Illumination source radiance distribution has a significant impact on vertical-horizontal bias and critical dimension (CD) variation on production wafers. In this article, the impact of source displacement error (SDE) on sub-90 nm patterning is studied. A Fresnel zone plate (FZP) is adopted as a metrology to quantify the amount of SDE. Experimental data show that some of pupil-fills may have more than 50 mrad of SDE and it could cause a serious pattern shift and/or CD asymmetry. A detailed description of FZP design specifications and application results are presented. Finally, SDE tolerance limits to print sub-90 nm features are discussed. (c) 2005 American Vacuum Society.