Inverse lithography technology (ILT)
Beginner
Letting a computer work backwards from the shape you want on the wafer to the best possible stencil.
Novice
Treating mask design as an inverse problem: change the mask, often pixel by pixel, until a simulation of the printed result matches the shape you want.
Expert
Gradient-based optimization of mask pixels through a differentiable lithography model. More precise than edge-based OPC, but the curved results must be broken into shapes a mask writer can draw, which raises cost, so it is used selectively.
Explained in GDS & tapeout (Design Flow).
See also: Optical proximity correction (OPC).