To provide the resolution necessary to generate images that can be statistically analyzed, FEI had to modify its detectors.
"The high energy primary electron beam causes low energy (secondary) electrons to emerge from the specimen and are then accelerate
(pulled) toward the detector," says Ben Lich, strategic marketing manager of SEM and DualBeam at FEI. Using a standard secondary
electron detector the particles in the image appear to be illuminated from one side only, resulting in a dark and a bright
side. The company engineers created a special symmetrical secondary electron detector that eliminates this effect. "But when
you do that, normally you will get a dark side of the object and a bright side of the object, with the bright side being the
side facing," says Lich. "For particle detection this is not desired because you want a good impression of the outer area
of the particle so you can do a precise measurement." Engineers then made a special detector that collects a secondary electron
symmetrically around the column and around the primary beam, thereby providing uniform illumination of the entire specimen
surface and a good definition of the edge around the entire particle.
The advances in SEM instrumentation are making it easier for tablet manufacturers looking into adding SEM instrumentation
to their quality laboratories. "With a low-entry type of tool, people get exposed to electron microscopy and may then follow
with a higher-end instrument," says Lich. "They find that it is not as hard as some people think it is."
1. All You Wanted to Know about Electron Microscopy (FEI Company, Hillsboro, OR).
2. D.A. Muller et al., "Atomic-Scale Chemical Imaging of Composition and Bonding by Aberration-Corrected Microscopy," Science
319 (5866), 1073–1076 (2008).
3. Advances in Optical Imaging (D0D2) and Emerging Technologies in Microscopy (D353) (Frost & Sullivan,