Methods are presented of forming sub-lithographic patterns using double exposure. One method may include providing a photoresist layer over a layer to be patterned; exposing the photoresist layer using a first mask having a first opening; developing the photoresist layer to transfer the first opening into the photoresist layer, forming a boundary in the photoresist layer about the transferred first opening that is hardened; exposing the photoresist layer using a second mask having a second opening that overlaps the boundary; and developing the photoresist layer to transfer the second opening into the photoresist layer, leaving the boundary, wherein the boundary has a sub-lithographic dimension.