Nuclear fusion research helps advance computer chips

West Lafayette, Indiana – Researchers are adapting methods used in fusion-energy research to create extremely thin plasma beams for a new class of “nanolithography” for use in making smaller computer chips.

Currently, the fine features in computer chips are created through photolithography – using ultraviolet light to project the image of a mask onto a light-sensitive material, then chemically etching the resulting pattern.

However, according to Ahmed Hassanein, head of Purdue’s School of Nuclear Engineering, “We can’t make devices much smaller using conventional lithography, so we have to find ways of creating beams having more narrow wavelengths”. 

The new plasma-based lithography under development generates “extreme ultraviolet” light having a wavelength of 13.5 nanometers, less than one-tenth the size of current lithography, Hassanein said.

Nuclear engineers and scientists at Purdue and the US Department of Energy are working to improve the efficiency of two techniques for producing the plasma. One approach uses a laser and the other an electric current.

“In either case, only about one to two percent of the energy spent is converted into plasma,” Hassanein said. “That conversion efficiency means you’d need greater than 100KW of power for this lithography, which poses all sorts of engineering problems. We are involved in optimizing conversion efficiency.”

Critical to the research is a computer simulation, called HEIGHTS – for high-energy interaction with general heterogeneous target systems – developed by Hassanein’s team.

The laser method creates plasma by heating xenon, tin or lithium. The plasma produces photons of extreme ultraviolet light.

Because of plasma’s electrical conductivity, researchers are able to use magnetic fields to shape and control it, forming beams, filaments and other structures. In experimental fusion reactors, magnetic fields are used to keep plasma-based nuclear fuel from touching the metal walls of the containment vessel.

HEIGHTS simulates the entire process of the plasma evolution: the laser interacting with the target, and the target evaporating, ionizing and turning into a plasma. The simulation also shows what happens when the magnetic forces “pinch” the plasma cloud into a smaller diameter spot needed to generate the photons.

Simulations match data from laboratory experiments recently built at Purdue, Hassanein said.

“It was very exciting to see this match because it means we are on the right track,” Hassanein said. “The computer simulations tell us how to optimize the entire system and where to go next with the experiments to verify that.”

Findings are detailed in a research paper scheduled to appear in the October-December 2009 issue of the Journal of Micro/Nanolithography, MEMS, and MOEMS.