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Industry players discuss standard for Open DFM

Posted: 04 Oct 2006     Print Version  Bookmark and Share

Keywords:foundries  fabs  EDA  DFM flow  DFM models 

Representatives of foundry, fabless and EDA providers began to brought forth the requirements for a standard, open design-for-manufacturability model. Last week, participants of the Open DFM Model Workshop have explored issues ranging from DFM flows to encryption and identified possible next steps.

The workshop was hosted by the Fabless Semiconductor Association (FSA), the Silicon Integration Initiative (Si2) and the Common Platform of Chartered Semiconductor, IBM and Samsung. The objectives, said Walter Ng, senior director of platform alliances at Chartered, were to reach "some alignment" on what is needed, explore possible directions and discuss how the open model effort can proceed.

Participants generally agreed that the industry is moving to model-based DFM, making an open DFM model essential. They also agreed that the model must support encryption and suggested approaches for doing that. Some called for creating a DFM Council to make further progress with open models.

An open DFM model would provide a standard way to exchange information between the IC design and manufacturing environments. It would work across different foundries and EDA tool environments, perhaps fostering what Si2 president Steve Schulz called a "virtual re-aggregation" of the semiconductor supply chain.

"Many [fabless] companies outsource to different foundries," noted Lisa Tafoya, vice president of global research at the FSA. "Do you want different avenues of working with each foundry partner, or do you want a standard, open DFM model?"

Models needed
A strong driver behind an open DFM model is the realisation that design rules alone are not enough and that a model-based approach will be needed at 45nm and below. "Design rules remain a key process-to-design link, but they can no longer describe all the interactions between process and design," said Peter Rabkin, program director for advanced technology development at Xilinx Inc. "We need something else to enhance manufacturability." Rabkin said DFM should be an "iterative" process that takes place throughout the design flow. Systematic DFM checks and layout modifications are needed at the cell, block and, ultimately, full-chip level, he said.

Among the challenges of DFM at 45nm, Rabkin said, is the integration of physical and electrical DFM. Users need to address the impact of process variation on design, he said. For example, lithography and etch effects cause transistor critical-dimension variation, which in turn impacts performance and leakage. Characterisation and modelling are needed to predict such effects, he said.

Rabkin's conclusion: "We need sophisticated modelling, simulation, analysis and design-modification DFM capabilities that can work reliably under conditions of incomplete process technology data." He described an "integrated DFM" approach that requires a sharing of information and optimisation across the design house, mask shop and foundry.

'Nervous' about DFM
Tim Farrell, manager of IBM's computational lithography project technology group, confessed to being "nervous" about the potential workload that DFM would bring to his group.

Nevertheless, Farrell acknowledged that DFM will be needed. Showing a road map for the transition to model-based DFM at 65nm and 45nm, Farrell said DFM model kits will include defect, lithography, chemical metal polishing (CMP) and statistical models, and will cover random, systematic and parametric yield.

DFM models work across tool flow

Model-based DFM, Farrell said, is "essentially a disruption of what we're doing today." He said it involves an "additional degree of freedom" that results in an increasing quantity of applications, suppliers, foundry data, information flow and tool interactions. It also requires foundries to release sensitive process data, Farrell said.

The good news is that open standards can reduce the number of tool combinations requiring support, he said, enabling the introduction of model-based DFM. This will lower costs for EDA, foundry and fabless providers, Farrell said, and enable innovation and collaboration.

Si2's Schulz said that users, foundries and EDA vendors have some differing perspectives on open DFM models. All need better support for model-based DFM, Schulz said. But users want portability across all foundries and EDA vendors; foundries want common use across EDA tools; and EDA vendors want common use across foundries.

"These appear to be contradictory requirements, but the key may be scoping the solution space," Schulz said. There's agreement, he said, over the need for model-based DFM and the importance of a "create-once, use-many-times" open-model paradigm. The key open DFM model considerations, he said, are scalability between speed and detail, flexibility over known variations and extensibility for future use.

Defining an open model doesn't solve the problem by itself, Schulz noted. There's still the question of how the model fits into real flows and how sensitive intellectual property can be protected. Thus, three key areas must be considered: models, design flows and encryption.

Working model-based flows, Schulz said, require semantic consistency, an ability to balance accuracy and performance, ease of creation and ease of run-time creation and use. "You'd like to have a lithography model guide place and route, yield and sensitivity analysis, and then OPC [optical proximity correction] and lithography simulation, but this needs to be architected up front," he said.

Encryption a must
Encryption must be an integral part of an open DFM model architecture, Schulz said. The best approach, he said, is to create a model once, and then let it be unlocked by unique keys, addressing different levels of security for different users. Access to models should be granular and hierarchical, he said, and encryption should be based on existing technology.

Setting forth requirements
Breakout sessions allowed participants to further define requirements for an open DFM model. In one session, participants concluded that some agency or organisation should play a role similar to the Compact Model Council's function in over- seeing Spice models. Participants also said that the relative importance of standardising models and interfaces needs to be determined, and that the physical modelling space should include mask, photoresist and CMP.

In this session, participants also called for a taxonomy definition of modelling terms, and suggested the need for a DFM Council with a set of formal deliverables. This effort could involve Si2's Design to Manufacturing Coalition, some said.

A session on encryption called for separate delivery vehicles for the encryption keys and model data. Participants said that data should be decrypted on the fly, that different encryption keys should be used for each release of DFM model data and that it should be possible to trace the full delivery path of process models. Vendors should provide 128- and 256-bit encryption keys, even though this would require ex- port controls, session members concluded.

Flow considerations
Another breakout session, looking at the design flow, listed requirements for library design, architectural design, physical design and the post-tapeout phase. Examples include extraction tools that are DFM-aware, a full 3-D field solver with a DFM simulator, an ability to model transistor and CMP variations for physical design, DFM-aware routing, postlayout shape analysis and improved information from metrology.

"We made progress today," Schulz of the Si2 concluded. "I think we added a broader understanding of the things that need to be considered, and I'm feeling more comfortable that there's a better sense of direction." Open DFM modelling will be taken up again in a panel discussion and working session next week (Oct. 11) at the FSA Supplier's Expo and Conference in San Jose, California.

- Richard Goering
EE Times




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