'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? Now, Virtuoso supports RF designs, and the RF designers measure the physical and radiation effects by using the 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.

Hi Folks, I hope you have been enjoying our Virtuoso Meets Maxwell series and maybe even had a chance to read my last blog titled Help With Electromagnetic Analysis - Part 1. In that blog, I introduced the different steps involved in Electromagnetic (EM) analysis from Virtuoso and then dove into some of the details of setting up the process stackup. Today, let’s discuss the next step, which is to select the geometry that you want to analyze.

How do you decide what geometries to analyze? Well, let me ask you this. Why do you even want to analyze a part of your layout using an EM analysis engine? If you’re following a somewhat traditional design flow, you may have already put together a schematic for your design, let’s say a 2.4 GHz Low Noise Amplifier (LNA), set up your testbench in Virtuoso Schematic Editor (VSE), your analyses and measurements in Analog Design Environment (ADE), run your circuit simulations using Spectre, and verified that the LNA is meeting the specification (Gain, Noise Figure, Rejection Ratio, etc.). In parallel to all of this, you or your layout engineer may have started assembling the layout of the LNA. How will you guarantee that your layout will also meet the specification that your circuit did? In other words, how do you do post layout verification? You can use an RC extraction engine like Quantus^TM for most of the layout. But, for inductors and RF input/output nets, RC extraction is not going to be accurate enough and you need to analyze/model those using an EM solver.

There are other scenarios. For example, let’s say you’re designing a filter or a passive network and you may choose to start with the layout, i.e., you may not even have a schematic. In this case, you will need to create a model of your layout by using an EM solver.

Regardless of your design flow, if you are designing something for multi GHz or high clock rate, you have to take RF or high frequency effects into account.

What do you mean I have to take high frequency effect into account? Aren’t circuit simulators already modeling my circuit? True, circuit simulation using device models and even basic net models (with Rs and Cs) will take the first order effects into account. However, as your operating frequency or clock speed increases (and if you take up to the third harmonics into account, this becomes even more crucial), the wavelength decreases and hence the electrical size of your layout becomes larger and larger, to the point where Kirchhoff’s Current and Voltage (KCL and KVL) laws will not work anymore. Depending on how tightly electrical and magnetic fields are coupled (and this increases as electrical size of the design increases), a quasi-static or complete solution of Maxwell’s equations may be needed to accurately predict high frequency behavior.

By the way, sorry for taking a slight detour and I don’t care if you think I am a geek, but James Clerk Maxwell is a personal hero of mine! He formulated the theory of electro-magnetism and demonstrated that electricity, magnetism, and light are different manifestation of the same phenomenon.

OK, fair enough, we need to analyze our layouts using EM solvers, but which parts of the layout? Why, the passive parts, of course. EM solvers don’t mess with active devices. And, even within the passive parts, you only want to analyze the paths that carry RF/high frequency. Remember that EM analysis can be computationally costly. So, you just don’t want to throw entire layouts to your EM solver. Typically, I’d start with the obvious choices: individual inductors, then multiple inductors to understand if they are coupling, and then gradually add nets. In other words, you will need to find out how you can partition your layout for EM analysis and RC analysis, and this is not always obvious. It takes some experience with EM tools before you can make educated guesses, but regardless of whether you’re an expert or a beginner, a unified flow like Virtuoso RF can definitely help. And, I am here to tell you how.

Wait a second, wait a second, what is this coupling you’re talking about? High frequency coupling is what causes all the problems, and this is why I consider Maxwell to be my hero! Maxwell’s equations tell us that as electrical size of a layout increases, current going through one metal will induce electrical and magnetic fields in a nearby metal. This, in a nutshell, is coupling or cross-talk. In other words, for RF circuits, RF signals do not stay confined within their intended paths. Let me give you two specific examples of high frequency coupling.

Let’s say, you have two inductors in your layout. You have models for each inductor. The models predict the behavior of the inductors at high frequencies. However, in your layout, if you are not careful and haven’t put sufficient space between the two inductors, at high frequencies, the behavior of the two inductors will not follow what their individual models will predict; it doesn’t matter how good the individual models are. This is because of any unintended coupling between the two coils.

Another example of high frequency coupling is having two metal routes lying side by side and only one of them being excited with current will demonstrate current in the other route as well.

OK, so we may have coupling, fine, then why not simply space these far apart so that there will be no coupling? Yes, if you could do that then all your problems would be solved. However, remember that feature sizes are shrinking, operating frequencies are increasing, and layouts are getting denser and denser, not only in 2D, but also in 3D. High frequency coupling is inevitable, in fact, it is almost impossible to predict high frequency coupling in the absence of using sophisticated EM solvers. Let me give you another example. Let’s say there is an inductor in your IC. This IC sits on a PCB. How will you make sure that a route on the PCB will not couple with the on-chip inductor and cause its performance to degrade? Remember, your responsibility may only be to design the IC, but at the end of the day, when your IC is part of a bigger system, if it fails because you did not account for this unintended coupling between the PCB route and the IC inductor, then you have failed to deliver! That is why EM analysis is a very important part of a design flow.

So, how does Virtuoso RF help me in selecting the geometry for EM analysis? I am glad you finally asked that! The EM solvers (we have 3) integrated within Virtuoso RF have a very deep integration with the layout environment and this makes the difference. Because of this integration, we allow you, the user, to select geometries (instances and nets) from your layout without having to modify the layout. In other words, the flow does not mandate you to have a different layout for manufacturing, and a “cleaned” layout for running EM analysis. You may wonder what is wrong with having multiple layouts. The problem is after running 20-30 different EM simulations and creating as many layout variants, at some point you will lose track of which layout is golden anymore, which one did you change based on analysis, which one is truly the one that should be sent to manufacturing. If you have ever gone through a similar exercise you will sympathize with me!

In Virtuoso RF, there are several techniques that will help you easily select the nets and instances and put them in an EM model. Please refer back to Kris’ Add Some MAGic to Your ElectroMAGnetic Analysis blog where she walks through the entire flow and shows you how to use the Electromagnetic Solver Assistant to select the structures. Let me highlight three main steps.

Selecting a Layer

Your design may have shapes on a variety of layers. However, remember that EM analysis typically includes layers from M1 and above. But what happens if there are other layers under the structures of your interest? Virtuoso RF has a layer mapping capability that lets you define the EM layers. Only geometries from the mapped layers will be sent to the EM solver. For example, let’s say in your inductor Pcell, you have some shapes on a layer called CA, but you don’t want those shapes to be sent to the solver. All you have to do is not define CA as an EM layer.

Selecting Nets and Instances

Because the EM model can be populated based on selected nets and instances, you have full control on what gets sent to the EM solver. The selection can be done directly on the canvas or from the Navigator. So, if you are worried about not including active devices in your EM analysis, then don’t worry because you will analyze only what you add to the EM model. But, this also means that you don’t have to “clean” or purge your layout to make it EM friendly. You can simply select nets and instances, and add them to your EM model.

But, wait, it gets even better! We offer something called a cutting boundary.

Drawing a Cutting Boundary

The cutting boundary can be a rectangle or a polygon. By applying this cutting boundary, you can define an inclusion box. Let’s say my EM model contains a net called netA and inductor. If I want to take a slice of netA and the inductor (why would I want to do that? Hint, that’s the slice that will most likely couple with the inductor), I can simply apply a cutting boundary and limit the extent of the net.

These three techniques (defining EM layers, selecting nets and/or instances, and applying the cutting boundary) give you the maximum flexibility for selecting the geometry for EM analysis.

In the next part, I will discuss shape pre-processing. This will be another important aspect of geometry selection. Until then, like my other hero Tigger says, I am going to say TTFN. Take care folks. Long live Maxwell.

Related Resources

• Virtuoso RF Solution
• What’s New in Virtuoso (ICADVM18.1 Only)
• Video - Running an Electromagnetic Analysis Using AXIEM
• Virtuoso Electromagnetic Solver User Guide

For more information on Cadence circuit design products and services, visit www.cadence.com.

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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Sutirtha Kabir