Patent Number: 062394304
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a particle beam apparatus, and more particularly to a particle beam apparatus with an energy filter. 2. Discussion of Relevant Art Particle beam apparatuses in the form of transmission electron microscopes with energy filters are known, for example, from U.S. Pat. Nos. 4,740,704, 4,760,261 and 5,449,914. The energy filters described in these documents are dispersive, that is, a charged particle entering the filter undergoes, on passing through the filter, a deflection which depends on the particle energy. The filter described in U.S. Pat. No. 4,740,704 is used by the inventors employer' in the transmission electron microscope 912 Omega manufactured and sold by LEO Elektronenmikroskopie GmbH. In the 912 Omega, the filter is arranged in the imaging beam path between the specimen to be investigated and the projection screen or the camera on which the specimen is electron-optically imaged. With such an energy filter on the imaging side, the energy loss which the particles have undergone in the specimen can be analyzed. At the same time, the imaging errors which depend on the energy, the chromatic aberrations, are reduced in the imaging beam path, since only particles with a reduced energy bandwidth contribute to the imaging. For the correction of chromatic imaging errors, both in scanning electron microscopes and also in transmission electron microscopes, it is known from U.S. Pat. No. 5,319,207 to provide a mirror corrector in the illumination beam path between the electron source and the specimen to be investigated. The mirror corrector consists of a magnetic beam deflector and an electrostatic mirror which images into each other the two planes of symmetry within the magnetic beam deflector. Although the beam deflector has dispersive properties, the corrector is non-dispersive overall, that is, particles entering the corrector undergo, after passing completely through the corrector, no deflection which is dependent on the particle energy. Such correctors are however relatively expensive and up to now have not been commercially offered. As an alternative to a corrector, it is known from an article by H. Rose in Optik (Optics), Vol. 85 (No. 3), pp. 95-98 (1990), to provide an energy filter in the illuminating beam path of a transmission electron microscope. The energy filtering which is effected permits at least the energy-dependent errors to be reduced, because of the small energy bandwidth of the particles that contribute to subsequent imaging. Although here also the filter has dispersive elements for the splitting of the particle beam according to energy, the filter is overall free from dispersion, so that the particles entering the filter again undergo, after completely passing through the filter, no deflection which depends on energy. The freedom from dispersion of the whole filter is attained in that the filter is symmetrical about a midplane, and the dispersion in both of the mutually symmetrical filter portions is exactly opposed. This freedom of the filter from dispersion insures that small voltage fluctuations at the filter do not lead to a drift of the beam behind the filter. Dispersion-free filters however have the disadvantage that the dispersion that can be attained in the energy selection plane, in which the energy selection takes place by means of a slit diaphragm, is relatively small. And since the dispersion is in general dependent on the particle energy and decreases with increasing particle energy, the particle energy within the filter has to be relatively low when high energy sharpness is to be attained. In the article, the starting point was a particle energy of 3 keV, and in later work by H. Rose a significant energy region of 3-5 keV was specified. At low particle energies within the filter, however, a broadening of the energy bandwidth results because of the so-called Boersch effect. Since the Boersch effect has significant effects particularly in intermediate images of the particle source within the filter, because of the higher particle density in such intermediate images, the use was already proposed by H. Rose of a filter with exclusively astigmatic intermediate images within the filter. Furthermore, a raster electron microscope with a dispersive energy filter between the source and the objective is known from Japanese Patent JP 62-93848. In the system described there, the filter is however only used for the production of a relative signal, so that the negative influence of the noise of the electron source on the subsequently produced picture can be eliminated by quotient formation between the actual secondary electron measurement signal and the relative signal. SUMMARY OF THE INVENTION The present invention has as its object to provide a particle beam apparatus in which the particle beam that is used for further imaging or picture production can have a high energy sharpness, and in which the influence of the Boersch effect is small. This object is attained by a particle beam apparatus having a particle beam producer, an objective, and an energy filter that has dispersion and is arranged between the particle beam producer and the objective. The energy filter images a first input plane achromatically into a first output plane and a second input plane dispersively into a second output plane. The particle beam producer is imaged into the first input plane. In the particle beam apparatus according to the invention, an energy filter is arranged on the illumination side, between the particle beam producer and an objective, as in the above-mentioned article. In contrast to the arrangement according to the above-mentioned article, this energy filter has a dispersion: that is, the particles that have passed through the whole filter have, at the end of the filter, a deflection which is dependent on their kinetic energy. A so-called imaging energy filter is concerned here, which images a first input plane achromatically into a first output plane, and simultaneously images a second input plane dispersively into a second output plane. The particle beam producer--or, more precisely, the surface of the particle beam producer that emits particles--is imaged in the first input plane of the energy filter, in the particle beam apparatus according to the invention, so that in spite of the dispersion of the energy filter, energy fluctuations of the particle beam do not lead to any drift of the image of the particle beam producer in and beyond the second output plane. Since dispersive energy filters have a higher dispersion than dispersion-free energy filters, the average particle energy in the particle beam apparatus according to the invention can be chosen to be higher, at the same energy sharpness of the energy-filtered particle beam, than according to the state of the art. Because of this higher average particle energy, which can be between 5 and 35 keV, and should preferably amount to about 8-20 keV, the negative influence of the Boersch effect is markedly reduced. The energy selection by a corresponding slit type selection diaphragm can take place, in the particle beam apparatus according to the invention, in the output side region of the energy filter or beyond the energy filter in the second output plane. The imaging of the particle beam producer in the first input plane preferably takes place with enlargement, such that by means of the energy selection beyond the energy filter, no cutting down of the aperture of the particle beam takes place in the subsequent beam path. In an advantageous embodiment example of the invention, the particles are already accelerated to a relatively high energy before they enter the energy filter, and pass through both the energy filter and the succeeding imaging stages with the same energy, and are braked to the smaller desired end energy only in the objective, or between the objective and the specimen to be investigated. This embodiment of the particle beam apparatus according to the invention can in particular be constructed as a low voltage scanning electron microscope, in which the particle beam is focused by the objective on the specimen to be investigated. To scan the specimen, a deflecting device is then provided in the region of the objective, and with it the particle beam focus can be deflected in two mutually orthogonal directions. The target energies in such low voltage scanning electron microscopes are between 10 eV and 10 keV. A detector for the detection of secondary electrons emitted from the specimen to be investigated is provided between the objective and the filter in such a low voltage scanning electron microscope. A further detector can be provided for the detection of back-scattered particles from the specimen, the beam path of these back-scattered particles being preferably coupled sideways out of the energy filter. For the separation of the directly back-scattered particles from those particles which have undergone an energy loss, a further slit diaphragm can be arranged between the filter and the detector for the detection of the back-scattered particles. As an alternative to the embodiment as a low voltage scanning electron microscope, the particle beam apparatus according to the invention can also be constructed as a high energy transmission electron microscope. In this case, the particle beam would be accelerated to the desired high target energy directly after exiting the energy filter. The dispersion of the energy filter should be in the region between 5-20 .mu.m/eV, preferably between 10 and 15 .mu.m/eV, at the average particle energy within the filter. If the dispersion of the filter is less than 5-10 .mu.m/eV, no sufficient energy sharpness can be attained, or slit widths of the selection diaphragm are required which are too small. If the upper boundary value of 15-20 .mu.m/eV is exceeded, the aperture of the particle beam behind the selection diaphragm then becomes too large, with the consequence that the subsequent electron optical imaging elements produce greater aperture errors, so that the gain in resolution possible by the reduction of the chromatic errors is further compensated or even over-compensated.