Research unveils microcontamination mystery

Hank Hogan

BUFFALO, NY—Microrganisms that survive in ultra-pure water found in cleanrooms may pave the way toward the development of transistors that combine living cells with semiconductors and lead to more effective microcontamination control, according to recent research conducted at the National Science Foundation’s Industry/University Center for Biosurfaces (IUCB).

A scanning electron photomicrograph of the extremophilic bacteria on a polished germanium substratum, isolated from the University of Arizona ultrapure water system. Living microbes and their endospores encased within semiconductor crystals can now be manufactured deliberately for experimental measurements.
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The impetus for the research was based on the fact that, despite ultraviolet light and bubbling ozone, waterborne microorganisms continue to survive. For semiconductors, that leads to circuit defects and yield losses. The IUCB, along with aligned research organizations, set out to discover what made survival possible.

“The microorganisms protect themselves from assault by encasement inside a rock, basically,” says Robert Baier, a biomaterials professor at the State University of New York at Buffalo and director of the IUCB.

These protective shells are constructed out of flecks of silicon and other semiconductors found in the ultrapure water. The living cells attach themselves to the partially etched bits of semiconductor and then the cells grow a crystal. The result is a icroorganism encased in a semiconductor. Subsequent cleaning doesn’t remove these biocrystals, and they can eventually cause shorts.

“There certainly are microbes found in the ultrapure water system,” comments Kimberly Ogden, an associate professor in the Department of Chemical and Environmental Engineering at the University of Arizona in Tucson. Ogden supervised the biological studies done as part of NSF-funded research.
Ogden notes that the same microbes have been found in research and industrial facilities. Of particular interest is that some of these microorganisms fix nitrogen. Hence, a nitrogen purge of ultrapure water may actually be feeding the living cells.

While there is some speculation that cells may be evolving to exploit the cleanroom environment, Ogden doesn’t think this is so.

“Cells do adapt to live in this atmosphere, but I think they always have. We’re just getting better at finding them,” she says.

As for what can be done on the contamination control front, Baier says it’s essential to remove floating semiconductor bits. Without a rock to hide in, the microorganisms are easier to kill. Furthermore, Ogden points out that on-going research is aimed at establishing how effective different treatments are, such as using argon instead of nitrogen for a purge.

Most intriguing, however, may be exploiting this ability to combine semiconductors with living cells. This could be particularly useful because living cells possess photonic, biological, and chemical characteristics that semiconductors do not. Therefore, the biological contaminant may turn into an asset.

As Baier says of the biosemiconductor prospects, “There’s actually an absolutely unique combination of living and nonliving substance, both of which use single electron transfers as their mode of activity. So here we were at the beginning of the next revolution in transistors.”

In addition to the IUCB, the NSF’s Center for Microcontamination Control at the University of Arizona and Rensselaer Polytechnic Institute in New York and the University of Arizona-based Center for Environmentally Benign Semiconductor Manufacturing took part in the research, as did Queen’s University of Belfast, Northern Ireland.


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