Contributions of stepper lenses to systematic CD errors within exposure fields

We have studied systematic line width (CD) errors as functions of field coordinates for three late-model i-line steppers from different manufacturers by measuring electrical resistance of lines patterned in poly-silicon. The combination of reticle errors with non-linear imaging accounts for a significant fraction of the total line width errors. After removing the effects of reticle errors, CD contour maps are consistent with aberration patterns in which the Strehl intensity is highest at the center of the field.