DALLAS In a decision that will intensify the debate on the merits of silicon-on-insulator (SOI) technology, Sun Microsystems Inc. and Texas Instruments Inc. have elected not to use SOI for Sun's next-generation microprocessor, which will be manufactured by TI.
TI had indicated last year that it was considering adding an SOI module to its 0.13-micron technology road map for Sun's UltraSparc processors, but now that won't happen. "We don't have any plans to use SOI," said Dennis Buss, vice president of silicon technology development at TI.
TI has manufactured Sun's microprocessors for 13 years, and has brought up its latest process technologies for initial use by Sun, then spread out the technologies later to make its own digital signal processors.
"We've done some early work on the Spice models so that Sun engineers could evaluate the technology, and together we've come to the conclusion that the advantages of SOI get smaller and smaller as you shrink [geometries]," Buss said.
Other chip makers, including IBM Corp.'s Microelectronics Division and Advanced Micro Devices Inc., have argued that SOI's advantages improve as a device's operating voltage drops, due in part to reduced current leakage at the device junctions.
But TI's technologists came to a different conclusion, Buss said. "For the same off leakage, you lose some of the gain in performance by going to SOI," he said. "We looked at it, and the Spice models showed modest improvements in performance. At the end of the day, when you add in the additional R&D costs, and the additional costs of designing with SOI, we decided we are better off sticking with bulk and pushing that as fast as we can."
David Yen, recently named vice president and general manager of Sun's processor products group in Palo Alto, Calif., said Sun looked at more than TI's SOI technology.
"After looking at several companies we decided there is no justification in going to SOI," Yen said. "What other companies are saying about SOI is definitely market hype. IBM's statements that the industry needs to go to SOI are just designed to get publicity in favor of SOI. You have to look at what they can do, not what they say."
IBM has been vocal in its support for SOI. Three years ago, the company said it would apply SOI technology throughout its processor line. Indeed, IBM will use SOI to manufacture its upcoming 0.13-micron Power 4 processor, a mammoth chip that will be used in IBM's servers. The Power 4 will compete with Intel Corp.'s Itanium and Sun's UltraSparc processors at the highest end of the computer market.
IBM's decision to use SOI in its server processors has sparked a ferocious debate, with IBM as SOI advocate and Intel as its major opponent. AMD threw its weight behind SOI in July, when president Hector Ruiz said AMD planned to use SOI technology across all of its microprocessors, ranging from the mobile to the server segments.
AMD has co-developed its logic technology with Motorola, and also licensed SOI design-related intellectual property to ease the transition to SOI-based designs. AMD will make its processors at its own fabs in Texas and Germany.
Motorola is also developing an SOI module on its 0.13-micron technology platform and is developing a G4 PowerPC design in SOI technology.
Intel has remained unswayed by the arguments put forth by AMD, IBM and Motorola. Intel technologists have published technical papers arguing that the merits of SOI technology decline as scaling progresses to 100 nm, 70 nm and beyond. And the processor behemoth has not budged from its position, despite rumors and speculation that Intel also would implement SOI technology, beginning with its mobile processor line.
Intel's Mark Bohr, director of process architecture and integration; associate Kaizad Mistry and others have said that as CMOS scaling continues, the reduced leakage possible with SOI diminishes. Buss said TI's researchers came to similar conclusions. "Generally, as you go down in voltage, the body coupling effect becomes more important, and a number of other factors come into play."
Moreover, TI, like Intel, is moving to 300-mm wafers. TI has already started prototype 300-mm wafer production at its DMOS 6 fab (the D stands for Dallas), beginning with 0.18-micron design rules. SOI wafers are available now at 200 mm, though the cost per wafer is roughly four to six times higher than a bulk silicon wafer, said Tak Ogawa, silicon analyst at Dataquest Japan. That cost may decline to a factor of three times more as SOI volumes increase, he said.
"In the near future the ratio could be less than three times. The proper target for an SOI wafer at the 200-mm size would be $430. At that level, processor vendors could expand SOI into the market."