Lithographic Projectors with Dark-Field Illumination

A new resolution enhancement technique (RET) for photolithography that allows the illumination of each and every feature on a reticle to be customized is described. This is important because different feature types, e.g., contact holes, isolated lines, dense gratings, etc., must be illuminated differently if the best possible image and the largest depth-of-focus (DOF) are to be produced. Examples of the imaging performance of a conventional lithographic projector retrofitted with dark-field illumination for three feature types commonly encountered in semiconductor lithography are presented.