Optical proximity correction (OPC)
Beginner
Changing the stencil shapes on purpose so that, after the blurry printing, the shapes on the chip come out right.
Novice
Pre-distorting the shapes on a mask to make up for blurring in lithography: moving edges, widening line ends into serifs or hammerheads, and adding thin assist bars too narrow to print themselves.
Expert
Model-based OPC splits polygon edges into segments, simulates the printed contour with a calibrated optical and resist model, measures the edge placement error at evaluation points, and moves each segment along its normal until the error converges. Full-chip runs need large compute farms.
Explained in GDS & tapeout (Design Flow).
See also: Inverse lithography technology (ILT), Mask data preparation (MDP).