20 August 2009

Nuclear fusion could advance computer chips

By adapting the same methods used in fusion-energy research, it may soon be possible to create extremely thin plasma beams for a new class of "nanolithography" that will be able to manufacture future computer chips.

Right now, making a chip involves using ultraviolet light to create the fine features in a process called photolithography, which involves projecting the image of a mask onto a light-sensitive material, then chemically etching the resulting pattern.

Ahmed Hassanein works on nanolithography to manufacture more advanced computer chips.

To continue advances in computer technology and to extend Moore’s law, an unofficial rule stating the number of transistors on integrated circuits, or chips, doubles about every 18 months, nanolithography will need to be the one of the manufacturing options.

“We can’t make devices much smaller using conventional lithography, so we have to find ways of creating beams having more narrow wavelengths,” said Ahmed Hassanein, the Paul L. Wattelet Professor of Nuclear Engineering and head of Purdue University’s School of Nuclear Engineering.

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 U.S. Department of Energy’s Argonne National Laboratory are working to improve the efficiency of two techniques for producing the plasma: One approach uses a laser, and the other “discharge-produced” method uses an electric current.

“In either case, only about 1 to 2% of the energy spent is converted into plasma,” Hassanein said. “That conversion efficiency means you’d need greater than 100 kilowatts of power for this lithography, which poses all sorts of engineering problems. We are involved in optimizing conversion efficiency—reducing the energy requirements—and solving various design problems for the next-generation lithography.”

Critical to the research is a computer simulation, called HEIGHTS—for high-energy interaction with general heterogeneous target systems—developed by Hassanein’s team. Computations for a single HEIGHTS simulation using Argonne supercomputers can take several months to finish, said Hassanein, a former Argonne senior scientist who led work there to develop HEIGHTS.

The laser method creates plasma by heating xenon, tin, or lithium. The plasma produces high-energy packets of light, called photons, of extreme ultraviolet light.

Plasma is a partially ionized gas-like material that conducts electricity. Because of this electrical conductivity, researchers are able to use magnetic fields to shape and control plasmas, forming beams, filaments, and other structures. In experimental fusion reactors, magnetic fields keep plasma-based nuclear fuel from touching the metal walls of the containment vessel, enabling the plasma to heat to the extreme temperatures required to maintain fusion reactions.

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.

“The computer simulations tell us how to optimize the entire system and where to go next with the experiments to verify that,” Hassanein said.

One design challenge stems from the fact lenses absorb the photons that make up light, so they cannot focus the beam. Instead, researchers work mirrors into the design. However, plasma condenses on the mirrors, reducing their reflectivity and limiting the efficiency of the process.

“We are trying to help find innovative ways of producing these photons, optimizing the production and mitigating the effects of the plasma on the mirrors,” Hassanein said. “So we are trying to improve the entire system.”

The simulation tool combines computations in plasma physics, radiation transport, atomic physics, plasma-material interactions, and magnetohydrodynamics, or what happens when a target heats, melts, and turns into a plasma.

For related information, go to www.isa.org/manufacturing_automation.