Masks for semiconductor devices and methods of forming masks of semiconductor devices are provided which are capable of improving line resolution. A disclosed mask includes: a first mask pattern disposed on a first side of the mask. The first mask pattern includes light-blocking patterns and light-blocking fine auxiliary patterns within a light-transmitting region. The mask also includes a second mask pattern disposed on a second side of the mask. The second mask pattern includes light-transmitting fine auxiliary patterns within a light-blocking region. The light-transmitting fine auxiliary patterns are disposed at positions corresponding to the light-blocking fine auxiliary patterns to facilitate an overlapping exposing process. The second mask has the opposite tone of the first mask, and the second mask is disposed at a position horizontally-translated from a position of the first mask. Accordingly, pattern bridge regions in repeated patterns of a poly-cell transistor device can be selectively removed.