Thick specimens in the CEM and STEM. Resolution and image formation.

A theory of resolution and image formation is presented for thick amorphous specimens in transmission electron microscopes. Eight modes of operation are considered, four in the scanning transmission electron microscope (STEM) and four in the conventional electron microscope (CEM). A thick specimen is defined here as one in which the resolution of detail is limited by plural scattering of the electron beam. In practice this includes films on the order of a micron in thickness. An analytic theory of plural incoherent scattering is developed which is general with respect to material and beam voltage. The theory gives the distribution of elastically scattered electrons as a function of transverse coordinate and angles, and is directly applicable to optical systems. The theory applies to all thicknesses normally encountered, and includes thin specimens as well as thick specimens. Criteria are proposed for evaluation of the quality of microscope images, and the modulation transfer function is applied to determine some practical estimates of picture quality. The STEM is found to have distinct advantages over the CEM for thick specimens. For a carbon specimen one micron thick a STEM operating in bright field at 90 keV produces an image which is roughly equivalent to that of a CEM operating in bright field at 1 MeV. Improvement can be obtained in the CEM by filtering out eneryg-loss electrons which degrade resolution due to chromatic aberration. This results in a reduction in signal intensity and usable thickness, however.