Suppressing ringing effects from very strong off-axis illumination with novel OPC approaches for low k1 lithography

Abstract

With the delay in commercialization of EUV and the abandonment of high index immersion, Fabs are trying to put half nodes into production by pushing the k1 factor of the existing scanner tool base as low as possible. A main technique for lowering lithographic k1 factor is by moving to very strong offaxis illumination (i.e., illumination with high outer sigma and a narrow range of illumination angles), such as Quadrapole (e.g., C-Quad), custom or even dipole illumination schemes. OPC has generally succeeded to date with either rules-based or simple model-based dissection together with target point placement schemes. Very strong off-axis illumination, however, creates pronounced ringing effects on 2D layout and this makes these simpler dissection techniques problematic. In particular, it is hard to prevent overshoot of the contour around corners while simultaneously dampening out the ringing further down the feature length. In principle, a sufficiently complex set of rules could be defined to solve this issue, but in practice this starts to become un-manageable as the time needed to generate a usable recipe becomes too long. Previous implementations of inverse lithography demonstrated that good CD control is possible, but at the expense of the mask costs and other mask synthesis complications/limitations. This paper first analyzes the phenomenon of ringing and the limitations seen with existing simpler target placement techniques. Then, different methods of compensation are discussed. Finally, some encouraging results are shown with new traditional and inverse experimental techniques that the authors have investigated, some of which only demand incremental changes to the existing OPC framework. The results show that new OPC techniques can be used to enable successful use of very strong off-axis illumination conditions in many cases, to further reduce lithographic k1 limits.