PDF asks software to improve chip yields

PDF asks software to improve chip yields
SAN JOSE, Calif. — Taking its semiconductor yield enhancement technology into the chip design world, PDF Solutions Inc. has rolled out Design-Based Yield Improvement (DBYI), a services and software offering aimed at fabless semiconductor providers. The company intends to help designers produce chip layouts that minimize manufacturing difficulties.

Designing for manufacturability is quickly becoming a concern at production geometries of 0.13 micron and below, where decisions made at the layout level can have a profound impact on chip yields. Chip manufacturability was a big topic at the International Symposium on the Quality of Electronic Design, where a panel examined the impact of process variations on chip design.

PDF today works primarily with process engineers and library developers inside integrated device manufacturers (IDMs). The company provides detailed process characterization and design evaluation technology that's offered as a service, using software that's run by PDF personnel. Much of the company's income represents a percentage of the cost savings realized by IDM customers due to increased yields.

Tackling yield losses

With the new DBYI offering, said Dave Joseph, PDF vice president of marketing, the intended customers are fabless semiconductor vendors and the focus is on design. "The whole intent is not to improve the process but to characterize it, and make designs more manufacturable," he said.

It's an important step, Joseph said, because somewhere around 0.18 micron, the yield loss associated with chip layout features begins to exceed yield loss due to contamination. What happens with contacts and vias becomes the primary determinant of yields, he said.

"A designer may want to use stacked vias to get a more compact chip, but it may yield very poorly, so he's gained area but lost a lot in yield," Joseph said. "But in another process, stacked vias may work quite well."

Just adhering to design rules doesn't guarantee good yields, Joseph said. "If designers could increase chip size by 1 percent and get 10 percent better yields, they'd do that. But right now they have no information about how to make such a trade-off," he said.

That's where the DBYI offering comes in. With it, PDF will use its universal characterization vehicle (UCV) test chips to calibrate yield and statistical Spice models for selected foundry processes. PDF personnel will apply these models in the company's yield and performance simulation tools. The result will be specific information about the impact of chip layout on yield.

But that doesn't mean designers will shoulder an extra burden. "We look for ways we can impact design without actually getting the designer involved," Joseph said. "For example, we can look at what standard cells the designer uses the most, and optimize those cells for the process. From the designer's perspective, he doesn't do anything — he just gets a better standard cell."

Making this all possible, Joseph said, is a new "full flow" characterization approach that does not require proprietary foundry information. The "short flow" approach used with IDMs, he said, requires PDF to have extremely detailed knowledge about process information. It provides a quick way to characterize individual modules, but demands "very intensive" interaction with process groups, he said.

Yield enhancement is indeed becoming as important an issue as design-for-test and verification for deep-submicron design. ASIC vendor LSI Logic Corp., for one, often uses diodes, flip-flops or SRAM structures to monitor yield. Fabless semiconductor companies that now rely on customer-owned-tooling have much to gain by paying close attention to yield, said Ronnie Vasishta, vice president of technology marketing at LSI Logic (Milpitas, Calif.)

"If you're a fabless semiconductor company and someone can provide a canned solution for yield management without having exhaustive yield solution technology then you'd probably be intrigued by it," Vasishta said.

DBYI service from PDF is available now. The company will charge an up-front nonrecurring engineering fee to customers, but PDF will derive much of its income from a customer's demonstrated cost savings, Joseph said.