A design structure, method, and system for partitioned dummy fill shapes for reduced mask bias with alternating phase shift masks, or with other two-mask lithographic processes employing a trim mask. A design structure is embodied in a machine readable medium used in a design process, the design structure comprising regions in a finished semiconductor design that do not contain as-designed shapes. The design structure additionally includes dummy fill shapes in the regions at a predetermined final density, wherein the generated dummy shapes are sized so that their local density is increased to a predetermined value. Moreover, corresponding trim shapes act to expose an oversized portion of the dummy shape, effectively trimming each dummy shape back to the predetermined final density.