Impact of illumination source symmetrization in OPC

It is well known that optical proximity effects are highly dependent upon the details of the illumination source. Tremendous effort is taken to match illumination source profiles between tools, as well as to appropriately represent the source intensity distribution in the models used for OPC and post-OPC verification. OPC software typically models the intensity profile in such a manner that empirical fitting of the CD data during model calibration can result in a representation of the "effective" source. In some cases, an actual measured source profile is available and can be referenced directly in the OPC recipe. However, it is common to average the 4 quadrants of a measured source profile such that the source representation is symmetrical about the x and y axes. This is done so that optical proximity correction can be applied hierarchically, with a single correction applied to a cell which may be instantiated in multiple orientations within the chip. It has generally been accepted that the positive runtime benefit accompanying this symmetrization is beneficial relative to any potential accuracy loss for cells oriented in different directions. In this paper, we investigate the impact of real source profile asymmetries on identical features with different orientations.