US Patent 7394080 Mask superposition for multiple exposures

An exposure system includes a mask stage module adapted for holding a first mask and a second mask, wherein the first mask is configured for illumination by a first beam to form a transformed first beam having a first pattern from the first mask and the second mask is configured for illumination by a second beam to form a transformed second beam having a second pattern from the second mask. The exposure system also includes a beam combiner configured to combine the transformed first and second beams to form a resultant beam, wherein the resultant beam is projected into a substrate coated with a photoresist layer.