NEWS OF THE WEEK the weapons—which, if used, would have had a devastating effect on poorly protected U.S. troops. Gas masks supplied to U.S. troops would have protected them from biological agent inhalation, the primary exposure route for such weapons. But the U.S. had no real-time biological agent detectors capable of warning troops when to don the masks. In addition, only oneseventh of the nearly 700,000 U.S. troops sent to the Gulf region received prophylactic vaccines against anthrax and botulinum toxin. And even troops so protected might have been overwhelmed by high bacteria or toxin levels. Anthrax and botulinum toxin are deadly agents that can kill at very low exposure levels. Aflatoxin is a carcinogenic mycotoxin produced by molds in corn, wheat, and other plants. However, says UNSCOM spokesman Tim Trevan, aflatoxin "causes short-term gastric effects" and thus can be used as an incapacitating agent. Trevan stresses that if killing is the aim, bombs are not the most effective way to deliver proteinaceous substances, because such agents are mostly destroyed by the heat of explosion. But the bombs can be effective in terrorizing enemy troops, he adds. Besides the three biological agents put into weapons, Iraq says it also developed a pathogen to destroy wheat and trichothecene (yellow rain) mycotoxins that were not placed in weapons. And it was researching—but had not yet developed—weapons-quality viral agents. Trevan expects more information about "how far Iraq got on the research and development of the plant pathogens and viruses" as UNSCOM studies the thousands of documents Iraq has turned over. UNSCOM now is selecting the highest priority documents for translation and analysis and will report its findings to the UN Security Council. At a recent press conference, Ekeus said UNSCOM's primary aim is to ensure that Iraq destroyed all its biological arms in July and August 1991, as it claims. The commission will focus on anthrax because botulinum is unstable and difficult to preserve. Until UNSCOM verifies that Iraq has destroyed all its weapons of mass destruction—chemical, biological, nuclear, and missile delivery systems—the Security Council is expected to maintain trade sanctions against Iraq. Lois Ember 8
SEPTEMBER 4,1995 C&EN
age alkanethiolate self-assembled monolayers that cover an etchable base. The new method has several advantages, the researchers say. Diffraction limits conventional photolithography A new microlithography technique resolution to around 100 nm. But neutral that uses atoms instead of light to cre- atom beams, because of their extremely ate patterns on an etchable substrate short wavelengths, show very little difhas the potential to create far tinier and fraction. Additionally, lithography techmore unusual structures than standard niques that use beams of electrons or ions suffer from electrostatic interactions photolithography. The new method—described in last that limit the extent to which the beam week's Science [269, 1255 (1995)] by sci- can be focused. Beams of neutral atoms, entists at Harvard University and the however, can theoretically be focused to National Institute of Standards & Tech- the size of the atom itself. Such control nology (NIST)—exploits the wavelike could lead to improved, more econominature of atoms to do tasks more com- cally manufactured chips and other microelectronic devices. monly thought of as suited for light. The team uses a substrate of thin gold Conventional lithography is performed by projecting a pattern onto a light- on a titanium-primed silicon wafer, coatsensitive surface to selectively remove ed with a 1.5-nm-thick self-assembled the surface layer, exposing a substrate monolayer of dodecanethiolate (DDT). beneath that can then be chemically The image is "developed" with a beam of argon atoms, a small percentage of etched. The Harvard/NIST team—led by which are metastable—that is, with Harvard chemistry professor George M. higher internal energies. This higher enWhitesides and Harvard physics profes- ergy is what causes damage to the sor Mara Prentiss—finds that metastable monolayer, whereas ground-state atoms neutral argon atoms can be used to dam- simply bounce off the surface, leaving it unscathed. And because argon is neutral, it is free from the pitfalls of other elements such Argon beam creates path as chromium or aluminum, which can in monolayer for etching end up plating out on the substrate, Prentiss notes. DDT The researchers experimented with monolayer two types of masking. In one case, they Gold layer Titanium layer placed a mechanical mask—a triangleSilicon wafer shaped aperture with an area of about 3 sq mm—over the wafer and then exposed the substrate to argon. The second type of masking employs the ability of the metastable atoms to be returned to the ground state, or quenched, by light. When the beam of argon atoms is crossed by a beam of light, the metastable argon atoms in the regions exposed to the light return to their harmless ground states and so do not damage the DDT layer. "The light acts like a mask by removing the argon's internal energy," Prentiss says, A self-assembled monolayer of dowhile unlighted areas of the beam redecanethiolate (DDT) on a thin gold tain their damaging properties. film supported on a titanium-primed The team is continuing to experiment silicon wafer is exposed to a beam with different light masks. Because of containing metastable argon atoms nonlinearities in the reaction of the atom (Ar*). Exposure to the Ar* beam rebeam with the light field, the scientists sults in disordered damage to the monolayer, creating a path to etch believe it may be possible to make feathe gold and titanium layers with an tures much smaller than the wavelength aqueous ferricyanide solution. of the light, Whitesides says.
Microlithography uses metastable argon atoms
Elizabeth Wilson
