Critical dimension (CD) control for isolated lines has been experimentally studied as a function of aerial image slope and proximity effects on a photomask. The aerial image slope was controlled by defocusing a 50 kV shaped beam. Isolated crosses of 0.2, 0.3, 0.5, and 1.0 mu m were written on a chrome on glass mask. When all of the features are written at the same dose a nonlinearity of 100 nm and a uniformity of 3 sigma=65 nm were observed for an aerial image slope of 120 nm. These were reduced to a nonlinearity of 20 nm and a uniformity of 3 sigma=18 nm when a dose correction was performed for each feature size. This suggests that proximity correction and not small aerial image slope is the most critical improvement needed to achieve good CD control.