Pupilgram imperfections and their effect on lithography

Over the past three years or so, as lens manufacturing has improved and lens aberrations have been continuously reduced, the effects of imperfections in the pupil fill, or the partial-coherence pattern produced by the illuminator of a lithographic tool, are rising up out of the noise level and becoming important. Tools for quantitatively measuring illuminator pupil fill are now becoming increasingly widespread, and the user community is collecting data and asking lithographic tool suppliers what the data mean. Previously, we measured imperfections in the pupil fill of Nikon S204-generation scanners by generating pupilgrams, analyzed them in a manner so as to extract their underlying structure, and compared the results to scaling laws derived from lithographic calculations run with Prolith. At that time, we found that the imperfections seen in that generation of lithographic tools were small enough to have only a minimal effect on litho performance. Now, pupil-fill effects in new-generation high-NA scanners have been measured and characterized. We present a systematic study of pupilgrams, measured with a pinhole reticle while exercising illuminator adjustments, and correlate them with simultaneous measurements of CD uniformity and V-H bias to evaluate the importance of the illuminator in overall CD performance. We have also developed a more sophisticated analysis method to predict the effects of the pupilgrams on litho performance, and we present the correlation between experiment and theory. This work should allow both lithographic tool suppliers and end users to inspect a pupilgram and determine whether it will adversely impact their lithography.