A study of linewidth variations across commercial stepper exposure fields suggests that variations in local value of partial coherence across the exposure field might be responsible for the excessive character of linewidth variance. A methodology to estimate the local value of partial coherence (LVPC) at a random point in the stepper exposure field is proposed and discussed. Estimation of the local value of the partial coherence summarized across the tested exposure field is represented as a 'Coherence map' of the stepper field. For a stepper under test, such a map revealed significant deviation in the local values of the partial coherence from the designed value. Estimated LVPC variation fully accounted for 'excessive' across field linewidth variation as well as for its orientational dependency. Several means of improving line width control in the presence of variance in LVPC are suggested. The impact of the presence of localized partial coherence in the exposure systems used with PSM and OPC technologies is analyzed briefly. Sensitivity to LVPC variations with the suggested monitor design and ways to improve t are also discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.