It is very important to understand how the interface interprets the layer sequence. The interface assumes that the technology is planarized, thus for a conducting layer, places where the conductor is absent are assumed to be filled with the dielectric material which is above the conductor. A side effect is that if a conductor is entirely absent, the effective dielectric thickness is actually the dielectric layer thickness plus the thickness of the missing conductor.
It is also worth noting that DarkField layers are polarity-inverted in the interface. If a DarkField layer is missing from the display, within the interface the layer is assumed to be everywhere present. Via layers are always DarkField, as are GroundPlaneClear conductors.
Objects are added to the interface database with the Save Selections button in the RLC Extraction panel General page, or with the corresponding !fx save command. The interface database is entirely separate from the geometry database displayed on-screen. Once a figure is placed in the interface database, any modification or deletion of the object shown on-screen will have no effect on the saved representation in the interface. There is no way to modify the shape of the object within the interface once it has been saved. The interface database can be cleared with the Clear Saved button in the panel.
When an object is saved in the interface, its geometry is split into a trapezoid representation. When running FastHenry, this representation must be ``Manhattanized''. Thus, non-Manhattan geometry is supported, but through this approximation only in FastHenry. The granularity of the Manhattanization is controlled by the FhMinRectSize variable or the corresponding text entry field in the RLC Extraction panel Partition page. The Manhattanized trapezoid list is saved in one long list of rectangles from all objects saved for the layer. When the geometry is reconstructed prior to processing for FastHenry, the entire list will be joined into a minimal set of disjoint Manhattan polygons, each of which is a separate conducting object as seen by FastHenry. Thus, overlapping and touching figures in the main database are effectively merged in the interface database.
When running FastCap, the Manhattanizing step is not done, but the trapezoid list is similarly combined into a minimal set of disjoint polygons, again effectively merging the original database geometry.
Geometry from layers with the Contact technology file keyword is added to the layer that is the target of the Contact specification, if it is included in the layer set.
Only conducting objects are actually added to the interface database. Attempts to add other objects will (silently) have no effect. The related insulators are automatically extracted from the layout when the conductor rectangle lists are joined into polygons. First, a list of vias is extracted, Manhattanized, and clipped to the associated conductor polygons. The vias are extracted to all depths in the cell hierarchy. The resulting list of rectangular vias represent the connection points between conductor layers.
The vias are then represented internally by a column of conductor material of the same type as the upper conductor, which extends down through the insulator and makes contact to the lower conductor. Thus. planarization is maintained at the via.
For FastCap, the dielectric interface planes are then defined. These are the planes at the bottom of each conductor layer, outside of the conducting objects, which represent the interface between the lower dielectric material and the upper dielectric material (recall that the upper dielectric material fills the parts of the conductor layer where the conductor is absent). These plane areas extend outside of the bounding box of all conductors by some distance, which is given (in microns) with the FxPlaneBloat variable, or equivalently with the text field in the RLC Extraction panel Partition page. This distance should be chosen so that the truncation of the dielectric is far enough away from the conductors that the error introduced in capacitance calculation is minimal, which generally means that the distance is large compared to layer thicknesses.
The bounding box is the minimum-sized rectangle that encloses all conducting figures added to the interface. For conducting layers that are DarkField, the layers are polarity-inverted within this rectangle extended by the FxPlaneBloat setting. Thus, a dark field ground plane, for example, will extend beyond the boundaries of the other conductors by this amount. When using DarkField conductors, one must bear in mind that they are always present, as they will be invisible in the display.