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The CMOS-mobile apps connection

Posted: 04 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:CMOS? logic chips? VLSI?

At the 2012 Symposium on VLSI Technology, the future of CMOS technology was the main topic. The event focused on Intel Corp.'s release of the technical specs of its new 22nm tri-gate transistor, issues such as 3D transistor design, and a panel also explored FinFET device technology.

Surprisingly, there are vastly different views on the best direction forward. I have never seen a time in the industry where the views varied so much. However, it is possible that none of the experts are either right or wrong. It is very likely that each expert is using different "value metrics" which I define as some weighting of chip metrics (cost, power and performance) and chip design metrics (design cost, complexity and time-to-market).

Engineers are using different metrics since companies are targeting different markets. The two biggest markets for logic chips are, of course, mobile (smartphone and tablet) and PC devices. These markets are now steering the technology direction and defining the winners and losers. Even within a given market segment like tablets, companies use different metrics (some think the big tablet market is at a $199 price point (versus $499$999). As an educator with a five-tablet family, I tend to agree with the former).

For those stuck in their cubicle or fab back home doing real work, here are my top ten insights from the VLSI conference.

  • Intel's tri-gate is an impressive engineering feat. On a single 300mm wafer close to a 1 trillion fins are fabricated (about 3 billion fins in each single CPU; each fin at the top ranges only about 6 to 9nm in width which is only about 12 to 18 silicon lattice spacings). That would be the smallest lateral feature ever patterned with lithography to enter high volume CMOS manufacturing. Nearly each single fin is important to yielding a 22nm CPU and a single broken fin would make the CPU unsellable. I continue to believe Intel is five years ahead of the industry with tri-gate technology.
  • Intel is currently shipping tri-gate CPUs, and though yield appears challenging most think Intel and its limited CPU offerings will be successful with tri-gate. In these ICs, performance has historically been valued over cost and power. However, the foundry and fabless mobile chip engineers are quick to point out that for mobile SOCs, there is no debate who has silicon technology and product leadership. Foundry 28nm is far better on the chip metrics of cost, power and performance than the 32nm technology used to fabricate Intel's mobile Atom processor chips (Medfield and Cedar Trail). Foundry also has a "rich" set of 28nm IP design shipping, such as the integration of baseband and application processor on a single 28nm monolithic SOC. I sense foundries and fabless companies were pointing that out in response to Intel's recent bold statement that the "Fabless Model is Collapsing!"
  • Before the conference, Intel's fin profile was reported by reverse engineering firm Chipworks to be trapezoidal, and that was one of the key topics before, during and even after the conference. Experts all agree that the ideal FinFETs should have a rectangular fin and yield issues likely drove Intel to alter the fin shape at the end of the technology cycle (potentially a tradeoff resulting from a schedule slip). During the Intel paper Q/A, the presenter (outstanding presentation by Chris Auth) was asked about the fin profile. Many in the audience were surprised that the answer was "performance" (lower external resistance). The leading hypothesis for the fin shape was that it is meant to fix a yield problem related to clearing the low k spacers material off the fins. The general consensus was that the fin profile (though fine for Intel) would have too much variation for mobile parts which are not speed or leakage binned (typically designed for > 98 percent yield at worse case speed and leakage variation). Most also thought that at 14 nm Intel would go back to a rectangular fin profile.
  • A general bulk trapezoidal FinFET (with single n and pFET work function and threshold voltage adjusted via halo doping design point) was also discussed. There was consensus on Intel's design point along with discussion of what lessons could be extracted from the interesting Intel work and applied to the mobile market. There was a high degree of concern about the non-fully depleted transistor formed in the bottom, thicker part of the fins since it will cause an additional off-state leakage. Intel in its paper only reported leakage down to 1nA/um which is an appropriate CPU target, but mobile devices require leakage about 100x lower in the 10pA/um range for the always on and footer/header power gating devices.
  • Additional gate work functions would be another approach to lower off-state leakage. However, the complexity of doing four or more work functions (for just two threshold voltage types, 2n and 2p) on a 3D structure was thought to be too costly and complex for the mobile market.

    On the design side, the discussion centered on a bulk-only FinFET approach that would likely require system repartitions. The general thinking was that the power management unit, RF and even analog circuits would need to be off chip. For low cost mobile solutions, a single chip SOC is almost a requirement, thus FinFET would not be very attractive in the large middle range like the 3G SOC China market.

  • Device experts almost universally viewed FinFET devices as: The "Good": Ideal FinFETs have outstanding DIBL and swing for the top device (Intel's published numbers are impressive when the threshold of the top device is low and the current is dominated by the top part of the fin);
  • The "Bad": Real FinFETs on bulk have sub-fin leakage and high transistor variation resulting from fin height control, fin doping, fin shape and all the 3-D process steps. Device and process engineers generally agree that "ideal FinFETs are great but production FinFETs lose some to all of the advantage compared to planar devices," depending on how implemented.

  • Many FinFET researchers papers were presented. Experts in mobile were clear that "cost was going to be higher than the market could take." These technologies look interesting for $200 (CPU) to plus-$1,000 (server) chips, but "my high-end mobile chips sell for $20" or "A lot of my mobile chips need to sell between $1 and $5."
  • The cost of lithography at 20nm and 14nm was also a hot topic. Industry thinking is that double patterning at 20nm is of marginal value and triple patterning at 14nm is technically viable for ~50nm metal pitch and ~70nm gate pitch, but it is not economical in the mobile market. The industry will pass on costly pitch scaling and focus on making better chips at the 20- and 28nm nodes.
  • With the introduction of the third generation iPad, greater attention must be paid technologies to GPU power/performance and cost metrics. Peak power in Apple's A5X chips is about 5W; and about 80 percent of that is GPU power. The GPU die area is also a much larger part of die compared to the CPU. A5X is the "new benchmark" to compete with in the tablet market.
  • Professor Tsu-Jae King Liu of the University of California at Berkeley discussed how planar CMOS can be extended to the "end of the CMOS" roadmap. IBM also published a pre-VLSI paper in June on extending planar to 14 nm. Along with Professor King, other experts think planar can be scaled to end of CMOS roadmap. FinFETs certainly have better electrostatics but porting and designing with transistors with high process variation and lots of design restrictions is a major issue especially for the mobile market. Just as important as the silicon technology is the design IP inside an SOC and how quickly the design IP ecosystem can be setup around a new technology. (This was covered in depth in "How will the chip wars be won?" published earlier this year:
  • Two illustrations of this can be seen in the hot products of 2012: The new iPad does not use the decade-old concept of strained silicon or high k metal gates; Qualcomm winning the design for the communication SoC in the new Samsung Galaxy S3 is due to Snapdragon's integration with LTE at low die cost and less about silicon technology features (another product without high K gates).

  • By the end of the conference, there was one point which we could all agree on: "It is hard to beat a great conference on Waikiki Beach."
  • - Scott Thompson
    ??SuVolta Inc.

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