We have developed a process method to suppress the effect of phase errors in alternating phase shift masks. Our method uses double exposure at reversed focus offsets to nullify the intensity imbalance caused by the phase errors. We have evaluated our technique using 120 nm half-pitch line space patterns and found it very successful with remarkable improvement in usable depth of focus without loosing exposure latitude. We also observed that our technique could bring immunity against the lens aberrations such as defocus and astigmatism. (c) 2005 American Vacuum Society.