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.

Single net tracing

This 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 or use "Edit Layer Stack" in the "Tools" menu. 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.

The layer stack (also referred to as "Connectivity") is also a technology component. This means if you select a technology for layout, the technology specific stack is selected. To edit the stack for all technologies, use the Technology Manager ("Tools/Manage Technologies") and use the "Connectivity" section from the technologies.

A connectivity definition can specify multiple stacks, out of which one has to be selected. This is useful, if your technology setup features multiple routing metal schemes for example. You do not need to setup different technologies for each scheme. If you have declared multiple stacks, you can choose one in the "Trace Net" dialog in the drop-down button right of the "Layer Stack" and the "Configuration" button at the bottom left.

Within the stack definition, 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.

Once a layer stack has been defined and selected, 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.

Sometimes you encounter large nets (e.g. power nets). When you click on such a net, the tracer will start running and the extraction will take a long time. If you're not interested in the details of such nets, you can limit the depth of the net tracing. This means, after a specified number of shapes is encountered, the tracer will stop and report the shapes collected so far as an incomplete net.

To configure the depth, enter the desired number of shapes in the "Trace depth" box at the bottom of the trace dialog. NOTE: the actual number of shapes in the net may be a litte less than the specified depth due to internal marker shapes which are taken into account, but are not delivered as parts of the net.

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.

Tracing all nets

This algorithm is borrowed from the LVS feature, where the scenario is extended by device recognition and netlist formation. In the context of the net tracer, nets consist of the connected shapes but don't attach to devices. As LVS extracts all nets in one sweep, using this feature in the net tracer will deliver all nets at once. Although this is a richer information output, the tracing of all nets is typically faster then tracing a single, big net such as power nets. The LVS net extractor also supports hierarchical processing which gives a considerable performance improvement and more compact net representations.

To extract all nets, use "Trace All Nets" from the "Tools" menu. It will start extracting the nets immediately. It will take the connectivity definition from the standard, single-net net tracer. You can edit this layer stack either from the single net tracer UI, from the technology manager or "Edit Layer Stack" from the "Tools" menu.

After the net tracer has finished, the netlist browser dialog opens. In this dialog you can:

Extracted nets are written and read in a KLayout-specific format called "L2N" ("layout to netlist"). This format contains both the nets and the shapes that make up a net. This way, the traced nets can be saved and retrieved later.