Exit surface image

Modem electron microscopes with field emission electron sources provide brighter and more coherent electrons. Images with information of crystal stmctures up to 1 A can be achieved. A through-focus exit wave reconstmction method was developed by Coene et al. (1992 1996) to retrieve the complete exit wave function of electrons at the exit surface of the crystal. This method can be applied to thicker crystals which can not be treated as weak-phase object. It is especially useful for stud5dng defects and interfaces (Zandbergen etal, 1999). 

X is measured along the exit surface in the direction of g further, the following abbreviations are used /p = TT Is = SS. This expression represents sinusoidal fringes with a period I/I9I they can be considered as forming an image of the lattice planes g. 

Let the crystalline object be a thin foil characterized by a two-dimensional transmission function q (jc. y) that describes at each point of the exit surface of the specimen the amplitude and phase of the electron beams emerging from the column situated at (.r, y) after dynamic diffraction in the foil. The diffraction pattern can be described to a good approximation as the Fourier transform Q (if. ) of the object function q (x, y). This diffraction pattern acts in turn as a source of Huyghens wavelets, which interfere to form the image, after linear magnification by the optica) lens systems the image is, in turn, the Fourier transform ip (x, y) of the diffraction pattern. 

One further breaks down the secondary electron contributions into three groups SEI, SEII and SEIII. SEIs result from the interaction of the incident beam with the sample at the point of entry. SEIIs are produced by BSE s on exiting the sample. SEIIIs are produced by BSEs which have exited the surface of the sample and further interact with components on the interior of the SEM usually not related to the sample. SEIIs and SEIIIs come from regions far outside that defined by the incident probe and can cause serious degradation of the resolution of the image. 

Figure 9. The entrance and exit pupils are the surfaces where the entrance and exit rays coming from the different field positions cross each other. In different terms, the entrance pupil is the aperture of the optical system as seen by an observer located at the position of the object, or at the location of the image for the exit one.

The result of the PAM reconstruction is, in general, only an approximation of the exit wave function. Some non-linear terms may be present exactly on the paraboloid surfaces, and, thus result in artifacts for the PAM reconstruction. However, the PAM result is a good approximation to the exit wave function, which, in the present implementation, is used as a starting point for a maximum likelihood (MAL) reconstruction that takes the non-linear image contributions fully into account (Coene et al. 1996, Thust etal. 1996a). 

The STM images of the reaction of CO and 02. (a) An 02 molecule (oval) and a CO molecule (circle) on a surface. As the two molecules approach each other, a reaction occurs (b) to form a 0—CO—0 complex (c). After an electron pulse is applied to the complex, a newly formed C02 molecule exits the surface, leaving behind a single 0 atom (d). 

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