A new form of microlithography that uses neutral atoms instead of light to write patterns on silicon has been demonstrated at the Commerce Department's National Institute of Standards and Technology by scientists from Harvard University and NIST.
The new method offers the promise of one day manufacturing integrated circuits or other microfabricated objects about 10 times smaller than is possible with light-based lithography methods. The neutral atom technique also has several advantages over X-ray, electron beam, and ion based lithography, which are under development for next-generation lithography systems.
The scientists' results are reported in the Sept. 1 issue of the journal Science. Funding for the collaborative project was provided by the National Science Foundation, NIST, and the Alexander von Humboldt Foundation of Germany.
"One of the exciting things about this work," says Karl Berggren of Harvard, "is the prospect of combining our technique with recent advances in focusing atoms. It allows us to consider doing lithography on the 10 to 50 nanometer size scale that previously could only be done with charged particles." (Neutral atoms are easier to form in the dense, broad beams best suited for semiconductor production because they do not repel each other like charged particles.)
Working with Berggren most closely on the experiments were Andreas Bard, a guest researcher at NIST supported by both Germany's Alexander von Humboldt Foundation and NIST, and James Wilbur, a National Institute of Health postdoctoral fellow in the chemistry department at Harvard. Also collaborating were John Gillaspy, Steven Rolston, William Phillips and Jabez McClelland from NIST's Physics Laboratory, and Andreas Helg, George Whitesides, and Mara Prentiss, principal investigator for the NSF grant, from Harvard University. McClelland had earlier filed a patent disclosure on a system concept similar to that used in the experiments.
Current semiconductor lithography involves shining short wavelength visible or ultraviolet light through detailed masks (stencils) of circuit patterns. The light pattern is reduced in size with optical lenses and projected on silicon covered with a photosensitive material called a resist. Areas exposed to the light are damaged and then are selectively removed or etched with chemicals. The process is repeated many times with different masks and material layers to build the complex structures needed for advanced computer chips and other microcircuits. The smallest circuit linewidths possible with light-based lithography are on the order of 100 to several hundred nanometers (billionths of a meter).
"Our technique," says Bard, "uses metastable, noble gas atoms to pattern a very high resolution lithography resist made with a single layer of molecules." Noble gases in their ground state are inert and don't interact with other elements, but when the atoms are excited to a metastable state, their electrons carry stored energy. Upon impact with a surface, the atoms release their stored energy to break chemical bonds. In lithography terms, a metastable, noble atom is a kinder, gentler writing tool than X-rays, electron beams or ion beams. Furthermore, the wavelength of individual metastable atoms is less than 0.01 nanometer, which makes it a much sharper "writing tip" than light wavelengths which are more than a 1000 times bigger.
The resist used in the Harvard/NIST method is a self- assembled monolayer of organic molecules called alkanethiolates which is adsorbed on the surface of the gold. An alkanethiolate molecule can be thought of as a ball and chain in a weightless environment. The thiol ball of the molecule bonds strongly to the gold, while the hydrocarbon chain floats away from the surface. On gold and a number of other metallic and oxide surfaces, these and other appropriate compounds will self assemble into a single layer of tightly packed molecules a few nanometers thick, much thinner than a typical photoresist.
The research group's experiments involved directing a beam of metastable argon atoms through a copper grid or screen with holes about 10 micrometers (millionths of a meter) across. Wherever the metastable atoms hit the self assembled monolayer resist, they released their energy and broke hydrocarbon bonds. Areas of gold underneath the weakened and damaged bonds were then washed away with a chemical bath. The result is a grid of gold lines a few micrometers wide with extremely sharp edges (less than 100 nanometers roughness). The gold features can then be chemically transferred into the silicon base, leaving a pattern in the silicon.
Ultimately, says Berggren, the group hopes to replace the physical screen in their experiments with an interference pattern created by standing waves of laser light or possibly by a laser hologram. In the current experiments, the group used laser light to selectively quench or turn off metastable atoms. The laser light releases a metastable atom's stored energy before it reaches the resist surface. A beam of laser light intersecting the metastable atoms' path was used to protect certain areas of a resist, while exposing others. According to Berggren, the combination of laser patterning with a metastable atom/self- assembled monolayer resist system may produce circuit features only 10's of nanometers wide.
A non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry, universities and other government agencies to develop and apply technology, measurements and standards.