SOI: It’s Elementary, My Dear Watson
Posted date : Feb 18, 2011

The other night, IBM’s “Watson” supercomputer beat the world’s two best players in the popular “Jeopardy” game show, in which the contestants are told the answer, but then have to figure out the right question.  In that spirit, here’s an ASN spin on the game:

Category: semiconductor design & manufacturing.

Answer: The starting substrate for the Power7 processors in IBM’s Jeopardy-winning “Watson” supercomputer.

Question: What is SOI?

Yes, Power7 is the 4th generation of Power processors that IBM has based on SOI. (The first was the Power4 – dual core, 174 million transistors/processor; 0.18µm copper + SOI – that IBM introduced 10 years ago.)

So why SOI? Well, first consider the size of the thing, and the performance and power involved. Then read on, because there’s more to it than that.

Here are the Power7 specs:

  • 45nm SOI
  • 1.2B transistors
  • 32MB onchip eDRAM shared L3
  • 8 processor cores
  • 12 execution units per core

For Watson, you have to keep multiplying. Watson is a cluster of 90 standard, commercially available IBM Power 750 servers – each with four Power7 processors. Each of those processors has eight 3.55-GHz cores, for a total of 2880 Power7 cores. The system has a combined total of 16 Terabytes of memory and can operate at over 80 Teraflops (trillions of operations per second).

OK, so SOI is helping with speed and power – that’s clear.

But in an ASN article five years ago (!), IBM Fellow Subramanian S. Iyer explained the importance of SOI in the memory part of the chip design equation. At that time, memory (mostly SRAM) was taking up almost 75% of the chip, so the push was on to shift at least some of that toward smaller, more stable and power-efficient embedded DRAMs (eDRAM).

As Dr. Iyer said, “The complexity adder is about half in SOI compared to bulk for deep trench based eDRAMs. […] We expect the use of eDRAM to proliferate to SOI in the 45nm generation.”

Here is a cross section of IBM’s 45nm SOI trench cell. The buried oxide is used to completely isolate the capacitor plate from the device.

What do you think of that? (Of course, if you’re a long-time ASN reader, you saw it here first.)

(Photos: Courtesy IBM)

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