Tomorrow's Lithography: It's All Wet

June 27, 2005

10 Min Read
Tomorrow's Lithography: It's All Wet

Later this year, semiconductor companies will begin volume production of chips with 65 nm circuits, just two years after the first 90 nm chips hit the market. At the same time, they're already moving from research to development for the next great leap in semiconductors: Shrinking the width of circuit lines to 45 nm.

It's all part of the relentless march of the industry to double the number of transistors packed on a chip every two or three years—a trend that Intel icon Gordon Moore predicted 40 years ago. Intel's Itanium 2 processor, introduced in 2003, contains an astounding 410 million transistors, nearly doubled that of the first Itanium chip, unveiled in 2002. Both these products totally eclipse the Intel 386 introduced just 20 years ago with its 275,000 transistors.

But what will be the technology that will take semiconductors to the next level?

More and more industry insiders are betting that the answer will be immersion lithography. By adopting a system that deposits a fluid between the lithography lens and the wafer, experts see two major benefits: an enhanced depth of focus (DOF) and new lens designs with numerical apertures (NA) larger than 1.0. Result: substantially improved resolution as circuit patterns are etched onto silicon.

In fact, many experts believe that immersion is the only practical method for reaching feature sizes of 45 nm and below in production quantities. A 2004 survey of 350 participants in a forum sponsored by International SEMATECH found that the immersion method in combination with a 193 nm light source was the most likely candidate for semiconductor manufacturing in 2007 and 2008. "The world decided to take the plunge, and the ripples are still spreading out," said one participant, Ralph Dammel of Clariant, a major photo resist manufacturer.

Adds Michael Tittnich, director of Technical Operations at Albany NanoTech: "I don't think 45 nm chip features can be reached by dry 193 nm lithography in a high-volume manufacturing environment. Immersion appears to be the key."

Other industry sources estimate that sales of immersion lithography tools could hit $230 million in 2005—starting from virtually zero in 2004. Systems are expected to cost around $25 million each.

Over the last 40 years, semiconductor companies have managed to double the number of transistors on a chip every two or three years.

Race is On for Equipment Makers

With the direction of the industry clear, makers of semiconductor scanners/steppers for lithography, such as ASML, Nikon, and Cannon, are moving ahead aggressively on new generations of equipment that incorporate immersion. Their efforts, in turn, are spawning new technology from suppliers of products ranging from lenses to photoresists to wafer stages.

The Netherlands-based ASML, the world leader in semiconductor lithography equipment with more than 50 percent of the market, already has preproduction systems running at various research sites around the world, including Albany NanoTech in New York, Applied Materials' Maydan Technology Center in California, and IMEC, Europe's largest independent research center in nanotechnology. Taiwan Semiconductor Manufacturing (TSMC) also took delivery of an ASML XT:1250i machine this past November and is running pilot programs.

While incorporating the new immersion technology, ASML's new 1250i and 1150i machines build on the company's TWINSCAN design (see past Design News coverage at The "dry" version of the TWINSCAN XT:1250, with an NA of 0.93, uses a 193 nm wavelength light source to produce 65 nm chips.

The TWINSCAN design features a two-stage system in which one wafer is situated under the lens for exposure to the reticle or photomask pattern to be etched on a chip, while a second stage moves another wafer through alignment and measurement processes prior to exposure. Among the key ASML design partners in this process: Philips (wafer stages), Agilent (interferometry), and Carl Zeiss (projection lenses).

In ASML's new immersion lithography system, wafer measurement is performed in a dry environment. In this stage, the unit makes a map of the wafer that the system uses during the exposure process to keep the wafer in focus. When the wafer moves to the exposure stage, the system scans each die position across a wafer. A typical scan or "shot" exposes a die size area measuring about 26 mm by 33 mm. The immersion fluid flows throughout this exposure process, but it is precisely contained in a thin film through a system of vacuum and air curtains. The efficiency of this design is such that by the time the system steps to the next die position for exposure, the previous die is essentially dry.

Assessing the Potential ASML's Christian Wagner notes that the use of immersion fluid in the XT:1250i preproduction machine improves the depth of field by a factor of 1.7 over the standard dry system at full step-and-scan speed. This allows for a throughput of 85 300-mm wafers an hour, a pace that will be boosted substantially in production models. The system uses a Carl Zeiss 1250i lens with an NA of 0.85 as well as a resolution down to 70 nm. However, experts agree that an NA of 1.0 or more is essential for production lithography immersion systems. "To go below a 65 nm node, you need better than a 1.0 NA," according to Bernie Wood, marketing director for Nikon Precision, ASML's chief competitor. ASML's newest TWINSCAN platform—the XT:1400—is designed to run in either a wet or dry mode. It comes with an "immersion conversion kit" that transforms the 193 nm scanner for volume production of 65 nm chips to a preproduction machine for developing new 45 nm technology. The kit's three main components: immersion-compatible wafer stages, immersion infrastructure, and a new Carl Zeiss Startlith 1400i lens with an NA of 0.93 and a DOF twice as large as a comparable dry scanner. So far, researchers who are working with ASML's preproduction immersion units seem pleased with the results, although they admit that they must answer many questions about the immersion method. "There are a number of things that we want to know," says Tittnich, the technical operations director at Albany NanoTech, which is partnering with IBM to study the new technology with an ASML 1150i machine. "For example, is the water going to act negatively with the photoresist materials on the wafer to create more particles and defect?" Tittnich adds that he is encouraged with the progress his team has made in minimizing defects and that results look "very promising." Partner IBM last December announced that it used the immersion technology to build processors around its Power architecture. "This advancement holds great promise for continuing progress in chip manufacturing technology well into the future," said IBM chief technologist Bernard Meyerson. Also in December, Taiwan Semiconductor said that it, too, had made SRAM ships using ASML immersion technology. Bern Lin, senior director of TSMC's micropatterning division, reported that yield, device characteristics, and defect levels were comparable to that of dry scanners.
Schematic demonstrates the key changes in ASML's TWINSCAN system as it moves to immersion technology. With ASML's dual-stage concept, the wafer first is measured in a dry environment, then transfers to a new wet exposure stage where it is scanned with a new lens with a high numerical aperture. Others in the Hunt ASML is by no means the only player in the immersion game. Rather than release preproduction lithography tools to research sites, Nikon Precision plans to introduce the first production machine to the market—its S609F—by the end of this year. Marketing Director Wood says the company already has firm orders in hand for this machine, which the company says is the first to have an NA of more than 1.0, using Nikon's home-grown projection lens system. Wood adds that the productivity of Nikon new immersion technology system is consistent with its dry systems. The wafer stage on the S609F, for example, moves at 500 mm per second. That translates into a production rate of about 100 to 120 wafers an hour. Another plus for the Nikon: Its POLANO polarized illumination system, which the company believes will improve image contrast in lithography by 20 percent. This tool enhances resolution, depth of focus, and the critical dimension uniformity of the line widths. Elsewhere, International SEMATECH and Exitech are working together to develop a new immersion lithography research tool with an ultra-high numerical aperture lens (NA of 1.3) from Corning Tropel and a 193 nm ArF light source from Lambda Physik. Rather than a full scanning field of 26 × 33 mm as in production models, the research machine will feature a small field of 0.5 × 0.5 mm, more suitable for testing resolution issues. "Now that the industry is running with immersion technology, we've shifted our focus to investigating how far we can push 193 nm immersion lithography at the more aggressive feature sizes," says Andrew Grenville.
Optics manufacturers must design new projection lenses for immersion lithography with numerical apertures (NA) greater than 1.0 to achieve the depth of field (DOF) and resolution needed for immersion lithography. Corning Tropel's AcquaCAT 193 immersion lens has a 1.05 NA. In addition to testing immersion photoresists at the chip feature sizes needed in production, the MS-193i will show chipmakers how imaging using photoresists with the hyper-NA immersion lenses is affected by such factors as polarization, illumination geometries, and photomask architectures. The tool also can be upgraded for even higher NA immersion lenses in the future. In the coming months, researchers at SEMATECH and other semiconductor centers will be tackling many issues surrounding immersion technology, such as possible defects caused by microbubbles and development of photoresist materials that will prevent water damage during the etching process. And while dionized water is the fluid of choice in current 193 nm systems, researchers are investigating other liquids with refractive indexes higher than water's 1.44 as the NA is pushed higher to produce chips down to the 32 nm feature size. The challenges on the mechanical side, too, are enormous. These range from the nozzle design for delivery of the immersion fluid to special stages sealed around the edges to contain any excess water. Even so, in a very short period of time immersion has become the consensus manufacturing tool for tomorrow's semiconductor production. "Both Nikon and ASML have proven the feasability of immersion technology and shown the improvement in depth of focus and other performance issues," says Nikon's Wood. "Virtually all of the top 20 IC companies, which control about 70 percent of the market, are installing immersion systems—or planning to get them soon." Web Resources For more on immersion lithography: »ASML Lithography equipment »Nikon Precision lithography equipment »Carl Zeiss projection lenses »Corning Tropel projection lenses »Albany NanoTech research center »Philips wafer stages. »SEMATEC research center    

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