Multiple patterning
Beginner
Printing one very dense wiring layer in two or more steps because its lines are too fine to print at once.
Novice
Printing one very dense layer with two or more masks (the stencils light is shone through), because its lines are too close together to print in one exposure. Shapes that are too close must go on different masks, so the router labels each shape with a “color” for its mask.
Expert
Litho-etch-litho-etch (LELE) needs a valid two-coloring of the conflict graph; an odd cycle cannot be two-colored, so a shape must be split or a wire rerouted. Spacer-based patterning (SADP, SAQP) makes regular rows of lines that favor on-track, one-direction wiring, with line ends set by separate cut masks.