A new double exposure technique using two alternating phase shift masks (APSMs) is developed for patterning clear field designs at ultra small critical dimensions. It is called Clear Field Dual APSM Lithography. It is based on the hypothesis that phase conflicts can be avoided for both masks if apertures oriented along the vertical direction are assigned to one mask, and those along the horizontal direction to the other. It is expressed as a heuristic mask synthesis strategy in which each mask is imaged at the full exposure dose. The strategy was validated using aerial image simulations of various design configurations. Interesting results were obtained regarding image stitching, tight patterns, design rules, image resonance and its mitigation, and the beneficial properties of phase edges.