Next Generation of Light-based Microscopes

Harnessing the power of ultraviolet and soft x-ray light.

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By Marlene Cimons, National Science Foundation

Remember the microscope you used in high school science? Chances are it was a light-based, or optical microscope, one that uses visible light and a system of lenses to magnify images of small samples. It was the first microscope ever invented, and it’s still quite popular.

But optical microscopes have limitations. Their ability to “see” depends on the wavelength of the light; the shorter the wavelength, the better the resolution. With visible light, the resolution is about 1/100 the size of a human hair.

Scientists have shown they can do better by using a different kind of light, and are developing new types of microscopes that rely on extreme ultraviolet light. Extreme UV has wavelengths that are between 10 and 500 times shorter than visible light, which puts the resolution on a nanometer scale, that is, the size of a collection of molecules.

“It’s a perfect match for nanotechnology,” says Jorge Rocca, director of the Center for Extreme Ultraviolet Science and Technology and professor of computer and electrical engineering and physics at Colorado State University, where the center has its headquarters. “If you use extreme UV or soft x-ray light, you can, for example, make microscopes with spatial resolution on the nanoscale.”

The sun is a source of the full spectrum of ultraviolet radiation. UV radiation, while not visible to the human eye, is familiar to most of us as the source of an occasionally painful sunburn. It is named ultraviolet because the spectrum consists of electromagnetic waves with frequencies higher than those that people can identify as the color violet.

Researchers studying astronomical objects typically refer to four subdivisions of UV radiation: near ultraviolet, middle ultraviolet, far ultraviolet and extreme ultraviolet. Near UV is the light closest to visible light, while extreme UV and soft x-rays light, which is more energetic, has a shorter wavelength, and is closer to so-called “hard” X-rays, such as those used in medical diagnostics.

Harnessing the power of extreme ultraviolet and soft x-ray light not only holds promise in the research lab, but also within industrial and scientific labs, with potential for wide-ranging applications. In fact, scientists are developing EUV light sources for the lithography of integrated circuits in order to create the tiniest and fastest next-generation computer chips.

“You can use this light to see smaller features than you can see with visible light, and you can write smaller features as well, such as those in computer chips,” Rocca says.

Rocca and Margaret Murnane, the center’s deputy director, also from the University of Colorado, and their collaborators are part of a center that is one of the National Science Foundation’s Engineering Research Centers. NSF has funded the center with an estimated $29 million during the last eight years. The center, based at Colorado State University in Fort Collins, also includes research partners at the University of Colorado at Boulder, the University of California at Berkeley and the Lawrence Berkeley National Laboratory.

Center scientists also are studying “soft” X-rays (SXR),” which are in between extreme UV and the hard X-rays you see in hospitals,” Rocca says.

“For some applications, incoherent extreme UV light produced by an ‘extreme UV light bulb’ can be sufficient,” Rocca says. “Plasmas created by heating materials with a laser or electrical discharges can produce such light. However, for other applications, a laser or ‘laser-like’ source of coherent light is needed. Our main goal at the center is to develop compact laser-like beams in the extreme UV.”

The hope is “to make EUV and SXR laser light, now mostly limited to a handful of large national facilities, available routinely in a broad variety of laboratory settings for applications such as high-resolution imaging, material analysis tools at the nanoscale, elemental and bio-microscopy, and nano-fabrication,” Rocca adds.

Center researchers at the University of Colorado and Colorado State University already have developed very compact sources of extremely bright coherent extreme UV light, and are using them to solve several important scientific and industrial problems. These include creating microscopes with extreme UV optics that have the ability to see much smaller objects, developed in collaboration with Berkeley scientists.

To be sure, light-based microscopes are among several different types of microscopes in use. Experts sometimes classify microscopes by the source they use to interact with samples to produce images, such as light, a scanning probe, or electrons.

An electron microscope, for example, uses a beam of electrons as its energy source, which has an exceptionally short wave length—about 100,000 times shorter than visible light - and, as a result, at present, provides significantly better resolution. But there are significant differences between electron microscopes and the new types of extreme UV/soft X-ray microscopes currently under development at the center.

In fact, imaging with light and with electrons are complementary technologies rather than competing technologies, Rocca says. “Both have a role to play,” he says. “For example, to use electron microscopy, the samples need to be electrically conductive, and if they are not, they need to be coated with a thin metal layer to make them so.”

Light microscopes, on the other hand, can image both conductive and insulating samples without the need of a coating. “Also, electron beams cannot be used to image samples that are subject to strong electric or magnetic fields or thick samples,” he says. “In contrast, electromagnetic radiation is not affected by electromagnetic fields. In summary, each might have advantages and disadvantages depending on the application--in other words, they are complementary techniques.”

The extreme UV microscopes developed by center scientists “can currently image nanostructures with 20 nanometer resolution and make movies of nanomachines in action,” Rocca says.

Moreover, “in the future, powerful new microscopes that can image the nanoworld with less than ten nanometer spatial resolution, and that even can capture the fleeting motions of electrons, will be possible,” he adds.

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