Enclosure

Enclosure (sometimes called overlap) is the rim by which one shape must extend past another, the band of leftover material after the smaller shape is placed.

A via that sits flush with metal can miss after etch and alignment, so the metal has to remain around the cut. A contact needs both active and metal to hang over it. A 0.05 micrometer enclosure is the difference between a connection and an open. Alignment error is why the rim exists. If the cut can miss by 20 nanometers, the metal must still cover it. A via 0.15 micrometer square inside a 0.25 micrometer metal cover leaves 0.05 micrometer on each side. On screen that pair is a small square inside a larger square, with an even border, and a rule marker appears if the border shrinks on one side.

The smaller square sits inside the larger one, with a rim of the larger shape on every side.

Enclosure is a cousin of width and spacing. Width is how fat a shape is. Spacing is how far it sits from a neighbor. Enclosure is how much of one shape surrounds the other. A design-rule check hunts all three. Putting the rim on the wrong layer fails the pair the rule actually named. Metal around a via is not the same requirement as via around metal. DRC records the pair as M1.ENC.V1, which is not V1.ENC.M1. One-sided enclosure, flush on one edge, is a special rule with its own name.

The articles enclosure, vias and contacts, and design rules and DRC follow that rim.