Researchers Reproduce Bugs in Chips that Foul Up Fabrication, But Could Lead to Biophotonic Transistors

GLASGOW, Scotland -- Extremely hardy bacteria that contaminate computer-chip fabrication facilities and mean nothing but trouble for chip manufacturers have been reproduced under controlled conditions by University at Buffalo researchers, who believe they could be the basis for potentially powerful biophotonic materials.

The research, presented here today (July 23, 2001) at the First International Conference on Semiconductor Photochemistry at the University of Strathclyde, demonstrates the researchers' success in deliberately producing these microbes encased in semiconductor prisms, an important first step toward exploiting these invaders for biophotonic applications.

It also describes the finding that the bacteria make sufficiently close contact with the semiconductor crystals in which they are encased so that electrons could easily be passed between them, forming the basis of a transistor made partly out of biological materials.

The work could lead to development of a biophotonic switch, a biologically based switch with the potential to harness the power of light and turn it into electricity, explained Robert Baier, Ph.D., UB professor of oral diagnostic sciences, director of the National Science Foundation-funded Industry-University Cooperative Research Center on Biosurfaces at UB and principal investigator.

Last year, Baier and his collaborators at the University of Arizona and at Queens University in Belfast, Ireland, made the discovery that Pseudomonas syzgii, which find their way into the semiconductor manufacturing process through ultrapure water, cannot be destroyed despite the best efforts to do so because they becomes embedded in nearly-perfect layers of crystals that grow on top of silicon and germanium.

"These are bacteria that live in ultrapure water," said Baier. "Fabricators treat the water with everything ranging from ozone to ultraviolet light in an effort to keep these bacteria out, but still they get in and in this very hostile environment, they adapt."

According to Baier, the bacteria chew away a little of the material of the semiconductor material and then use it to build a tiny 'house' around themselves. In their experiments, the UB researchers could produce these protective shells in sizes ranging from 5 micrometers to 100 micrometers, which is about the width of a human hair.

That shell, which protects the microbes from the harsh environment outside, is actually a new transistor because electrons can flow across its surface, said Baier, while the presence of the bacteria means that now there is a variable negative charge to boost or limit that electron flow.

Since some bacteria are so sensitive to light, that current flowing inside the tiny crystal of germanium or silicon might be controlled by the pigments in a single cell of the primitive bacteria, making it capable of amplifying an electron signal the way an ordinary transistor amplifies electrical current.

"Everything that lives on earth's surface is essentially a parasite, feeding off photonic processes," said Baier. "What we are doing here, hopefully, is finding a more effective way to harness the power of light."

The current limitation in the field of photonics is that light is a very hard thing to grab onto, Baier said.

"Light is so elusive, but some biological organisms are so amazingly sensitive to light that once you shine light on them, they may have enough energy to function as biophotonic circuits," he continued.

Baier presented data from laser confocal microscopy and atomic force microscopy experiments conducted at UB's Institute for Lasers, Photonics and Biophotonics that demonstrated that these bacteria are in sufficiently close contact with the shell they build around themselves that electrons can be passed, the prerequisite for any material to function as a semiconductor.

"We have demonstrated that the skin of the bug comes right up to the edge of the semiconductor crystal," said Baier. "This is a true intimacy of contact, where these organisms seem to have uniquely nucleated inside of the crystals."

Baier said the researchers were lucky to have found that the microbes did in fact self-fluoresce under laser illumination.

That property allowed the scientists to make observations without having to add extraneous materials to the microbes in order to make them fluoresce.

"It also increases the prospects that these bacteria are electronically 'rich' and will be able to perform important functions," he said.

The next step for the researchers is to attach micro-wires to the new 'biochip' and monitor how electron-hole flow is modulated by light-stimulated bacterial activity, work that will again involve UB's Institute for Lasers, Photonics and Biophotonics.

"We don't know how to manipulate light, but with this kind of 'biochip,' we hope that we will be able to find a more efficient way to convert light waves to electricity and we do know how to manipulate electricity," he said.

"The question we originally set out to answer, with support from the NSF, was 'how can these bacteria live in ultrapure systems and contaminate these semiconductors'," Baier said. "We were asking the question to figure out how to eliminate these bacteria, but it turned out we discovered something that could be the beginning of an entirely new field."

The work was co-authored by Robert L. Forsberg of UB and Jan Sjogren of the University of Arizona.