Everyone in the chip industry knows that the giddy, exponential curve they've been riding for decades can't go on forever. Some day a ”showstopper” will finally appear, signaling an end to the amazing pace at which microprocessors, memory, and other chips have become denser and faster without getting more expensive. Nobody ever expects that dreaded day to be right around the corner. But now, sobering revelations about a futuristic, multibillion-dollar chip-making initiative have thrown a shiver through the industry, raising concerns that the showstopper may be closer than anyone had thought.
As recently as March, researchers were still confident that a technique called extreme ultraviolet (EUV) photolithography would be ready in 2011 to start churning out cutting-edge logic chips. But at an advanced lithography symposium held that month by the photonics society SPIE, experts from IBM and its development partners AMD, Micron Technology, and Qimonda said they do not expect EUV to be ready for its intended debut. Others in the industry, though less blunt, say progress made in the coming year will make or break the deadline.
Historically, each generation of photolithography technology has remained useful for about six or seven years, spanning three size reductions, or nodes, in chip processing. Today's technology uses light with a wavelength of 193 nanometers to produce chips with key parts, or features, that measure just 65 nm. If the seven-year rule holds true, 193-nm lithography will need a replacement by 2012 or 2013.
Before anyone panics, it's important to note that the industry has been consistently wrong about when any particular production technology will hit its limits. But with six years to go, it's clearly crunch time for this technology. ”The next year or so is going to be crucial,” says Michael C. Mayberry, vice president of Intel's technology and manufacturing group.
Until recent years, semiconductor road maps anticipated challenges developing masks and photoresists capable of handling EUV, but not problems generating EUV light as such [see sidebar, ”EUV: Expectations vs. Realities”]. Only in 2005 did the road map spell out the hurdles that would have to be surmounted for EUV lithography to work. Since then, contrary to expectations, obtaining an adequate light source has turned out to be the biggest stumbling block.
A chip's vast profusion of transistors is created by a process of depositing successive layers of metals, insulators, and other materials on a wafer of semiconductor, and then etching away the part of each layer not wanted. The process of defining what goes and what stays is known as photolithography. First the wafer is covered with a chemical called a photoresist. The circuit pattern to be projected on the wafer is drawn on a transparent photomask. The photolithography system shines UV light through the photomask, projecting a shadow of the circuit pattern on the wafer. The photoresist reacts to the light. The parts of the photoresist that react harden and protect the areas directly beneath, allowing everything else to be etched away.
The shorter the wavelength of the light used in the projection, the smaller you can make the transistors and wiring on a chip. EUV sources aim to operate at 13.5 nm--technically, past the ultraviolet part of the spectrum and into the low-energy end of the X-ray band. Transistor features on today's best chips are as small as 65 nm--less than 1 percent the width of the cotton fiber in your shirt. By the time EUV was supposed to come online, they were expected to be around 22 nm.
The EUV wavelength was chosen many years ago, not because there was a good source of 13.5-nm light at hand but because there were good reflectors and filters available. Chip makers expected that, over the years, all the other pieces of the technology would fall into place as they had for other new photolithography systems. But some of those pieces still don't fit. ”The biggest challenge is the source,” says Michael Lercel, lithography director at Sematech--an independent, nonprofit consortium with a charter to help develop new chip-manufacturing technologies. And the source is intrinsically tied to another problem: a light source has to be paired with new photoresists sensitive to it. But the development process for photoresists, too, has been more painful than predicted.
A commercial EUV lithography system will almost certainly need a source that can operate steadily and reliably at 150 to 200 watts. But in practice, developers of sources for photolithography systems--such as Cymer, Gigaphoton, Philips Extreme, Starfire Industries, and Xtreme Technologies--are still struggling to achieve 10 W on a consistent basis. At a Sematech workshop in late May, Gigaphoton reported an EUV source capable of a record 130 W, but only in short bursts, which suggests but does not prove that it could be made to provide 40 W of usable light. That's good for testing prototype systems, but it's still far too dim and intermittent for commercial use.