TOLLEY Alfredo Juan
congresos y reuniones científicas
Foil Thickness Determination Based on CBED Patterns.
Congreso; Congreso CIASEM 2013; 2013
Several methods have been developed to determine the local thickness of thin foils using Transmission Electron Microscopy. The method reported by Kelly [1], uses 2-beam Convergent Beam Electron Diffraction (CBED) patterns, in which the local thickness and extinction distance are determined from the positions of the minimum intensity fringes in the diffracted disc. Delille [2] showed that better precision can be obtained by fitting the intensity profile of the 2-beam CBED pattern to the 2-beam dynamical theory with the thickness and extinction distance as adjustable parameters after background subtraction. Castro Riglos [3] proposed another method that uses on-axis CBED patterns in which local thickness is determined by comparison between experimental and simulated patterns. In this presentation the results from the three approaches are compared. On-axis [001] and 2-beam (220) CBED patterns were obtained from the same location in specimens of a heat treated age-hardenable Al alloy, with a Philips CM200UT microscope operated at 200 kV. On-axis CBED pattern simulations were carried out with the JEMS software package [4], using the Bloch wave formalism and Doyle-Turner scattering amplitudes. The maximum of the cross correlation between experimental and simulated patterns was used to determine the best match. Figure 1 shows a 2-beam CBED pattern, the intensity profile of the diffracted disc together with the best fit using the method of Delille, and the plots of the Kelly method. Figure 2 shows the experimental on-axis CBED pattern, the simulated patterns for different thicknesses around that of the best match, and the plot of the maximum of the cross correlation function between simulated and experimental images as a function of thickness. Table I compares the thickness values and extinction distances obtained using the three methods. The thickness results of the three methods show good agreement, within 3% [5].