To Step Up, Semiconductor Lithography Steps Down – Way Down

Hank Hogan, Contributing Editor, hank.hogan@photonics.com

For semiconductors, smaller is better. That’s why the industry may soon take a big step down. On the horizon is a switch from 193- to 13.5-nm-wavelength lithography. This extreme-ultraviolet (EUV) technology eventually could allow an almost tenfold shrinking of current chip features.

The surest sign that EUV is about to be deployed after years of development is money, said Stefan Wurm. He is associate director of lithography at the Albany, N.Y., branch of Sematech, the Austin, Texas-headquartered semiconductor research consortium.

In July 2009, leading lithography toolmaker ASML Netherlands BV of Veldhoven announced the availability of EUV scanners for process development. That milestone has been followed by another.

“As of late February this year, six companies have committed to buy such tools. That’s really significant,” Wurm said. “Each of those companies – to buy one of those tools, install it and get it running – is probably looking at an investment that exceeds $100 million.”

But to paraphrase Mark Twain, rumors of the imminent death of 193-nm lithography have been greatly exaggerated. It – and even longer wavelengths – will be around for years.

Repeating history

In the past, semiconductor makers have moved down in lithography wavelength, when necessary, to pattern finer features. For example, before 193-nm lithography, which is powered by ArF lasers, the industry used KrF lasers at 248 nm. Thus, some argue, EUV is really in keeping with tradition.

This lithography source for extreme semiconductor manufacturing uses a laser-induced plasma to produce extreme-ultraviolet, or 13.5-nm, photons, which will be used to pattern future semiconductor chips. Courtesy of Cymer Inc.

“What we’re doing is nothing different than what we’ve done before. It’s just a factor of 14 instead of a factor of 1.5,” said Vivek Bakshi, president of EUV Litho Inc., also of Austin. The company promotes its namesake through consulting, workshops and education.

In some ways, the new lithography and its deployment are really business as usual. Bakshi noted that many of the current lithography modeling tools and techniques still apply because EUV still projects photons to print a pattern.

The switch does demand some changes from current practice, however. EUV lithography must be done in a vacuum because the photons are absorbed by air. Routing the light around and focusing it on the wafer must be done via reflection, not refraction. Thus, mirrors are required, and the masks must be reflective, not transmissive.

Scalable sources

EUV also requires a completely new yet cost-effective and reliable source. Versions have been demonstrated that hit 100-W output power, enough for minimal production. High-volume manufacturing will require doubling that figure.

The EUV source from Cymer Inc. of San Diego uses a high-powered infrared laser to bombard a microscopic molten tin droplet as much as 50,000 times per second. The resulting plasma radiates photons over a range of wavelengths. The 13.5-nm light is collected and directed into the scanner illuminator.

Because 13.5-nm photons are absorbed by air, and lenses that focus them are lacking, sources for extreme-ultraviolet lithography must operate in a vacuum and route light using mirrors. Courtesy of Cymer Inc.

Cymer is a leading supplier of deep-ultraviolet light sources. For EUV, the company went with a laser-produced plasma rather than generating one between electrodes because of some fundamental advantages.

“It’s a scalable power solution. The plasma is in space. It’s not close to any of the hardware,” said Nigel Farrar, Cymer’s vice president of global lithography applications and technical and strategic marketing.

Besides keeping the plasma at a distance and minimizing damage from it, this scheme also allows for collecting photons over a wider angle. A downside is that it required development of a suitable laser. Further advances in source power could come from the laser as well as in more efficient conversion of its pulses into EUV photons.

Cymer is supplying the sources for ASML’s prototype production EUV scanners. Farrar said that the source deliveries are on track.

Gigaphoton Inc. of Oyama, Japan, also is pursuing a laser-based approach. The company has announced plans to start shipping EUV light sources next year.

Mirror, mirror on the scanner

Carl Zeiss SMT AG of Oberkochen, Germany, makes another key lithography component for ASML’s scanner. Zeiss supplies the projection optics, a set of six mirrors that bounce the light from the mask onto the wafer. The mirrors must be polished precisely, with a roughness of about 100 pm across more than 300 mm.

ASML technicians assemble the first preproduction extreme-ultraviolet patterning scanner in a cleanroom in Veldhoven, the Netherlands. Courtesy of ASML BV.

Over that span, the reflectivity, which is about 70 percent, can vary only in the hundredths of a percent at the central wavelength. What’s more, the mirror pointing and positioning are exacting, with the latter tolerance better than 1 µm.

Winfried Kaiser, senior vice president for product strategy at Carl Zeiss SMT, noted that the tug of the earth cannot be neglected. “Gravity can introduce aberrations. So the art is to hold this heavy mirror without deformation.”

ASML director of corporate communications Lucas van Grinsven said that the company has commitments from six customers for its EUV scanners. The six potential buyers are scattered over all parts of the world and represent all segments of the semiconductor industry.

The first of the preproduction systems is slated to ship this year. In mid-June, van Grinsven said: “Just last week, we’ve integrated the first complete system. So that all looks beautifully on track.”

By 2012, he continued, ASML must be able to provide production systems that could then be churning out chips in volume a few years later. The likely insertion point, he said, will be around the 20-nm half-pitch or smaller memory technology node, which will have considerably finer features than today’s 45- and 32-nm nodes.

In its new scanners, ASML is trying to change only what it must. The company, for example, is employing the same wafer movement mechanism in the new tools that it does in the current ones.

Inspecting masks and other hurdles

This march to EUV does face some significant hurdles. Making masks of sufficiently high quality, for instance, is still a challenge. Another is checking for defects in those masks initially and after use.

Brian Trafas is chief marketing officer for KLA-Tencor Corp. of Milpitas, Calif. The company makes mask inspection tools, including one capable of handling 22-nm logic node EUV masks. The tool does this using a 193-nm source, but Trafas said that this would not work for the 16-nm-and-smaller nodes.

To make working chips, you need defect-free – or defect-free enough – masks. As chip features get smaller, verifying that becomes harder to do. A new reticle defect inspection platform from KLA-Tencor Corp. does this for 2x-nm, such as 22-nm, lithography through innovations in imaging and computational lithography. Courtesy of KLA-Tencor Corp.

“We need to inspect the EUV masks at the same basic wavelength. So today we’re OK, but as we look toward the future, we believe actinic is necessary for success,” he said.

Developing such a capability will not be cheap. The new tool, for instance, will need its own EUV source and associated vacuum-based materials handling. To help defray that cost, KLA-Tencor is seeking outside funding from other industry players.

Carl Zeiss SMT likewise makes an inspection tool, said the company’s Kaiser. The company also is looking into building an inspection tool with an EUV source.

A double take on alternatives

There is the chance that EUV will prove to be too expensive. There are alternatives to it, such as extensions to current technology.

However, 193-nm lithography can print only 22-nm half-pitch features through double patterning. As the name implies, this puts product through lithography twice. Further shrinks could necessitate double-double patterning. Because it requires only a single pass, EUV has cost and space advantages.

In this decision about when to deploy the new lithography, it helps that EUV need be employed only initially on the most demanding layers. What’s more, some of the earliest uses of EUV probably will be to pattern dense contact hole layers.

That mitigates against the need to produce masks of the lowest defect level, Sematech’s Wurm said. “You don’t have a large open area, so defects don’t matter that much.”

Finally, in the semiconductor industry, keeping up with the Joneses, innovation-wise, is a requirement. Thus, the industry may hesitate while alternatives are evaluated but then move quickly to embrace a solution. If companies do otherwise, they risk missing a competitive advantage. That may be the case with the new lithography.

As EUV Litho’s Bakshi said, “Nobody wants to be left behind in this industry. You don’t get a second chance.”