Abstract:
A particle-optical apparatus for changing trajectories of charged particles of a divergent particle beam oriented along a longitudinal axis is proposed, comprising: 
     an inner electrode arrangement which is at least partially transparent for the particles, engages at least partially around the longitudinal axis with a radial distance and extends along the longitudinal axis, 
     an outer electrode arrangement which engages at least partially around the inner electrode arrangement with a radial distance and extends along the longitudinal axis, and 
     a voltage source for providing a potential difference between the inner and the outer electrode arrangements, wherein the voltage source provides such a potential difference that a kinetic component of a particle traversing the inner electrode arrangement is reversible, said kinetic component being oriented orthogonally to the longitudinal axis. 
     Moreover, an illumination system and an imaging system as well as a manufacturing method employing said particle-apparatus are proposed.

Description:
FIELD OF THE INVENTION 
     The invention relates to a particle-optical apparatus for changing trajectories of charged particles of a beam of particles. Furthermore, the invention relates to an illumination apparatus and a projection system comprising such a particle-optical apparatus as well as a method for device manufacture. Such method comprises a photolithographic step in which the particle-optical apparatus is employed. In particular, the particle-optical apparatus is provided for use in an projection electron-beam lithographic system as well as for use in a method for device manufacture by means of projection electron-beam lithography. 
     BACKGROUND OF THE INVENTION 
     The so-called SCALPEL method (Scattering with Angular Limitation in Projection Electron-beam Lithography) is known as a method which employs a beam of electrons for imaging and exposing a radiation sensitive layer. This method is described in the white book “SCALPEL: A Projection Electron-Beam Approach to Sub-Optical Lithography”, Technology Review, December 1999, by J. A. Liddle, Lloyd R. Harriott, A. E. Novembre and W. K. Waskiewicz, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, N.J. 07974, USA. The entire disclosure of said document is incorporated in this description by reference. Furthermore, U.S. Pat. Nos. 5,079,112, 5,130,213, 5,260,151, 5,376,505, 5,258,246, 5,316,879 as well as European patent applications nos. 0,953,876 A2 and 0,969,326 A2 relate to the SCALPEL method. The entire disclosures of the above-mentioned patent documents are likewise incorporated in this description by reference. 
     A conventional projection lithographic system is used, for example, for the manufacture of a semiconductor device. Here, the structures to be formed on a semiconductor wafer are defined in a mask, the mask is illuminated by a beam of electrons and the structures defined on the mask are imaged onto the semiconductor wafer. The semiconductor wafer is provided with a radiation sensitive layer. After having been exposed by the electron beam, the radiation sensitive layer as well as the semiconductor wafer are subjected to further steps for forming the structures in the wafer material. 
     FIG. 1 schematically shows an illumination apparatus for illuminating a mask  3  with charged particles. The charged particles are electrons which are emitted by an electron source  5  in a beam direction  7 . The particle beam emitted by the source  5  exhibits little divergence which, for reasons of illustration, is, however, shown in FIG. 1 enlarged in size. A maximum angle α of the electrons with respect to the beam direction  7  is about 5 mrad. 
     The source is imaged by a first electron-optical focusing lens  9  into a front focal plane  11  of a second electron-optical focusing lens  13 . The focusing lens  13  acts to shape the electrons divergently traversing the focal plane  11  such that a substantially parallel particle beam  15  with extended beam cross-section is formed in order to illuminate a field  17  on the mask  3  of a size of about 1 mm transverse to the beam direction  7 . 
     The maximum illumination aperture which is attainable with this type of illumination is determined by the spatial dimension h of the source  5  transverse to the beam direction  7  as well as by the focal length f 1  of the lens  13 . The maximum angle β of the particles with respect to the beam direction  7 , when the same impinge on the mask  3 , is determined by        β   =       h     2   ·     f   1         .                            
     For small dimensions of the source  3  (FIG. 1 shows a dot-shaped source), the illumination aperture is thus low. However, a high illumination aperture is desirable in order to be able to transfer also small structures defined on the mask to the wafer with precision. 
     It is conceivable to increase the spatial dimension of the source transverse to the beam direction in order to increase the illumination aperture. However, it is problematic for sources of charged particles to increase the source dimension if the field illuminated on the mask is to be uniformly illuminated as well. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a particle-optical apparatus which contributes to the increase of an illumination aperture in a particle-optical illumination system. 
     Moreover, it is an object of the present invention to propose a particle-optical apparatus for changing trajectories of charged particles of a particle beam. In this respect, it is, in particular, an object of the invention to propose a particle-optical apparatus which selectively changes the trajectories of the charged particle, i.e., which acts only on trajectories of specific charged particles and not uniformly on the trajectories of all particles of a particle beam. 
     Furthermore, it is an object of the present invention to propose an illumination apparatus for illuminating a field which is to be illuminated and is spatially extended transverse to the beam direction with a comparatively high illumination aperture or/and comparatively uniformly. 
     Moreover, it is an object to propose a projection system, the illumination apparatus of which exhibits the above-mentioned advantages. It is a still further object of the invention to propose a method for manufacturing in particular miniaturized devices which enables the devices to be manufactured with increased precision. 
     To this end, the invention is based on the following consideration: 
     In an imaging illumination system, as it has been described above with reference to FIG. 1 by way of example, the light-transmitting value or emittance is a conservative quantity. This quantity is defined as the product of the square root of the illuminated area and the illumination divergence (numerical aperture). In an imaging illumination system, an increase of the illumination divergence is thus not achievable without decreasing the illuminated area. Therefore, the invention is based on the idea to develop a particle-optical apparatus which does not act as an imaging system but changes the trajectories of the charged particles traversing the particle-optical apparatus in a different way. The trajectories of different groups of particles are to be changed differently such that, all in all, an increase of the light transmitting value or emittance of the beam passing through the apparatus is achieved. 
     In particular, the invention proposes a particle-optical apparatus comprising two cylindrical electrode arrangements which are fitted into one another, said electrode arrangements being disposed relative to a particle beam entering the apparatus such that the beam direction is oriented approximately parallel to the direction of extension of at least one of the cylindrical electrodes. Moreover, an inner one of the two electrode arrangements is of such a length and has such a diameter that trajectories of at least those particles which enter the apparatus at an angle with respect to the beam axis which is larger than a minimum angle traverse the inner electrode arrangement radially with respect to the beam direction. To this end, the inner electrode arrangement is at least partially transparent for the charged particles. There is an electric potential difference between the inner electrode arrangement and the outer electrode arrangement such that a kinetic component of the particles traversing the inner electrode arrangement is reversed, said kinetic component being oriented transversely to the beam direction. 
     The inner and outer electrode arrangements together act like a cylindrical, internally mirrored tube which encloses the particle beam and reflects particles which want to escape from the interior of the cylinder back into the same. 
     For a group of particles of the particle beam which enter the apparatus with little divergence, the apparatus is preferably not effective, that is, this group of particles traverses the apparatus straightly, so that an observer positioned at the exit side of the apparatus perceives these particles as emerging from the particle source. 
     For another group of particles with increased divergence, the apparatus is preferably effective such that the particles are reflected once by the reflecting tube. The observer perceives this group of particles as emerging from a spatially distributed source which appears to be disposed beside the actual source. 
     For a still further group of particles with a still higher divergence, the apparatus is effective such that these particles are reflected twice or more by the reflecting tube so that the observer perceives this group of particles to emerge from further spatially distributed sources which appear to be spaced apart by a still greater distance from the actual source. 
     Accordingly, the effect of the apparatus of the invention is such that even a small radiation source is perceived by the observer as a radiation source which appears to have a substantially increased radiation emitting area. 
     If such a particle-optical apparatus is used in an illumination system, it contributes to an apparent increase of the spatial dimension of the radiation source transverse to the beam direction. This results into an increase of the light-transmitting value or emittance of the illumination system. For this reason, the apparatus of the invention also enables the illumination aperture to be increased for an illuminated field which is extended transverse to the beam direction. 
     Due to the potential difference between the inner electrode arrangement and the outer electrode arrangement, there is provided a space between these two electrode arrangements with an electric field therein which renders it possible to reverse the transversal kinetic component of the particles which enter said space. As this field is limited to the space between the inner electrode arrangement and the outer electrode arrangement, the charged particles must be enabled to enter this space. For this reason, the inner electrode arrangement is at least partially transparent for these particles. Preferably, this property of the inner electrode arrangement, namely partial transparency, is achieved in that the inner electrode arrangement is divided into a plurality of sub-electrodes which are spaced apart from one another. Preferably, the individual sub-electrodes are on a common equal electric potential, and a material-free space is provided between the sub-electrodes so that the particles pass through two adjacent sub-electrodes and can enter the space between the inner electrode arrangement and the outer electrode arrangement which provides the reflecting electric field. However, it is also possible that particles impinge directly on the sub-electrodes and thus cannot enter the space between inner and outer electrode. 
     In order to obtain a transparency as high as possible for the charged particles, the sub-electrodes preferably extend substantially parallel to the longitudinal axis of the apparatus or/and substantially parallel to the main direction of the particle beam entering the apparatus. 
     If the above-described particle-optical apparatus is employed in an illumination apparatus for illuminating an object, it acts there as an emittance changing apparatus which is preferably disposed between a particle source and the object plane. Preferably, such an illumination apparatus also comprises an imaging condenser system which is disposed between the emittance changing apparatus and the object and directs the particles emerging from the emittance changing apparatus to the object. 
     Preferably, the particles emitted by the particle source pass directly into the emittance changing apparatus. It is, however, also preferred to provide an optical system between the particle source and the emittance changing apparatus for producing an image of the source between the actual source and an entrance cross-section of the emittance changing apparatus. 
     Preferably, the condenser system comprises a focusing lens which images the source itself or the image of the source into an intermediate plane disposed between the emittance changing apparatus and the object plane. As a result, preferably several images of the source itself are produced in the intermediate plane if the first focusing lens images the source in the intermediate plane, and several images of the image of the source are produced if the first focusing lens images an image of the source in the intermediate plane. This plurality of images of the source or of the images of the source images are distributed in the intermediate plane, in particular, adjacent to one another. 
     However, it is also preferred that the first focusing lens does not produce an exact image of the source or of the source image in the intermediate plane. In this case, it is essential for the first focusing lens to direct the particles which have passed through the emittance changing apparatus in such a way through the intermediate plane that they are spaced apart in the intermediate plane from the longitudinal axis of the apparatus or the beam center by a distance which increases the more often the trajectory of a particle has been changed by the emittance changing apparatus. As compared to a situation in which the emittance changing apparatus is not disposed in the beam path, there is thus provided in the intermediate plane an extended illuminated area or an extended area which is traversed by particles. The particles traversing this extended area are directed to the object plane preferably by means of a second focusing lens such that in the object plane a field is illuminated which has a dimension in a direction transverse to the beam direction which is smaller than the dimension of the illuminated area in the intermediate plane. However, as compared to the situation in which the emittance changing apparatus is not disposed in the beam path, the illumination of this area in the object plane is then effected with an increased numerical aperture. 
     The invention also provides for a method for device manufacture, such devices being preferably highly miniaturized devices, such as micro-mechanical structures or integrated circuits. As far as integrated circuits are concerned, a mask includes a circuit pattern which corresponds to a single layer of the circuit to be formed on a suitable substrate, for example, a silicon wafer. In order to image the pattern onto a target area, also referred to as die, of the substrate, the latter is first covered with a radiation sensitive layer, also referred to as resist. Subsequently, the radiation sensitive layer is exposed or irradiated in that the pattern of the mask is imaged by means of charged particles, for example, electrons or ions, onto the radiation sensitive layer. The radiation sensitive layer is then developed and either the irradiated or exposed or the non-irradiated or unexposed regions of the irradiated layer are removed. The remaining structure of the radiation sensitive layer is then used as a mask, for example, in an etching step, an ion implantation step, a material deposition step or the like. 
     According to the invention, it is provided for that the mask and the structure which is defined on the mask and which is to be imaged onto the substrate are illuminated by the above-described illumination apparatus in a photolithographic step of the method. 
     Exemplary embodiments of the invention will be described below in further detail with reference to the accompanying drawings, wherein 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an illumination apparatus, without a particle-optical apparatus according to the invention being provided in the beam path thereof, 
     FIG. 2 is a schematic perspective view of an embodiment of a particle-optical apparatus of the invention, 
     FIG. 3 is a cross-sectional view of the particle-optical apparatus according to FIG. 1, 
     FIG. 4 is a view reflecting the function of the apparatus shown in FIGS. 2 and 3, 
     FIG. 5 is an enlarged view of a section indicated by reference sign V in FIG. 4, 
     FIG. 6, is a further illustrative view reflecting the function of the apparatus shown in FIGS. 2 and 3, 
     FIG. 7 is an embodiment of an illumination system according to the invention and 
     FIG. 8 is a partial view of an embodiment of a projection system according to the invention. 
    
    
     DETAILED DESCRIPTION 
     An emittance changing apparatus  21  shown in FIG. 2 serves to change a beam  25  of charged particles entering on an entrance side  23  of the apparatus  21  such that the emittance of the beam is increased when the same emerges from an exit side  27  of the emittance changing apparatus  21  opposite to the entrance side  23 . 
     A cylindrical outer electrode  29  extends between the entrance plane  23  and the exit plane  27 , the cylinder direction and axis of symmetry  35  of said outer electrode coinciding with a direction  31  of the beam  25 . The outer electrode  29  is formed by four plate electrodes  33  which are disposed in pairs orthogonally to one another, said plate electrodes being disposed spaced apart from the longitudinal axis  35  at equal distance. Each of the plate electrodes  33  is formed of a material which is electrically conductive all-over. 
     The emittance changing apparatus  21  further includes an inner electrode arrangement  37  comprised of a plurality of wires  39  each extending parallel to the longitudinal axis  35 . The wires  39  are disposed within the hollow space formed by the plates  33 , each wire  39  being spaced apart from a directly adjacent plate  33  by a distance of d=0.2 mm. The wires  39  are thus disposed on four planes  41  each extending parallel to a respective plate  33 , each plane  41  being spaced apart from the longitudinal axis  35  by a distance of D=0.5 mm. The wires  39  are tightened between a frame mount  43  disposed on the entrance side  23  and a frame mount  45  disposed on the exit side  27  and electro-conductively connected to same. 
     The emittance changing apparatus  21  further comprises a voltage source  47  which is connected, through a line  49 , to the outer electrode arrangement  29 , on the one hand, and, through a line  51  via one of the frame mounts  43 ,  45 , to the inner electrode arrangement  37 , on the other hand, the inner electrode arrangement  37  being on ground potential. The voltage source  47  produces a potential difference of 10 V between the inner electrode arrangement  37  and the outer electrode arrangement  29  so that the outer electrode arrangement  29  has a potential of =10V as compared to ground. 
     Accordingly, an electric field is produced in a space  61  between the inner electrode arrangement and the outer electrode arrangement, the shape of this field being schematically indicated by field lines  53  in FIG.  3 . The effect which the latter has on a space  57  inside the inner electrode arrangement  37  is that of a Faraday cage so that there is substantially no field in this space. 
     FIG. 4 shows two trajectories  54  and  55  of electrons of the beam  25  which traverse the emittance changing apparatus  21 . When entering the apparatus  21 , the trajectory  54  has a small divergence or angle with respect to the central axis  35  and thus extends entirely in the inner space  57  and, accordingly, it traverses the apparatus  21  straightly. In contrast to that, the trajectory  55 , when entring the apparatus  21 , extends at a larger angle with respect to the longitudinal axis  35  so that it penetrates the inner electrode arrangement  37  at a location  59  in that the corresponding electron flies through adjacent wires  39  and enters the space between inner electrode  37  and outer electrode  29 . This is shown in enlarged view in FIG.  5 . Due to the electric field present in the space  61  (see field lines  53 ), a force which is directed opposite to the field lines  53  acts on the electron, which force is indicated in FIG. 5 by arrows  63 . This force  63  reduces a kinetic component oriented transversely to the longitudinal axis  35  of the electron moving in the space  61  and, eventually, reverses the kinetic component oriented transversely to the longitudinal axis  35  which was present when the electron entered the space  61  at the location  59 , so that the electron leaves the space  61  again at a location  65  in that it flies through two wires  39  of the inner electrode arrangement  37  and returns into the inner space  57  in which there is no field and further moves straightly in the same and emerges from the emittance changing apparatus  21  at the exit side  27  thereof. The space  61  between the inner electrode and the outer electrode thus acts on an electron traversing the inner electrode  37  like a plane mirror. 
     FIG. 6 shows a particle source  67  disposed concentrically with respect to the central axis  35  of the emittance changing apparatus  21  and spaced apart from the entrance plane  23  of the apparatus  21 , said particle source having a dimension h transverse to the longitudinal axis  35 . FIG. 6 further shows, by way of example, a plurality of trajectories for electron paths, namely two trajectories  69  and  69 ′ which extend at such an angle with respect to the longitudinal axis  35  that electrons are reflected once by the electrode arrangements. Furthermore, two trajectories  71  and  71 ′ are shown which extend at a larger angle with respect to the longitudinal axis  35 , so that the respective electrons are reflected twice by the electrode arrangements. Viewed from the eye of an observer  75  positioned, in beam direction, behind the exit end  27  of the apparatus  21 , the electrons moving on the trajectories  69 ,  69 ′,  71  and  71 ′ appear to emerge from virtual beam sources  77 ,  77 ′,  79  and  79 ′, respectively, which are disposed off-set in the direction transverse to the longitudinal axis  35  beside the actual beam source  67 . Here, the virtual sources  79   79 ′ which are produced by the trajectories  71  and  71 ′, respectively, and are reflected twice by the apparatus  21  are disposed further away from the longitudinal axis  35  than the virtual sources  77  and  77 ′ which are produced by the trajectories  69  and  69 ′, respectively, and are merely reflected once by the apparatus  21 . 
     Accordingly, from the point of view of the observer  75 , the emittance changing apparatus  21  acts such that, all in all, the observer perceives radiation which appears to emerge from a radiation source which has a dimension A transverse to the longitudinal axis  35 . In the example illustrated in FIG. 6, wherein, due to the limited beam divergence, maximally two reflections occur in the emittance changing apparatus  21 , said dimension A is five times as large as the dimension h of the actual source  67 . Here, the divergence, that is, the angle from the longitudinal axis  35  or the angular spread of the beams emerging from the exit end  27  of the apparatus  21 , as compared to the entry of the beams at the entrance end  23 , is not changed by the apparatus  21 . As, however, the apparent area of the source is increased by the apparatus  21 , the apparatus  21  has increased the light transmitting value or the emittance of the beam which is defined as the product of the square root of the illuminated area and the illumination divergence. 
     FIG. 7 shows an embodiment of an illumination system  83  in which the emittance changing apparatus  21  is integrated. The longitudinal axis  35  of the emittance changing apparatus  21  is oriented such that it coincides with the optical axis of the illumination system  83 . 
     The illumination system  83  is part of a lithography system which, furthermore, comprises a projection system  84  which is described in further detail herein below with reference to FIG.  8 . Here, an illuminated object  85  is a mask having patterns defined thereon which are projected onto a radiation sensitive layer  105  of a semiconductor wafer  107  by means of the projection system  84 . The radiation source  67  is a thermal electron source, wherein the electrodes emitted by the cathode heated up to about 2000 K are accelerated by a voltage of about 100 kV in the direction of the optical axis  35 . The kinetic energy of the electrons in the direction of the axis  35  is thus 100 keV, while the kinetic energy perpendicular to the axis  35  has a maximum of about 0.2 eV. A maximum angle α which the trajectory of an electron emitted by the source  67  maximally assumes with respect to the optical axis  35  is thus about 1.4 mrad. 
     A focusing lens  87  produces an image of the source  67  in a plane  89  reduced in size by a factor of ten. A further focusing lens  91  is disposed on the optical axis such that it images the image of the source  67  in the plane  89  into an intermediate image plane  93 . Between the plane  89  and the focusing lens  91 , there is disposed the emittance changing apparatus  21  which has a length of 200 mm in the direction of the optical axis  35  and an edge length of about 10 mm*10 mm transverse thereto. 
     The intermediate image plane  93  is imaged by a third focusing lens  95  to infinity. FIG. 7 merely shows some exemplary trajectories of electrons on their way from the source  67  to the mask  85 . A group  97  of the trajectories shown in FIG. 7 belongs to the electrons which are emitted by the source  67  at an angle α with respect to the axis  35  which is so small that they traverse the emittance changing apparatus  21  without reflection. For these electrons, the focusing lens  91  produces an intermediate image  100  of the source  67  in the intermediate plane  93  which is disposed on the longitudinal axis  35 . These trajectories are rendered parallel by the focusing lens  95  so that, when they impinge on the mask  85 , they enclose a very small angle β with respect to the axis  35 . 
     A further group  98  of trajectories belongs to those electrons which are emitted by the source  67  in such a range of the angle α with respect to the axis  35  that they are reflected once in the emittance changing apparatus  21 . For these electrons, the focusing lens  91  produces an intermediate image  101  in the intermediate image plane  93  which is spaced apart from the optical axis  35 . The focusing lens  95  also directs these electrons onto the mask  85 , where they overlap with the electrons of the bundle  97 , but impinge on the mask at a considerably larger angle β with respect to the axis  35 . 
     A still further group  99  of trajectories belongs to the electrons which are emitted by the source  67  at an almost maximum emittance angle α. These electrons are reflected twice in the emittance changing apparatus  21 , and the focusing lens  91  produces for these electrons an image  102  of the image of the source  67  in the plane  89 . These images  102  of the source  67  produced after dual reflection in the emittance changing apparatus  21  are spaced apart from the optical axis  35  by a still larger distance than the images  101  of the electrons which have only been reflected once in the emittance changing apparatus  21 . 
     The images  102 , too, are directed by the focusing lens  95  as a substantially parallel bundle of beams onto the mask  85  where they overlap with the electrons of the bundles  97  and  98  and impinge on the mask  85  at a still larger angle β with respect to the axis  35 . 
     Here, the angle β at which the last-mentioned group  99  of trajectories extends with respect to the longitudinal axis  35  constitutes the maximum illumination aperture and is considerably larger than the maximum angle of divergence α of the source  67 , although a quadratic field having an edge length l of about 1 mm is illuminated on the mask  85 . Despite its relative large dimensions as compared to the dimensions of the source  67 , this field is illuminated substantially homogeneously. It is also apparent from FIG. 7 that the edge length l of the illuminated field is smaller than the distance of the intermediate images  101  and  102  from the longitudinal axis  35 . 
     The projection system  84  shown in FIG. 8 images the mask  85  onto a radiation sensitive layer  105  which is applied to the semiconductor wafer  107  on the side facing the mask  85 . To this end, the projection system  84  comprises a doublet of magnetic lenses  109  and  111  which images structures and patterns defined on the mask onto the radiation sensitive layer  105  reduced in size by a factor of four. FIG. 8 shows three exemplary groups of trajectories with a different distance M from the optical axis  35 . It is also apparent from FIG. 8 that the mask  85  is illuminated by the illumination system  35  shown in FIG. 7 with an illumination aperture β. 
     The doublet of focusing lenses  109  and  111  guides the trajectories through a symmetry plane  113  where a cross-over is formed for the beams, that is, the trajectories intersect the optical axis  35  at this location. In this symmetry plane  113 , there is also provided an aperture filter  115  which is traversed by the beams which have traversed the mask  85  unscatteredly, while the electrons which are slightly deflected from their original path by the mask  85  are absorbed by the filter  115  and thus do not reach the radiation sensitive layer. 
     In FIG. 8, the reference numbers  121 ,  123  and  125  designate secondary coils which provide magnetic fields in addition to the focusing lenses  109  and  111  and can be driven such that aberrations are reduced when the mask  85  is projected onto the radiation sensitive layer  105 . 
     In the above-described embodiments, the emittance changing apparatus was used in an illumination system and a projection system which serves to image a mask structure onto a substrate. However, the emittance changing apparatus  21  is not limited to such an application. It can be employed whenever trajectories of charged particles are to be influenced, the effect or the change of the trajectories being dependent upon an angle of the trajectories with respect to an optical axis prior to their entry into the emittance changing apparatus. 
     Moreover, the emittance changing apparatus has been described above to comprise an inner electrode arrangement and an outer electrode arrangement, each having a quadratic cross-section. However, other geometries are conceivable in this respect as well. In particular, the inner and outer electrode arrangements can be of circular cylindrical shape or any other shape. 
     The arrangement of the cylindrical electrode arrangement of the emittance changing apparatus has been described above as being symmetric with respect to the incident beam. However, it is also possible to provide an angle between incident beam and longitudinal cylinder axis or to shift the longitudinal cylinder axis parallel to the incident beam. 
     In the embodiment described in FIG. 7, an image of the source is formed by the condenser system in the intermediate image plane. However, it is also possible to image the source itself into the intermediate image plane. 
     In the above embodiments, the sub-electrodes of the emittance changing arrangement have been described as tightened wires. Alternatively, it is also possible to provide the sub-electrodes by other measures. One possibility in this respect is to lithographically produce the sub-electrodes in that the recesses between adjacent sub-electrodes are etched from a solid material in an etching process.