Focus sensitivity of CD extreme values demonstrated on a 40 nm pitch DRAM hexagonal contact hole use case printed with 0.33 NA EUV lithography

Abstract

The effect of exposure defocus on the extreme values of the CD distribution of a DRAM hexagonal contact hole use case (capacitor patterning) was studied. Yield-relevant numbers of contact hole CD measurements were extracted from images collected with an e-beam inspection tool capable of massive data collection. The resultant CD distributions were analyzed for their focus sensitivity. It was found that the tails of the distributions (extreme values) respond stronger to defocus than the mean or 3-sigma. A statistical analysis of the extreme values was performed using either the raw data or analytically using a model distribution fitted to the data sets. When the available CD budget is based on the extreme values (99% spec) rather than the average CD and LCDU the focus range for which the process is in spec is impacted. Lastly, a methodology based on the extreme value distributions was introduced to evaluate the contribution of focus control over the CD budgets.