A 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. The method and system comprises locating regions in a finished semiconductor design that do not contain as-designed shapes. The method and system generates dummy fill shapes in the regions at a predetermined final density and sizes the generated dummy shapes so that their local density is increased to a predetermined value. The method and system further creates corresponding trim shapes that act to expose an oversized portion of the dummy shape, effectively trimming each dummy shape back to the predetermined final density. The method and system can be implemented on a computer program product comprising a computer useable medium including a computer readable program.