2009

Micro Electron MicroProbe and Sample Analyzer

EDX and high-resolution microscopy could be performed in the field.

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.

altThe 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.