New concept of information resolution in scanning electron microscope images

As a new means to evaluate SEM images from the viewpoint of image information, we have proposed information resolution, Rinf, defined as a spatial length per bit in information-passing capacity (IPC). The IPC densities (per basic area) were carried out by both taking into account the signal-to-noise ratio of the statistical mean power densities and applying a previously reported fitting functions to an optical system with an arbitrary size of aberration, diffraction (including lambda = 0), and source. Here, the nu-independent specimen contrast of tau(s)(nu)= 1 [used in Eq. (9)] is assumed. General characteristics on Rinf are typically plotted as a curve of log (Rinf/R(inf, ideal)) vs. log T, where T is (Bm2 + Cm4)(1/2), Bm and Cm are the modified spherical and chromatic aberrations, respectively. The subscript "ideal" represents an aberration-free optical system. The values of Rinf [calculated by Eq. (11)] are compared to those of the conventional beam size d [calculated by Eq. (1)] and the previously reported modified resolution Res [calculated by Eq. (7)]. It was found that R(inf, min) value is 0.3-0.9 times as small as the d(min) value and its corresponding beam semi-angle alpha(inf, min) is 1.2-1.8 times as large as the alpha(conv, min) value (depending on S/N).