Hyper-NA imaging of 45nm node random CH layouts using inverse lithography

Abstract

The imaging of Contact Hole (CH) layouts is one of the most challenging tasks in hyper-NA lithography. Contact Hole layouts can be printed using different illumination conditions, but an illumination condition that provides good imaging at dense pitches (such as Quasar or Quadrupole illumination), will usually suffer from poor image contrast and Depth of Focus (DOF) towards the more isolated pitches. Assist Features (AF) can be used to improve the imaging of more isolated contact holes, but for a random CH layout, an AF placement rule would have to be developed for every CH configuration in the design. This makes optimal AF placement an almost impossible task for random layouts when using rule-based AF placement. We have used an inverse lithography technique by Mentor Graphics, to treat a random contact hole layout (drawn at minimal pitch 115nm) for imaging at NA 1.35. The combination of the dense 115nm pitch and available NA of 1.35 makes the use of Quasar illumination necessary, and the treatment of the clip with inverse lithography automatically generated optimal (model-based) AF for all geometries in the design. Because the inverse lithography solution consists of smooth shapes rather than rectangles, mask manufacturability becomes a concern. The algorithm allows simplification of the smooth shapes into rectangles and greatly improves mask write time. Wafer prints of clips treated with inverse lithography at NA 1.35 confirm the benefit of the assist features.