Intensive Optimization of masks and sources for 22nm lithography

Traditional OPC is essentially an iterated feedback process, in which the position of each target edge is corrected by adjusting a controlling mask edge. However, true optimization adjusts the mask variables collectively, and in so-called SMO approaches (for Source Mask Optimization) the source variables are adjusted as well. SMO's exploitation of complex source designs is shown to provide superior solutions to those obtained by mask optimization alone. Moreover, in development work the ability to adjust the source opens up new options in process engineering, and these will become particularly valuable when future exposure tools provide greater flexibility in programmable source control.