Virtuoso Meets Maxwell: Keeping Things Simple in the Virtuoso RF Solution

13 Apr 2020 • 5 minute read

'Virtuoso Meets Maxwell' is a blog series aimed at exploring the capabilities and potential of Virtuoso® RF and Virtuoso MultiTech. So, how does Virtuoso meet Maxwell? The Virtuoso platform supports RF designs, and RF designers can measure physical and radiation effects by using Maxwell's equations. In addition to providing insights into the useful software enhancements, this series broadcasts the voices of different bloggers and experts about their knowledge and experience of various tools in the Virtuoso IC-Packaging world along with the nuances of RF, microwave, and high frequency designs. Watch out for our posts on Mondays.

We have all heard the sayings “Less is more” and “Keep it simple”. Electromagnetic simulation is an activity where following that advice has enormous payoffs. If you’re a regular reader of the Virtuoso meets Maxwell blogs, you might remember Kabir’s multi-part series Help With Electromagnetic Analysis. In Help With Electromagnetic Analysis - Part III he introduces Virtuoso RF Solution’s automatic shape pre-processing. If you have not read this, I highly recommend it.

In this blog I’ll talk about some of my experiences with how Virtuoso RF Solution’s shape simplification feature has helped my customers get significant performance improvements with minimal impacts on accuracy.

Let’s first review the reason that you would want to perform shape simplification. Think about a component that you want to analyze in an EM solver – an inductor for example. Along with the structures that comprise the coil itself, you probably have some type of metal fill in the center of the coil to make sure your layout obeys metal density rules. The metal fill will most likely have a minimal impact on your electromagnetic results, but it will almost certainly have a dramatic negative impact on simulation time. Removing or merging such a structure requires making a unique copy of the layout, perhaps flattening the shapes of your PCell, and maybe a good chunk of tedious hand editing.

Virtuoso RF Solution provides you with an automated way to do all of this within the context of the original design. All you need to do is to formulate a set of simplification directions, or a ‘recipe’, that describes how the shapes are to be processed. Virtuoso will do the hard work for you, with speeds that are orders of magnitude faster than any manual method. Let me share a couple of examples with you.

My first example of shape simplification use is very similar to the metal fill use case described above. The customer’s goal was to analyze a handful of inductors from their foundry PDK that included fill:

The above screenshot is from a generic case but is a good example of the real-world problem.

Acting on the knowledge that this fill would have only a minimal impact on simulation results, I helped the customer create a recipe file that removes all metal on the fill layers smaller than 2umx2um, which was the size of the shapes in the actual design:

This very trivial recipe took a simulation of three inductors from just over ten minutes without simplification down to just under 15 seconds with simplification. While we can all agree that ten minutes is not a huge block of time, a simple simplification step yields results 40x faster. It took me less than a minute to set up. And the results? Nearly identical (<1%) return and insertion loss, Q and Leff from DC to anticipated operating frequency. And the best part? As the size and complexity of the simulated structures grows, the potential performance improvements can improve dramatically. Let’s look at another real case.

My second example is a far more complex case where a multiple metal layer ground mesh is used as a shield structure. While the details of the design are confidential, I can share that the structure was made up of a very intricate pattern easily containing a few million shapes. No EM simulator could be used to analyze the design as-is, and so the customer was faced with the painstaking processes of manually merging and smoothing the shapes. When we first started our evaluation of Virtuoso RF Solution with this customer, they were working on manual simplification of a small subset of the design in hopes of being able to use it with a non-Cadence EM tool. I wrote a simple recipe file to perform a per-layer merge of the mesh shapes and also clustered the associated vias into 4x4 arrays. With this change, Virtuoso was able to leapfrog the customer’s progress with an accurate EM simulation in 95 minutes, including the time to perform simplification. In this case we’ve shortened the run time from multiple days down to mere hours, again with acceptable results. With these types of improvements, it’s feasible to do multiple design iterations in a day rather than only one or two a week. You will also likely find that you are able to tackle structures that are too large to realistically simulate with your legacy EM flow.

I hope this has motivated you to take a closer look at Virtuoso RF Solution's shape simplification flow. Whether tackling a handful of passive devices in your design or analyzing structures previously out of reach, this capability will significantly improve your electromagnetic simulation throughput.

Related Resources

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About Virtuoso Meets Maxwell

Virtuoso Meets Maxwell series includes posts about the next-generation die, package, and board design flow with a focus on reinventing and optimizing the design process to ensure that the designer remains a designer! Keep watching!

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Kristin Fullerton