Sapphire Semiconductors? Gem-Like Material May Promise Faster, Smaller, More Reliable Circuits, UD Prof Says

Alumina-better known as sapphire or ruby in its mineral form-may promise faster, smaller, more reliable computer circuits, thanks to University of Delaware technology described in the July 1998 Journal of Electronic Materials, tentatively scheduled for mailing July 13.

Developed by UD Electrical Engineering Prof. James Kolodzey and colleagues, a new technique produces extremely thin, alumina films offering an electrical storage capacity three times greater than silicon dioxide, the material most commonly used in existing transistors-the 'on/off' switching devices in semiconducting circuits.

"We've created alumina films demonstrating a capacitance or dielectric constant of around 12, so they can hold 12 times more electrical charge than air-and roughly three times more than silicon dioxide," Kolodzey says. "If these films can be successfully integrated into a device, it may be possible to make them three times thicker, which should eliminate reliability problems."

As electronic circuits continue to shrink, Kolodzey explains, electrons moving near silicon dioxide films thinner than about 3 nanometers-roughly the width of 15 atoms-often begin to "tunnel" or leak from designated pathways. Whenever tunneling occurs, circuits lose efficiency, just as a leaky cup wastes water.

In addition to their enhanced electrical storage, the UD alumina films exhibited device-grade material characteristics, with relatively few current-blocking flaws found in surface regions, according to Kolodzey and his coauthors, including Johnson O. Olowolafe, UD associate professor of electrical engineering. (Net oxide-trapped-charge density was measured at ~10¹¹ cm^-2.)

"Alumina films aren't going to turn your PC into a Cray supercomputer anytime soon," Kolodzey cautions. "But other researchers have predicted that circuits based on thin-film alumina transistors might be one thousand times faster at performing 'flash memory' or rapid-recall tasks." (See reference.)

Generating Jewels

How did Kolodzey and his colleagues grow such promising, thin alumina films? The new UD process involves indirectly or reactively sputtering aluminum onto a positively charged silicon substrate in the presence of nitrogen and argon gases, then exposing the material to air and heat.

Specifically, the silicon substrate is secured on a mounting device inside a vacuum chamber, along with argon and nitrogen gas and an aluminum "target," says Kolodzey's graduate student, Thomas N. Adam. When subjected to high-voltage electricity, positively charged atom clusters or ions from the argon begin to bombard the negatively charged aluminum target. As ions pummel the target, aluminum atoms are dislodged, react with nitrogen, and then are drawn onto the silicon substrate.

In addition, Adam explains, "We oxidize aluminum nitride. We basically replace the nitrogen with oxygen to form aluminum oxide, or alumina."

Once coated, the substrate is placed inside a small, cylindrical furnace. Heating a sample for one hour at 800 degrees Celsius (1475 degrees Fahrenheit) produces alumina layers with a thickness of 33 nanometers, UD researchers report. Setting the temperature to 1000 degrees C (1832 degrees F) for the same period of time generates films 524 nanometers thick. Because alumina films store more electricity, Kolodzey notes, they could be made thicker than the silicon dioxide layers in existing transistors.

While the UD technique requires an additional processing step to oxidize the alumina films, Kolodzey says their potential for improved reliability in semiconducting circuits should prove worthwhile. "If silicon dioxide layers within transistors become too thin, they'll eventually fail," he says. "We can't shrink existing materials much more before we're going to begin seeing significant problems."

Other members of the UD research team included students Mike Dashiell, Guohua Qiu, Ralf Jonczyk and Dave Smith; as well Karl Unruh, an associate professor of physics and astronomy at UD; Charles P. Swann, a professor emeritus with the Bartol Research Institute at UD; John Suehle of the National Institute of Standards and Technology; and Yuan Chen of the Center for Reliability Engineering at the University of Maryland, College Park. Research described in this news release received support from the U.S. Army Research Office and from the Defense Advanced Research Projects Agency.

* Reference-W.H. Lee, J. T. Clemens, R. C. Keller, and L. Manchanda, "A novel high K inter-poly dielectric (IPD), Al₂O₃ low voltage/high speed flash memories: erasing in msecs at 3.3 V.," Tech. Dig. 1997 Symp. on VLSI Technology, Feb. 1998, paper 9A-4, pp. 117-118.