Taking the Jiggles Out of One Angstrom Microscopy

December 19, 1997

By Paul Preuss, [email protected]

When you're trying to achieve good image resolution at one angstrom -- which is roughly the distance between atoms -- getting rid of jiggles becomes a major undertaking.

John Turner of the National Center for Electron Microscopy describes the environment of the new One Ångstrom Microscope (OÅM) and its neighbors as "the most extensively designed anywhere." The OÅM, based on a CM300 Field-Effect-Gun microscope built by Philips Electronic Instruments, is closeted inside an isolated environment planned from the ground up.

The pixel detector system
Concrete vibration mounts for the microscopes
Before construction could begin, Turner needed to find out what vibrations the OÅM would undergo when no machinery was running, no trucks were driving by, and no people were walking the halls. "There are built-in mechanisms to damp high-frequency vibration," he explains, "but they don't help at very low frequencies that resonate with the two-meter length of the microscope itself" -- vibrations, that is, which can cause the microscope column to resonate as an inaudibly low-pitched organ pipe might do.

Turner visited the site on Christmas Day, 1995, and what he saw astonished him. By his standards, the vibration was extreme, especially in the crucial wavelengths of one to five hertz. The readout of a seismic station a quarter of a mile away confirmed his observations; he called the UC Berkeley Seismological Laboratory and found that he was observing evidence of a storm track -- the crashing of breakers against Pacific beaches dozens of miles away.

In order to damp the "micro-seisms" caused by storm winds and distant surf, and building-borne vibrations too, the OÅM and its neighbors -- a CM200 also built by Philips, and the custom-built Spin-Polarized Low Energy Electron Microscope -- rest on massive reinforced concrete slabs which in turn rest on undisturbed earth. The slabs are a meter thick and more than three by four meters wide; each weighs 34 tons. To prevent any contact of the slabs with the surrounding foundation and to aid in damping vibration, the spaces between them are filled with neoprene, the same material used in wet suits.

On these solid foundations are built rooms that minimize not only mechanical vibration but acoustic noise -- the walls are sound-proofed, and pumps and other machinery are in a separate sound-proofed room. Stray electromagnetic fields are minimized, too -- the main transformer is 20 meters away, outside the building, and the wiring runs through the second floor ceiling and down to the microscope equipment rooms. As to variations in temperature, "The temperature in here changes at most 0.15deg. C," says Turner. "We're aiming to get that down to 0.1deg.C."

For the success of the isolated-environment project, Turner credits Charles Allen of the Lab's Facilities Department and Project Manager Greg Raymond, who sadly died of an unexpected illness during construction.

Not long after the OÅM was installed, researchers confirmed the hoped-for one-angstrom information limit with a test specimen of microscopic particles of gold on a thin layer of amorphous tungsten. That goal could not have been achieved without an almost ideally vibration-free, noise-free environment.

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