How much is enough? An analysis of CD measurement amount for mask characterization

Albrecht Ullrich; Jan Richter

doi:10.1117/12.830671

23 September 2009 How much is enough? An analysis of CD measurement amount for mask characterization

Albrecht Ullrich, Jan Richter

Proceedings Volume 7488, Photomask Technology 2009; 74881K (2009) https://doi.org/10.1117/12.830671
Event: SPIE Photomask Technology, 2009, Monterey, California, United States

Abstract

The demands on CD (critical dimension) metrology amount in terms of both reproducibility and measurement uncertainty steadily increase from node to node. Different mask characterization requirements have to be addressed like very small features, unevenly distributed features, contacts, semi-dense structures to name only a few. Usually this enhanced need is met by an increasing number of CD measurements, where the new CD requirements are added to the well established CD characterization recipe. This leads straight forwardly to prolonged cycle times and highly complex evaluation routines. At the same time mask processes are continuously improved to become more stable. The enhanced stability offers potential to actually reduce the number of measurements. Thus, in this work we will start to address the fundamental question of how many CD measurements are needed for mask characterization for a given confidence level. We used analysis of variances (ANOVA) to distinguish various contributors like mask making process, measurement tool stability and measurement methodology. These contributions have been investigated for classical photomask CD specifications e.g. mean to target, CD uniformity, target offset tolerance and x-y bias. We found depending on specification that the importance of the contributors interchanges. Interestingly, not only short and long-term metrology contributions are dominant. Also the number of measurements and their spatial distribution on the mask layout (sampling methodology) can be the most important part of the variance. The knowledge of contributions can be used to optimize the sampling plan. As a major finding, we conclude that there is potential to reduce a significant amount of measurements without loosing confidence at all. Here, full sampling in x and y as well as full sampling for different features can be shortened substantially almost up to 50%.

Citation Download Citation

Albrecht Ullrich and Jan Richter "How much is enough? An analysis of CD measurement amount for mask characterization", Proc. SPIE 7488, Photomask Technology 2009, 74881K (23 September 2009); https://doi.org/10.1117/12.830671

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