Optimization of the Signal to Noise Ratio in EFTEM ElementalMaps with Regard to Different Eonization Edge Types

An energy filtering transmission microscope can provide information about the spatial distribution of an element by selecting anenergy loss corresponding to a characteristic inner-shell excitation. One parameter affecting the obtainable resolution, as well as thedetection limit of such core-loss images (elemental maps), is the signal to noise ratio. Statistical noise due to low inelastic crosssectionsmakes weak-contrast features invisible, leading to a loss of resolution and a deterioration of the detection limits. Theinformation content therefore is largely determined by the noise level in the recorded maps. As a consequence, improving the detectionlimits 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 directlyapplicable to core-loss imaging. In this paper we describe the procedures which have been employed for these calculations and discuss theresults, giving useful information about some setup parameters for elemental mapping with different edges. The calculated imaging parameterscan then be interpreted in terms of the attainable spatial resolution in the elemental maps. q1998 Elsevier Science Ltd. All rights reserved