Gigascale integration hitting the wall?

Gigascale integration hitting the wall? (1997 Hot Chips Symposium in Palo Alto) (Industry Trend or Event) (Electronic News)

Palo Alto, Calif.--For almost four decades now we have enjoyed exponential improvements in the productivity and performance of semiconductor technology. How much longer can we expect this to continue? That was the question posed by James Meindl from the Georgia Institute of Technology during his keynote address at last week's Hot Chips IX Symposium here at Stanford University. The answers he postulated are disturbing and, in some cases, seem to contradict the long-accepted Moore's Law that IC performance roughly doubles approximately every 18 months.

According to Dr. Meindl, future opportunities for gigascale integration, or GSI, will be governed by a hierarchy of limits. The biggest challenge Dr. Meindl sees is interconnect technology.

"The intrinsic switching delay of a 1-micron MOSFET is 10 picoseconds. For a 100-nanometer (or 0.10-micron) MOSFET, it's 1 picosecond. If I look at the response time of a 1 millimeter long low volt interconnect- -or minimum geometry interconnect--for a 1-micron technology, it's 1 picosecond. In other words, the transistor takes 10 times longer to switch than the interconnect.

"But for 100-nanometer (0.10-micron) technology, the interconnect is going to take 100 times longer to switch than the transistor. Right now, we're in a transition time where we're going from being transistor switching time dominated to interconnect switching time dominated, and we need some clever new architectures to overcome these problems."

In addition to interconnect technology, the chip system itself is subject to numerous limits, according to Dr. Meindl. "System level limits are countless. But I'd like to suggest to you that--after looking at the various limits in the context of the hierarchy at the system level--I think there are five generic limits. The first limit is imposed by the architecture of the chip. A second is imposed by the switching energy of semiconductor technology, the third one is imposed by the heat removal capability of the packaging, the fourth by the clock frequency or timing you'd like to achieve. And finally there is a limit imposed by the sheer size of the chip."

Where we have headroom, he said, is to improve switching performance because transistors are going to get better and better. But, where we don't have any headroom is in improving the technology of interconnects. "What's needed is very smart ideas and architectures, for one thing, to keep interconnects short."

Dr. Meindl said that he sees a "flattening out" of the downward curve of minimum feature size that has enabled regular, predictable die shrinks and consequent performance improvements and cost reductions. Discussing the industry's track record in reducing minimum feature size (in microns), by calendar year, he noted that "It was about 25 microns in 1960, 2- 1/2 microns in 1980, and there's no doubt that the minimum feature size in the year 2000 for commercial products is going to be in the 0.25-micron to 0.18-micron range.

"Now after that I show three possible scenarios which I call the 125- nanometer pessimistic scenario, the 62-nanometer realistic scenario, and the 31-nanometer optimistic scenario. (Regarding the 62-nanometer scenario) when we reach a 62-nanometer minimum feature size, corresponding to a 50-nanometer minimum channel length, for bulk technology we're going to flatten out and stop scaling down. Why? Because we will be softly colliding with the minimum allowable dimensions of bulk MOSFETs and no matter how much money we invest we're not going to defeat the laws of physics."

The long-discussed move away from optical lithography is cited as another factor for the impending slowdown in the industry's ability to reduce feature size.

"I'm showing a slowdown in the rate of scaling after the 125-nanometer generation, and this slowdown is to about half the rate of the historical scaling. Why is this slowdown starting at the 125-nanometer level? The reason is that, I don't know of anyone in the microlithography community who is saying we can carry optical lithography to feature sizes below 125 nanometers.

"I would say that the biggest mistake I've seen repeated every few years for many years now is a projection of when optical lithography would run out of gas. But once again, I think that it's so universal now, it's so agreed upon," that it is going to happen in the near future, Dr. Meindl said.

"I'm suggesting that once we get to the 125-nanometer dimension we' re not going to be able to use optical lithography. That means we will need a new lithography technology." That, in turn, means the industry will need, "a new masking technology, new resist technology, and even new metrology. I think those technological problems and associated economic problems are going to slow down the rate of scaling after we hit 125 nanometers."

Finally, if physics and lithography don't slow things down, Dr. Meindl said, economics might.

"If we feel very confident that physics allows us to continue to scale downward, and we feel very confident that we have a sub-optical lithography that is manufacturing worthy, then is it going to be a tolerable business risk to invest $5 billion in a new factory in the year 2000? And it will only go up after that."

Despite this pessimistic-sounding projection, Dr. Meindl also predicts that the industry will achieve a 1 billion transistor chip by the year 2000 and a trillion-transistor device by 2020.

"Even for the pessimistic scenarios we're going to have a billion- transistor chip by the year 2000. We're going to see gigabit memory chips by that year. As we look out at the year 2020, were going to be approaching a trillion transistor chip or terascale integration. I think that's the prospect that we have."

Also at the ninth annual Hot Chips Symposium:

* During the High-End CPUs session, Brad Burgess, chief architect for Motorola at the Somerset Microprocessor Design Center, provided additional technical details on the recently-introduced MPC750 PowerPC processor (EN, Aug. 4). Mr. Burgess noted that while the device has four stages, "from a programming perspective it's a three-stage pipeline." In the device's load-store unit, store gather is supported for word size, cache inhibited and write-through stores. "This is to help the bandwidth of the 64-bit architecture get utilized better," Mr. Burgess said.

* David Papworth at Intel provided a deeper look at the Pentium II introduced earlier this year, what can be described as Intel's implementation of MMX multimedia technology on the Pentium Pro architecture. MMX consists of 57 new instructions that were added to provide multimedia capability. When asked whether Intel is looking at the possibility of adding more multimedia instructions to the MMX set, Mr. Papworth responded, "Yes, we are always looking at ways to enhance performance." He noted, however, that "We have to consider the relative cost and inertia effects," of changing the MMX stack at this point.

* Kevin Normoyle, lead engineer for the "I" series UltraSparc microprocessors at Sun Microsystems, discussed Sun's planned UltraSparc IIi (EN, Antenna, Oct. 7, 1996) code-named "Sabre". The UltraSparc IIi is the first in Sun's "I" series chip designed to provide a balance of price/performance and ease of use. The UltraSparc IIi features four-way superscalar instruction issue, synchronous, external L2 cache, and two separate clock domains with the internal clock logic running at 132MHz.

* John Wharton, head of Applications Research of Palo Alto, acted as panel moderator for a session titled "If I Were Defining 'Merced' " during which panelists Keith Diefendorff, director of microprocessor architecture at Apple; Bruce Lightner, VP of development at Metaflow, John Novitsky, VP of marketing at MicroModule Systems; Martin Reynolds, senior analyst at Dataquest; and Pete Wilson from Motorola took a sometimes serious and sometimes tongue-in-cheek look at the planned Intel-Hewlett Packard CISC/RISC architecture due out in the next couple of years.

For example, Mr. Lightner noted that Microsoft (and its customers) are still using 16-bit code by and large; that full 32 to 64-bit conversion will be hard, slow, impossible; that software will have to be re-written for 64-bit APIs; that we need to start sometime and that 64 bits STILL won't be enough.

DeTar, Jim, Gigascale integration hitting the wall? (1997 Hot Chips Symposium in Palo Alto) (Industry Trend or Event)., Vol. 43, Electronic News, 09-01-1997, pp 14(2).