US Patent 7494749 Photolithography using interdependent binary masks

The invention, in its various aspects, is an interdependent binary photomask for use in a photolithography operation in a semiconductor fabrication process, a method for fabricating these interdependent photomasks, and a method of using the same. The photomask comprises a first binary reticle and a second binary reticle. Each binary reticle includes a pattern formed on a plate, but the pattern formed on one plate is interdependent with the pattern formed on the other plate so that the reticles are used in tandem to transfer the pattern onto wafers having features residing in different focal planes. The method of fabricating the interdependent binary photomask consequently includes specifying a first and a second portion of a circuit layout, the first and second circuit portions being interdependent. The first and second portions are digitized and used to form first and second interdependent patterns on separate reticles. In use, the first reticle is aligned with a portion of a wafer and the wafer portion is then exposed. The second reticle is then aligned with the same wafer portion and the wafer portion exposed.