By default, a layer named ``FCAP'' will serve as the masking layer. If, however, the FcLayerName variable is set to a layer name, that layer will provide the masking function. We will refer to this layer as the ``mask layer''.
If no objects are found on the mask layer, all geometry in the current cell will be treated in the interface. Be advised that the capacitance extraction will very rapidly become untenable if too much geometry is included. The interface itself is not designed to handle large object collections, though it will remain snappy while generating files that may take weeks to run. More than 100 objects is probably pushing things. The effective area of interest (AOI) is the bounding box of the current cell.
If objects are found on the mask layer, then the mask layer pattern is anded with the other layers, and the resulting geometry is processed by the interface. The bounding box of the mask layer patterning becomes the effective AOI, which can be much smaller than the cell bounding box. In any case, the AOI bounds all geometry in the problem.
The geometry is as shown in the drawing window, though geometry is saved in an internal representation that removes any overlap of objects in the original layout. Outside of the AOI, and above all geometry and below the substrate, vacuum (relative permittivity of 1.0) is assumed.
The substrate is included in the calculation as follows. A variable named SubstrateThickness can be set to specify the assumed substrate thickness in microns. If not set, a thickness of 75 microns is assumed. Typically, the substrate thickness would be set in the technology file with the SubstrateThickness keyword, which sets the variable. It can be set interactively from the Params page of the Cap Extraction panel (see 16.17.2).
If the substrate has nonzero thickness:
The boundary of the substrate is taken as the AOI, bloated by the value given by the FcPlaneBloat variable. This is generally desirable to move the substrate edge effectively away from structures of interest. If not set, a value of 0.0 micron is assumed.
Interface panels will be created on the sides and bottom of the substrate when the input list file is generated. If a positive FcPlaneBloat is given, dielectric interface panels will also cover the top of the substrate outside of the AOI.
If the substrate has zero thickness:
This is obtained by setting the SubstrateThickness variable to 0. We attempt to treat the substrate as filling the infinite half space, though is is not clear how to convey this to FasterCap. Outside of the AOI, the substrate/vacuum interface extends to infinity. We approximate this with finite panels extending a distance given by FcPlaneBloat out of the AOI.
NOTE: the original interface made no attempt to deal with the substrate. This is reasonable, as the different substrate treatments should have little effect on results in most cases.
Note also that the FcPlaneBloat parameter extends the substrate only, and not the geometry. This is different from the original interface, which would also extend the dark-field layers. To effectively bloat the geometry as well as the substrate, one can use the FCAP layer in most cases to enlarge the AOI.