LVS Debug: The Good, The Bad, and The Future

Extraction errors
Extraction errors
Texted shorts
Shorts found in the extraction process are often just text conflicts that occur due to different text -names being present on the same net. With traditional LVS technology, when shorts are identified after an LVS run, the errors must be corrected and the extraction process run after each error correction, each time lengthening the total time required for the LVS process to achieve an LVS-clean result.

By distinguishing between extraction errors and true comparison errors, advanced LVS debugging techniques enable designers to identify and correct these errors before moving on to the debugging of the true comparison errors. In our example, once Calibre nmLVS has run the LVS process and generated a shorts database, the results are displayed in Calibre RVE, a results viewing tool. The first thing you notice is that extraction errors are now separated from the comparison errors (Figure 1). This separation enables designers to analyze and correct errors created in the extraction netlist, such as shorts created by text conflicts.

Using a Calibre RVE feature called Interactive Short Isolation (ISI), designers can now fix multiple top-level power-ground shorts without having to run the extraction process after each error correction. With ISI, designers identify the polygons constituting a short. They then highlight the shorted path in Calibre DesignRev (a layout viewing tool), step through the polygons, and assign net name text (VDD or VSS) to each of the polygons, based on their knowledge of the design layout (Figure 2). In this way, they can quickly and progressively get to the location of a texted short.

Once designers locate the shorted polygon, they can virtually remove this polygon from the shorts database and run ISI to see if the short is corrected. During ISI, Calibre nmLVS does not launch the extraction process. Instead, it utilizes the extraction information present in the shorts database, which allows ISI runs to generate results very quickly. This speed allows designers to conduct a what-if analysis on their designs to rapidly deduce the optimum solution to fix the shorts. Designers can quickly make a few changes in the design to fix the short, run ISI, and see the results of those changes. At any stage of the analysis, designers can revert back to the original shorts database generated from the Calibre nmLVS run.

If the change made to the shorts database fixes the short, then Calibre RVE displays the next shorted path between the two nets (Figure 3) and the process is repeated until no additional shorts are found (Figure 4).


Of course, at this point, the changes made to fix the shorts are virtual changes to the shorts database, not actual changes made in the design. To actually fix the shorts, the designer must make permanent changes in the design environment to remove the shorted polygons.