validLayers – Stop Layers Specification

You can also use the validLayers constraint to specify the layers that stop another layer, such as when a transistor is implemented as a single piece of diffusion cut into two parts by a poly shape. The two parts in this case, form the drain and the source of a MOS device and the poly shape forms the transistor gate.

A stop layer is defined using a derived layer with a NOT operation between two layers.

For accurate connectivity extraction, if the validLayers constraint contains a poly and a diffusion layer, Cadence recommends that you check that the poly is intended to stop the diffusion and the derived layers are appropriately defined in the validLayers constraint. If the constraint is not appropriately defined in the technology file, a warning message is issued and the extractor may report false shorts.

Consider the following derived layer definitions in the validLayers constraint:

(StoppedLayer 10001 (Oxide 'not Poly))

(validLayers (StoppedLayer))

The constraint definition indicates that the:

Poly and Oxide are extractable layers.
Oxide is being stopped by Poly.

Consider another example where a layer-purpose pair “stops” another purpose pair on the same layer:

(D1 10001 (Metal 'select slot))

(StoppedLayer 10002 (Metal 'not D1))

(validLayers (StoppedLayer))

The constraint definition indicates that the:

All purposes of Metal1 are extractable.
(Metal1 slot) “stops” all other purposes of Metal1

In addition to enabling poly or metal shapes be stopped by shapes on another valid layer, Layout XL supports that standalone cut shapes or cut shapes inside vias be stopped by another shape. For this, the cut layer and the stop layer should be defined in the validLayers constraint.

Consider the following derived layer definitions in the validLayers constraint:

(StoppedVia1 10040 ( Via1 'not StopVia1 ))

(validLayers (StoppedVia1))

The constraint definition indicates that Via1 is being stopped by StopVia1.

For information on how stop layers can be set at the global level so that all the layout users are benefited by the global settings, see Issues Faced when Setting up the PDK to Improve Layout XL Compliance.

Virtuoso Layout Suite XL: Connectivity Driven Editing User Guide
Product Version IC23.1, November 2023