We have studied variations of the widths of polysilicon lines patterned to nominal dimensions of 0.25 mu m with an i-line stepper. The analysis is based on measurements of electrical resistance of the etched polysilicon patterns. Systematic dependence of linewidths on exposure field coordinates accounts for a large fraction of their total variance. We have expressed the systematic intrafield variance as a sum of three terms : one attributed to the stepper, another to the reticle, and a third to both. The systematic intrafield dependence is most significant for small isolated lines; this term alone had a 3 sigma value of 25 nm at a nominal dimension of 0.25 mu m.