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Ka-Ngo Leung (second from right) with Ye Chen, Lili-Ji, and Qing Ji beside their test apparatus. (Berkeley Lab photo)

UC Berkeley nuclear engineer Ka-Ngo Leung has developed a highly-efficient ion beam technology that could edge out tried-and-true methods for fabricating myriad microscale products. Medical implants that are currently manufactured one at a time could be batch produced in bulk. The new ion beam technology could also boost production at microchip fabrication facilities while helping keep Moore's Law on track, says Leung, the head of the Plasma and Ion Source Technology Group at Lawrence Berkeley National Laboratory.

Ion beams that deliver a steady stream of positively-charged particles are already common tools in the semiconductor industry. Engineers use the focused energy to add impurities that change a semiconductor's electrical properties, characterize the material, and pattern structures on it just nanometers in size. (A nanometer is one-billionth of a meter.)

The problem, Leung explains, is that when the ion beam hits a non-conductive material, for example a silicon wafer's insulating substrate, that material becomes charged by the positive ions. As the charge accumulates, it repels the ions and causes the beam to lose its pinpoint focus.

A double-chamber plasma source forms beams of positive ions and electrons that can simultaneously be delivered to a target. (Berkeley Lab photo)

"This becomes an increasingly serious problem when you're talking about shrinking the features on the circuit to even smaller length-scales," says Leung, a professor-in-residence in the Department of Nuclear Engineering. . "The charge can also jump from one point to another on the sample, perhaps damaging the circuit."

Collaborating with his graduate students and Qing Ji, a guest researcher at the Berkeley Lab, Leung designed a system that adds a flood of electrons to the ion beam. The negatively-charged electrons neutralize the ions, preventing the sample from building up a problematic charge. Balancing out the ion beam with an electron beam is common practice, Leung points out, but combining the two into a single beam is entirely novel.

Integrating the two beams was a necessity for the researchers' ion beam imprinter, a system that enables hundreds or thousands of ion beams to be generated using a single ion source. With so many ion beams, Leung explains, it would be virtually impossibly to align each one with a separate electron beam.

According to Leung, the combined electron and ion beam can remove several steps from traditional lithographic techniques used to fabricate integrate circuits. During the fabrication process, ions, known as dopants, are implanted in the silicon wafer to change way electricity is conducted. In order to implant ions in some specific regions of the wafer and not others, a mask, or stencil, is applied to the wafer to protect the regions that shouldn't be hit with the beam. The ion imprinter negates the need for the wafer masking process by putting the mask at the source of the beam. The technique is not unlike making hand-shadows on a wall with a flashlight.

"The ion beams are coming right out of the mask, enabling you to implant the ions in the shape you desire in just one step," Leung says.

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Outside of the semiconductor industry, the researchers believe that their technology could even replace lasers for certain micromachining applications. Because the ion beam is compatible with curved masks, the system is capable of cutting three-dimensional shapes.

For example, cardiac stents--mesh-like structures used to expand clogged arteries--are machined from steel with a laser beam that must be carefully steered. As a result, each beam can only carve out one stent at a time. According to Leung though, the ion beam imprinter "would allow hundreds or thousands of stents to be cut in one shot."

Currently, the researchers are developing ion sources to produce a variety of metallic ions, opening up even more applications for the new beam. Someday, Leung says, it could even become possible to use the system to fabricate magnetic quantum dots, nanometer-sized semiconductor crystals. Quantum dots may eventually be the basis of nanomemory chips that pack dozens of gigabits of data into a square centimeter.

Ka-Ngo Leung's home page

Plasma and Ion Source Group at Berkeley Lab

"Next Generation Semiconductors May Rely On Ion Beam Lithography" by Paul Preuss (Berkeley Lab)

"Ion Beams Wield a Sharper Scalpel Caring New Niches for Focused Ion Beams" by Paul Preuss (Berkeley Lab Currents, 29 October 2004)

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