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DOF enhancement for contact holes by using Nikon's CDP option and its introduction into production

Historically the primary methods used to achieve the industries ever-tightening resolution requirements were reduction of exposure wavelength and increased projection lens NA. Today however, photo engineers are pushing optical lithography well beyond the realm of what was once considered practical. Specific scanner exposure tool features have to be implemented to achieve the aggressive imaging objectives. One such example is to use focus drilling to expand the depth of focus for contact layers. This paper describes the implementation of focus drilling through the Continuous DOF expansion Procedure (CDP). In CDP, the wafer is tilted along the scanning direction, while the wafer stage continuously moves upward or downward during exposure. CDP technology provides an enhanced process window with initial data showing a 30% improvement in DOF for 250-nm contact holes. It also eliminates the need for double exposures and therefore maintains high throughput, comparable to standard wafer exposure.