Telecentricity has long been recognized as an important property of lithography-tool optics. "Shift" telecentricity error,
an angular misalignment of the illuminator and projection lens, manifests itself as an unwanted translation of the aerial
image through focus. At best focus, no effect is visible. Generally, litho tool acceptance tests measure directly the
translation through focus, although it is also possible to use pupil-imaging techniques to observe the error by measuring
offset of the pupil fill with respect to the projection lens NA.
Even with a perfectly aligned system (i.e. a perfectly centered pupil fill), it is still possible to induce a translation
through focus. This arises especially in multipole pupil fill patterns (dipole, quadrupole, etc.) which have one pole
brighter than the others. We refer to this effect as "quasi-telecentricity."
With the tight pitches and extreme polar illumination patterns coming into increasing use, this effect will become more
important in the next few years. We have calculated the size of the effect and placed limits on the illumination patterns
that can be used for this type of printing.