APPARATUS FOR ENERGY-SELECTIVE VISUALISATION

Abstract: An apparatus for energy-selective visualisation of a beam of charged particles emerging from an object is equipped with a double-­ focusing energy analyser (16) which is provided with a two-dimensional entrance diaphragm (18). For the visualisation of both an image and a spectrum an optical imaging system (24) behind the analyser is preferably equipped with one or more quadripole lenses. An illumination system for an object to be examined is preferably equipped with a system for a sequential partial illumination of the object.

Description: The invention relates to an apparatus for forming an energy-selective image of an object comprising: imaging means for forming with a beam of charged particles an image of an object; a deflection system comprising first and second deflection means for deflecting said beam of charged particles in two directions; energy analyzing means for analyzing energy or velocity spectra of a partial image that is selected by an entrance diaphragm of said energy analyzing means, the energy analyzing means being situated between the first and the second deflection means; and detection means having a detection plane for detecting charged particles leaving said energy analyzing means, and comprising at least one charged particles lens situated between the energy analyzing means and the detection plane. An apparatus of this kind is known from the Journal Phys. Soc. Japan 17, 1962, pp. 569-570 (see also US-A-3 256 432). An apparatus described there for energy-selective visualisation of an object irradiated by an electron beam comprises, successively positioned after an imaging lens, a line deflector, an energy analyzer with an entrance slit and an exit slit, a cylindrical lens and a second line deflector. For the visualisation of an object, a line is selected by the first slit diaphragm for the formation of an energy or velocity spectrum of that part of the image that is allowed to pass through by the slit. With the second slit diaphragm a relatively narrow energy band is selected from this and an image is formed from this with the aid of a cylindrical lens. By moving the image of the object transversely across the first slit and shifting the image on the final screen in accordance therewith, an image of the object is formed for a predetermined energy band. Often it is desirable to not only obtain image information but to also be able to observe an energy spectrum of the electrons coming from the object. It is an object of the invention to provide for an apparatus in which both energy-selective imaging and spectral observation can easily be performed. An apparatus according to the invention is characterized in that said detection means comprise control means connected to said charged particles lens for adjustment of a focal distance of said charged particles lens to form for one range of focal distances a filtered partial image on the detection plane and for another range of focal distances a spectrum of said partial image on the detection plane. In the energy analyzing means, the partial image appears in an intermediate image plane. Upon imaging the partial image onto the detection plane by the charged particles lens that is placed between the energy analyzing means and the detection plane, only electrons having a predetermined energy reach the detection plane, the paths of the electrons having other energies being changed such that they are for instance cut off by an exit aperture of the energy analyzing means and a filtered partial image appears in the detection plane. By changing the focal distance of the lens while keeping a constant distance to the detection plane, the object plane of the lens is shifted to coincide with a plane of dispersion in the energy analyzing means in which electrons coming from a common point which is situated in the image plane of the energy analyzing means or is situated outside the entrance diaphragm of the energy analyzing means, are dispersed. Now an image consisting of a number of spots of different intensities appears on the detection plane, thus forming a spectrum of the partial image. The images to be analyzed for an object can be formed by releasing charged particles, normally electrons from the surface of an object, for example by irradiating this with electromagnetic or corpuscular radiation. Such pictures can also be formed by irradiating a preparation with an electron or ion beam, in which respect the difference in interaction between individual particles of the beam and the preparation results in differences in velocity for the particles behind the preparation. In a preferred embodiment, the detection means comprise a television camera tube to which a monitor is connected for image reproduction. A spectrum analyzer can be connected to the camera tube for spectrum analysis, but a spectrum analyzer can also be connected directly for this purpose, the focal distance of the charged particles lens being set by the control means such that the spectrum appears at the detection plane of the detection means. For the detection of spectra or images use can also be made of a film camera, with the advantage of the production of a direct hard copy, or of a preferably self-scanning CCD matrix. For the projection of a spectrum or an image on the detection plane use is made in a preferred embodiment of a quadrupole lens. Such a lens can be placed in front of and/or behind the plane of dispersion of the energy analyzing means in which a diaphragm having an exit slit can be placed. With a quadrupole lens positioned close in front of the plane of dispersion (as seen by the electron beam) an enlarged spectrum can be projected on the detection device at a given energisation of the lens. The action of the lens in the dispersion direction is then strongly negative. Varying he energisation then results in a variation in the enlargement of the spectrum. Since the strength of a quadrupole lens in the negative direction can be made much greater than in the positive direction, a marked enlargement of the spectrum can be achieved in this way with a single optical element. A weak positive energisation of the quadrupole lens in the dispersion direction results in an enlarged energy-filtered image on the detection plane of the detection means. Because of the positive lens action the image appears sharp in the dispersion direction, while crosswise to the dispersion direction the size of the partial image at the object plane of the quadrupole lens is relatively small, as a result of which there is therefore a greater depth of focus for that direction and hence a relatively sharp visualisation can be achieved. The enlargement of the image formed in this way is not variable because then the sharpness would be lost. The size of the image can, however, be varied with the electron microscope itself. With a quadrupole lens positioned close behind the plane of dispersion of the energy analyzing means in which a diaphragm having an exit slit is placed, all the required image formations can be achieved merely by varying the energisation in a single direction, and with a positive lens action in the dispersion direction. With a weak energisation a filtered image is achieved, while stronger energisation gives a visualisation of the spectrum which can be varied in size. The enlargement of the spectrum is then restricted, however, by the limited strength of the quadrupole lens. Greater flexibility can be obtained by adding one or more extra quadrupole lenses between the analyzing means and the detection plane. The second deflection means are positioned between the analyzing means and the detection plane and can form part of the quadrupole lenses. Some preferred embodiments in accordance with the invention will be described in further detail below on the basis of the drawing. The only figure in the drawing shows a schematic representation of an energy-selective imaging apparatus according to the invention. In an apparatus for energy-selective visualisation the drawing shows a schematic representation of an electron microscope 1 incorporating in a housing 2, to be evacuated, a cathode 3, an anode 4, a condenser system 6, a beam deflection system 8 and an objective lens 10. An image of a preparation 12 incorporated in the apparatus is normally formed by means of an end lens 11 on an exit screen 14. A transmission image 13 of a preparation on the exit screen 14 is formed here by an electron beam 15 emitted by the cathode which, after passing the preparation, contains electrons of mutually differing energy, which would also be the case if the beam was completely monochromatic before interacting with the preparation. The difference in energy arises as a result of the difference in the degree of interaction of the individual electrons from the electron beam with the preparation. Energy analyzing means in the form of an energy selection filter 16 are coupled under the removable exit screen 14 for energy-selective imaging. Here the filter is a double-focusing, curved filter. The double focusing action can, for example, be obtained by using tilted, conical or bevelled pole pieces or an entrance and/or exit plane of the filter sloping with respect to the direction of the main ray of the electron beam. An example of a doublefocusing filter of this kind is described in Optik 71, No. 1, 1985, page 11 to 14, particularly page 12. The embodiment of the filter is not significant for the invention. The filter 16 is fitted with an entrance diaphragm 18 limiting the image field, positioned optically close to a plane in which an image of the preparation 12 is formed by the end lens 11. At the site of a plane of dispersion in which a diaphragm with an exit slit 20 is placed a reduced image of the crossover which is therefore in fact a reduced image of the source, is formed by the filter. An intermediate image of the preparation is formed in an image plane 22 of the filter, coinciding with its achromatic plane. The energy-resolving power is now largely determined by the transverse dimension of the image at the site of the plane 22, which dimension is determined by the diaphragm 18. By means of a quadrupole lens 30 or 32 close to the exit slit 20 the achromatic image in the plane 22 is visualised on an entrance target 26 of a television camera tube 28. In this way the diameter of the image to be detected and the energy-resolving power of the filter are related. If a narrow energy-selecting slit 20 is used in order to achieve a high energy resolving power, then only an appropriately small image field can also be permitted because of this. If a larger image field is required, it is necessary to correct the filter for errors which occur in this respect, which, as already mentioned, is only possible to a certain extent. For the analysis of a larger total image an x-y deflection system 25 is incorporated after the end lens and an x-y deflection system 27 after the filter. By energising both deflection systems synchronously, the whole object can again be imaged on the target 26, albeit sequentially in partial images. With partial images of, for example, 50 x 50 pixels, and a line scanning and an image scanning in ten steps each, a total picture in a hundred partial pictures is analyzed into 250,000 pixels. It is advantageous for the scanning to take place synchronously with locally occurring interference fields, hence for example with a line frequency of 20 msec, or a whole number times that frequency, as a result of which an image repetition frequency occurs in whole multiples of 0.2 sec. Here consideration can be given as to whether or not to use interlacing in the imaging in order to prevent the occurrence of frame flicker. In the drawing, a quadrupole lens is incorporated both in front of and behind the exit slit 20 (as seen by the electron beam). It is, however, possible to use only a single quadrupole lens 30 in front of the exit slit 20. With the quadrupole lens 30 having a strong negative lens action in the dispersion direction a spectrum is projected in enlarged form on the target 26 the exit slit 20 being enlarged or removed. Variation in the strength of the lens gives a direct variation in the enlargement of the spectrum and hence in the mutual line intervals in this. Since the negative action of a quadrupole lens can be made relatively strong, a large enlargement range can be covered in this way. With a relatively weak lens action in opposite energisation, hence positive in one direction of the dispersion, the achromatic image is visualised sharply on the target 26 from the image field 22 with the quadrupole lens 30. Thanks to the low cross-dimension of the beam in the direction crosswide to the dispersion direction a sharp visualisation can be achieved in both directions. It is now not possible to vary the image size with the quadrupole lens, for then the image sharpness would be disturbed, but provision can be made for this by an earlier image size variation in the system, or possibly by using another quadrupole lens. Quadrupole lenses have the advantage that when imaging no image rotation takes place with varying lens strength, that they take up little space, are light, and have a strong lens action even at a relatively low energisation. With a quadrupole lens 32 positioned behind the filter all the visualisations mentioned can be achieved by varying the lens strength within a single mode. With a weak positive energisation in the dispersion direction the object plane of the lens 32 coincides with the achromatic plane 22 and a filtered picture is formed on the target 26, while stronger energisation in the same direction results in a shift of the object plane of the lens 32 and an image of the spectrum which can be varied in size appears on the target. For the control of the whole apparatus a central control unit 34 has been added to which an image registration unit and a hard copy unit 36, respectively, have been connected here. A monitor 38 has also been incorporated here for image reproduction. In order to avoid overloading the preparation during partial sequential imaging which applies here, it is advantageous to irradiate momentarily only that part of the preparation of which an image is then formed. With a scanning in the 10 x 10 steps already mentioned, only about a hundredth part of the prepartion is then irradiated each time and the total load on the preparation can therefore be reduced by a factor of 100. A beam-shape scanning system 40 in the illumination system for the preparation can be used for this scanning. After that, a beam with an adapted transverse cross-section is moved over the preparation in the sequence of the above-mentioned steps. This scanning is controlled synchronously with the beam scanning for the energy selection or the visualisation. In an electron beam apparatus equipped with a matrix semiconductor emitter as the electron source it is also possible to use a synchronised activation control of individual emitters or groups of emitters for scanning the preparation.

Claims: An apparatus for forming an energy-selective image of an object comprising: imaging means (1) for forming with a beam (15) of charged particles an image (13) of an object (12); a deflection system comprising first (25) and second (27) deflection means for synchronously deflecting said beam of charged particles in two directions, whereby the first deflection means (25) permits division of the said image (13) into partial images in two dimensions; energy analyzing means (16) for analyzing energy or velocity spectra of a said partial image that is selected by an entrance diaphragm (18) of said energy analyzing means, the energy analyzing means being situated between the first (25) and the second (27) deflection means; and detection means (28) having a detection planet (26) for detecting charged particles leaving said energy analyzing means, and comprising at least one charged particles lens (30, 32) situated between the energy analyzing means (16) and the detection plane (26), characterized in that said detection means comprise control means (34) connected to said at least one charged particles lens (30, 32) for adjustment of a focal distance of said charged particles lens to form for one range of focal distances an energy-filtered partial image on the detection plane (26) and for another range of focal distances a spectrum of said partial image on the detection plane. An apparatus according to Claim 1, wherein said imaging means include means for directing electromagnetic radiation onto said object to form said beam of charged particles. An apparatus according to Claim 1, wherein said imaging means include means for irradiating said object with a substantially monochromatic electron beam to form said beam of charged particles. An apparatus according to Claim 1, 2 or 3, wherein said imaging means include means for selectively and sequentially illuminating portions of said object, a partial image being formed of each portion. An apparatus according to Claim 1, 2, 3 or 4, wherein at least two quadrupole lenses (30, 32) are provided, one (30) of said at least two quadrupole lenses being disposed between said energy analyzing means (16) and an exit slit, (20) thereof, another (32) of said at least two quadrupole lenses being disposed between said exit slit (20) and the second deflection means (27). An apparatus according to Claim 1, 2, 3 or 4, wherein the charged particles lens (30) is disposed between the energy analyzing means (16) and an exit slit (20) thereof, said charged particals lens being being negatively energized in a direction of dispersion of said analyzing means for imaging the spectrum. An apparatus according to Claim 1, 2, 3 or 4, wherein the charged particles lens (30) is disposed between the energy analyzing means (16) and an exit slit (20) thereof, said charged particles lens being positively energized in a direction of dispersion of said analyzing means. An apparatus according to Claim 1, 2, 3 or 4, wherein the charged particles lens includes an integrated structure of a lens and beam-deflection element. An apparatus according to any one of the preceding Claims, wherein said detection means include camera means for monitoring spectrum analyses. An apparatus according to any one of the preceding Claims, wherein said detection means include semiconductor means for detecting said partial images, said semiconductor means acting as a target for said partial images.

Inventors: LE POOLE, JAN BART, VAN DER MAST, KAREL DIEDERICK
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01J37
IPC codes: H01J37, G01Q30