Off-grid

Off-grid is a corner or edge that does not sit on the allowed crossings the factory published.

Mask tools and rule decks reject that miss, or they round it and change a space that looked available. Off-grid appears after a scale, after a rotation by a messy angle, after an import from mechanical CAD, or after a copy from a cell drawn on a finer grid than the target process. The evidence is a rule marker on a vertex, or a coordinate such as 1.2037 µm when the manufacturing grid is 0.005 µm. A 0.002 micrometer miss is enough.

The gold rectangle sits between the dots. No corner meets a dot.

Snap prevents new misses. It does not heal old ones. The stored tick is a DBU. Off-grid means “not a legal multiple,” not “smaller than a DBU.” Some all-angle edges can still have endpoints on grid. The color fill is innocent. The vertex list is where the miss is stored.

The repair is mechanical. Move the vertex, or run a snap-to-grid command, then recheck width, because a 0.002 micrometer move can shrink a space. A rotated instance at 15° can sprinkle off-grid corners through a whole array. Rotate on allowed angles, or flatten and snap once, on purpose. Moving a vertex onto a legal point is snapping and off-grid. The points that count as legal are the grid. A shape that misses them can fail a check even when it looks wide enough, which is design rules and DRC.