Bulk Area Connectivity Extraction - Identify the Bulk Area

For the extractor to recognize the bulk area of a device placed over the substrate or a well area, the bulk terminal should either have:

Physically drawn shapes that represent the areas to be extracted.
A derived region that represents the areas to be extracted.

In both cases, the bulk terminal must be explicitly set as soft-connect for the extractor to recognize it as a bulk connection.

If the bulk terminal is implementing a tap to the substrate or the well, it is being used to polarize the substrate or the well, so the bulk terminal should not be set as soft-connect.

You can create a soft-connect bulk terminal by using the dbSetTermSoftConnect SKILL API. Because the terminal is soft-connect, it does not draw current and is used only to polarize the bulk area of the corresponding device. The supported direction for polarization is from the substrate or well layer toward the bulk instance terminal. This allows the bulk area of the device to be polarized from an appropriate external source, but does not allow the connectivity to propagate from the bulk instance terminal to the outside of the device.

As usual, the connectivity of a bulk terminal is provided by the associated top-level net attached to the instance terminal.
Even though it is recommended to explicitly set bulk area terminals as soft-connect, a terminal that has only pins on well layers is considered to be effectively soft-connect even if the bulk is not explicitly defined.

The connectivity extraction of a bulk terminal varies on a case basis:

If the bulk terminal is implemented by at least one extractable pin figure, the extractor uses the corresponding geometries to detect any top-level overlap on these physical layers.
If the bulk terminal is not implemented by any extractable pin figure (shapeless bulk terminal), the extractor derives the bulk area for that terminal using a derived layer. The derived layer represents the intersection between two or more physical layers, typically poly and diffusion. To set the connection between the bulk terminal and this derived layer, use the dbSetSoftConnectTermPinlessLayer SKILL function.
For example, the bulk area of an NMOS device placed on a p-type substrate can be a shapeless bulk for which the bulk area is derived based on the “gate area”. In that case, you can use the following definition to derive the bulk area:
```
techDerivedLayers((gateArea 10001 (PO 'and OD)))
```
For the extractor to recognize the derived bulk area for connectivity extraction, the derived layer that represents the bulk area must be defined as a valid layer and used as a parameter for dbSetSoftConnectTermPinlessLayer. By default, a shapeless bulk terminal connects to the substrate. However, if you need to redefine this connection, you can do so using the dbSetSoftConnectTermConnectToLayer SKILL function.
For a device having several bulk areas, either on the same bulk terminal or on different ones, the bulk areas should be physically connected so that they are properly polarized. Otherwise, the extractor creates open markers to indicate that the bulk areas should be connected. In the case of a device having several bulk terminals, such as when implementing multi-fingered devices, all the bulk terminals must be set as must-connect terminals to allow for proper detection of the unconnected bulk areas by the extractor.

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