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Virtuosity: Support for Stacked Devices in Modgen

Advanced node processes for IC device manufacturing provide tremendous advantages in terms of power, performance, area utilization, and the overall cost. To keep up with the growing demand of reducing the area and cost of chips, we have been trying to squeeze in more and more devices into the already shrunken chips. This myriad of devices makes it challenging for layout designers to visualize and customize their circuits. So, there has been a growing need to develop efficient ways for layout automation.

In my previous blog, I gave a brief introduction to Virtuoso® Module Generator (Modgen), an important tool for analog layout automation. I had also hinted about my upcoming blogs that will talk about the new features we have introduced for Advanced node PDKs in Modgen. Here's my next blog in the series, in which I'll focus on one such key feature, Support for Stacked Devices in Modgen.

What are Stacked Devices?

Stacked devices are a set of devices that are placed in sequence and wired in series. So, the source of one device is connected to the drain of the adjoining device. Also, stacked devices have a common super-master and share a common gate and bulk connection (if available).

Now that you know what stacked devices are, let's see how they are created. Here are a few commonly-used methods to create device stacks:

  • Series-connecting the devices explicitly in the schematic editor (flat or with the schematic hierarchy)
  • Setting a series-factor (s-factor) on a device
  • Using the lxCombination property
  • Creating iterated instances

 

Editing Stacked Devices in Modgen

You can use the grid pattern assistants—the Grid Pattern Editor (GPE) and the Grid Pattern Mapping (GPM) assistants—to visualize and edit stacked devices. The assistants display each stacked device as a single symbol. So, it is easier to create and manipulate large patterned arrays of devices.

Leonardo Da Vinci's famous quote "Simplicity is the ultimate sophistication" aptly describes our approach to how stacked devices are represented inside Modgen. Here's what I mean. The following circuit contains two instances, each with an s-factor of 40 and an m-factor of 5.

The traditional representation of this circuit in the GPE and GPM assistants would have a total of 400 symbols (200 for each instance). Now, with the support for stacked devices enabled, the total number of symbols for both the instances put together has been reduced to 10 (5 for each instance). Isn't it easy to understand the layout of the stacked devices and manipulate them in this mode?

You can use the options in the GPE and GPM assistants to manipulate the stacks. For example, you can:

  • Change the orientations of stacks
  • Name stacks
  • Add dummies on one or more sides of stacks
  • Use built-in pattern presets to place stacks

I hope you found this feature interesting. 

Stay tuned for my next blog, in which I'll talk about another exciting new feature available in Modgen for advanced node PDKs!

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About Virtuosity

Virtuosity has been our most viewed and admired blog series for a long time that has brought to fore some lesser-known, yet very useful software and documentation improvements, and also shed light on some exciting new offerings in Virtuoso. We are now expanding the scope of this series by broadcasting the voice of different bloggers and experts, who would continue to preserve the legacy of Virtuosity, and try to give new dimensions to it by covering topics across the length and breadth of Virtuoso, and a lot more… Click Subscribe to visit the Subscription box at the top of the page in which you can submit your email address to receive notifications about our latest Virtuosity posts.

Happy Reading!

Aneesh Shastry

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