New research sheds light on multi-talented UV

05/01/2003

By Hank Hogan

CERRITOS, Calif.—Seeing the ultraviolet light can be done in more ways than one, for not only does it kill, but it can also carve and characterize.

A team of researchers at the University of Colorado proves this, saying extreme ultraviolet (UV) beams can shed light on nanoscale features of desktop-dwelling bacteria, etch intricate sub-micron patterns on any given substrate and provide a portal to the a new world of pharmaceuticals.

The discovery, however, has not minimized prior UV methodologies, which effectively kills microbiological contaminants. That includes the severe acute respiratory syndrome (SARS) virus, according to John Simon, a British physician who is briefing doctors in Manila on how best to treat patients suffering from the deadly disease.

Killing viruses or etching substrates cannot, however, be accomplished with just one blanket UV-methodology.

It's all about the frequency

Take, for instance, the HVAC that provides chilled air for cleanrooms and the buildings that house them. The ductwork and cooling fans are dark and dank, a perfect recipe for certain types of microbial growth.

"The primary things that we'd worry about in an HVAC environment would be the mold because it is going to grow in any of those systems and contaminate the system by simply producing spores," notes Roger Stamper, vice president of sales and marketing for Steril-Aire, Inc.

Steril-Aire makes a line of ultraviolet lights intended for decontamination. Some of these, according to Stamper, are now being put to use in cleanrooms. The company's focus is on ultraviolet lights operating in the C band (UVC), which is from 200 to 280 nanometers (nm). Unlike other biological control mechanisms, ultraviolet light is not something to which organisms can develop immunity. Particularly in the C band, the light actually attacks the DNA in the cell and destroys the cell's ability to reproduce.

In theory, this contamination control idea is simple. Shine the light on those areas where mold and the like grow, and thereby eliminate the problem. For cleanrooms, this pays off in more efficient HVAC operation and less of a contaminant load for downstream HEPA filters.

In practice, implementation is a bit more difficult. One of the best sources for UVC light is a plasma mercury discharge at 257 nm. Commercial versions of this type of light have been around for decades. The challenge, notes Stamper, is the use of mercury in a cold environment. Below 68 degrees Fahrenheit, the mercury plasma starts freezing out into a solid. That cuts UVC output and also reduces the tubes' operational lifetime.

In developing solutions, Stamper says that Steril-Aire has put the latest technology to use. The result is a UVC light source that doesn't mind the cold.

"We now use electronic ballast whereas in the past it was magnetic," Stamper explains. "We use a thicker wall tube design, and the mixture of gas is designed for even as low as zero."

The company says that independent testing has shown these lights deliver up to seven times greater output than conventional UVC devices. Another aspect that's important for cleanroom operation is the use of filters to eliminate any light below 200 nm, which keeps ozone from being produced.

And while that sounds radical, researchers at the University of Colorado have found a way to create extreme-ultraviolet (EUV) wavelengths that are 10 to 100 times shorter than visible light waves, which they say will find application in such fields as microscopy, lithography and nanotechnology.

The project, funded by the U.S. National Science Foundation (NSF) and the U.S. Department of Energy, yielded a system comprised of a waveguide, a hollow glass tube with internal humps that coax light waves into traveling along at the same speed, and a table-top femtosecond (one quadrillionth of a second) laser amplifier.

According to the National Science Foundation, the system is small compared to the room-sized devices currently used to create EUV lasers. The beam's peak power is also higher than any other light source at the wavelengths it achieves, which range from the ultraviolet to the EUV region of the spectrum.

Firing the laser through the gas-filled waveguide creates EUV beams, according to researchers. The intense laser light rips the atoms apart, resulting in charged ions and electrons. The laser beam then accelerates the electrons to very high energies and slams them back into the ions, releasing electromagnetic radiation or, in this instance, photons at EUV wavelengths.

Ripples in the diameter of the waveguide coax the light waves from the laser and EUV beams into traveling at the same speed, preventing them from being out of sync, canceling one another out and weakening the strength of the output beam.

"These waveguide structures are amazingly simple—just a modulated, hollow glass tube," says Margaret Murnane, co-leader of the University of Colorado research team. "It is as if the laser beam surfs on the modulations and is slowed down—just as the speed bumps on the road slow a car down very simply and effectively."

The Colorado team hopes to extend the beam's range further into the EUV region of the spectrum, reaching below 4 nm, where the light is ideal for imaging biological structures.

Producing a beam in this region would allow researchers to build a small microscope for imaging living tissues on a desktop or for viewing objects at the nanoscale, the National Science Foundation indicates.

"In 10 years, laser light will span all the way to the x-ray region of the spectrum," says Henry Kapteyn, another research co-leader team. "The light will be used for the most precise microscopes that we can imagine, allowing real-time movies of the complex dance that atoms weave in chemical reactions, and in pharmaceuticals yet to be visualized."