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Atom by atom in 3-D

 

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In preliminary tests at the FEI Company, before the TEAM 0.5 was shipped, NCEM’s Christian Kisielowski tested the microscope’s ability to resolve individual atoms and precisely locate their positions in three dimensions. He made a series of images of two gold crystals connected by a “nanobridge” only a few dozen atoms wide. From each exposure to the next, individual gold atoms could be seen changing positions.

To achieve this extraordinary resolution, TEAM 0.5 embodies technical advances that have only recently become possible, including ultra-stable electronics, improved aberration correctors, and an extremely bright electron source.

Spherical aberration degrades images, making points of light look like disks, and correcting it can make dramatic improvements to image resolution. (This was famously demonstrated in 1993, when spherical aberration in the Hubble Space Telescope’s optical lenses was corrected in a special space mission.) In the case of electron microscopes, a series of multipole magnetic lenses of varying geometries shapes the electron beam.

“Correcting spherical aberration in an electron microscope has long been possible in theory,” says Dahmen. “But only recently has it become practical, because today’s stable electronics reduce drift and fast computers allow continuous adjustments in real time.” Corrector technology has even become available commercially, says Dahmen, “but no off-the-shelf corrector can match TEAM 0.5’s ability to compensate even higher-order aberrations.”

Correcting spherical aberration makes it possible to use the TEAM 0.5 not only for broad-beam, “wide-angle” images but also for scanning transmission electron microscopy (STEM), in which the tightly focused electron beam is moved across the sample as a probe, capable of performing spectroscopy on one atom at a time — an ideal way to precisely locate impurities in an otherwise homogeneous sample, such as individual dopant atoms in a semiconductor material.

Aberration correction is also essential for another advanced feature of TEAM 0.5: its ability to maintain high resolution with lower electron beam energies.

“Low energy electrons have longer wavelengths, so they are harder to focus,” Dahmen explains. “Aberration correction allows better than one-angstrom resolution with excellent contrast even at 80 kilovolts. This is important when you don’t want to damage the sample with a high-energy beam — in biological studies, for example.”

It’s not just high resolution that makes TEAM 0.5 the world’s best microscope, Dahmen says. When all the electrons in the beam focus at the same plane, image contrast and signal-to-noise ratio improve tremendously.

“It’s because the signal-to-noise ratio is so good that you can adjust focus atom by atom, with enough sensitivity to obtain information about the three-dimensional atomic structure of a single nanoparticle.” Dahmen adds, “This brings us within reach of meeting the great challenge posed by the famous physicist Richard Feynman in 1959: the ability to analyze any chemical substance simply by looking to see where the atoms are.”

The position of individual atoms in a structure can be determined by taking images at different angles, from which the computer reconstructs a 3-D tomograph of the sample, as in a CAT scan. To make this possible an innovative system capable of tilting and rotating the sample, and moving it up, down, or sideways under the electron beam, is also being developed at NCEM.

Much smaller than sample stages now in use, the new TEAM stage will be housed entirely inside the microscope column. Manipulating the sample by such methods as minute piezoelectric “crawlers” that change shape when electricity is applied, the new stage will be able to control and reproduce the sample’s position and attitude with an accuracy of less than a billionth of a meter.

Installation of the new stage must await the next phase of the TEAM Project: the TEAM I microscope, due to be set up at NCEM early in 2009.

While TEAM 0.5 corrects spherical aberration in both the “probe” beam (the electron beam before it strikes the sample) and the image beam (after it exits the sample, but before it reaches the detector), TEAM I will also correct chromatic aberration in the image beam, which has never beeen accomplished before. Spherical aberration is caused by the shape of a lens; chromatic aberration results when a lens refracts light or electrons of different wavelengths (different colors or energies) at different angles.

“Correcting chromatic aberration is harder and takes more space,” says Dahmen. “The chromatic aberration corrector will add two feet to the height of the TEAM I column. But the new configuration will also allow us to enlarge the gap between the pole pieces, into which the sample fits. In TEAM 0.5 this gap is only about two millimeters, so we have to use traditional outside-mounted sample stages, with limited space to manipulate the sample. In TEAM I the gap will be five millimeters; the sample stage will have much greater freedom of movement.”