Joe Kwan is the Product Marketing Manager for Calibre LFD and DFM Services at Mentor Graphics. He is also responsible for the management of Mentor’s Foundry Programs. He previously worked at VLSI Technology, COMPASS Design Automation, and Virtual Silicon. Joe received a BA in Computer Science from the University of California Berkeley and an MS in Electrical Engineering from Stanford University.
When to Farm Out Your DFM Signoff
The DFM requirements at advanced process nodes pose not only technical challenges to design teams, but also call for new business approaches. At 40nm, 28nm, and 20nm, foundries require designers to perform lithography checking and litho hotspot fixing before tapeout. In the past, DFM signoff has almost always been done in-house. But, particularly for designers who are taping out relatively few devices, the better path may be to hire a qualified external team to perform some or all of your DFM signoff as a service.
DRC and DFM have changed pretty dramatically over the past few years. At advanced nodes, you need to be more than just “DRC-clean” to guarantee good yield. Even after passing rule-based DRC, designs can still have yield detracting issues that lead to parametric performance variability and even functional failure. At the root of the problem is the distortion of those nice rectilinear shapes on your drawn layout when you print them with photolithographic techniques. Depending on your layout patterns and their nearby structures, the actual geometries on silicon may exhibit pinching (open), bridging (short) or line-end pull-back (see Figure 1).
|Figure 1: SEM images of pinching and bridging. LPC finds these problems and lets you fix them before tapeout. Litho checking is mandatory at TSMC for 40nm, 28nm and 20nm process nodes.|
In the past, these problems were fixed by applying optical proximity correction (OPC) after tapeout, often at the fab. But at 40nm and below, the alterations to the layout must be done in the full design context, i.e. before tapeout, which means that the major foundries now require IC designers to find and fix all Level 1 hotspots. TSMC’s terminology for this is Litho Process Check, LPC.
Usually, design companies purchase the DFM software licenses and run litho checking in-house. This approach has the obvious benefits of software ownership. Designers have full control over when and how frequently they run the checks. The design database doesn’t leave the company’s network. There is a tight loop between updating the design database and re-running verification.
But what if you have not yet set up your own LPC checking flow and need time to plan or budget for software and CPU resources? Or, if you only have a few tapeouts a year? In these cases, you would benefit from the flexibility and convenience of outsourcing the LPC check.
A DFM analysis service is an alternative option to software purchase by performing litho checking for you. Here’s how it works: the design house delivers the encrypted design database to a secure electronic drop box. The analysis service then runs TSMC-certified signoff—for example, Calibre LFD—in a secure data center. Your DFM analysis service should demonstrate that they have an advanced security infrastructure that can isolate and secure you IP. Access should be limited to only those employees that need to handle the data. You would get the litho results back, along with potential guides for fixing the reported hotspots. A cloud-based DFM analysis service for TSMC’s 40nm, 28nm, and 20nm process nodes is available from Mentor Graphics.
A DFM service can also be useful when you already have Calibre LFD licenses, but find yourself with over-utilized computing resources. Having a DFM service option gives you flexibility in getting through a tight CPU resource situation or can ease a critical path in your tape-out schedule. The DMF service can run the LPC while you perform the remaining design and verification tasks in parallel.
Whether you use a DFM services or run LPC in-house on purchased software, it is very important to run litho checking early and often. This lets you identify problematic structures early and allows more time to make the necessary fixes. But now you have more flexibility to make the right business decision regarding how to reach DFM signoff.