NASA Jet Propulsion Laboratory, California Institute of Technology

The first international workshop on Ballistic Electron Emission Microscopy (BEEM) was held March 9 at NASA's Jet Propulsion Laboratory. The workshop was hosted by JPL's Center for Space Microelectronics Technology.

BEEM is a technique, based on scanning tunneling microscopy, that permits probing interfaces, boundaries between different materials, below the surfaces of semiconductors and metals.

The technique was invented at JPL in 1988 by Drs. William Kaiser and L. Douglas Bell and is currently in use or is being developed at dozens of laboratories around the world.

About 65 scientific representatives from the United States, Europe and the Far East attended the workshop. The format was informal with discussion of preliminary results and work in progress.

Kaiser presented the keynote address reviewing the development of BEEM at JPL. Other major research reports were presented by Dr. Robin Williams of University of Wales at Cardiff, Dr. Young Kuk of AT&T Bell Laboratories, Dr. Rudy Ludeke of IBM, Dr. Hans Hallen of Cornell University, and Dr. Leo Schowalter of Rensselaer Polytechnic Institute.

The theoretical aspects of BEEM were discussed by Dr. L. Douglas Bell of JPL, Dr. Mark Stiles of the National Institute of Science and Technology, and Dr. L. Craig Davis of Ford Research Laboratories. The workshop was chaired by Dr. Michael Hecht of JPL.

BEEM is based on scanning tunneling microscopy, or STM, which won its creators the Nobel Prize in Physics in 1986. It enabled the imaging of metal and semiconductor surfaces at the resolution of an atom.

STM uses electron tunneling across a vacuum gap between an atom-sized probe tip and a target surface. This tunneling process only occurs when the tip is within about one nanometer (one billionth of a meter) of the surface. By holding the tunneling current constant and scanning the tip across the surface, the tip follows the surface without actually touching it, and an image at the resolution of an atom may be obtained.

BEEM uses an STM tip to inject a highly localized electron beam into a sample structure which has an interface below the surface. Electrons entering the structure propagate "ballistically," or without undergoing scattering or loss of energy, into the material a distance of 10 nanometers or more.

Analysis of the transmitted and reflected electron currents gives information on material quality and interface properties. By scanning the tip across the surface, not only is a surface image obtained as in conventional STM, but also an image of the electron transmission across the buried interface. The resolution of the probe is on the order of one nanometer.

BEEM, for the first time, allows detailed study of important device interfaces and it will impact the ability to manufacture novel electronic devices and detectors. The workshop was presented jointly by the JPL Center for Space Microelectronics Technology and the Southern California American Vacuum Society, with funding provided by the Strategic Defense Initiative Organization/Innovative Science and Technology Office, the Office of Naval Research, the Defense Advanced Research Projects Agency (DARPA), and the U.S. Army Laboratory Command.

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