The Payoff: How RV740 Saved Cypress

For its first 40nm GPU, ATI chose the biggest die that made sense in its roadmap. That was the RV740 (Radeon HD 4770):


The first to 40nm - The ATI Radeon HD 4770, April 2009

NVIDIA however picked a smaller die. While the RV740 was a 137mm2 GPU, NVIDIA’s first 40nm parts were the G210 and GT220 which measured 57mm2 and 100mm2. The G210 and GT220 were OEM-only for the first months of their life, and I’m guessing the G210 made up a good percentage of those orders. Note that it wasn’t until the release of the GeForce GT 240 that NVIDIA made a 40nm die equal in size to the RV740. The GT 240 came out in November 2009, while the Radeon HD 4770 (RV740) debuted in April 2009 - 7 months earlier.


NVIDIA's first 40nm GPUs shipped in July 2009

When it came time for both ATI and NVIDIA to move their high performance GPUs to 40nm, ATI had more experience and exposure to the big die problems with TSMC’s process.

David Wang, ATI’s VP of Graphics Engineering at the time, had concerns about TSMC’s 40nm process that he voiced to Carrell early on in the RV740 design process. David was worried that the metal handling in the fabrication process might lead to via quality issues. Vias are tiny connections between the different metal layers on a chip, and the thinking was that the via failure rate at 40nm was high enough to impact the yield of the process. Even if the vias wouldn’t fail completely, the quality of the via would degrade the signal going through the via.

The second cause for concern with TSMC’s 40nm process was about variation in transistor dimensions. There are thousands of dimensions in semiconductor design that you have to worry about. And as with any sort of manufacturing, there’s variance in many if not all of those dimensions from chip to chip. David was particularly worried about manufacturing variation in transistor channel length. He was worried that the tolerances ATI were given might not be met.


A standard CMOS transistor. Its dimensions are usually known to fairly tight tolerances.

TSMC led ATI to believe that the variation in channel length was going to be relatively small. Carrell and crew were nervous, but there’s nothing that could be done.

The problem with vias was easy (but costly) to get around. David Wang decided to double up on vias with the RV740. At any point in the design where there was a via that connected two metal layers, the RV740 called for two. It made the chip bigger, but it’s better than having chips that wouldn’t work. The issue of channel length variation however, had no immediate solution - it was a worry of theirs, but perhaps an irrational fear.

TSMC went off to fab the initial RV740s. When the chips came back, they were running hotter than ATI expected them to run. They were also leaking more current than ATI expected.

Engineering went to work, tearing the chips apart, looking at them one by one. It didn’t take long to figure out that transistor channel length varied much more than the initial tolerance specs. If you get a certain degree of channel length variance some parts will run slower than expected, while others would leak tons of current.

Engineering eventually figured a way to fix most of the leakage problem through some changes to the RV740 design. The performance was still a problem and the RV740 was mostly lost as a product because of the length of time it took to fix all of this stuff. But it served a much larger role within ATI. It was the pipe cleaner product that paved the way for Cypress and the rest of the Evergreen line.

As for how all of this applies to NVIDIA, it’s impossible to say for sure. But the rumors all seem to support that NVIDIA simply didn’t have the 40nm experience that ATI did. Last December NVIDIA spoke out against TSMC and called for nearly zero via defects.

The rumors surrounding Fermi also point at the same problems ATI encountered with the RV740. Low yields, the chips run hotter than expected, and the clock speeds are lower than their original targets. Granted we haven’t seen any GF100s ship yet, so we don’t know any of it for sure.

When I asked why it was so late with Fermi/GF100, NVIDIA pointed to parts of the architecture - not manufacturing. Of course, I was talking to an architect at the time. If Fermi/GF100 was indeed NVIDIA’s learning experience for TSMC’s 40nm I’d expect that its successor would go much smoother.

It’s not that TSMC doesn’t know how to run a foundry, but perhaps the company made a bigger jump than it should have with the move to 40nm:

Process 150nm 130nm 110nm 90nm 80nm 65nm 55nm 40nm
Linear Scaling - 0.866 0.846 0.818 0.888 0.812 0.846 0.727

 

You’ll remember that during the Cypress discussion, Carrell was convinced that TSMC’s 40nm process wouldn’t be as cheap as it was being positioned as. Yet very few others, whether at ATI or NVIDIA, seemed to believe the same. I asked Carrell why that was, why he was able to know what many others didn’t.

Carrell chalked it up to experience and recounted a bunch of stuff that I can’t publish here. Needless to say, he was more skeptical of TSMC’s ability to deliver what it was promising at 40nm. And it never hurts to have a pragmatic skeptic on board.

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  • Dudler - Sunday, February 14, 2010 - link

    Whoa... Just Whoa.
    Very insightful and interesting.

    Amd moving to GF next generation and Norther Islands this year?

    Whoa.

    Thx Anand :)
    Reply
  • carlhenry - Sunday, February 14, 2010 - link

    i really loved the eyefinity being so 'mission impossible'. awesome article! Reply
  • Patrick Wolf - Sunday, February 14, 2010 - link

    The first rule of SunSpot: You don't talk about SunSpot. :D

    Reply
  • mubeen - Tuesday, February 16, 2010 - link

    Such an informative article Mr. Anand. Really appreciate your efforts. Reply
  • just4U - Tuesday, February 16, 2010 - link

    I agree it's a great article, nice to see these ones pop up from time to time. It don't really matter to me if it's nvidia, amd or intel.. just having a more insightful read is refreshing. Reply
  • DearSX - Sunday, February 14, 2010 - link

    I really do a like article like this. I wonder how old others are cause I'm 25 and I did not even notice how much reading things like this mattered. Its more important than buying something or performance figures imo. Reply
  • Paulman - Sunday, February 14, 2010 - link

    I, too, will second that. Really cool. :P Reply
  • MrSpadge - Sunday, February 14, 2010 - link

    Many thanks for this interesting article and especially the in-dpeth information about TSMC, the process and how ATI deals with it. Thanks to you, Anand, for writing it and thanks to the good guys at AMD for sharing this information!

    Beyond excellence :)
    Reply
  • hyvonen - Monday, February 15, 2010 - link

    And NVidia's call for "zero defects, zero variability" was jaw-dropping. Do they have any idea how silicon is manufactured?

    Unbelievable cluelessness... with this attitude, they are out of business in the next two years. They are completely unprepared to harness the potential in the latest process nodes.

    Meanwhile, it also sounds like TSMC is way over its head in this - uncharacteristic screw-ups with the chamber matching, bad leakage etc. Not to mention the choice to delay HKMG until 28nm (and having to kill 32nm because of that). GF is going to eat them for lunch, and take over the foundry business.
    Reply
  • ET - Sunday, February 14, 2010 - link

    Indeed. One of the more interesting articles I've read in a while. Reply

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