Optimization of the signal to noise ratio in EFTEM elemental maps with regard to different eonization edge types

An energy filtering transmission microscope can provide information about the spatial distribution of an element by selecting an energy loss corresponding to a characteristic inner-shell excitation. One parameter affecting the obtainable resolution, as well as the detection limit of such core-loss images (elemental maps), is the signal to noise ratio. Statistical noise due to low inelastic cross-sections makes weak-contrast features invisible, leading to a loss of resolution and a deterioration of the detection limits. The information content therefore is largely determined by the noise level in the recorded maps. As a consequence, improving the detection limits means first optimizing the signal to noise ratio (SNR). Since electron energy-loss spectroscopy (EELS) is the basis for energy filtering, EEL spectra can be used as a basis to calculate optimal values of the SNR for different ionization edges and the results shall be directly applicable to core-loss imaging. In this paper we describe the procedures which have been employed for these calculations and discuss the results, giving useful information about some setup parameters for elemental mapping with different edges. The calculated imaging parameters can then be interpreted in terms of the attainable spatial resolution in the elemental maps. (C) 1998 Elsevier Science Ltd. All rights reserved.