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22 August 2001 Comprehensive analysis of statistical and model-based overlay lot disposition methods
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Overlay lot disposition algorithms in lithography occupy some of the highest leverage decision points in the microelectronic manufacturing process. In a typical large volume sub-0.18micrometers fab the lithography lot disposition decision is made about 500 times per day. Each decision will send a lot of wafers either to the next irreversible process step or back to rework in an attempt to improve unacceptable overlay performance. In the case of rework, the intention is that the reworked lot will represent better yield (and thus more value) than the original lot and that the enhanced lot value will exceed the cost of rework. Given that the estimated cost of reworking a critical-level lot is around 10,000 (based upon the opportunity cost of consuming time on a state-of-the-art DUV scanner), we are faced with the implication that the lithography lot disposition decision process impacts up to 5 million per day in decisions. That means that a 1% error rate in this decision process represents over 18 million per year lost in profit for a representative sit. Remarkably, despite this huge leverage, the lithography lot disposition decision algorithm usually receives minimal attention. In many cases, this lack of attention has resulted in the retention of sub-optimal algorithms from earlier process generations and a significant negative impact on the economic output of many high-volume manufacturing sites. An ideal lot- dispositioning algorithm would be an algorithm that results into the best economic decision being made every time - lots would only be reworked where the expected value (EV) of the reworked lot minus the expected value of the original lot exceeds the cost of the rework: EV(reworked lot)- EV(original lot)>COST(rework process) Calculating the above expected values in real-time has generally been deemed too complicated and maintenance-intensive to be practical for fab operations, so a simplified rule is typically used.
© (2001) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David A. Crow, Ken Flugaur, Joseph C. Pellegrini, and Etienne L. Joubert "Comprehensive analysis of statistical and model-based overlay lot disposition methods", Proc. SPIE 4344, Metrology, Inspection, and Process Control for Microlithography XV, (22 August 2001); doi: 10.1117/12.436735;


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