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.

Tuesday, January 6, 2015

Pectinariidae Tubes

In the following images the magnification and length scales are incorrect due to an instrument failure! There is a lesson in this: always check measurements to standards!

Pectinariidae are a family of marine polychaete or segmented worms that build conical trumpet or ice cream cone shaped homes. The conical homes are upside down in the marine sea bottom, with the small opening at the surface and the cone opening up wider and wider in the sediment. The pectinariidae or "ice cream cone worms" use their robust setae to dig into the sediment, while using tentacle like structures to bring food to their mouth. Grains of sand are passed through their guts or between the gap between the worm and its conical home. Much of the material it digs and sifts through is passed through its alimentary canal and expelled from the cone.

The conical homes are assembled by the worm using its mouth parts to select and locate grains according to shape. Since the cones are exactly one grain thick, the grains must be well fit for structural strength. EDS spectroscopy verifies the grains to be simple sand, quartz, and the presence of NaCl. The samples were found, dried, and flash coated with graphite without additional processing to remove salt or other debris such as the occasion diatoms that can be seen hitch-hiking.

Grains of sand are cemented together using a protein-based cement from worm's mouth. EDS spectroscopy shows that the interstitial regions contain NaCl and light-element material: carbon, oxygen, etc., which is consistent with an organic material. In this image some diatoms can be see near the top of the image. The cement appears to be porous, but upon further inspection it is shown to consist of bubbles. The formation of bubbles may reflect the production of the protein at the ice cream cone worm's mouth, and may be useful in the structural project of cementing these grains together as it allows the worm to fill as much gap-volume as possible with the smallest amount of actual cement material.

The final micrograph shows the edge of the tube at high tilt. The large depth of field at large working distance allows one to see much of the length of the tube in addition to the edge of the tube. The length scale of individual grains are clear, and the tube is clearly a single sand grain thick.