Design for yield trends

SARA VER-BURGGEN, contributing editor, SemiMD

Should foundries establish and share best practices to manage sub-nanometer effects to improve yield and also manufacturability?

Design for yield (DFY) has been referred to previously on this site as the gap between what the designers assume they need in order to guarantee a reliable design and what the manufacturer or foundry thinks they need from the designer to be able to manufacture the product in a reliable fashion. Achieving and managing this two-way flow of information becomes more challenging as devices in high volume manufacturing have 28nm dimensions and the focus is on even smaller dimension next-generation technologies. So is the onus on the foundries to implement DFY and establish and share best practices and techniques to manage sub-nanometer effects to improve yield and also manufacturability?

‘Certainly it is in the vital interest of foundries to do what it takes to enable their customers to be successful,’ says Mentor Graphics’ Technology Communications Manager, Gene Forte, adding, ‘Since success requires addressing co-optimization issues during the design phase, they must reach out to all the ecosystem players that enable their customers.’

Mentor refers to the trend of DFY moving closer to the manufacturing/foundry side as ‘design-manufacturing co-optimization’, which entails improving the design both to achieve higher yield and to increase the performance of the devices that can be achieved for a given process.

But foundries can’t do it alone. ‘The electronic design automation (EDA) providers, especially ones that enable the critical customer-to-foundry interface, have a vital part in transferring knowledge and automating the co-optimization process,’ says Forte. IP suppliers must also have a greater appreciation for and involvement in co-optimization issues so their IP will implement the needed design enhancements required to achieve successful manufacturing in the context of a full chip design.

As they own the framework of DFY solutions, foundries that will work effectively with both the fabless and the equipment vendors will benefit from getting more tailored DFY solutions that can lead to shorter time-to-yield, says Amiad Conley, Applied Materials’ Technical Marketing Manager, Process Diagnostics and Control. But according to Ya-Chieh Lai, Engineering Director, Silicon and Signoff Verification, at Cadence, the onus and responsibility is on the entire ecosystem to establish and share best practices and techniques. ‘We will only achieve advanced nodes through a partnership between foundries, EDA, and the design community,’ says Ya-Chieh.

But whereas foundries are still taking the lead when it comes to design for manufacturability (DFM), for DFY the designer is intimately involved so he is able to account for optimal trade-off in yield versus PPA that result in choices for specific design parameters, including transistor widths and lengths.

For DFM, foundries are driving design database adjustments required to make a particular design manufacturable with good yield. ‘DFM modifications to a design database often happen at the end of a designer’s task. DFM takes the “ideal” design database and manipulates it to account for the manufacturing process,’ explains Dr. Bruce McGaughy, Chief Technology Officer and Senior Vice President of Engineering at ProPlus Design Solutions.

The design database that a designer delivers must have DFY considerations to be able to yield. ‘The practices and techniques used by different design teams based on heuristics related to their specific application are therefore less centralized. Foundries recommend DFY reference flows but these are only guidelines. DFY practices and techniques are often deeply ingrained within a design team and can be considered a core competence and, with time, a key requirement,’ says McGaughy.

In the spirit of collaboration

Ultimately, as the industry continues to progress requiring manufacturing solutions that increasingly tailored and more and more device specific, this requires earlier and deeper collaboration between equipment vendors and foundry customers in defining and developing the tailored solutions that will maximize the performance of equipment in the fab. ‘It will also potentially require more three-way collaboration between the designers from fabless companies, foundries, and equipment vendors with the appropriate IP protection,’ says Conley.

A collaborative and open approach between the designer and the foundry is critical and beneficial for many reasons. ‘Designers are under tight pressures schedule-wise and any new steps in the design flow will be under intense scrutiny. The advantages of any additional steps must be very clear in terms of the improvement in yield and manufacturability and these additional steps must be in a form that designers can act on,’ says Ya-Chieh. The recent trend towards putting DFM/DFY directly into the design flow is a good example of this. ‘Instead of purely a sign-off step, DFM/DFY is accounted for in the router during place and route. The router is able to find and fix hotspots during design and, critically, to account for DFM/DFY issues during timing closure,’ he says. Similarly, Ya-Chieh refers to DFM/DFY flows that are now in place for custom design and library analysis. ‘Cases of poor transistor matching due to DFM/DFY issues can be flagged along with corresponding fixing guidelines. In terms of library analysis, standard cells that exhibit too much variability can be systematically identified and the cost associated with using such a cell can be explicitly accounted for (or that cell removed entirely).’

‘The ability to do “design-manufacturing co-optimization” is dependent on the quality of information available and an effective feedback loop that involves all the stakeholders in the entire supply chain: design customers, IP suppliers, foundries, EDA suppliers, test vendors, and so on,’ says Forte. ‘This starts with test chips built during process development, but it must continue through risk manufacturing, early adopter experiences and volume production ramping. This means sharing design data, process data, test failure diagnosis data and field failure data,’ he adds.

A pioneer of this type of collaboration was the Common Platform Consortium initiated by IBM. Over time, foundries have assumed more of the load for enabling and coordinating the ecosystem. ‘GLOBALFOUNDRIES has identified collaboration as a key factor in its overall success since its inception and been particularly open about sharing foundry process data,’ says Forte.

TSMC has also been a leader in establishing a well-defined program among ecosystem players, starting with the design tool reference flows it established over a decade ago. Through its Open Innovation Platform program TSMC is helping to drive compatibility among design tools and provides interfaces from its core analysis engines and third party EDA providers.

In terms of standards Si2 organizes industry stakeholders to drive adoption of collaborative technology for silicon design integration and improved IC design capability. Forte adds: ‘Si2 working groups define and ratify standards related to design rule definitions, DFM specifications, design database facilities and process design kits.’

Open and trusting collaboration helps understand the thriving ecosystem programs that top-tier foundries have put together. McGaughy says: ‘Foundry customers, EDA and IP partners closely align during early process development and integration of tools into workable flows. One clear example is the rollout of a new process technology. From early in the process lifecycle, foundries release 0.x versions of their PDK. Customers and partners expend significant amounts of time, effort and resources to ensure the design ecosystem is ready when the process is, so that design tapeouts can start as soon as possible.’

DFY is even more critically involved in this ramp-up phase, as only when there is confidence in hitting yield targets will a process volume ramp follow. ‘As DFY directly ties into the foundation SPICE models, every new update in PDK means a new characterization or validation step. Only a close and sustained relationship can make the development and release of DFY methodologies a success,’ he states. •

SARA VER-BURGGEN is a contributing editor for Semiconductor Manufacturing and Design, www.semimd.com.

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