Miniature Scanning Electron Microscope

NASA's Jet Propulsion Laboratory, Pasadena, California

A miniature scanning electron microscope (SEM) with a capability for x-ray microanalysis has been proposed. This SEM would be particularly suitable for analyzing samples of dust, soil, drill tailings, and other finely divided solids collected in various environments. Designed for use in the robotic exploration of remote planets, asteroids, and comets, this instrument could also be used on Earth, where it could be operated in the field as well as in the laboratory.

The miniature SEM (see figure) would include an electron-beam column comprising a highly integrated assembly of electrostatic electron optics; as a result, this electron-beam column would be much smaller and less complex, in comparison with the electromagnetic-optics-based electron-beam column of a conventional laboratory SEM. An electrostatic deflection system in the column would be used to raster-scan the electron beam across a sample.

The Miniature SEM would have dimensions of only a few centimeters and would consume much less power than does a conventional SEM.

A coaxial microchannel plate would be used to detect secondary and back-scattered electrons. A small, highly pure, deeply depleted silicon detector, with sensitivity from <1 to >10 keV, would be used to analyze the electron-beam-induced x- rays emitted by the sample, for identification of chemical elements in the sample. The field of view of the SEM would measure about 100 by 100 µm, and the spatial resolution would be about 10 nm.

The vacuum needed in the interior volume of the electron-beam column and sample chamber would be provided by a dedicated vacuum system comprising a small sputter ion pump and roughing pump. Samples would be brought into the chamber by a sample-acquisition system that would include (1) a rod equipped with O-ring vacuum seals near its outer end and with detents on its side to hold samples and (2) drive assemblies that would rotate and translate the rod. A typical sample-acquisition sequence would begin with extension of the rod to expose the detents. Sample material would be dropped into upward-facing detents, optionally with the help of a spill tray containing holes to guide the samples into the detents.

The rod would then be retracted to bring the sample-filled detents into the chamber. The rod could be further translated and rotated to bring a desired sample or part of a sample into registration with the electron-beam column. To prevent sample material from fouling the O-ring vacuum seals, small brushes would remove any sample material protruding from the detents and the rod would be made slightly narrower in the detent region than in the vacuum-sealing outer region. Once the samples had been analyzed, the rod would be extended, then rotated to dump the samples from the detents and to bring a set of empty detents to the top to receive the next set of samples.

This work was done by John L. Callas of Caltech for NASA's Jet Propulsion Laboratory. NPO-20499