Friday, October 5, 2012

Field Emission SEM

This is an image of the Au cluster on HPOG standard used to check the resolution of the JEOL 5900 SEM. This image was taken using an FEI Nova 400 nanoSEM field-emission FE-SEM in the FSU BSIR. A comparison can be made with an image taken with the JEOL 5900 in a previous post.

While the JEOL 5900 SEM image taken at CMMP was not under optimal conditions-- it was a particularly bad day for building noise-- it should be noted that not all electron microscopes are the same! SEM's and FE-SEM's are different instruments with unique capabilities and limitations.

One of the big differences is the type of emitter. SEM's using tungsten filaments use thermionic emission to generate electrons. Such emitters are spatially extended and subject to thermal drift while producing a fairly low brightness source. Field emitters used in FE-SEM's extract electrons using electric field as well thermal energy, and thus produce very small high brightness sources.  The FE-SEM emitter is always on so these sources are less subject to thermal drift. On the other hand, field emission sources are much more expensive and prone to destruction by poor vacuum.

Electron optical column design aside, there are other important differences.  Because of the high spatial resolutions possible with FE-SEM's, images are often not taken with the traditional ET-detector. Instead, a "through lens" detector or TLD is used, where secondary electrons are detected in the objective lens. In this case, because of the small working distances typically used for high resolution work, a significant solid angle of secondary electrons can pass into the objective lens. The detection efficiency is increased in field immersion mode, where the sample area is embedded the magnetic field of the objective lens, as this increases the number of electrons that make it up the column to the TLD.

This last image at 500,000X shows the capabilities of the Nova 400 FE-SEM on this Au cluster sample.  This image was taken at 15 kV with TLD and field immersion.  Individual nanoparticles are visible in the gaps that are unresolvable in the JEOL 5900.

For more information about the BSIR FE-SEM look here.