The Net Tracing Feature
The net tracing function allows tracing of a net by detecting touching shapes that together form a conductive region. It features specification of a stack of metal (or in general "conductive") layers optionally connected through via shapes. The net tracing algorithm will follow connections over the via shapes to form connections to other metal layers. The material footprint can be derived from single layout layers or a boolean combination of several layers. For example, this allows selecting source and drain regions of transistors by subtracting the poly region from the active area region.
The algorithm is intended for extracting single nets and employs an incremental extraction approach. Therefore extraction of a single small net is comparatively fast while extraction of large nets such as power nets is considerably slower compared to hierarchical LVS tools currently.
The net tracing function can be found in the "Tools" menu. The user interface allows tracing of multiple nets which are stored in a list of nets extracted. If labels are found on the nets, these are used to derive a net name. Beside that, the cells which are traversed in the net extraction are listed, so the cells being connected by this net can be identified.
Before nets can be extracted, a layer stack must be specified. Press "Layer Stack" on the user interface to open the layer stack dialog. Layers must be specified in the "layer/datatype" notation. The via specification is optional. If no via layer is specified, both metal layer shapes are required to touch in order to form a connection. If a via layer is specified, a via shape must be present to form the connection.
KLayout allows specification of symbolic layers and to use boolean expressions. That way it is possible to assign meaningful names to layers (i.e. "POLY" or "VIA1") and to use derived layers (i.e. "ACTIVE-POLY" for the source and drain regions of a CMOS transistor). Read more about these features in Connectivity and Symbolic Connectivity Layers.
If a layer stack has been defined, a net can be traced by pressing the "Trace Net" button and clicking on a point in the layout. Starting from shapes found under this point, the net is extracted and listed in the net list on the left side of the net tracing dialog. If "Lock" is checked, another net can be traced by clicking at another point without having to press the "Trace Net" button again.
The "Trace Path" function works similar but allows specification of two points and let the algorithm find the shortest connection (in terms of shape count, not geometrical length) between those points. If the points are not connected, a message is given which indicates that no path leads from one point to the other.
The display of the nets can be configured in many ways. The configuration dialog is opened when "Configure" is pressed in the trace net dialog. Beside the color and style of the markers used to display the net it can be specified if and how the window is changed to fit the net.