Abstract
The electron image contrast of tungsten atom clusters changes remarkably depending on the spatial frequency, scattering factor phase, lens aberration and defocusing, but it is confirmed that the image appears in negative contrast and satisfies an anastigmatic condition when recorded under optimum focus. When based on a cluster model, the spatial frequencies can be divided into individual atom clusters by successive band pass filters. An electron micrograph taken at the optimum focus condition is selected from a defocus series of micrographs and is used for digital processing. In order to measure the exact size of the clusters, the enhancement of filtered images by their gradient images to extract edge information and the subsequent superimposition of both images are derived. Based on Fourier analysis, it is proposed that tungsten films consist of double cluster layers which correspond to a thickness consisting of 3–5 single atoms. In addition, their three-dimensional localization can be determined more precisely than by earlier computer-aided reconstruction methods. The cluster models are in good agreement with the processed data suggesting that the amorphous phase is a distinct phase and not microcrystalline.
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