With the expected continual progress of micro-electronics scaling, low k1 techniques may be required even with EUV
lithography. One of important techniques of low k1, the off axis illumination (OAI) in combination with sub-resolution
assist features (SRAF) on reticles, has been used extensively in optical lithography. Use of assist features combined with
off axis illumination typically requires extremely small pattern sizes. The assist pattern enables printing dense and
isolated lines simultaneous.
In a low k1 region of around 0.4, assist features will increase depth of focus (DOF) of isolated and semi-isolated lines
even in EUV. Since EUVL process operates at a relatively higher k1 value than that for the optical lithography, the assist
feature size needed is relatively smaller. In addition, with the mask shadowing effect of EUVL, all horizontal lines
should be biased thinner by a couple of nanometers, and horizontal assist features will need to do the same. Fabricating
such narrow features on masks is challenging, and could potentially limit the application of SRAF in EUVL in the low k1
A novel approach is proposed to create assist features with similar width as the main critical dimension features. The
proposed technique creates assist patterns using thinner absorber which would have higher reflectance than normal
absorber. Thinner absorber assist pattern can perform similarly with narrower assist pattern and easier to fabricate. With
off axis illumination in EUVL and assist patterns, process margin of semi-isolated and isolated lines can be increased for
k1 lower than 0.4.