IBM fabs 90nm G5 using strained silicon

IBM's first 90nm chip, the G5-class PowerPC 970FX, incorporates a key technique used by Intel to improve processor performance, it has emerged, courtesy of a small mention tucked away at the end of a Reuters story.

Like Intel's Prescott Pentium 4, the 970FX uses Strained Silicon. By stretching - or 'straining', as it's called in the trade - a lattice of silicon atoms, chip makers improve the ability of electrons to flow through the semiconducting material.

Transistors made from strained silicon are said to get a 35 per cent performance boost over regular transistors of the same size.

The technique works by adding a layer of silicon over a layer of silicon germanium (SiGe). The top layer's silicon atoms align themselves with those in the SiGe layer's wider-spaced crystal lattice, pulling them apart. The trouble is - as Intel has found - that it's hard to integrate the addition of that SiGe layer into existing fabrication processes.

Now, where IBM differs from Intel is in the use of silicon-on-insulator (SOI), which conveniently aids the implementation of strained silicon. As it announced last September, IBM removes the SiGe layer before fabrication, after applying the strained silicon onto the insulator. The upshot: it gains benefits of strained silicon using what is essentially its standard SOI process. By removing the SiGe layer, it doesn't have to integrate that material into the chip fabrication process per se. It calls the new technique, Strained Silicon Directly on Insulator (SSDOI).

The presence of strained silicon in addition to SOI may well explain how IBM has been able to bring the 970FX's power consumption to well below that of the 130nm 970 - in direct contrast to Prescott, which, clock for clock, has a higher power consumption than its 130nm predecessor, Northwood. ®

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