Beam blurring due to Coulomb effects is very important in cell projection lithography. This article reports on Coulomb effects in cell projection lithography by simulation and experiment, and shows some important results. First, the Coulomb effect in a single cell is substantially uniform in conventional cell projection optics. Second, refocusing is effective in reducing the space charge effect even in Koehler illumination with a Gaussian crossover. Third, the optimum cell size in the case of a large aperture percentage depends on the ratio of the total exposure time in the peripheral circuit to the total settling time in the memory cell mat. In addition, a modified mask technique is proposed to decrease the influence of Coulomb effects on throughput.