Radiant energy – Electron energy analysis
Reexamination Certificate
1998-08-24
2001-02-06
Anderson, Bruce C. (Department: 2881)
Radiant energy
Electron energy analysis
C250S307000, C250S311000
Reexamination Certificate
active
06184524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention of the present application relates in general to energy filtering transmission electron microscopes and, more particularly, to an automated adjustment or set up of energy filtering systems of energy filtering transmission electron microscopes.
2. Description of Prior Art
Modern energy filtering transmission electron microscopes (EFTEMs) are able to form images from energy resolved subsets of an electron beam which has traversed a specimen. However, the images produced by energy filtering microscopes will be aberrated and distorted unless optical defects inherent to an energy dispersing element of an energy filtering system are precisely compensated by additional electron lenses within the energy filtering system. Making the needed adjustments is a task requiring a high level of theoretical knowledge and experimental skill which restricts the availability of modern EFTEMs to a handful of experts.
Accordingly, there is a need for an arrangement for automatically performing the adjustments needed to set up EFTEMs for effective usage such that the power of modern EFTEMs would become available to a much wider range of scientific researchers.
SUMMARY OF THE INVENTION
This need is met by the invention of the present application wherein electron optical aberrations of an energy filtering system of an energy filtering transmission electron microscope (EFTEM) are automatically corrected under computer control to set up the EFTEM for use.
Optics of the electron microscope preceding an energy filter are used to scan the beam at the entrance to the filter in a pattern corresponding to a defined geometry. The beam can either be finely focused to yield a spot at each position visited during the pattern scan, or the beam can be spread out and imprinted with a well-defined intensity distribution, such as normally occurs due to passage of the beam through a specimen, so that its relative scanned displacements can be assessed using cross-correlation techniques. In the case of the finely focused beam, electron images of the scanned pattern, as recorded at some point beyond the energy filter, directly yield a spot pattern image. Deviations of the recorded spot pattern image from the defined scan geometry reflect the imaging aberrations introduced by the energy filter. In the case of the spread out beam, post-filter electron images of the scanned beam are cross-correlated with an image of the beam taken without scanning, i.e, an undeflected beam image, yielding cross-correlation peak images that give the effective displacement of each scanned beam position due to the aberrations/distortions of the filter. Summing the cross-correlation peak images again yields a spot pattern image that is equivalent to that obtained in the focused beam case. Deviations of the recorded spot pattern image from the defined scan geometry are analyzed to assess and subsequently correct aberrations introduced by the energy filter.
Optics of the electron microscope preceding an energy filter are used to finely focus or spread the beam at the entrance to the filter with the beam then being scanned in a pattern corresponding to a defined geometry with the scanned beam images being used to determine points of relative scanned displacement. For correction of achromaticity, an offset is applied to the energies of the beam electrons by changing the accelerating voltage at an electron gun of the transmission electron microscope (TEM). The known positions of the scanned beam image are again located and the differences between the current positions and those measured before the change in beam energy are determined. The differences are averaged and the net average displacement is taken as a measure of the energy filtering system's departure from true achromatic imaging. If the net average displacement is not within operator specified tolerances, the effect on average displacement to changes in the current of a chromatic adjustment quadrupole lens of post-slit electron optics of the energy filtering system is assessed. The current in the chromatic adjustment quadrupole lens is changed and the measurement of net average displacement of scanned beam image positions is repeated. These operations are iteratively performed if more precise achromatic adjustment is required or until the measured average displacement is within specified tolerances at which time the image formed at the beam detector is deemed to be achromatic.
For correction of magnification and aspect ratio, the scanned beam image positions are located. The average distance between positions along the top and bottom of the scanned beam image is determined and the average distance between positions along the right and left sides of the scanned beam image is determined with these two distances being used to calculate the overall magnification, M, of the image, and the ratio of the vertical to the horizontal magnification is utilized to obtain the aspect ratio, A, of the image. The incremental effects on M and A of currents flowing through first and second magnification adjusting quadrupole lenses of the post-slit electron optics of the energy filtering system are computed and used to determine current changes which are applied to the magnification adjusting quadrupole lenses. The measurements of M and A are repeated and, if the measured values still deviate from preferred values by more than operator specified tolerances, the entire procedure, starting with the controlled changes to the quadrupole currents, is iterated until the specified tolerances are met.
An image of the scanned beam is captured by an electron camera and the location of the center of each scanned beam illumination within the image is determined. The position data is analyzed relative to nominal positions using least squares fits, one for horizontal measured coordinates and the other for vertical measured coordinates. Aberration coefficients are determined and nullified by adjustments to currents passing through three sextupole lenses of post-slit electron optics of the energy filtering system.
It is, thus, an object of the present invention to provide an autoadjustment set up method and apparatus for an energy filtering transmission electron microscope which can adjust an energy filtering system of the microscope using images of strategically placed scanned beam illumination; to provide an autoadjustment set up method and apparatus for an energy filtering transmission electron microscope which can adjust an energy filtering system of the microscope to consistently achieve and exceed required precision for the adjustments; and, to provide an autoadjustment set up method and apparatus for an energy filtering transmission electron microscope which can adjust an energy filtering system of the microscope with computational demands which can be met by a conventional personal computer interfaced to energy filtering system hardware.
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pat
Brink Henri Adriaan
Hunt John Andrew
Kundmann Michael Karl
Anderson Bruce C.
Gatan Inc.
Killworth, Gottman Hagan & Schaeff, L.L.P.
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