In the lab

• By Jim DiPeso
• Sep 01, 2000

A greener dip for chips

Integrated circuits (ICs) are miniature assemblies of electronic components vital to the electronics industry. In order to function properly, they must be ultra clean. Of the 400 process steps necessary to make a typical integrated circuit, about 50 to 60 of these involve cleaning the film and substrate surfaces.

Traditional methods used to clean ICs included liquids — typically acids and bases. These processes had to interface with vacuum chambers, where most IC fabrication steps occur. After the liquid cleaning of IC substrates, it is necessary to insert a drying step before starting vacuum processes.

Dr. Dennis Hess, a professor at the Georgia Institute of Technology, has devised a new routine for bathing ICs that eliminates the drying step, consequently streamlining the fabrication process and making it more environmentally friendly.

Dr. Hess has been experimenting with a liquid-phase water cleaning process that can interface directly with vacuum processes. Water is heated to temperatures above the boiling point while pressure is simultaneously added to keep the water in the liquid phase. After the cleaning is complete, pressure is reduced and flashes the liquid off the surface.

"This new process takes advantage of what we know about liquid cleaning, but modifies the approach to be compatible with vacuum processes," said Dr. Hess.

Sanitary solutions aside, the new process is greener than the old because it uses water instead of the corrosive, toxic chemicals previously used for IC cleaning.

The technique has shown promise, but Hess does not yet know if it will be feasible for production processes.

"At this point in our work, we need to collaborate with IC equipment manufacturers or IC device manufacturers to try out the method on actual IC wafers so we can assess the stability of the approach for large-scale fabrication," said Dr. Hess.

Abundant research has been done on cleaning wafers using vapors, but the technique has proven successful because of the extreme complexity. Additionally, the cleanliness level may not be equivalent to that of traditional techniques.

In spite of the advantages of using water to clean surfaces, there is a downside: The reactivity of the water is very high and can actually etch the silicon wafer. Therefore, Dr. Hess is looking at other options, including the possibility of using additives to mediate the reactivity.

For more information, check out the Georgia Institute of Technology Chemical Engineering Website at www.che.gatech.edu.

Do plants need sunscreen?

The damaging effects of excessive ultraviolet radiation (UV-B) for humans are well known, but until recently, there was little evidence that the sun's damage to plants was more than leaf-deep.

Researchers at the Friedrich Miescher Institute in Basel, Switzerland have found that plants' susceptibility to the sun's damaging rays accumulates over generations and could worsen if the depletion of the earth's protective ozone layer continues at its current pace.

"As far as we know this is the first time it has been shown that UV-B has an effect on the stability of the genome," said molecular biologist Barbara Hohn.

She and her colleagues discovered that plants exposed to UV-B have rearranged or changed DNA. Higher levels of exposure led to the biggest changes.

"This particular kind of rearrangement in DNA is increased under UV-B. Whether that is positive for a plant or negative for a plant, we cannot yet tell," said Hohn.

What is known is that exposure to sunlight and resulting genetic damage can lead to skin cancer in humans. Hohn has found that plants endure similar changes but do not develop tumors. She and her colleagues note the effect on the plant's genome increased with each generation so the plant somehow remembered earlier DNA damage.

"The DNA rearrangements in skin cells can lead to cancer. Such DNA rearrangements have now also been found in plants. Whether this is any danger for humans we do not know. The research only shows that the effects are more general than we know," said Hohn.

In a commentary on the research, Anne Britt of the section of Plant Biology at the University of California at Davis, said the research suggests the depletion of the ozone layer could have a measurable impact on the mutation rate of some plants.

"But further work to determine the mutation rate and the spectrum of mutations caused by UV-B exposure are required before we can fully assess the significance of these observations," she added.

For more Information check out the Friedrich Miescher Institute at www.fmi.ch.

Writing a new chapter on drinking water purification

Researchers have developed a battery-powered disinfecting "pen" that electrochemically generates mixed oxidants from a salt solution to purify drinking water.

Tests of the invention recently took place at the University of North Carolina at Chapel Hill's (UNC-CH) Environmental Health Microbiology Laboratories. Dr Mark D. Sobsey led the effort with doctoral students Maren E. Anderson and Julie A. Kase.

Los Alamos Technical Associates and MIOX Corp of Albuquerque developed the pen, and the latter funded the UNC-CH research.

Investigators evaluated the new battery-powered tool to see how well it inactivated waterborne parasites, viruses and bacteria. Several alternative pen cell designs also were tested by seeding oxidant solutions generated from the pen with microbes — including highly chlorine-resistant Cryptosporidium parvum, a major source of contamination in water.

"There was dramatic — more than 99.99 percent &3151; reduction of all test bacteria and viruses within one to ten minutes," said Sobsey. "Considerable inactivation of C. parvum was also achieved within 90 minutes, the amount depending on the design of the pen cell."

The way the miniature mechanism works is that it electromagnetically generates a mixture of oxidants from a salt solution that was able to inactivate C. parvum eggs, as well as bacterial spores, bacteria and viruses to produce safer drinking water in minutes.

"The pen cell makes it possible for people to easily and quickly have safe supplies of personal drinking water in remote and isolated areas and during situations where water supplies are at risk of being contaminated," said Sobsey. "There is a very big need for this, and it almost certainly will save lives."

Users put a small amount of water into the pen, salt pellets dissolve in the water and the solution comes in contact with battery-powered electrodes. Within 30 seconds of flicking a switch, a chemical reaction known as electrolysis generates chlorine and other oxidants from the saltwater.

"The solution contents are then added to a quart bottle of water or a canteen," said Sobsey. "This delivers enough oxidants to disinfect the water in 10 minutes even better than plain chlorine does, and then the water can be drank safely."

The pen is seven inches long and weighs four ounces. A lighter, less expensive version is in the process of design for the outdoor recreation market.

For more information check out the UNC-CH website at www.unc.edu.

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This article originally appeared in the 09/01/2000 issue of Environmental Protection.