Chapter 5
Computational Lithography with Coherent Illumination
Due to resolution limits of optical lithographic systems, the electronics industry has relied on RET to compensate and minimize mask distortions as they are projected onto semiconductor wafers [92]. Resolution in optical lithography obeys the Rayleigh criterion resolution 
, where 
is the wavelength, NA is the numerical aperture taking on values around 0.9 for most lithography systems used today, and 
is the process constant that can be minimized through RET methods [11, 37, 76, 77]. OPC methods modify the mask amplitude by the addition of subresolution features to the mask pattern. PSMs, commonly attributed to Levenson et al. [35], induce phase shifts in the transmitted field that have a favorable constructive or destructive interference effect. Thus, a suitable modulation of both the phase and the intensity of the incident light can be used to effectively compensate for some of the resolution-limiting phenomena in optical diffraction.
Several approaches of OPC and PSM optimizations have been proposed in the literature. These range from heuristic and empirically based design rules to computationally expensive optimization-based ...
