The photoresist of a column spacer is very sensitive to UV light, and its uniformity depends on the exposure process. In a proximity-type aligner, the exposure gap can control the light diffraction and intensity, which affect the column spacer thickness and the critical dimension. The limitation of non-flat mask and work stage causes a non-uniform exposure gap. Applying an attenuated phase shift mask can compensate for this handicap. By using commercial software, we analyzed the difference between a conventional mask and a phase-shift mask as a function of the exposure gap and examined changes in the column spacer's thickness loss and the critical dimension. In order to reduce the diffraction effect and to get a uniform intensity, we varied the transmission in an opaque mask area and found that the thickness uniformity could be improved by about 30 \%. Consequently, we expect the attenuated phase shift mask to have a larger possibility of improving the column spacer's thickness uniformity without any mechanical alteration.