Electron Holography Lab Pushes Resolution Limit

Scientists always strive to reduce vibrations and other disturbances to their electron microscopes. But the Triebenberg lab, which was completed just over a year ago, is the first to have been designed from the outset to minimize electromagnetic, mechanical, acoustic, and thermal interference. Power supplies, air conditioning, heating and cooling units, and other utilities are structurally isolated from the electron microscopes. “Lichte has recognized that the building is an integral part of the whole instrument,” says Abbas Ourmazd, director of the Institute for Semiconductor Physics in Frankfurt on the Oder, Germany. “The time is ripe. Instruments and algorithms only recently got to the point that they are limited by buildings. Now he is truly limited by the aberrations of the instruments.”

Lichte aims to take electron holography to the limits of resolution, to define those limits, and to refine the method so that it becomes attractive for widespread use. Analogous to optical holography, electron holography involves combining a beam—in this case, of electrons—that has passed through the sample with one that has not. The resulting interference fringes yield phase as well as amplitude information, and so offer more scope than traditional electron microscopy for correcting aberrations. Just moving the electron microscopes into the Triebenberg lab more than doubled the fringe contrast, says Lichte. “The results are overwhelming. To judge the progress, you should know that formerly we had to struggle for every percent in improvement.”

“We have 0.9 Å resolution. We want to get to 0.8 Å—that could take another five years,” Lichte adds. It’s a matter of optimizing spherical aberration, axial coma, astigmatism, and a half dozen other parameters.

Improving resolution, says Uli Dahmen, who heads the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory, “is extremely important in terms of materials science—to help us understand how materials behave on the atomic scale.” At 0.85 Å, Dahmen’s newest microscope is among the highest resolution machines in the world but, he says, reconstructing images takes a few hours, whereas “electron holography has the potential to do nearly real-time images.”

That’s not electron holography’s only advantage: “The phase lets you extract the electric and magnetic fields present inside a sample. No other technique does it with anywhere near the same spatial resolution,” says David Smith, director of Arizona State University’s Center for High Resolution Electron Microscopy. Visualizing these fields is expected to become increasingly important for applications involving microelectronics, superconductors, and other materials. “Electron holography is a wide open field, one of the frontiers. There is a great deal to be done, and great opportunities,” says Dahmen. “As far as sites for electron microscopy, [the Triebenberg lab] is the ultimate—it’s the site that all microscopists would want. I’m eager to see what it does.”

Lichte and his colleagues are still outfitting the Triebenberg lab. So far, they have three electron microscopes, with space for three more. The microscopes have electron energies of 100 keV, 200 keV, and 300 keV. A few other labs, in particular Akira Tonomura’s at Hitachi’s Advanced Research Lab in Hatoyama, Japan—the other leader in electron holography—use energies around 1.0 MeV (see Physics Today, June 2000, page 9), which give inherently better resolution, but can also do more damage to samples.

Making the lab happen took the persistence of Lichte and the connections of Schulze, a longtime Dresden resident who knows the ropes in the former East Germany. They got support from Triebenberg’s local authority, who, Schulze says, was interested in a swap: If the university promised to remove a telescope from a 100-year-old tower, the region would officially support the electron microscopy lab’s coming to Triebenberg. The other key to landing the former Soviet radar station was a conversation with the head of the Real Estate Bank of the State of Saxony. “After an hour, we agreed,” says Schulze. “He said, ‘I’ll save the land for you.’ We owe him thanks.” Lichte won’t say how much the new building cost, except that it came to roughly the same as two of his electron microscopes.

And, although the lab is up and running, it’s not quite in the clear: The little road leading to the lab crosses private land, and wrangling over the form and amount of compensation to the farmer who owns it is holding up a long-term permit for using the Triebenberg site and delaying the lab’s opening celebration. In the meantime, a small telescope to look for asteroids is planned for the site, and the rest of the 5.5 acres is growing wild and will be studied by botanists from the Technical University of Dresden.

“It was extraordinarily difficult getting the lab together,” says Smith. “Hannes now needs people with cutting-edge problems to reward his extraordinary efforts.”