Long arm of Moore's Law

In 1965 an engineer at Fairchild Semiconductor named Gordon Moore noted that the number of transistors on a chip doubled every 18 to 24 months. A corollary to 'Moore's Law', as that observation came to be known, is that the speed of micro-processors, at a constant cost, also doubles every 18 to 24 months. Moore's Law has held up for more than 30 years. For users, it's been a fast, fun and mostly free ride. But can it last?

In 1965 an engineer at Fairchild Semiconductor named Gordon Moore noted that the number of transistors on a chip doubled every 18 to 24 months. A corollary to "Moore's Law", as that observation came to be known, is that the speed of micro-processors, at a constant cost, also doubles every 18 to 24 months.

Moore's Law has held up for more than 30 years. It worked in 1969 when Moore's start-up, Intel, put its first processor chip — the 4-bit, 104KHz 4004 — into a Japanese calculator. And it still works today for Intel's 32-bit, 450MHz Pentium II processor, which has 7.5 million transistors and is 233,000 times faster than the 2,300-transistor 4004.

Intel says it will have 100-million-transistor chips on the market in 2001 and a 1-billion-transistor powerhouse performing at 100,000 mips in 2011.

For users, it's been a fast, fun and mostly free ride. But can it last?

Although observers have been saying for decades that exponential gains in chip performance would slow in a few years, experts today generally agree that Moore's Law will continue to govern the industry for another 10 years, at least. Nevertheless, it does face two other formidable sets of laws: those of physics and economics.

A mind-numbing variety of things get exponentially harder as the density of circuits on a silicon wafer increases. The Semiconductor Industry Association's (SIA) 1997 Technology Roadmap identified a number of "grand challenges" as the width of individual circuits on a semiconductor chip shrinks from today's 250 nanometres (or billionths of a metre) to 100 nanometres in 2006, four product cycles later. One hundred nanometres is seen as a particularly challenging hurdle because conventional manufacturing techniques begin to fail as chip features approach that size.

And it isn't just making the chips that's getting more difficult — as Intel discovered in 1994 when an obscure flaw in its then-new Pentium processor triggered a firestorm of bad publicity that cost the company $US475 million. Modern chips are so complex that it's impossible to test them exhaustively. Increasingly, chip makers rely on incomplete testing combined with statistical analysis. The same methods are used to test complex software, such as operating systems — but for whatever reason, users who are willing to put up with software bugs are intolerant of flaws in hardware.

At the present rate of improvement in test equipment, the factory yield of good chips will plummet from 90% today to an unacceptable 52% in 2012. At that point, it will cost more to test chips than to make them, the SIA says.

Chip makers are hustling to improve testing equipment — and are extremely reluctant to discuss the matter, which they see as vital to their future competitiveness.

Although the cost of a chip on a per-transistor or per-unit-of-performance basis continues to fall smartly, it masks a grim reality for chip makers: A fabrication plant costs about $US2 billion today, and the price is expected to zoom to $US10 billion — more than a nuclear power plant — as circuit widths shrink below 100 nanometres. Significantly, "scaling" isn't one of the SIA's grand challenges. "Affordable scaling" is.

Indeed, the industry's progress may eventually be slowed by a lack of capital, says James Clemens, head of large-scale integration research at Bell Laboratories, the New Jersey research and development arm of Lucent Technologies. "Social and financial issues, not technical issues, may ultimately limit the widespread application of advanced [sub-100 nanometres] integrated circuit technology," he says.

As an analogy, Clemens points to the airline industry, which knows how to routinely fly planes faster than sound but, due to the cost and technical complexity, doesn't do it.

"A lot of people are worried about cost," says John Shen, a professor of electrical and computer engineering at Carnegie Mellon University in Pittsburgh.

Transistors are etched on to silicon by optical lithography, a process by which ultraviolet light is beamed through a mask to print a pattern of interconnecting lines on a chemically sensitive surface. The conventional approaches that work at 250 nano-metres probably can be refined to etch features as small as 130 nanometres: 400 atoms wide, which is a thousand times thinner than a human hair. But at 100 nanometres and below, where the wavelength of light exceeds the size of the smallest features, entirely new methods will be needed.

An Intel-led consortium is working on "extreme ultraviolet" lithography, which uses xenon gas to produce wavelengths down to 10 nanometres. An approach favoured by IBM uses X rays with a wavelength of 5 nanometres. Meanwhile, Lucent is developing lithography that uses a beam of electrons. These and other alternatives are complex, costly and still unproven.

Continued progress in processor speeds will require better ways of designing and making chips, but the biggest obstacles to higher performance may currently lie just off the chip: in the motherboard and in the logic that connects the chip to cache memory, graphics ports and other things.

"We do not have the design or manufacturing capabilities in those off-chip structures to keep up with the rapid growth in processor clock speeds," says Bruce Shriver, a New York-based consultant, and a computer science professor at the University of Tromso in Norway. "Unless the design and implementation capabilities in those areas catch up, they will be a critical limiting point."

But Albert Yu, general manager of Intel's microprocessor products group, says Shriver is worried about a "very temporary problem". Increasingly, off-chip units such as cache will become integrated on to the processor chip, allowing them to work at the same high frequencies as the processor and eliminating the bus between them, he says.

In just the past few months, a number of promising announcements have come out of US research labs:

q Last month IBM began shipping 400MHz PowerPC chips that use copper wiring instead of the conventional aluminium, which doesn't perform as well but is easier to manufacture. As circuits shrink, the performance and cost advantages of copper grow.

q IBM announced last month that it could boost transistor switching speeds 25% to 35% by putting an insulating layer of silicon dioxide — called "silicon-on--insulator" — between the transistor and its silicon substrate. Refinements of the technology, which reduces distortion and current drain, could push feature widths down to 50 nanometres, IBM says.

q In February a graduate student research team at the University of Texas, working with the industry consortium Sematech, printed 80-nanometre features (a third the size of today's) on a semiconductor wafer. Remarkably, the tiny features were produced with conventional deep ultra-violet light. The advance was due to a special etched-quartz mask developed by DuPont Photomasks in Texas.

None of these is the breakthrough that will buy another decade for Moore's Law. But they illustrate the kinds of advances that chip away at the brick wall toward which Moore's Law is habitually said to be headed.

Says Carnegie Mellon's Shen: "We've always said there's this wall out there, but when you get closer to it, it sort of fades away or gets pushed back."

Anthes is Computerworld US's editor at large. Email him at gary_anthes@cw.com.

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