The HF-2000 Cold Field-Emission Electron Microscope

The HF-2000 cold field-emission electron microscope is a hot new item in ORNL's High Temperature Materials Laboratory (HTML). Users at the the Materials Analysis User Center in the HTML like its remarkable capabilities. So do its sponsors in DOE's Office of Transportation Technologies, which reports to the Assistant Secretary for Energy Efficiency and Renewable Energy.

Bernhard Frost (front) and Edgar Völkl collect electron holograms at the HF-2000 cold field-emission electron microscope.

This instrument is the culmination of two decades of development by Hitachi Ltd., in Japan. It is the only transmission electron microscope currently available that operates at 200,000 volts and uses cold field- emission technology. "Cold field" refers to the fact that the filament in the instrument's electron gun operates at room temperature, limiting the energy spread of the electron beam to about 0.5  electron volts. Such a low energy spread coupled with the small diameter of the electron source generates the highly coherent beam needed for recording high-quality electron holograms.

This microscope is the only commercial instrument that is constructed using expensive Permalloy housings for the electromagnetic lenses. Permalloy construction provides extra shielding against the effects of external magnetic fields, which can distort images and degrade holograms. It has an advanced dry-pumped vacuum system designed partly to our specifications, eliminating the possibility of contamination of the inside of the microscope with oil vapors from standard vacuum pumps.

Another feature of this microscope is the high electron current available in the finest beam sizes (e.g., 1 nanoamp current in a beam 1 nanometer in diameter). This fine probe can generate X rays from nanometer-sized areas of the specimen. The X rays, which are detected using a solid-state detector system mounted on the microscope, have energies characteristic of the particular elements present in the irradiated microarea. A spectrum of these energies is displayed directly on an Apple Macintosh computer, allowing rapid correlation of specimen features with the elements they contain.

The HF-2000 is also equipped with a digital camera system from Gatan, Inc. The camera operates with a charge-coupled device (CCD) chip, similar to the technology used in modern camcorders (except that our camera system cost $100,000). The CCD camera records images up to 1000 ¥ 1000 pixels (picture elements) in size, giving us the advantage of immediate viewing, processing, and analysis of images as well as rapid hard-copy output. This system offers greatly enhanced data collection for this instrument as well as for all the other electron microscopes in our laboratory that are equipped with digital recording devices. Most important, the digital camera eliminates film handling and associated chemical wastes of old-fashioned electron microscopy.

A newly developed spectrometer system, now being installed, will permit digital images to be made from electrons that have lost specific amounts of energy as they pass through the specimen. This "imaging filter," also provided by Gatan, Inc., fits below the microscope and accepts the electron beam into a magnet that bends the beam around into a detector. The electrons that lose energy are bent through a greater angle, enabling them to be selected appropriately by a slit system and used to form the "energy loss image." With the low energy spread of the cold field emitter, we expect to be able to generate images that will show, for example, the dispersion of nanocrystals of diamond, the crystalline form of carbon, on an amorphous carbon film. These "maps" of the locations of elements can be generated with just a few seconds' exposure at nearly the resolution of the standard microscope image.

Operators in other cities can control our microscope,
right from their lab computer.

The digital imaging capabilities available on the HF-2000 electron microscope have opened a new and exciting possibility for extending the usefulness of the instrument and for improving the quality and quantity of our research results. We have developed both an interface to the microscope and software that is integrated into the camera control software, allowing an operator to control the microscope to a large extent directly through the local computer. This control capability has been extended to allow operators in other cities to have the same control, right from the computer in their own office or laboratory. An inexpensive (about $200) commercial remote operation program called TimbuktuPro and a small digital camera such as the Connectix QuickCam (about $90) permit remote users to access the local microscope and see its display on their terminals. Each user also can communicate in a "TelePresence" mode with local operators via the QuickCam camera system, much like scattered participants in a teleconference who "see" each other on a computer screen. When the remote user operates the computer mouse, the same motion springs up on the local computer, thus providing all of the microscope control functions directly to the remote user.

A number of demonstrations of the remote operation of our HF-2000 have already been conducted, from locations such as San Diego, Washington, Nashville, and Detroit. Many industrial and university users are already excited about the possibility of interacting with us on a more frequent and timely basis. In many instances, this capability will eliminate the need to travel long distances to complete research, saving time and money.

International Workshop on Electron Holography

One of the missions of ORNL's LDRD electron holography project was to promote and popularize the use of electron holography for materials and biological science research. In addition to developing new techniques and applications and publishing the results of the research, ORNL researchers organized three international symposia/workshops, which were sponsored by the project. The first symposium, which was held in June 1992 in Knoxville, Tennessee, focused on coherent beam microscopy techniques, including electron holography. The second was a 2-day symposium on coherent beam imaging and digital microscopy; it was held in conjunction with the Microscopy Society of America (MSA) meeting in August 1993 in Cincinnati. Papers from this symposium were subsequently published in 1994 as a special issue of the MSA Bulletin.

One of the missions of ORNL's LDRD electron holography project was to popularize the use of electron holography for materials and biological science research.

The third and most significant workshop was co-sponsored with the Tonomura Electron Wavefront project in the Experimental Research for Advanced Technology Program of the Japan Research Development Corporation (JRDC). The Japanese government provided $20 million for this 5-year program of research in electron holography, which was directed by Akira Tonomura of the Hitachi Advanced Research Laboratory, one of Japan's leading candidates for the Nobel Prize for physics. Most of the funding for the workshop was provided by the Japanese government through the JRDC, which wanted to publicize further the outstanding results obtained during the run of the Tonomura project. Because of the recognition achieved by the ORNL project during its brief run, the JRDC management decided that a joint workshop held in the United States would be the best way to celebrate the end of both holography projects. The 3-day workshop was held in August 1994, also in Knoxville. It had nearly 30 invited speakers, including 18 from overseas.

More than 35 full papers were published as a hardbound proceedings from this workshop by Elsevier North-Holland. Two of the five editors of this book, Electron Holography, are associated with ORNL Larry Allard of the Metals and Ceramics Division and David Joy, ORNL-UTK Distinguished Scientist.

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