For tight pitch patterning with sub-wavelength mask features, simulations and wafer data show that many mask stacks
that provide superior image contrast, can provide inferior MEEF performance. For example, 6% MoSi EPSM is found
to have higher MEEF than binary masks despite having better contrast and exposure latitude when equal lines and spaces
on the mask are used to pattern equal lines and spaces on the wafer. Likewise, the deposition of SiO2 on-top of the
chrome surface of a binary mask improves contrast but degrades MEEF compared to a binary mask. When contrast is
varied by mask stack or by print bias, MEEF is poorly correlated with contrast and often increases with increasing
contrast. The optimal print bias for exposure latitude is significantly different than the optimum print bias for MEEF.
MEEF, on the other hand, is highly correlated with the difference between maximum and minimum intensity when one
varies mask stack, print bias and illumination. Analytical MEEF equations are derived that support this strong
relationship between MEEF and the difference between maximum and minimum intensity.