Analysis of electron intensity as a function of aperture size in energy-filtered transmission electron microscope imaging

The dependence of electron intensity on condenser and objective aperture si ze in energy-filtered transmission electron microscope imaging was investig ated using high-purity silicon. Results show that a reduction in intensity occurs in pre-edge and post-edge images due to a reduction in aperture size and that this effect depends more strongly on the objective aperture size (collection angle) than on the condenser aperture size. This reduction in i ntensity due to the use of a smaller objective aperture causes the signal i n elemental maps of the Si L-2,L-3 edge to decrease. However, although the signal decreases in both the pre-edge and post-edge images as either the co ndenser or objective aperture sizes decrease, the ratio of post-edge intens ity to pre-edge intensity increases with decreasing objective aperture diam eter, causing the value of the jump ratio to increase. This occurs because the effects of plural scattering on the background intensity decrease with objective aperture size and because the intensity reductions in the numerat or (post-edge image intensity) and denominator (pre-edge image intensity) a re similar, causing their ratio to increase as each decreases. (C) 1999 Els evier Science B.V. All rights reserved.