Transmitted electrons show diffraction patterns in an SEM

Scientists at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, have blurred the lines between TEM and SEM by developing a technique they call transmission electron backscatter diffraction (t-EBSD), which uses an SEM with a strategically placed sample to obtain diffraction patterns from electrons that are transmitted through a sample.

Experimental configuration of an SEM for t-EBSD analysis.  Image credit: Roy Geiss, NIST.

Traditionally, SEM has analysed electrons reflected from a sample while TEM has examined electrons that have passed through one. By mixing the two techniques, Robert Keller and Roy Geiss of NIST have actually improved the spatial resolution of electron diffraction on individual nanoparticles in an SEM by an order of magnitude: t-EBSD can resolve nanoparticles as small as 10 nm in diameter, compared to the 120-nm minimum particle size previously demonstrated with traditional EBSD. The results were reported in a recent issue of the Journal of Microscopy.

''The difficulty with getting diffraction from a non-smooth surface in the normal reflection EBSD mode is that you have difficulty getting a pattern because you have so much specular scattering into many directions,'' Geiss says. ''With transmission EBSD, the problem is removed, so it opens a whole new world of studies within the SEM.''

This whole new world opened up when Geiss tried to perform traditional EBSD on some InGaN nanowires. He had deposited the nanowires on a TEM grid, but wanted to see what information he might gain from the large angle diffraction patterns that EBSD affords.

Of the 100 or so nanowires on the grid, he saw diffraction patterns from only two or three of them because of randomly oriented surface normals, which directed the signal away from the detector. ''I thought, why can't we just do it in transmission mode?'' Geiss says. ''Nobody had done that before, absolutely nobody.'' He rotated the grid 90 degrees to put it in transmission mode, and obtained diffraction patterns from almost every wire on the sample grid.

''We moved the sample way up high in the SEM column and tilted it away from the camera, and it turns out that you get a lot of high angle scattering directly into the EBSD camera, without added noise from the sample holder,'' Keller says, describing the modification process in more detail. ''It didn't intuitively make sense to us, but it was happening.''

To prove to themselves they were not seeing artifacts, they ran Monte Carlo simulations that showed that at least several percent of the incident beam electrons that traveled through the sample scattered through large enough angles to hit the detector. Further validation of the technique came when they obtained transmission diffraction patterns from electrons passing through a 40-nm thick Ni sample. Eventually they showed that a standard 20-25 keV SEM electron beam could pass through more than a quarter micron of copper and still produce a good diffraction pattern.

''We're hoping to bring the general concept of quantitative transmission analysis from the TEM to the SEM,'' says Keller. ''There is probably well over an order of magnitude more SEMs in the field than there are TEMs. We're hoping this opens the door to more quantitative analysis, including diffraction and potentially spectroscopy, to those that don't traditionally have access to TEMs.''