The STEM at UIC is used to map atomic structures to within one-tenth of a nanometer, and its electron beam must cover a distance of several feet in order to do so. This extreme requirement for stability means that almost everything interferes with its operation: vibration, small variations in stray magnetic fields, temperature changes, and even changes in atmospheric pressure.

"We can actually tell when a storm front is passing through just by looking at the picture on the microscope," says Charlie Brown, Ph.D., adjunct professor of mechanical engineering and chemistry for the University of Illinois at Chicago and director of the University's Research Resources Center.

Electron Microscopy: A Primer

Transmission electron microscopy (TEM) is performed by directing a highly focused (but relatively large diameter) beam of electrons through a very thin sample and onto imaging sensors below. As the beam passes through the sample, some electrons strike the atoms which compose it and are deflected away. The deflection (i.e., diffraction) of the electrons is detected and used to define contrast that results in a picture of the micro-structure of the sample. Contrast is defined by a complex mathematical calculation, which makes interpretation of complex structures difficult. TEM is used extensively to characterize biological samples and materials such as nanotubes.

STEM is performed in a similar manner, but with a much smaller beam of electrons—roughly one-tenth of a nanometer in diameter, or one ten-billionth of a meter—that is rastered across the sample to essentially trace an image of the beam passing through the sample pixel by pixel. As the STEM at UIC is used primarily to characterize semiconductors and superconductors, the samples in question consist of ordered crystalline lattices, and the extent to which electrons are deflected away by atoms in the sample correlates with the atomic numbers of the atoms in individual atomic columns. The gray-scale contrast in these images is an indication of the atomic composition of the sample, which makes interpretation of the pictures straightforward.

A 200 kilovolt electron beam was chosen for the STEM at UIC because at this voltage the crystal lattices of the samples being examined can reside in the beam for days without damage.

Vibration Interference

As if the contingencies of weather were not enough to deal with in accommodating such a sensitive instrument into the UIC campus, Brown and his team had to deal with the fact that the campus is located in downtown Chicago, near the Sears Tower and wedged between two major expressways, one of which also incorporates the infamous Chicago "L", an elevated, 600-volt-DC electric train that rumbles past the campus every ten minutes.

After considering other options on campus, Brown and his team eventually chose a two-story wing of a five-story building called Science and Engineering South, located in the east section of the campus.

Designed by architect Walter Netsch—perhaps most famous for his design of the Air Force Academy Chapel in Colorado Springs, Colo.—the 300,000-sf building is unconventional in design. Based on overlapping squares, with one essentially rotated one-eight of a turn, the building footprint incorporates numerous angles of 45 and 135 degrees. Above-ground floors are supported or suspended by irregularly spaced beams radiating out from their central columns.

While investigating the possibility of locating the STEM in what was originally a cafeteria, Brown discovered that the floor in that part of the building was, as far as vibration isolation is concerned, "phenomenal." The space has floating reinforced concrete floors and is partially underground. It is also essentially isolated from the rest of the building, with its own power substation and ground.

"What we've discovered is that the floors under those parts of the building where you have these radiating beams are actually very good," says Brown. "The floors nearby that don't have the radiating beams above tend to be poor."

Because water chillers also produce vibration, the team placed the chiller outside the building on its own isolated concrete pad, with parallel dual water pumps suspended from the second floor deck.

Vibration-mounted air handlers are located on the roof, but draw air from the building rather than from outside. Since this air has already been filtered and either heated or cooled to normal office temperatures, tighter tolerances can be maintained in the finished, polished air that is sent back down into the lab.

The original cafeteria also had an observation area above, allowing ductwork to be placed high over the lab, behind a suspended ceiling. The ductwork is oversized to reduce microphonic noise, although Brown notes that this is not really adequate to the STEM's requirements. All services to the lab, including ductwork, come only to the edge of the room, not into it.

Movement through the facility is restricted via an access-control system to minimize foot traffic near the lab, and thus avoid vibration.

"Everyone is required to walk the long way around to get to sample prep. There are no shortcuts," says Brown.

A small storage room adjacent to the lab further limits pedestrian traffic near the microscope, thereby reducing vibration and noise. Double doors from this small storage room into both the lab and the hallway allows the high voltage tank, which requires regular maintenance, to be removed from the lab without disturbing the microscope.

Having avoided or eliminated as many sources of vibration as possible, the team still opted to purchase a vibration compensation system; they chose one made by Integrated Dynamics Engineering (IDE). The instrument sits on a series of compressed air jacks, and the control system adjusts air pressure in the jacks to compensate for vibration.

"Although our floor moves, our instrument does not," says Brown. "The reason seems to be that although the floor oscillates, it undulates at a very low frequency, low amplitude, and low bandwidth, which is ideal for these sorts of systems. The idea is not to have a floor that doesn't move. It is to have one that moves in a highly predictable fashion and then use an active compensation system to flatten that out."

Magnetic and Other Types of Interference

As if the raft of external and building-related complications were not enough to deal with, the refit project included plans for a separate NMR lab housing massive, 80,000-gauss vertical and horizontal magnets for performing nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) research. Because electron microscopes like the STEM use magnets to focus their electron beam, even small variations in magnetic fields can interfere with their operation. The magnetic field requirement for the STEM is a variation of one milligauss or less--roughly 100-millionth the static field produced by the NMR and EPR magnets. A person walking through the field can cause that much variation to occur.

Not surprisingly, the design solution involved placement of the NMR lab at the opposite end of the building from the STEM.

The University also purchased an active magnetic compensation system, but it has turned out to be inadequate to the STEM's exacting requirements.

"We've only used it once to handle a highly predictable field from a faulty floor heating system, but researchers still couldn't do really high-resolution work until the cause of the interference was fixed," says Brown. "We've found it doesn't help with unpredictable field variations because more than one part of the instrument needs compensation at the same time."

The project team has made several other adjustments to eliminate various other sources of interference. For instance, in place of normal monitors all computers now have flat monitors. Low-intensity working light is incandescent rather than fluorescent; the latter produced interference even with special low-RF ballasts. They also are preparing to move all computer keyboards and monitors outside the room, because operator body heat proved sufficient to destabilize the microscope.

Special Considerations for Installation, Maintenance and Use

Brown renovated hallways in the space that houses the STEM to provide access to a remote full-height dock at the same elevation. Access hallways are now at least six feet wide, with no center supports in double doors, allowing the STEM to be rolled into place.

"This is important because these machines are very heavy and very bulky, and you can't tip, rattle, or shake them," he says.

The facility also has a low loading dock for smaller, routine deliveries. The low edge of the delivery platform helps prevent drivers from accidentally bumping the dock when backing into the space, eliminating another potential source of vibration interference.

Brown also recommends installing a high-capacity fiber-optic link to support digital imaging. The fiber-optic link would also support operation of the instrument from long distance when this option becomes commercially available.

The Bottom Line: "Avoid and Control"

Brown advises planners facing installation of similar instruments to think about everything in and near the space that might interfere—from the person down the hallway who is putting in a new lab, to the wiring in the room, to the traffic outside.

"For maximum success, we've learned you have to stay as far away from even your own clients as possible," he says. "Also, maintain control of the space around you, including the basement, the floor above, and the roof."

By John Treat

We welcome your Questions and Comments

Copyright 2008 Tradeline Inc.
All Rights Reserved
ISSN: 1096-4894