Custom Layout Insights
Graham Etchells, Director of Product Marketing at Synopsys
Graham Etchells started in EDA before it was termed EDA. He has held marketing and sales positions at several companies and has been chasing the holy grail of analog/custom layout automation ever since he was a marketing director at Cadence in the mid-1990s. He says past experience indicates we may … More »
Custom Compiler In-Design Assistants (Part 3)
September 20th, 2016 by Graham Etchells, Director of Product Marketing at Synopsys
In the blog ‘Custom Compiler In-Design Assistants (Part 2)’, I outlined how we can use StarRC to report capacitances on critical nets in the layout even when the design is still in flux and not completely LVS-clean. In addition to capacitance reports, we also showed resistance reporting which is critical for FinFET-based layouts. At advanced nodes, the impact of parasitics, electromigration (EM) and restricted design rules drive critical layout choices. Interconnect that does not meet resistance, or EM criteria and unbalanced capacitances on matched nets, can and often does adversely impact layout schedules. So the earlier in the layout phase the layout engineer can address these issues, the sooner he or she can close the design.
EM in particular is a notorious problem in the FinFET process due to the high drive of the transistors and thin metals. So let’s say, for example, the layout engineer has to route a critical net which could be susceptible to the impact of EM. This is a non-trivial task that could be quite challenging. However, if you use Custom Compiler, there are some very cool capabilities that make laying out interconnect that meets EM criteria very quick and very easy.
First, the interactive router can be used to quickly route the net in question and because this is a smart router the pins are tapped to automatically as the trunk is extended, so with only a few clicks the task is done (see Figure 1).
Figure 1. Interactive router with automatic pin tapping
In order to check for electromigration issues, the EM checker needs to know what the currents are on the pins. To do this, the layout engineer simply back-annotates the currents onto the pins from one of the prior simulation runs. The next task is to select the net that was just routed and invoke the EM checker. The results are shown in the electrical reporter pane as shown in Figure 2.
Figure 2. Electromigration report
The report is a ‘pass/fail’ style, where the nets shown in green are passing and those in red are failing. For wires that are failing, the layout engineer can query the report to display additional information on what wire width would remove the violation. In Figure 3, the EM reporter shows that if the wire width on metal 2 and metal 3 was 0.0505µm instead of 0.04µm the violations would be resolved. So by simply increasing the width of the interconnect by 0.01µm the layout engineer can fix the EM issues.
Figure 3. Recommended width to fix EM violations
Again, the interactive router in Custom Compiler’s Layout Assistant makes this task quick and simple. Using the Track Pattern Assistant, the layout engineer chooses the next available wire width for metals 2 and 3 that is greater than 0.0505µm, which, as shown in Figure 4, is 0.06µm.
Figure 4. Track Pattern Assistant
The previously routed net with the old width is deleted and the interactive router is used to quickly re-route the net with the new 0.06µm width, which is automatically snapped to the correct routing track and mask color. When the EM report is re-run, all segments of the net are returned green, indicating a pass for the EM rules.
So, although electromigration can be a critical issue when dealing with FinFET designs, Custom Compiler’s In-Design Assistant for EM reporting and the Layout Assistant for interactive routing make the task of dealing with EM simple and routine.
Check out the Custom Compiler Webisode 4 to see Custom Compiler’s In-Design Assistant EM reporting in action.