effectiveness of excursion control, and it is important that the respective metrology needs be quantified and validated using actual fab production data. In the following sections, we present the overlay excursion management strategies and analysis steps, and validate the methodology with 300 mm fab overlay data. In particular, the wafer-to-wafer and within-wafer sampling needs, and impact of cycle time on excursion risks will be addressed. Summer 2003
Yield Management Solutions
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