Sensitivity of hyper-NA immersion lithography to illuminator imperfections

The perfect top hat illumination source never exists but is widely assumed and used in lithography simulations for scanner performance investigation, process development and OPC verification. As line width shrinks below 45nm, the simulation error caused by using this idealized top hat source is no longer negligible in the hyper-NA immersion lithography. In this paper, we aim to make a systematic study of the lithography difference between the realistically smoothed and sloped illumination source (smooth source) and the top hat source. The simulation results consist of two parts. In the first part, we carried out a numeric investigation of the lithographic sensitivity for the commonly assumed source imperfections: center-shift, intensity imbalance, geometric ellipticity and energetic ellipticity. In the second part, we investigated the impact of the slope of smooth sources in both radial and azimuthal direction. A smooth source model was used to generate the smooth and top hat sources with such imperfections, and then imported them into simulation software SOLID E for computations. The CD and pattern shift were calculated through pitch and focus. The simulation results showed that the lithographic sensitivity to illuminator imperfection is pronounced. An error up to 5nm CD difference was observed between smooth and top hat sources. This study demonstrates that the prediction accuracy can be significantly improved by using smooth source in simulations in hyper-NA immersion lithography.