The proposed electron microscope would function without the need for external vacuum pumps and thus have a significant reduction in size, mass, and power consumption, as compared to conventional (vacuum-pump-equipped) electron microscopes now used in many laboratories. These devices could be used for both imaging as well as chemical-composition determination, in laboratory and field applications. There may be a significant potential market for these devices in applications now served by conventional scanning and transmission electron microscopes in physical and biological sciences, engineering, medicine, and chemistry.
Because the proposed devices could operate in air, it would not be necessary to prepare specimens for examination in vacuum; this is a decisive advantage in situations in which vacuum or the preparation process could damage specimens (e.g. biological specimens). Vacuum pumps are used in conventional electron microscopes because vacuum enables the lossless propagation of electrons over required distances. In the presence of a gas (e.g., air), electrons propagate over short distances, with loss of kinetic energy. In the operation of the proposed devices, working distances to specimens would be made small enough to limit attenuation of electrons to acceptable levels. The spatial resolution is determined primarily by the properties of the electron-transparent, atmosphere isolation membrane that encapsulates the electron column. The best achievable spatial resolution is expected to be at the micron level, whereas conventional electron microscopes give nanometer resolutions. Nevertheless, the advantages may outweigh the loss of resolution in many applications.
In addition to the advantages mentioned above, the proposed electron microscopes offer the great advantage of mass-producibility at relatively low cost by microfabrication techniques established for silicon micromachining. The fabrication process for the proposed electron microscopes would also exploit the recent development of low-voltage, low-power arrays of field-emission electron sources, the miniaturization of high-voltage electronics, and the development of devices that can detect secondary electron emission in the presence of gases.
A typical microfabricated electron microscope column is expected to be a few millimeters thick and about a centimeter square. The evacuated column will consist of a stack of microfabricated chips with metal-film apertures that will serve as electrodes for acceleration, deflection, and focusing of the electrons (see figure). The electron sources will either be an array of thermionic or field emitters, depending on the vacuum level maintained by an integral ion pump (not shown). Although typical field-emission sources require ultra-high vacuum [~10 -10 torr] for operation, the development of diamond-based field emitters promises much less stringent vacuum requirements [as low as 10 -4 torr] for operation.
The key to the self-contained, atmospheric operation is the electron-transparent membrane that encapsulates the electron column. Recently, high-quality thin films of materials such as silicon nitride, boron nitride, and diamond have been developed. These materials have a low average atomic number and are mechanically very robust. Thus, extremely thin films of these materials offer low electron attenuation with the ability to withstand over one atmosphere of differential pressure.
The detectors for the electron microscope will be mounted outside the encapsulating membrane. These detectors will measure fluxes of characteristic x-rays, backscattered electrons and secondary electrons via gas ionization, emitted by the sample in response to the primary electron irradiation.
This work was done by Thomas George of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.