A phase shifting mask set and method of suing the phase shifting mask set to pattern a layer of negative photoresist. The mask set comprises a first phase shifting mask and a second phase shifting mask. The first phase shifting mask has regions of 90° phase shift and −90° phase shift in the contact hole regions of the mask. The second phase shift mask also has regions of 90° phase shift and −90° phase shift in the contact hole regions of the mask. In the second phase shift mask the 90° phase shift regions are rotated 90° spatially with respect to the 90° phase shift regions of the first phase shift mask and the −90° phase shift regions are rotated 90° spatially with respect to the −90° phase shift regions of the first phase shift mask. A layer of negative photoresist is exposed with the first and second phase shift masks and developed to form the photoresist pattern used to form contact holes.