Probabilistic process window: a new approach to focus-exposure analysis

BackgroundFocus-exposure process window measurement and analysis is an essential function in lithography, but the current geometric approach suffers from several significant deficiencies.AimBy clearly identifying the problems with the geometric process window approach, a process window measurement and analysis method will be proposed to address these problems.ApproachThe probabilistic process window (PPW) proposed here takes metrology uncertainty into account and rigorously calculates the expected fraction of in-spec features based on settings for the best dose/focus and presumed random errors in dose and focus. Using the fraction of in-spec features thus calculated, a much more rigorous determination of the trade-off between exposure latitude and depth of focus (DOF) can be performed.ResultsThe PPW approach is demonstrated on focus-exposure data generated from a standard extreme ultraviolet lithography process at three different pitches, showing the value of this method.ConclusionsThe PPW approach offers clear advantages in accuracy for both DOF determination and the best dose/focus determination. Consequently, its use is preferred both for process development applications and high-volume manufacturing.