Overcome the process limitation by using inverse lithography technology with assist feature

Patterning of contact hole using KrF lithography system for the sub 90nm technology node is one of the most challenging tasks. Contact hole pattern can be printed using Off-Axis Illumination(OAI) such as dipole or Quasar or Quadrupole at KrF lithography system. However this condition usually offer poor image contrast and poor Depth Of Focus(DOF), especially isolated contact hole. Sub-resolution assist features (SRAF) have been shown to provide significant process window enhancement and across chip CD variation reduction. The insertion of SRAF in a contact design is mostly done using rule based scripting. However the rule based SRAF strategy that has been followed historically is not always able to increase the process window of these 'forbidden pitches' sufficiently to allow sustainable manufacturing. Especially in case of random contact hole, rule-based SRAF placement is almost impossible task. We have used an inverse lithography technique to treat random contact hole. In this paper we proved the impact of SRAF configuration. Inverse lithography technique was successfully used to treat random contact holes. It is also shown that the experimental data are easily predicted by calibrating aerial image simulation results. Finally, a methodology for optimizing SRAF rules using inverse lithography technology is described. As a conclusion, we suggest methodology to set up optimum SRAF configuration with rule and inverse lithography technology.