|Comparison of vibrations using the anti-vibration slab within Lawrence Berkeley National Laboratory’s One Angstrom Microscopy laboratory (left column) as compared to not using the slab (right column). Vertical (top) and horizontal (middle and bottom) vibrations are both mitigated, by factors ranging up to 10 times, by the slab. Frequencies are measured from 1-100 Hz and velocities are measured from 0.1µm/sec to 10µm/sec. Click to enlarge.|
Over the years, scientists have been developing instrumentation capable of probing the structure, composition and morphology of materials on an ever-decreasing scale. While electron microscopes a decade ago offered resolutions down to ~ 0.3 nm ( ~ 3 Ångstrom), it is almost routine today for scientists to purchase an instrument that hits or even breaks the 0.1 nm barrier. Tomorrow’s aberration-corrected transmission electron microscopes (TEM) will have even better resolution.
To that end, researchers at the U.S. Department of Energy’s Argonne National Laboratory (ANL), Illinois; Lawrence Berkeley National Laboratory (LBNL), Calif.; Brookhaven National Laboratory, Upton, N.Y.; and Oak Ridge National Laboratory (ORNL), Tenn.; have teamed up with Frederick Seitz Materials Research Laboratory at the Univ. of Illinois to design and build the highest-resolution microscope in the world by the end of the decade through the $25 million Transmission Electron Aberration-corrected Microscope (TEAM) project. That instrument is expected to break the 0.05 nm barrier—more than a million times smaller than the diameter of a human hair (ca. 100 µm).
But any microscope can be limited by its environment. To measure and capture images of a structure or event at the sub-angstrom level requires that the atomic-level feature being studied must demonstrate unprecedented lack of motion during exposure. “Both spectroscopic analysis-type resolution and visual resolution can be limited by environmental factors,” says Nestor Zaluzec, senior scientist at ANL’s Electron Microscopy Center, “and this is increasingly the case as we push the boundaries of science.” While magnetic fields, temperature change, air pressure fluctuation, and mechanical and acoustic vibrations must be minimized when using any modern TEM or STEM, unprecedented parameters must be met to achieve imaging at the scale to be delivered by tomorrow’s electron microscopes. The first place that such tight specifications can be addressed is through the instruments themselves.
Built to fit
FEI recently introduced its Titan system, a sub-angstrom TEM that has been designed to work in existing EM laboratories. According to Dominique Hubert, director of the company’s worldwide Nano Research Business Unit, the instruments are capable of delivering 0.7Å resolution in facilities originally designed to support microscopes that deliver 1.5 Å resolution. “We wanted to be sure the constraints accepted by the community for a non-corrected microscope would work for the Titan,” says Hubert. “We can’t succeed if we ask people to build a more special room than they already have with each new instrument.
That said, Hamish Fraser’s basement EM laboratory, at Ohio State Univ., Columbus, was renovated, but not completely rebuilt, for the installation of his Titan instrument. “Locating a laboratory in the basement has advantages in terms of limiting susceptibility to vibration,” says Fraser. The facility’s floor and foundation were removed, and padding and insulation were added for sound and other vibration proofing before new concrete was poured. The new foundation within a foundation effectively isolated the room from building vibrations caused by HVAC units and other equipment.
Indeed, most researchers agree that the better the room, the better the instrument’s performance. The issues facing laboratory designers include vibrations coming both through the solid surfaces of the room (mechanical) and through the air (acoustical). Sources can be remote, like traffic from nearby highways, trucks making deliveries, speed bumps in the parking lot outside, rotary vacuum pumps down the hall, or overhead air traffic. They also can include vibrations occurring in the room or nearby that can affect imaging, like doors opening and closing, HVAC vents resonating, and footsteps of workers. The key to solving many of these problems does indeed lie in making the floor of the room as stable as possible. Tomorrow’s facilities, according to Zaluzec, will have floors with unprecedented stability—vibration levels of less than 0.25 µm from peak to peak. “But instrument manufacturers can’t assume that the client is going to dig a 500-foot hole and isolate the instrument inside it,” says JEOL’s V P Mike Kersker. “The manufacturer needs to make specifications for room environments that are reasonable, and provide help in meeting those specs where necessary.” JEOL, for instance, hermetically seals the goniometers in its instruments, which diminishes the effects of slight pressure changes.
Acoustic vibration, on the other hand, is often tackled by remediation. Michael O’Keefe at LBNL, designed a lab to house his 1 Å microscope. All noise-making equipment in the laboratory is banished to a separate room, and the microscope room is lined with 5 cm of sound-absorbing material on the ceiling and all four walls. “My design horrified an acoustic consultant who exclaimed that the room as I designed it would be so acoustically ‘dead’ that it would be impossible to work in,” he says, “but our microscope users don’t seem to mind.”
|At ORNL, air enters the microscope instrument room through two cylindrical perforated ducts and diffuses through a porous ceiling (blue arrows). Air is removed from the room via two plenum walls completely covering the two side walls of the room (red arrows). This schematic also shows the room-in-room construction and the isolated floor pads for the microscopes. Also note the gap between the lab wall and the exterior building wall. Click to enlarge.|
While most sources of mechanical and acoustic vibration can be addressed, electron microscopes are especially susceptible to low-frequency (high-energy) vibrations, and these prove to be the most intractable. For instance, Larry Allard, at ORNL’s High Temperature Materials Laboratory, has built the facility’s Advanced Microscopy Laboratory from the ground up to accommodate his JEOL 2200 FS microscope and other specialized equipment. But solving the problems of low-frequency (30 Hz and lower) vibrations remains a work in progress. “We are well within the parameters needed to do imaging at unprecedented levels, but still detect some low-frequency vibration despite all of the special measures we’ve undertaken. It may be related to the long runs of ductwork, which may be acting like organ pipes, even when the air conditioning is off,” he says.
Magnetic fields are another problem for designers of tomorrow’s electron microscope facilities. Magnetic field cancellers such as those manufactured by Integrated Dynamics Engineering, Randolph, Mass., are a start. But it often is necessary to work on the emissions problem where it occurs. At some new facilities, designers have gone so far as to completely eliminate iron rebar from the buildings, replacing it with fiberglass. Walls of existing facilities are opened up, extraneous electrical outlets and wiring are removed, and the remaining electrical wiring is twisted and held in place with plastic ties to cut down on the generation of magnetic fields.
Stabilizing room temperature and airflow is another key measure. Temperatures of some facilities are controlled to well under 1° C change per hour, and, according to some experts, may eventually have to be controlled to a standard as much as 20 times more exacting—within 0.1° F change per hour. On the other hand, blasting cool air into the room is not a solution, for air movement itself is a powerful detriment to the performance of the microscope. At ORNL, careful study has indicated that they can halve the number of air changes per hour (to five) without detriment to system performance, reducing unwanted airflow. Their lab uses two large perforated metal ducts, with 50% open area, to supply air into a mixing plenum above the drop ceiling of the room. Air flows down uniformly from the entire area of the ceiling, leading to a very low flow rate.
Not only are new approaches to air conditioning vital, but many facilities have opted to completely banish the microscope user from the room. “The operator of the instrument provides unwanted heat,” says Mike Kersker of JEOL, “and requires a comfortable temperature in which to work. Removing the operator from the operating environment of the microscope is one way to eliminate this problem.
One example of a new facility designed to accommodate the demands of the future is the Triebenberg Laboratory near Dresden, Germany. Hannes Lichte and his team literally walked away from vibration issues by situating the facility far from power lines, roads, air traffic routes, and even gusts of wind. They separated the building housing the microscopes from another building that held the power supply and control, heating and air conditioning units. The microscopes were placed on three layers of concrete foundations that, in effect, created a building within a building within a building. Power cables were twisted and shielded to avoid AC-stray fields. Today, water-cooled walls help draw the heat generated by the microscopes, and a hollow floor serves as an air channel.
Oh, the places we go
Indeed, most researchers agree, the environment is now part of the microscope. Where electron microscopes are located will, in the future, control to a significant extent the quality of the results obtained. As such, researchers will be increasingly willing to take additional steps to isolate and control their research environments.
Ilene Semiatin is a freelance writer based in White Plains, N.Y.