THE number of transistors that can be put on a computer chip tends to double about every two years, just as Intel Corp.'s co-founder Gordon Moore predicted more than 20 years ago. But lately, experts have been worried that Mr. Moore's law might falter as the laws of physics catch up with the ability to cram more and more on a chip. That's why Lawrence Livermore--with its skills in optics, precision machining, multilayer coatings, miniaturization, and a host of other capabilities developed in its weapons work--teamed up with fellow DOE laboratories Lawrence Berkeley and Sandia to literally shed new light on the subject. They also teamed up with three of the biggest names in the semiconductor industry--Intel, Motorola, and Advanced Micro Devices. The result is a $250-million project, funded by corporate money, to continue developing extreme ultraviolet (EUV) lithography technology.
The team's strategy has been to focus on specific technologies for EUV lithography, including multilayer coatings, projection optics, optical substrates, mask and optical design, and metrology (Figure 1). Livermore brings expertise in optics, precision engineering, and multilayer coatings; Sandia's part includes the systems engineering, the resists (protective coatings that prevent unwanted etching), and the light source. Berkeley provides its Advanced Light Source to characterize the optics and resists in the EUV range.
Traditionally, advances in optical lithography--a photography-like technique of using light to carve channels on silicon wafers--have relied on using shorter and shorter wavelengths of light, which can produce smaller features much as a razor can make a finer cut than a hacksaw. State-of-the-art techniques use ultraviolet (UV) light, and experts believe that chips will continue to follow Moore's law for another 10 years as even shorter wavelengths are used.
Current systems use UV light with a wavelength of 0.248 micrometer, to image a master pattern through lenses onto a silicon wafer that is covered by a resist. This technology can produce features of just 0.25 micrometer, or about one four-hundredth the width of a human hair. In less than 10 years, engineers plan to build chips with features measuring about 0.13 micrometer by using wavelengths of 0.193 micrometer. But beyond that point, physics intervenes and light shorter than that--called extreme ultraviolet light--will be absorbed, rather than refracted, by a conventional quartz lens. The result: no image.

Enter Multilayer Coatings
To solve the absorption problem associated with lenses, Livermore researchers turned to mirrors that reflect and focus the light on the chip. Called extreme ultraviolet lithography, the technique bounces EUV photons off an elaborate setup of mirrors, including a mask made of reflective materials, that ultimately focuses the photons on a resist-coated silicon wafer. By doing so, the Laboratory and its partners have designed an EUV system that can pattern features smaller than 0.05 micrometer.
But this method is not without its own technical challenges. The typical EUV mirror made with coatings of alternating films of silicon and molybdenum can reflect only about 65% of the photons that hit them. Because the photons in the EUV system are reflected nine times before they hit the wafer, the losses mount until only 1 to 2% of the original photons hit the target, which makes for long, costly exposure times. To make the system cost-effective, researchers must boost the reflectivity of the mirrors to at least 72%. However, Don Sweeney, the acting program leader for Advanced Microtechnology, says he thinks that goal can be reached in a few years. "Our researchers have already had some success in making higher-reflectivity mirrors from new combinations of materials."

The Incredible Shrinking Chip

Why are smaller computer chips better and faster? It might seem a paradox, but as the size decreases, the chips become more powerful. It's as simple as getting to grandma's house faster if she lives next door rather than across town: the electronic signals zipping around the circuitry to solve computing problems have less distance to travel. Today's chip contains about 3,260 times more transistors than the chip of 1971. Here's a chronology of the shrinking of chips as the semiconductor industry became a $150-billion enterprise.

Boosting EUV Lithography
Another important advance that assists the EUV lithography work is the development of the Ultra Clean Ion Beam Sputter Deposition System, the result of a collaboration between Lawrence Livermore and Veeco Instruments Inc. of Plainview, New York. That work earned an R&D 100 Award for the team in 1997 (see S&TR, October 1997, pp. 8-9).
The conventional method for making the reflective masks for EUV lithography is called magnetron sputtering. But the defect rate for the process is about 10,000 defects per square centimeter, far too many for successful EUV lithography. The new process, embodied in Veeco's IBSD-350, produces precise, uniform, highly reflective masks with 81 alternating layers of molybdenum and silicon, each 3 to 4 nanometers thick. As the machine directs a beam of ions at the masks, the ions physically collide with each mask and form a vapor, which is precisely deposited on it at a defect density of less than 0.1 per square centimeter--a 100,000-fold improvement over conventional methods. This process also holds great promise for a number of other applications using virtually any material or combination of materials including metals, semiconductors, and insulators. A near-term possibility is making very-low-defect-density films for ultrahigh-density heads for the magnetic recording industry.
Also boosting the EUV work is another R&D Award-winning Livermore invention--the Absolute Interferometer (see S&TR, October 1997, pp. 6-7)--which can measure large surfaces to an accuracy of less than 1 nanometer, the thickness of a few atoms. This capability is essential to fabricate the nearly perfect mirrors necessary for EUV lithography because the processing requires surface-measurement feedback during the polishing process at a level of accuracy that conventional devices cannot provide.
But any of these technologies would not ensure the longevity of Moore's law by itself. "Intel came to us because we are the world leader in technology for EUV lithography," Sweeney says (Figure 2). "Our work grew out of our national security mission, and our multidisciplinary workforce allows us to integrate the technologies needed for this project. The commercialization of EUV lithography through the consortium is an exciting prospect for everyone who's interested in smaller and faster computing."
--Sam Hunter

Key Words: Absolute Interferometer, Advanced Microtechnology Program, extreme ultraviolet (EUV) lithography, Ultra Clean Ion Beam Sputter Deposition System.

For further information contact Don Sweeney (925) 422-5877 (

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