Radiant energy – Electron energy analysis
Reexamination Certificate
2001-02-08
2003-05-06
Berman, Jack (Department: 2881)
Radiant energy
Electron energy analysis
C250S3960ML, C250S311000
Reexamination Certificate
active
06559445
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
Imaging electron energy filters are used in transmission electron microscopes in order to improve the contrast of object imaging or of diffraction diagrams by the selection of electrons of a given energy range. The recording of element distributions and energy loss spectra is also possible with such filter systems.
2. Background Art
Filter systems are known from German Patent document U.S. Pat. No. 5,449,914, U.S. Pat. No. 4,740,704 and U.S. Pat. No. 4,760,261 which use three or four homogeneous or inhomogeneous magnetic fields as dispersive elements. These energy filters are straight-vision, i.e., the optical axes of the incident and emergent electron paths are mutually coaxial. These direct-vision energy filters have the advantage of relatively simple adjustability, since the whole imaging system before and after the energy filter can be pre-adjusted with the energy filter switched off. This advantage is however achieved at the expense of a relatively large constructional height of the whole system of electron microscope and energy filter, since all the electron-optical components are arranged in series along a straight optical axis. Mechanical stability problems can arise from this, particularly with electron energies of 200 keV and higher, and with the relatively large filters, arranged asymmetrically with respect to the symmetry axis of the electron-optical column, which are required for these energies.
Moreover, energy filters are known, for example from U.S. Pat. No. 4,851,670, which have a single deflecting region as dispersive element, effecting a beam deflection through 90°. A single dispersive element however produces relatively large imaging aberrations, because of which a quite expensive imaging system has to follow the deflecting element. The 90° deflection of the optical axis by the dispersive element, and the following horizontal course of the optical axis after the energy filter, admittedly reduce the constructional height. However, mechanical stability problems very easily arise with this system also, since the expensive optical imaging system after the energy filter leads to quite large moments under the influence of gravity.
Electron energy filters are furthermore known from German Patent document DE 198 38 600-A1 which likewise produce a 90° total deflection of the optical axis between the filter input and filter output, but which nevertheless have a symmetrical structure with respect to the midplane by means of multiple beam deflection in opposite directions. It is known that the symmetrical structure of the energy filter enables some imaging aberrations to be avoided within the energy filter, so that improved imaging properties result overall. However, here also, the horizontal course of the optical axis after the filter output leads to mechanical stability problems.
The present invention has as its object an energy filter, particularly for electron microscopes, which on the one hand makes possible a small constructional height of the whole system of electron microscope with energy filter, and on the other hand leads to as few mechanical stability problems as possible. A further object of the invention is to provide an energy filter in which the imaging aberrations which arise electron-optically can be kept as small as possible.
The first-mentioned object is attained according to the invention by means of an energy filter with magnetic deflection regions wherein all the deflection regions in common produce a total beam deflection through an angle between 90° and 210°, and the second-mentioned object is obtained by an energy filter with magnetic deflection regions which are arranged symmetrically with respect to midplane (M) and wherein the Helmholtz length of the energy filter is greater than double the average value of the deflection radii in the deflection regions. Advantageous embodiments of the invention will become apparent both from the combination of the two measures and also from the features of the dependent claims.
The electron energy filter according to the invention has several magnetic deflecting regions. All four deflecting regions in common produce a total deflection of between 90° and 120°.
Because the total deflection of the optical axis between the filter input and filter output is more than 90°, an optical axis running obliquely upward after the filter output results when the optical axis runs vertically downward before the filter input. The moments arising under the effect of gravity on the electron-optical components arranged after the filter output are reduced by this obliquely upward course of the optical axis after the filter output. An optimum mechanical stability is of course attained when the optical axis runs vertically again after the filter output, and the filter thus produces a total beam deflection of 180°, deviations of the course of the optical axis by ±30° from a vertical course having an only slight adverse effect on the mechanical stability. The limit of the maximum possible total deflection is given by the conditions that the detector following the energy filter must not be situated above the energy filter in the beam path of the electron microscope, and that the beam path emergent from the energy filter is also not to intersect the beam path entering the energy filter.
In order to keep the unavoidable electron-optical imaging aberrations of the energy filter small, besides maintaining mechanical stability, the energy filter is on the one hand to be constructed symmetrically with respect to a midplane, and at the same time the Helmholtz length is to correspond to at least twice, preferably at least three times or even five times, the average of the deflection radii in the deflection regions. The Helmholtz length is the distance between two planes, imaged to scale by the energy filter at a scale of 1:1, in or before the input-side region of the energy filter. One of these two input-side planes, the input diffraction plane, is then imaged dispersively at an imaging scale of 1:1 into the so-called dispersion plane, and the second of these two planes, the input image plane, is achromatically imaged at an imaging scale of 1:1 into the so-called output image plane.
A portion of the second order errors are known to disappear due to the symmetrical construction of the energy filter. By means of combination with a Helmholtz length which is long within the energy filter in comparison with the deflection radii—or with the average value of the deflection radii when the deflection radii are different—a small ray pencil diameter results within the energy filter, so that it is furthermore attained that the unavoidable higher order imaging aberrations remain small.
A Helmholtz length which corresponds to at least five times the deflection radius or of the average value of the deflection radii is then particularly suitable for electron microscopes with a monocular head before the energy filter—seen in the direction of electron propagation—since the Helmholtz length then corresponds approximately to the usual constructional length of the monocular head, i.e., the distance of the last projective lens before the monocular head and the fluorescent screen or detector.
A beam deflection through an angle greater than 135° preferably takes place in the first and last deflection regions. The energy filter has a very high dispersion because of the relatively long path lengths in the magnetic field associated with this.
It is furthermore advantageous if the first and last deflection regions respectively consist of two magnetic partial regions separated by a drift path, with the deflection angle in the first partial region after the filter input and in the last partial region before the filter output corresponding to the deflection angle of the two middle partial regions. At the same time, the drift path between the second partial region of the first deflection region and the second deflection region is to correspond to the drift path between the first and second partial regions of the first
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