The effects of mask bias on the performance of an optical lithographic process are examined theoretically and experimentally. By studying the effects of bias on the aerial image, the fundamental reasons for process improvement with bias are made clear. Simulations are used to examine mask linearity and exposure and focus latitude as a function of bias. Experimental data confirm the improvement in latitude for isolated lines with bias. The proximity effect is investigated and shown to be not well understood. Two possible explanations for the proximity effect: image flare and developer depletion, are studied.