A Few Steps Towards a More Quantitative Understanding of Contrast in the Scanning Electron Microscope


The interaction volume of the electron beam with the specimen in a scanning electron microscope (SEM) is a highly complex function of the surface structure of the specimen, its chemical composition and the energy of the scanning electron beam.路 The video signals formed by secondary electrons (SE) or backscattered electrons (BSE) reflect this complexity insofar as they may contain not only information of the interior of the pixel which has just been scanned and its neighborhood, but may depend on surface details hundreds of microns apart from the impinging point of the electron beam. This leads to artifacts in scanning electron micrographs, e.g., edge brightening. The knowledge of the spatial distribution of the current density of the BSE and SE released by the impinging beam are the key for a more quantitative understanding of contrasts in scanning electron micrographs. In a first step, our emission microscopic method to visualize these distributions has been improved by substituting a photographic registration method by a charged couple device (CCD) densitometer. The resolution of our present densitometer (256 grey levels) is not sufficient to record the full dynamic range of the SE current density distributions. However, this will be possible in the near future with a state of the art CCD-camera and a 14 bit image processing system

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