The extraction algorithm can handle the situation where there is a single ground plane layer, either clear or dark field. Groups connected to ground are always assigned to group number zero. Group zero is only used when a layer has been identified as a ground plane through one of the keywords.
By default, handling of a GroundPlane (clear field) layer is the same as for other Conductor layers, however, in the top-level cell, the largest area group extracted on this layer is assigned to group 0, the ground group. There an alternative mode where all areas of the layer, in any cell, are assigned to the ground group.
There are two levels of support for a dark-field ground plane, indicated by the presence or absence of the MultiNet keyword following ``GroundPlaneClear''. The simplest situation is where the MultiNet keyword is absent. In this case, terminals and contacts with no connection, which would otherwise connect to the GroundPlaneClear layer if that layer were present, are assigned to group 0 (ground).
For example, suppose the technology file contained the following lines:
Layer M0
GroundPlaneClear
...
Layer I0
Via M1 M0
In this case, an area of I0 over an area of M1 and not over an area of M0 would indicate a connection of the M1 area to ground.
To repeat, if the MultiNet keyword does not appear, then all areas outside of the GroundPlaneClear layer geometry are assumed to be above ground. Vias and Contacts that have been specified for the ground plane layer will make contact to ground in the absence of the ground plane layer.
Although this sometimes works for simple cells, it can lead to trouble. Suppose that an island of ground plane metal is used as part of the metalization for the chip pads. This would appear as a hole in the displayed representation of the ground plane layer. Then each pad will be extracted as shorted to ground!
If the MultiNet keyword is given following the GroundPlaneClear keyword, then an internal layer, which is the inverse polarity of the ground plane layer, will be created and used for extraction purposes. The algorithm used for inversion can be specified by an integer 0-2 which optionally follows ``MultiNet''. There are also !set variables which parallel the technology file keywords. Complete information can be found in 16.8.