Focused ion beams are important in the semiconductor industry, where they are used to carve structures with dimensions measured in billionths of a meter, repair defects in masks used for photolithography, isolate and analyze elements of integrated circuits, "dope" semiconductors with specific atomic species, and perform other tasks.

Focused ion beams have also been used to create images of surfaces, pattern thin films for dense magnetic storage, analyze the chemical content of samples, and investigate biological systems. And because ion beams can shape materials with microscopic precision, they can micromachine miniature medical implants, such as cardiac stents that hold weak blood vessels open.

Complicating these applications, however, is the fact that "problems arise when positive ions are used for imaging or micromachining insulating materials," says Qing Ji, who authored the Applied Physics Letters report with her colleagues Lili Ji, Ye Chen, and Ka-Ngo Leung.

Qing Ji is a postdoctoral fellow in the Center for Imaging and Mesoscale Structures at Harvard University and a guest at Berkeley Lab. Coauthors Lili Ji and Chen are with Berkeley Lab's Accelerator and Fusion Research Division (AFRD) and the Department of Nuclear Engineering at the University of California at Berkeley, as is Leung, who heads the Plasma and Ion Source Technology Group in AFRD.

The trouble with using positive-ion beams on insulating samples, Ji explains, is that "the target material is charged by the positive ions; as the positive charge builds upon the sample it repels the ions and defocuses the beam."

Traditionally two methods have been used to keep a nonmetallic sample from acquiring charge from a positive-ion beam, she says. "One method is to pass the beam through a gas cell, where it is partially neutralized before it reaches the sample by acquiring electrons from the gas. The other is to train a separate beam of electrons on the sample."

"In fact our new beam system was inspired by one of our group's previous inventions, a multiple-ion-beam system that can steer hundreds of beamlets simultaneously," says Ji. "The device had no room for a neutralizing gas cell, and there was no way to use a separate electron beam to neutralize the sample."

A new way to neutralize ion beams:

The group came up with a novel solution: instead of a liquid-metal ion source, standard in many focused ion beam devices, the new system uses two chambers in which plasma is generated byradio-frequency electromagnetic fields, which separate gas molecules into their component electrons and positive ions. The two chambers are divided by an arrangement of electrodes that allows only high-energy electrons to exit the first chamber and at the same time keeps positive ions out.

In the second chamber an ion beam is formed and accelerated by a lower voltage, which does not impede the high-energy electron beam. Both beams combine in a single mixed beam and are extracted by the accelerator column. The self-neutralizing, mixed beam stays tight on its way to the target, because with electrons present there is little "space charge" -- the positive ions do not push one another apart -- nor does it charge the sample upon striking it.

The combined-beam system can accelerate numerous species of ions, including noble gases like argon, metals like manganese, and even molecular ions like carbon-60 "buckyballs," useful in biological studies because of their stability.

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