Imaging of multilayers with fresnel contrast by transmission electron microscopy

摘要

Diffraction contrast and high resolution imaging, which are generally used to visualize multiplayers, fail when they are applied to study metallic multilayers (MLs) composed of elements with close atomic number. In such a case (i.e. Co/Cu multilayers) neither the absorption nor the structure can give enough contrast between elements to observe the dtails of the mutlilayer stacking. This is illustrated in Figure 1.a which shows a bright field image of (NiFe/Cu/Co/Cu)x8 multilayers deposited on Si. Thus, although the existence of a multilayer structure is clearly demonstrated by both small angle X-ray diffraction and electron diffraction, this does not appear clearly in the images in diffraction contrast. In such systems, if the crystals in the layers are not in strong diffracting condition, absorption is the predominant cause of contrast, and since the constituent elements have extremely close Z, the absorption will be identical for all the layers leading to little or not contrast. However, the contrast can be enhanced by defocusifng the objective lens of the microscope leading to the appearance of Fresnel fringes at the interface between thelayers (Figure 1.b). In this study we investigate the field of application of this contrast mode to extract both qualitative and quantitative information in metallic multilayers. We show that the fine details of the Fresnel fringes can be described quantitatively in order to assess the thickness and dwavines of the layers, and the interface sharpness.