Optical lithography in the sub-50-nm regime

The use of immersion technology will extend the lifetime of 193nm and 157nm lithography by enabling numerical apertures (NA) much greater than 1.0. A definition of effective k1 is derived to assist in comparison of various technologies with differing optical characteristics. The ultimate limits of NA are explored by analysis of polarization effects at the reticle and imaging effects at the wafer. The effect of Hertzian or micro-polarization due to the size of the reticle structures is examined through rigorous simulation. For the regime of interest, 20nm to 50nm imaging, it is found that dense features on the reticle will polarize the light into the TE component upwards of 15%. Below this regime, the light becomes polarized in the TM direction. Additionally, oblique incidence on the reticle, resulting from large system NAs and 4x reduction, will cause PSM phase errors. The use of polarization in the illuminator for imaging will result in substantial gains in exposure latitude and MEF when the NA~1.3 with 45nm lines at 193nm. The end-of-line pullback for 2-dimensional patterns is reduced by the use of TE polarization in the illuminator. The overall polarization effects increase with decreasing k1. The lower limit of optical lithography can be extended by using source-mask optimization and double exposure to go below the classical resolution limit, i.e., k1<0.25.