Thin film interference effects on process latitude in an off-axis (quadrupole) illumination system are investigated with a bottom antireflective coating (ARC) or without the bottom ARC over SiO2substrate using an electrical linewidth measurement technique. Experimental results indicate that an off-axis illumination system can improve an increasing disadvantage in maximum depth-of-focus (DOF) range with increasing resist thickness if using a resist thickness corresponding to a top extreme along the swing curve on a sufficient bottom ARC compared with the standard illumination system. But more serious thin film interference such as the case without the bottom ARC provides less advantage in the maximum DOF range even when reducing resist thickness. It is also found that centered focus positions exhibit a trend to offset in the positive defocus direction with increasing resist thickness, even using a combination of the bottom ARC and the smaller incident angle of illumination light in an off-axis illumination system. The centered focus positions without the bottom ARC are found to show more pronounced swing curve-type behavior with an almost horizontal inclination than that with the bottom ARC.