Mask optimization to increase process margins of optical lithography

摘要

Optical lithography uses laser wavelength of 193 nm to print integrated circuit features as small as 45 nm. This sub-wavelength resolution is possible due to the various resolution enhancements "tricks" applied to the optics and to the mask. Sub-resolution assist features (SRAFs) belong to the class of "mask tricks". SRAFs are small non-printing features that help to increase process margins by leveling off image intensity and consequentially reducing process sensitivity to the focus and dose variations. Currently the placement and size of SRAfs is decided by complex set of rules, usually organized into tables that map 1D spacing between main features into the placement and size of SRAFs. We present here a study of the model-based technique for SRAF insertion, base on the ideas of inverse lithography. We show that it increases process margins and delivers solutions that are superior to the rule-based SRAFs.