Posted by Graham Etchells on March 15, 2016
Over the last series of blogs we have looked at what tools the layout engineer has available to him/her to help them deal with the complexity of doing layout with FinFETs. Even though there are tools that help, the fact is there is still a productivity hit when comparing the time it takes to do a FinFET based layout versus a planar CMOS layout. When I asked my layout colleagues “how much longer does it take to do a FinFET based design versus planar CMOS?” they said it takes 2-3X more time.
So if we are to recoup layout productivity when doing a FinFET based design, which areas should we focus on? Well let’s start at the very beginning, which according to Julie Andrews in The Sound of Music, is a very good place to start. The task of generating the devices and placing them such that they meet all the design rules and will produce a robust working design is about 30% of the layout time. So if we can speed this task up then we will gain back some of the productivity we lost due to the complexity of the FinFET process.
Because one device in the schematic can translate to hundreds of devices in the layout, the layout engineer needs a fast yet simple way to generate the device and get the individual transistors grouped together such that the device will work properly and adheres to the strict layout dependent rules that the FinFET process requires. Take a differential pair for example. In the schematic it is two symbols but in the layout it could be hundreds of devices. To ensure good analog layout that will work it is typical for these devices to be interdigitated in a specific pattern. Doing this by hand is a time consuming tedious and error prone task. This is where you need some automation. Not the ‘push the button’ and see what I get kind of automation, but a guided / interactive approach that is fast and easy to refine such that you get the result you want. This approach is especially important when implementing ECO changes.
Having the layout engineer make simple graphical choices of how the layout needs to look and then having the placement taken care of by the placement engine is the fastest way to achieving correct layout. Using this user guided interactive approach cuts the layout time from hours to minutes.
So where else can we look to claw back some layout productivity?
Well that’s another post.