Wednesday, January 7, 2015

The Choice of Beam Energy: The Letter "a"

Any SEM image is simply the output of some detector as a function of the scanned electron beam position. As such the X and Y axes relate directly to lateral position, and this position can be calibrated using a lithographed length standard. The image brightness and contrast relates to some probe-sample interaction that is being monitored using any number of detectors available to the microscope. The most common are secondary electron detectors (SEI), such as the ET-detector (Everhardt-Thornley detector), which detect low energy electrons generated through a large number of inelastic scattering events, and back-scattered electron detectors (BEI), which detect elastically scattered electrons that undergo a small number of elastic scattering events. The key to interpreting SEM images is an understanding of these probe-sample interactions, but the choice of operational parameters is also dependent upon the basic physics of electron-sample interactions.

In this example a 7-point Garamond letter "a" was printed on a laser printer and coated with a few 10's nm of graphite to suppress charging. In the first image this letter "a" was imaged at 20 kV using secondary electrons (SEI mode) where the presumption would be that the column performance is quite good. The performance of the JEOL 5900 is specified as a resolution of ~ 10 nm as measured imaging Au clusters on HOPG at 30 kV-- so this image should be near the optimal column performance. The fibrous structure of the paper is very clear, as are brighter non-fibrous structures that were shown to be surface treatment material through EDS. There is very little evidence of our little letter "a" even though we know it is there by visual inspection. The laser printer has clearly fused enough toner to locally change the optical properties of the paper surface, but those regions don't produce significant secondary electron contrast to see the "a" when imaging in SEI mode.

In the second image, taken at 10 kV, small amorphous blobs of fused toner start to become visible, especially on some of the larger flat fibers. Nevertheless, it's largely impossible to make out the "a" letter.

The third image is taken at 2.5 kV and the soft low-density toner produces enough secondary electron contrast to be imaged. An outline of the letter "a" is very clearly visible, and it is obvious that it is constructed of fused blobs of amorphous polymer bonded to the paper's cellulose fibers.

The point of this application note is that the beam energy must be selected depending upon the system to be imaged-- not just in consideration of electron-optical performance of the column.  In this case the actual part of the sample to be imaged does not require optimal column performance in terms of probe diameter, but sufficient secondary electron emission is required-- and that is possible only at lower beam energies.

The last image is a back-scattered compositional image (BEI COMPO) which reflects the local density or average atomic number of the printed "a" letter. This image was taken at the same beam energy as the first image where the "a" was invisible. The region of interest, the pattern provided by the fused toner, is very slightly less dense than the surrounding cellulose matrix and is easily visible in this imaging mode. The paper additives are also more obvious and appear as the very bright domains.

That is the other point of this application note. Not only does the beam energy need to be selected according to sample, but also the imaging detector. In many cases a sample is best suited imaged at several beam energies and using several detectors to fully understand the sample under investigation.

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