Abstract:
An energy filter receiving an electronic beam oriented along an optical axis. The filter includes a deflecting system which deviates in a dispersion plane not including the optical axis the received beam and a dispersing system which guides the beam sent by the deflecting system on an optical path inscribed in the dispersion plane and returning to the deflecting system. The deflecting system brings back in alignment with the optical axis the beam coming from the dispersing system. The deflecting system consists of a single element ensuring the inverse deviations of the beam whether outgoing or incoming. The invention is useful for transmission electron microscope.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Stage Application of International Application No. PCT/FR97/01555, filed Sep. 3, 1997. Further, the present application claims priority under 35 U.S.C. § 119 of French Patent Application No. 96/11146 filed on Sep. 12, 1996. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an energy filter, also called velocity fitter, a transmission electron microscope and associated method for filtering energy 
     2. Description of Background and Relevant Information 
     The invention is especially applicable to TEM (Transmission Electron Microscope) or to combined TEM-STEM (Scanning Transmission Electron Microscope) as well as to electron sources. It could be used for specific STEM microscopes. 
     A notable shortcoming of transmission electron microscopes, during the formation of images or of diffraction diagrams lies in the presence of chromatic aberrations. The latter, essentially due to faulty adjustable electromagnetic lenses of the microscope, are detrimental to contrast and to resolution. Chromatic aberrations can be reduced to a certain extent by applying an electron acceleration voltage that is both high and stable, and by observing very narrow samples. 
     However, a manner, particularly efficient and accurate, to improve the picture consists in eliminating a portion of the dispersed electrons in a non-elastic way using an energy filter. 
     The electrons having undergone a given loss of energy may also be employed to form the picture. By selecting a characteristic loss of a type of interaction or of a chemical element, we can obtain a filtered picture providing the mapping of this type of interaction or of this element. 
     Energy filtering also enables to form the picture of samples that would be too thick to be observed with a conventional transmission electron microscope. 
     An energy filter usually comprises spatial dispersion means for the electrons of the beam transmitted by the sample in relation to their energy, as well as a filtering slot enabling selection of an energy window. Besides filtering pictures or diffraction diagrams, energy filters are also employed for spectral analysis of energy losses. Energy filters can be installed in an electron microscope either inside the column of the microscope as an integral part of the instrument, or as an accessory below the visualisation screen. We shall find recent reports on several types of filters known in the articles by Bernard Jouffrey: &lt;&lt;Energy loss spectroscopy for transmission electron microscopy&gt;&gt; in Electron Microscopy in Materials Science, World Scientific, 1991, pp. 363-368 and by Harald Rose and Dieter Krahl: &lt;&lt;Electron optics of imaging energy filters&gt;&gt;, in Energy Filtering Transmission Electron Microscopy, Springer, 1995, pp. 43-55. 
     For example, the article in the magazine Optik, vol. 96, no4, pp. 163-178 by Uhlemann and Rose, describes a mandolin-type magnetic energy filter. 
     A parameter determining energy filters is energy dispersion D, expressed in μm/eV: the greater this parameter, the greater the selective power of the filter. In order to increase this dispersion D, various energy filters have been suggested, which cause the electrons of the beam to follow sufficiently long an optical path. Indeed, the dispersion D increases in particular with the length of the distance covered. Thus, in so-called Ω systems, while remaining in a fixed vertical plane, the beam propagating along the optical axis of the system is first deviated laterally, runs then along an optical path substantially parallel to the optical axis in the propagation direction, then is deviated towards the optical axis of the microscope before it is brought back in alignment with its initial direction. 
     The problem of the filters employed usually lies in their space requirements. Good dispersion D of the filter is indeed obtained by causing the electrons to run a distance over sufficient height. Vertical space requirements of the current filters range generally between 25 and 50 cm, for a dispersion D not exceeding 6 μm/eV. 
     The European patent application EP-40.538.938 suggested an electron beam instrument provided with an energy selective device. The latter causes the electrons to follow a path in a dispersion plane not containing the optical axis of the instrument. The vertical space requirements of the energy selective device are then reduced considerably for a given path length. In the specific embodiment disclosed in said document (FIG.  3 ), the energy filter comprises four beam deviating elements located in the dispersion plane, in the respective corners of a substantially rectangular figure which accepts two orthogonal planes of symmetry. The filter also comprises a first deflecting element deviating the beam of the optical axis of the microscope towards one of the deviating elements in the dispersion plane, and a second deflecting element deviating the beam coming from another deviating element in alignment with the optical axis. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an energy filter capable of producing a wide dispersion D while exhibiting small vertical space requirements, notably increasing the dispersion properties of the filter disclosed in the document EP-0.538.938. 
     Another object of the invention is such a filter that can be used in conventional electron microscopy at high as well as at low voltage, stigmatic in the first order and affected by small aberrations only. 
     An additional object of the invention is an energy filter enabling high acceleration voltages. 
     The invention also relates to a transmission electron microscope provided with an energy filter generating wide dispersion D while exhibiting reasonable vertical space requirements, whereas this microscope can be notably of TEM or TEM-STEM type. 
     The invention also relates to an energy filtering method for an electron beam propagating along an optical axis, generating wide dispersion over small height along the optical axis, whereas this method can be applied to imaging, diffraction or spectrometry. 
     To this end, the invention relates to an energy filter receiving during operation an electron beam oriented along an optical axis in a propagation direction. The energy filter comprises: 
     a deflecting system that deviates in a dispersion plane not containing the optical axis, the beam received along the optical axis, and 
     a dispersing system that guides the beam sent from the deflecting system on an optical path inscribed in the dispersion plane and returning to the deflecting system, and which generates a spatial dispersion of the electrons of the beam in relation to their energy, 
     whereby the deflecting system brings back in alignment with the optical axis in the propagation direction the beam coming from the dispersing system. 
     According to the invention, the deflecting system comprises a single deflecting element ensuring inverse deviations of the beam, whether outgoing or incoming. 
     The energy filter according to the invention is different with respect to the existing systems in that it comprises a single deflecting element which, both, deviates the beam in a dispersion plane not including the optical axis and provides inverse deviations of the beam, whether outgoing or incoming. The vertical space requirements of this energy filter are therefore reduced considerably, while the latter remains particularly efficient, thus ensuring wide dispersion, small aberrations and other suitable optical properties. 
     The expressions &lt;&lt;deflecting system&gt;&gt; and &lt;&lt;dispersing system&gt;&gt; are generic expressions referring to the main technical effect of each of both systems. It is however extremely difficult to prevent the deflecting system from also causing dispersion, even if the latter is rather reduced. Similarly, the dispersing system generates deflections of the beam that follow energy dispersion. 
     The outgoing and incoming paths between the deflecting and dispersing systems are generally collinear, although slight discrepancies may appear in relation to one another. 
     The energy filter may thus provide particularly satisfactory results, notably as regards dispersion D. 
     The dispersing system should advantageously cause the electron beam to describe a closed curve not surrounding the optical axis. 
     Thus, the optical path covered by the beam in the deflection plane accepts convexity changes, notably favourable to the limitation of second order aberrations. 
     Preferably, the deflecting and dispersing systems are symmetrical with respect to a plane of symmetry containing the axis and with respect to the dispersion plane. 
     This configuration of the systems enables to obtain correct stigmatism and achromatism properties. 
     According to an advantageous embodiment of the energy filter, the dispersion plane is perpendicular to the optical axis. 
     According to a preferred embodiment of the dispersing system of the. energy filter according to the invention, the latter comprises: 
     a first deviating element, which receives the beam coming from the deflecting system and which deviates the latter along an incoming direction, and 
     a second deviating element, which receives from the first deviating element the beam along the incoming direction, causes it to follow a circular path in the dispersion plane and sends it back towards the first deviating element along an outgoing direction, 
     whereas the first deviating element receives the beam coming from the second deviating element along the outgoing direction and deviates it towards the deflecting system. 
     It is then advantageous that the first deviating element is arranged between the axis and the second deviating element, whereas the second element comprises an external aperture in which the first element is installed partially. 
     In this particular embodiment, the dispersing system causes the electron beam to describe a closed curve not surrounding the optical axis. 
     According to a preferred embodiment of this lay-out: 
     the first element comprises a pair of polar parts parallel to the dispersion plane, each being hexagonal in shape and comprising a larger base perpendicular to the axis facing the deflecting system, two perpendicular sides connected at right angle to the larger base and parallel to the plane of symmetry, two oblique sides, respectively connected to the perpendicular sides and a smaller base; opposite and parallel to this larger base and connected to oblique sides, and, 
     the second element comprises a pair of polar parts, respectively coplanar with the polar parts of the first element, each crown-shaped, comprising a centre arranged in the plane of symmetry and whose external aperture reaches inside the crown through a passage facing the smaller base of the polar parts of the first element and is delineated laterally by two sides, respectively, facing the oblique sides of the polar parts of the first element. 
     Advantageously, the deflecting and dispersing systems comprising magnetic sectors which comprise, each, a pair of opposite polar parts, separated by an air-gap, connected to actuation and control means in order to create in each air-gap a requested magnetic field, whereby the magnetic fields are uniform in each air-gap. 
     The invention also relates to a transmission electron microscope provided with an energy filter according to the invention. 
     The invention also relates to a method for energy filtering of an electron beam propagating along an optical axis in a propagation direction. In this method: 
     the beam is guided on an optical path substantially inscribed in a dispersion plane not including the optical axis in order to generate dispersion of the beam electrons in relation to their energy, 
     the beam is re-directed in alignment with the axis in the propagation direction, and 
     an energy window is selected spatially. 
     According to the invention, the beam is caused to follow in the dispersion plane incoming and outgoing paths which are collinear as well as of opposite directions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be understood better when reading the description below of an embodiment of an electron microscope and of an energy filter according to the invention, given for exemplification purposes and not limiting in any ways, with reference to the appended drawings, on which: 
     FIG. 1 is a schematic representation of a longitudinal section of a transmission electron microscope according to the invention; 
     FIG. 2 shows in perspective an energy filter according to the invention employed in the electron microscope of FIG. 1; 
     FIG. 3 is a side view of the energy filter of FIG. 2; 
     FIG. 4 is a top view of the energy filter of FIG. 2; 
     FIG. 5 represents schematically the optical path covered by the electron beam in the energy filter of FIGS. 2 to  4 ; 
     FIGS. 6A and 6B represent the paths followed by the electrons in the energy filter of FIGS. 2 to  4 , respectively along a first and a second main section: 
     FIG. 7A shows an intensity curve I in relation to the energy loss E which can be obtained for a sample with the electron microscope of FIG. 1; 
     FIG. 7B shows an enlarged section of this curve; and 
     FIG. 8 is a schematic representation of a longitudinal section of another embodiment of the electron microscope of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A transmission electron microscope  1 , such as represented in FIG. 1, comprises conventionally a column  7  constructed around an optical axis  5 , an acquisition unit  3 , a processing unit  4 , a vacuum pump as well as electronic power supply and a control mechanism. 
     A transmission electron microscope  1 , such as represented on FIG. 1, comprises conventionally a column  7  constructed around an optical axis  5 , an acquisition unit  3 , a processing unit  4 , a vacuum pump as well as electronic power supply and control means. 
     The column  7  of the microscope  1  comprises upstream an electron gun  9 , having an electron source  10 , a cathode  11  and an anode  12 . An acceleration voltage V applied between the cathode  11  and the anode  12  enables to extract electrons at very high speed. The acceleration voltage V ranges normally from 80 kV to 300 kV, but some instruments operate at lower or higher voltages. 
     Assuming that the column  7  is vertical, the following elements can be found in succession from top to bottom in the column  7 : a first and a second sets of condensing lenses  13 A and  13 B, a sample-holder  19 , a set of objective lenses  14 , a set of diffraction lenses  15 , a set of intermediate lenses  16 , a first and a second sets of projection lenses  17 A and  17 B and a fluorescent screen  18  in the lower section of the column  7 . Such an assembly is suitable to imaging processes since an enlarged picture of a sample  6  placed on the sample-holder  19  is displayed on the screen  18 . It is also suitable to the diffraction or to the execution of a fine probe, which may be fixed or not. 
     The lenses  13 - 17  are variable focus electromagnetic lenses for which the current is caused to vary in their power supply coils. The electronic mechanism supplies the lenses  13 - 17  so that they may be able of focusing or deviating the electron beams, as well as the high voltage generator of the electron gun  9 . 
     Other lenses, not represented by reasons of clarity, may be advantageously added to the column  7 . 
     An energy filter  20  is interposed between the sets of lenses  16  and  17 A. This filter  20  comprises a dispersing mechanism  21 , which disperses spatially the electrons of a beam in relation to their energy, and a slot  22  downstream of the dispersing mechanism  21 , which selects an energy window in the beam going through the former. In FIG. 1, energy dispersion takes place essentially in the first order in a plane perpendicular to the plane of the sheet whereby the slot  22  is open in a direction perpendicular to the plane of the sheet The slot  22  is preferably adjustable in width and in position, in order to adjust the energy window as well as regards the average energy on which it is centered as regards its width. 
     The lower section of the column  7  forms a projection chamber  8 . 
     Conventionally, it is necessary to impose high vacuum inside the column  7  and the energy filter  20 , whereby this vacuum is typically in the order of 2.5×10 −5  Pa. 
     The processing unit  4 , connected to the screen  18 , makes it possible to obtain precise information on the image formed on this screen  18 . The microscope  1  enables measurement of the intensity I received by the screen  18  for a given energy loss E of an electron beam, and to visualize and store a corresponding spectrum, using the processing unit  4 . The energy filter  20  of the electron microscope  1  of the example may therefore have a double application: filtering the image formed on the screen  18  or analysis of the sample  6  by obtaining an energy loss spectrum. 
     In operation, the sample  6  suitably prepared is placed on the sample-holder  19 . An electron beam  70 , sent by the electron gun  9  along the optical axis  5  reaches the sample  6  in the form of an incident beam  71  after passing through the sets of condensing lenses  13 A and  13 B. The beam goes through the sample  6  and becomes a transmitted beam  72  which goes through the sets of objective lenses  14 , of diffraction lenses  15  and of intermediate lenses  16 , and is subject to energy dispersion in the dispersing mechanism  21 . The beam  70  then takes on the shape of a dispersed beam  73  at the outgoing of the dispersing mechanism  21 , whereas this dispersed beam  73  is aligned with the optical axis  5 . It is then filtered through the slot  22  and this beam, filtered  74 , after passing through the sets of projection lenses  17 A and  17 B reaches the screen  18 . Thus, an image or a filtered diffraction diagram or an energy loss spectrum of a given range of the sample  6  is obtained, which can be wide or narrow. 
     The spectrum is represented in the form of a curve  150  of intensity I in relation to the energy loss E, as can be seen on FIG. 7A, whereby the energy loss E and the intensity I are respectively associated with the axes  151  and  152 . In the case of a thin object, we can observe a high peak without any loss  154  centred on the axis  153  with zero loss, whereby most electrons are transmitted by the sample  6  without any energy loss E through elastic interaction with atomic cores. The loss-less peak  154  is generally followed by one or several secondary peaks  155 , whereby their heights depend on the thickness of the sample  6 , then by fluctuations admitting successive extremes and whose intensity I decreases gradually. The extremes  157 - 159  of the curve  150  (FIG. 7B) may also exhibit intensities I lower than that of the main peak  154  by several orders of magnitude and are therefore not visible unless longer counting takes place, thereby enabling to accumulate the electrons in a determined energy window  156 . 
     The dispersing mechanism  21  will now be described in more detail relative to its structure and function. These are essentially laid out on a dispersion plane  23  perpendicular to the optical axis  5  and therefore horizontal. All the elements of this dispersing mechanism  21  are symmetrical with respect to this dispersion plane  23 . They are also symmetrical with respect to a plane of symmetry  24  containing the optical axis  5  (FIG.  5 ). 
     The dispersing mechanism  21 , represented in FIGS. 2 to  4 , comprise a deflecting system  30  and a dispersing system  40 . The deflecting system  30  fulfills a double function: it deviates in the dispersion plane  23  the beam  72  transmitted along the optical axis  5  towards the dispersing system  40  and it re-deviates in alignment with the optical axis  5  in the initial propagation direction, the beam  70  coming from the dispersing system  40 . The deflecting system  30  thus constitutes an interface between the optical axis and the dispersion plane  23  as regards the path followed by the beam  70 . 
     The dispersing system  40 , for its own part, guides the beam  70  sent by the deflecting system  30  on an optical path  80  inscribed in the dispersing plane  23  and sends it back to the deflecting system  30 . The dispersing system  40  comprises a first and a second deviating elements  41  and  42 , receiving successively the beam  70  coming from the deflecting system  30 . 
     The deflecting system  30  and both deviating elements  41  and  42  constitute respectively three magnetic sectors of the dispersion mechanism  21 . 
     In the particular embodiment shown, the deflecting system  30  comprises two polar parts or electromagnets  31  and  32  parallel to the plane of symmetry  24 . Each part  31  and  32  is substantially triangular in shape, comprising a larger base  33  and two upper  35  and lower  36  oblique sides. This triangular shape is extended at the apex opposite to the larger base  33  by a rectangular protuberance comprising a smaller base  34  parallel and opposite to the larger base  33  and two sides  38  and  39  perpendicular to the smaller base  34  and linking the latter respectively to the oblique sides  35  and  36 . The bases  33  and  34  are parallel to the axis  5  and arranged on either side of this axis  5 . 
     The first deviating element  41  comprises two polar parts  43  and  44  parallel to the dispersion plane  23 . Each of them is hexagonal in shape, comprising a larger base  50  perpendicular to the axis  5 , facing the smaller bases  34  of the polar parts  31  and  32 . It also comprises two perpendicular sides  51  and  52  connected at right angle with the larger base  50  and parallel to the plane of symmetry  24 , two oblique sides  53  and  54  respectively connected to the perpendicular sides  51  and  52  and a smaller base  55  opposite and parallel to the larger base  50  and connected to the oblique sides  53  and  54 . The upper  43  and lower  44  polar parts are separated by an air-gap  47 . 
     The second deviating element  42  comprises, for its own part, two upper  45  and lower  46  polar parts, respectively coplanar with the polar parts  43  and  44  of the first deviating element  41 . The first element  41  is provided between the axis  5  and the second element  42 , whereas the polar parts  45  and  46  of the latter comprise external apertures  61  in which are respectively provided the polar parts  43  and  44  of the first element  41 , partially. Each polar part  45  and  46  is shaped like an open crown, comprising a centre  60  located on the plane of symmetry  24 . Its external aperture  61  reaches inside the crown via a passage  62  facing the smaller bases  55  and is delineated laterally by two sides  63  and  64  respectively facing the oblique sides  83  and  54  of the corresponding polar part  43 ,  44 . The polar parts  45  and  46  are separated by an air-gap  48 . The crown shape of the polar parts  45  and  46  enables to obtain a light structure and a good distribution of the magnetic field. 
     In operation, the electronic mechanism applies in each air-gap  37 ,  47 ,  48  of the magnetic sectors  30 ,  41 ,  42  a constant and preferably uniform magnetic field. The beam  72  transmitted by the sample  6  reaches the deflecting system  30  along the optical axis  5 . It is then deviated between the polar parts  31  and  32  in order to be inscribed in the dispersion plane  23 , following an incoming arc of circle  81  of radius RI. The beam  70  then covers the free space between the deflecting system  30  and the first deviating element  41 , along an incoming segment  82  at the intersection of the dispersion plane  23  and of the plane of symmetry  24 , whereas the incoming segment  82  defines a deflecting direction  25 . All the optical path covered then by the beam  70  until it returns to the deflecting system  30  is included in the dispersion plane  23 . 
     The beam  70  reaches after the incoming segment  82  the first deviating element  41  and describes there an arc of circle  83  inside the dispersing system  40 , of radius R 2 . The beam  70  leaves the first element  41  through one of the oblique sides  54  along a direction forming with this side an angle a 4 . It covers then, between the oblique side  54  and the second deviating element  42  ,a first segment  84  inside the dispersing system  40 . 
     The beam  70  then enters the second deviating element  42  via the side  64  of the aperture  61  facing the oblique side  54  and describes therein a circular path  85  centred on the centre  60  and of radius R 3 . 
     The rest of the path covered by the beam  70  follows the symmetry with respect to the plane of symmetry  24 . Thus, the optical path followed by the beam  70  comprises successively a second inner segment  86  between the side  63  of the second element  42  and the oblique side  53  of the first element  41 , an inner arc of circle  87  inside the first element  41 , and an outgoing segment  88 , superimposed on the incoming segment  82 . 
     Having reached the deflecting system  30 , the beam  70  is re-deviated to the axis  5  in the initial propagation direction, covering in the deflecting system  30  an outgoing arc of circle  89  symmetrical of the incoming one  81  with respect to the dispersion plane  23 . It comes out, in alignment with the axis  5 , in the form of the dispersed beam  73 . 
     All the path portions  81 - 89  followed by the beam  70  make up the optical path  80  covered by the dispersion mechanism  21 . This optical path  80  comprises mainly a closed curve  83 - 87  not including the optical axis  5 , a linear portion  82  and  88  between the axis  5  and this closed curve as well as connections  81 ,  89  of this portion linear with the axis  5 . The curve changes in the closed curve, i.e. changeovers between arcs of circle  83  and  85  on the one hand, and  85  and  87  on the other hand, exert a positive effect on the reduction of second order aberrations. 
     The lay-out of the incoming and outgoing segments  82  and  88  proves rather efficient in providing very satisfactory optical properties, notably a good dispersion D, if a satisfactory selection of the parameters of the filter  20  is provided. 
     The optical properties of the filter  20  depend mainly on nine parameters: the three radii R 1 , R 2  and R 3 ; the distance d 1  between the deflecting system  30  and the first flexing element  41 , and the distance d 2  covered by the beam  70  between the first flexing element  41  and the second flexing element  42 ; the angle a 1  defined for the deflecting system  30  between the normal to the upper oblique side  35  and the transmitted beam  72 , the angle a 2  defined for the first flexing element  41  as the deflection angle of this element, the angle a 3  between one of the oblique sides  53 ,  54  of the first flexing element  42  and the side  63 ,  64  of the second flexing element  42 , and the angle a 4 . 
     A particularly judicious choice of these parameters of the filter  20  is: 
     R 1 =2.70 cm, R 2 =5.40 cm, R 3 =9.98 cm; d 1 =3.51 cm, d 2 =1.5 cm; 
     a 1 =45.5°, a 2 =64.5°, a 3 =10°, a 4 =81.4°. 
     Other parameters involved are the widths e 1 , e 2  and e 3 , respectively of the air-gaps  37 ,  47  and  48 . These widths e 1 , e 2 , e 3  have been selected in order to make a compromise between opposite needs: the smaller they are, the less significant the stray field, hence the lower the aberrations, but the more problems raised by opening the beam  70 , notably in diffraction mode. The widths e 1 , e 2  and e 3  are traditionally comprised between 2 and 10 mm, advantageously in the order of 2 mm. 
     The length L is determined as the distance from the incoming point of the beam  72  transmitted in the deflecting system  30  and the position of the selection slot  22 . For illustrative purposes, the dispersion D is respectively equal to 6 μm/eV, 8 μm/eV and 10 μm/eV for the lengths L, respectively 18.7 cm, 23.9 cm and 29 cm. 
     The energy dispersion of the beam  72  transmitted by the sample  6  can be materialised while considering various electron paths, as represented on FIGS. 6A and 6B. In this example, the widths e 1 , e 2  and e 3  of the air-gaps  37 ,  47  and  48  are equal to 6.5 mm, the acceleration voltage V is 200 kV and the dispersion 8 μm/eV. The beam  72  received by the filter  20  has a 15 mrad divergence. 
     The projections of certain paths of the beam  70  are interesting along two main sections. The first main section is associated with the follow-up of the beam  70  in the dispersion plane  23  whereas the second corresponds to the follow-up of the beam  70  in a plane perpendicular to the dispersion plane  23 . The spaces and the angles are strongly emphasised on FIGS. 6A and 6B solely for clarification purposes. To simplify, let us assume that two families of electrons reach the filter  20 , each corresponding to a set energy loss. These losses are respectively equal to 340 eV and 2400 eV. 
     In the first main section (FIGS.  6 A), the electrons represented reach the deflecting system  30  along six distinct directions  91 - 96 . The directions  91 - 96  have an angular aperture Al equal to the divergence of the beam equal to 15 mrad, and to each incident direction correspond two paths of electrons belonging respectively to both families. The paths cross each other at an incoming point  90  located in the deflecting system  30 , whereby this incoming point  90  is the first convergence point or incoming cross-over. The paths along both families superimposed at the incoming point, come apart to form six paths  101 - 106  for the electrons of the first family and six other paths  111 - 116  for the electrons of the second family. After going through, in succession, the deflecting system  30 , the first deviating element  41 , the second deviating element  42 , then again the first deviating element  41 , the paths of the electrons pass the deflecting system  30  again and the paths  101 - 106  and  111 - 116  of both families join together to cross one another respectively at a first point  100  and at a second point, not represented, vertical to the first point  100 , whereas these points are called outgoing cross-overs and located at the outgoing of the deflecting system  30 . 
     A similar behaviour is observed in the second main section (FIG.  6 B). Thus, the electrons of the beam  72  reach the deflecting system  30  along six directions  121 - 126  of angular aperture A 2  equal to the aperture of the beam  72 , i.e. 15 mrad. The electron paths join together at an incoming point  120  of first convergence, then both energy families, superimposed at incoming for each path, come apart, thus generating six electron paths  131 - 136  for the first family and six others  141 - 146  for the second family. The paths  131 - 136  and  141 - 146  cover, in succession, the deflecting system  30 , the first deviating element  41 , the second flexing element  141 , again the first deviating element  41 , then the deflecting system  30  and they then converge at two image points  130  and  140 , vertical to one another, of the incoming point  120 , respectively, of both families. 
     In another embodiment of the microscope  2  according to the invention, represented in FIG. 8, the filter  20  is provided in the lower section of the column  7  of a microscope  2 . In FIG. 8, the elements identical to those of FIG. 1 are designated by the same references. 
     In this configuration, the following elements can be found in succession from top to bottom in the column  7 : the sets of condensing lenses  13 A and  13 B, the sample-holder  19 , the set of objective lenses  14 , the system of diffraction lenses  15 , the first and second sets of projection lenses  17 A and  17 B, the screen  18 , the set of intermediate lenses  16 , the filter  20  and a third and fourth sets of projection lenses  17 C and  17 D. The acquisition unit  3  is placed downstream of the column  7  and is connected to the processing unit  4 . An advantage of this embodiment of the microscope  2  is that the device with filter  20  may simply be added below an existing microscope. 
     Among the interesting embodiments within the scope of the invention, the magnetic field applied between the upper  45  and lower  46  prisms of the second deviating element  42  may be non-uniform, but it may accept a constant gradient. This gradient is applied in order to focus the beam  70  between the prisms  45  and  46 , in order to reduce the electron collisions on the polar parts and to thus diminish the losses of the beam  70 . 
     It is quite obvious that the particular forms given to the deflecting and dispersing systems only constitute a particular embodiment and that any other embodiment is possible as long as the latter is covered by the set of claims. Notably, the dispersing system  40  may be such that it causes the electron beam  70  to describe a closed curve surrounding the optical axis  5 . To do so, the dispersing system  40  is for instance composed of two deviating elements, one of which is located on a first side of the optical axis and the other on the is opposite side, whereby the first deviating element deviates the electrons towards the second deviating element which causes the former to cover an optical path of sufficient length to obtain the desirable dispersion D. 
     The description of a closed curve surrounding the optical axis  5  enables reduction of the space necessary for all the dispersion mechanisms  21 , but has the shortcoming of increasing the aberrations of the second order since the path in the dispersion plane  23  is always described in the same direction. 
     According to the embodiment exposed, the deflecting system  30  receives the beam  72  transmitted along the angle of incidence al, preferably equal to 45.5°. Other embodiments than that shown are acceptable, providing they offer this same angle of incidence a 1 . 
     However, the deflecting system  30  may also be composed of polar parts  31  and  32  receiving the beam  72  transmitted at normal Incidence. In such a case, a multipolar optical system is placed at the incoming of the deflecting system  30 , such as a quadripole or an octopole. 
     Generally speaking, the deflecting  30  and dispersing  40  systems must be designed so that the propagations of the beam  70  towards one another are collinear, or so that the deflecting system is formed of a single deflecting element. 
     Although the presentation of the invention is made for a transmission electron microscope or TEM, it is also suitable for a scanner transmission electron microscope or STEM or for a combined electron microscope.