Interaction between Illumination System and Object

The interaction between the incident electron beam and the object produces elastic and inelastic scattering. The coherent term of elastic scattering is responsible for the electron diffraction pattern. An additional incoherent term due to interaction with the nucleus of the atom is responsible for the real absorption factor. It produces a peak near 0 = 10 radian (i.e., very near the incident beam). From 100 to 400 kV, s varies in the range 0.01 to 0.06 A.  

Inelastic scattering is due to the electron cloud of the atom. An incident electron pulls out an electron of the sample (ionization). It produces only the incoherent term with an intensity also near the incident beam, which varies in the 0 range 10 radian (0.05 A ) to 2.10 radian (0.012 A ). In both cases the noticeable intensity of the signal is far from that of the 002 reflexion of graphite, 0.298 A (3.354 A), even 

It is thus absolutely necessary to measure the sample thickness at the point of interest. One such method is shadow-casting [21]. The thickness is particularly difficult to measure because grinding has to be avoided. Sample thickness cannot be evaluated by visual control of its transparency. Transparency increases as voltage increases, so a 100-A-thick particle is not detectable at 400 kV. 

It must also be considered that the illumination system is not perfect, and that it is subject to aberrations that limit the coherency of the incident beam. 

Two monochromatic waves having the same amplitude and the same frequency, but with ( ] (( 2, initially are coherent if (j)i - ( )2 is constant during time (monochromaticity or temporal coherence). A phase shift constant during time would also be constant in space (i.e., everywhere in the source there is no local variable intensity—spatial coherence) [22]. 

---