A proposed, low-power, backpack-sized instrument, denoted the micro electron microprobe and sample analyzer (MEMSA), would serve as a means of rapidly performing high-resolution microscopy and energy-dispersive x-ray spectroscopy (EDX) of soil, dust, and rock particles in the field. The MEMSA would be similar to an environmental scanning electron microscope (ESEM) but would be much smaller and designed specifically for field use in studying effects of geological alteration at the micrometer scale. Like an ESEM, the MEMSA could be used to examine uncoated, electrically nonconductive specimens. In addition to the difference in size, other significant differences between the MEMSA and an ESEM lie in the mode of scanning and the nature of the electron source.
The MEMSA (see figure) would include an electron source that would focus a beam of electrons onto a small spot on a specimen, which would be mounted on a three-axis translation stage in a partly evacuated sample-exchange chamber. Whereas the electron sources in other SEMs typically contain thermionic cathodes, the electron source in the MEMSA would contain a field-emission cathode containing a planar array of bundles of carbon nanotubes (CNTs). Cathodes of this type are capable of high current densities (tens of amperes per square centimeter) at very low fields (8 to 10 V/μm), and the arrays of bundles of CNTs in them are amenable to fabrication within designated areas of the order of a few square nanometers, making it possible to focus electron beams to small spots using simplified electron-beam optics. Another advantage of CNT field-emission cathodes is that they can tolerate operation in relatively poor vacuums [pressures of 10–5 to 10–4 torr (about 0.013 to 0.13 Pa)], which can be maintained by relatively small turbopumps, in contrast to the multistage pumps needed to maintain high vacuums required for thermionic cathodes.
As in an ESEM, the MEMSA would include a gaseous secondary-electron detector (GSED) for detecting secondary electrons excited by impingement of the electron beam on the specimen. Electrical charging of an electrically nonconductive specimen by the electron beam would be neutralized by impingement, on the specimen, of positive ions of the residual gas in the chamber. A Positive Intrinsic Negative (PIN) diode would be used as detector for energy-dispersive analysis of x rays generated in the impingement of the electron beam on the specimen.
Unlike in ESEMs and other SEMs, the electron beam would not be raster-scanned across the specimen. Instead, the electron beam would be focused on a fixed spot, through which the specimen would be moved in small steps by use of the translation stage to effect scanning. The amplified output of the GSED acquired at each step would be stored, along with the translation coordinates, in a digital memory, so that an image of the specimen could be reconstructed after completion of the scan. Omitted from the figure for the sake of simplicity is a context imager — essentially, a relatively-low- magnification electronic camera that would facilitate initial coarse positioning of the specimen in or near the electron-beam spot in preparation for scanning.
As in an ESEM, the sample-exchange chamber and the interior volume of the electron source would be separated by pressure-limiting apertures (PLAs), which are small apertures through which the electron beam passes on its way to the specimen. As in an ESEM, the PLAs would be sized to retard the flow of residual gas from the sample-exchange chamber to the electron source side so as to mainain the desired lower pressure on the electron source side and the desired higher pressure in the sample-exchange chamber. In the original intended application of the MEMSA, the PLAs would be four platinum disks containing apertures of ≈0.5-mm diameter, chosen to maintain a pressure between 5 and 7 torr (about 0.67 to 0.93 kPa) of residual Martian atmospheric gas comprising primarily CO2.
The MEMSA is expected to be capable of imaging at a spatial resolution of 40 nm or finer without EDX, or imaging at somewhat coarser resolution (of the order of 200 nm) with EDX in the energy range from 100 to 20 keV. The coarsening of resolution in the case of EDX would be a consequence of the need to use higher electron current. The maximum power demand of the MEMSA during operation has been estimated to be ≈5 W.
This work was done by Harish Manohara, Gregory Bearman, Susanne Douglas, Michael Bronikowski, Eduardo Urgiles, and Robert Kowalczyk, of Caltech and Charles Bryson of Apperati, Inc. for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-45389, volume and number of this NASA Tech Briefs issue, and the page number.