The effect of thin film interference on process latitude in deep UV lithography is investigated using a precise electrical linewidth measurement technique. The experimental results calculated by exposure-defocus plots show swing curve type dependencies of critical dimension variation, maximum depth-of-focus (DOF) range, and centered focus position. It is demonstrated that the maximum DOF ranges for equivalent incoupling points along the swing curve approximately follow a linear equation introduced in accordance with simulation results of aerial image contrast. It is also verified that a thinner resist with a resist thickness corresponding to an optimum incoupling point along the swing curve can provide a distinct advantage in maximum DOF range, in spile of a remarkably low aerial image contrast, especially in the case of critical patterns. Moreover, it is found that the centered focus positions of the critical patterns exhibit a trend to offset in the positive defocus direction with increasing resist thickness.