Messungen und Berechnungen zum elektronenmikroskopischen Streukontrast für 17 bis 1200 keV-Elektronen

Abstract

Electron transmission experiments under electron microscope conditions were done with C-, Ge- and Pt-films to get information about the influence of electron energy and objective aperture. One gets an exponential law of transmission for small mass thicknesses x of the objects and can calculate the “contrast thickness” xk (in T = exp(— x/xk)). Using the Lenz theory, two constants Xa and ϑ0 are determined, for each element and electron energy, from the measurements of xk at four different objective apertures. xa is related to the total elastic scattering cross section and ϑ0 is the half width of the atomic scattering amplitude ƒ(ϑ). The variation of xa and ϑ0 with electron energy is not in agreement with the theory using the first Born approximation and a simple screened atomic potential. But using these two experimentally determined constants in a plural scattering theory of LENZ, good agreement between calculated and experimental deviations from the exponential law of transmission up to mass thicknesses of 300 µg · cm-2 is obtained. To get better theoretical values of xk, the complex atomic scattering amplitudes were calculated quantum theoretically with the WKB-method and Hartree potentials. The values agree with the experimental results for Ge- and Pt-films. For carbon there is a large contribution by inelastic scattering making a direct comparison with experimental results difficult. The energy dependence of xa shows saturation for high voltages, as expected by theory. At high voltages the difference in the xk-values for films with different atomic numbers is larger, resulting in lower xk-values for platinum. But at very low voltages the xk-values of carbon are lower than those of platinum. Some measurements of xk at 60 keV on targets of noble gases confirm the absence of a large difference in contrast between atoms in the gaseous and condensed amorphous state.