Abstract:
A particle optical apparatus has a particle source for generating at least one beam of charged particles, and a magnet arrangement having two pole plates, which are arranged spaced apart from one another, such that the at least one beam of charged particles in operation passes through the pole plates, wherein trenches are provided in the pole plates, in which trenches coil wires are arranged. The trenches, when viewed in a cross section transverse to an extension direction of the trenches, have a smaller width in a region of a surface of the pole plates, than in a region arranged at a distance from the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This applications claims priority under 35 U.S.C. §119 to German Patent Application DE 10 2008 062 888.3, filed Dec. 23, 2008, the entire contents of which are hereby incorporated by reference. 
     FIELD 
     The present disclosure relates to a particle optical device, and in particular to an electron microscope, having a magnet assembly. 
     BACKGROUND 
     Electron microscopes are used for examining an object by directing a primary electron beam towards the object to be examined, whereupon electrons emanating from the object are guided to a detector as a secondary electron beam, and detected there. 
     For electron microscopes, it is generally desirable to deflect the primary and/or the secondary electron beam in a precise manner, e.g. about an angle of 10° or more; or to separate an electron beam from a counter-propagating electron beam, for being able to separately manipulate these beams. For an electron microscope operating in 180°-deflection, or an electron microscope with a mirror corrector, this typically means that both beams have to pass a deflecting field of a beam separator. 
     For beam guiding, magnetic fields are usually employed, which often have to fulfill high demands concerning their homogeneity, stability and edge effects. Metal plates with a high magnetic conductivity, usually of an iron-nickel-alloy, can be placed at a fixed distance opposite from one another as pole plates for generating these magnetic fields. Coils are fitted into the surfaces of the pole plates, which coils may be energized by a flow of current. A magnetic field which is homogeneous to a high degree of approximation is then generated on the areas surrounded by the coil wires. Such a device is known from German Patent No. 102 35 981. 
     The demands on the stability and accuracy of the geometry of the pole plates can be quite high, in particular for the electron microscopes with a mirror corrector mentioned above. 
     SUMMARY 
     Conventionally, the heat generated by energizing the coils is dissipated through the pole plates. The present inventors have now found that thereby, temperature gradients are caused, which affect the geometry of the pole plates. Additionally, temperature drifts may result, when the amount of energizing has to be changed. These influences result in undesirable variations of the deflecting field. 
     The present disclosure can overcome such drawbacks and deficiencies. 
     The present disclosure provides a particle optical device with
         a particle source for generating at least one beam of charged particles, and   a magnet assembly including two pole plates, which are arranged spaced apart from one another, so that in operation, the beam of charged particles passes between the two pole plates, wherein trenches are provided in the pole plates, in which trenches coil wires are arranged.       

     Generally, the concept contemplated in this application resides in reducing and/or in stabilizing the generation of heat in the coils, and/or in reducing the transfer of heat from the coils into the pole plates. 
     According to a first aspect, a particle optical device is provided, in which the coil trenches have a smaller width in surface regions of the pole plate, than in regions located at a distance from the surface. 
     Thereby, more space is provided in some embodiments for the total cross section of the coil wires, without substantially compromising the particle optical properties of the pole plates essentially determined by their surface contours. The heat generation for a like energizing current is reduced by the larger cross section of the conductor, and the accordingly lower (electric) resistance. 
     The enlargement of the trench cross section can, in some embodiments, be effected inwardly, outwardly or in both radial directions in relation to the conductor loop. 
     According to a second aspect, a particle optical device including a coil mount is provided, on which coil mount the coil wires are supported, wherein a gap is provided between the coil mount and the pole plate. 
     According to this structure, in some embodiments the heat transfer from the coil wires to the pole plates via heat conduction is reduced, particularly in a case where the pole plates and the coils are located in a vacuum. The heat dissipation from the coils may occur around the pole plates or through holes provided therein, respectively, e.g. using pillar-like stands made of a heat conducting material and extending through the holes. 
     In some embodiments, such an arrangement allows that generated heat is conducted to a cooling device bypassing the pole plates. 
     According to a third aspect, a particle optical device including two layers of an electrically conducting and nonmagnetic material is provided, which layers are arranged between the two pole plates, and which extend on either side of the beam of charged particles. 
     According to some embodiments, an environment for the beam may be provided, to which a high voltage can be applied, while the sample and the coils are on ground or near-ground potential. While the current supply for the coils is simple, an electrical voltage may in operation be applied e.g. to parts of the sample, and the electron energy at the sample is variable. The nominal electron energy between the pole plates, however, remains constant, and likewise the magnetic field needed for deflection and therefore the energizing current. As a consequence, the remaining heat input into the pole plates through heat loss of the coils (e.g. via heat radiation) remains constant and, thus, temperature drift is reduced. 
     According to a fourth aspect, a particle optical device is provided, in which the coil wires are isolated thermally and with respect to high voltage from the pole plates, and the latter are put on a higher electric potential than the coil wires. 
     According to some embodiments, this results that on the one hand, the coil wires do not undergo thermal variations, and on the other hand, that the heat input into the pole plates is small. In some embodiments, this structure can be combined with that according to the first aspect, such that the desire for increased space in the coil trench may not result in a reduced cross section of the coil wires. 
     According to a fifth aspect, a particle optical device having a permanent magnet is provided, which is located within the coil arrangement as projected onto the beam plane. 
     According to some embodiments, this arrangement results in the main portion of the magnetic field to be generated being provided by the permanent magnet, while only a comparatively smaller portion is provided by the coil arrangement. Thereby, the current flow through the coils, and accordingly the heat input into the pole plates is reduced, and as a result also the influence on the pole plate geometry. 
     In some embodiments, a magnetic flow density of the material used for the permanent magnet is particularly little influenced by the temperature. This small dependency on the temperature can, in embodiments, be compensated for using a control e.g. measuring the temperature and applying a corresponding correction current to the coils as described e.g. in German patent No. 102 35 455. 
     Generally, two or more or all of the above aspects may be combined with one another. 
     Hereinabove, the particle optical device is not restricted to electrons, but also ions, myons or others may serve as the charged particles. A preferred application of the particle optical device, however, is in the field of electron microscopy. 
     The particle optical device may be used in several different types of electron microscopes. These comprise, on the one hand, such devices in which the primary electron beam is an examining beam focused onto specific locations on the object, in particular focused successively onto distinct locations, and wherein a secondary electron intensity is detected integrally, i.e. is not spatially resolved. These types of microscopes are known in the art as SEMs (Scanning Electron Microscope). On the other hand, the devices comprise electron microscopes with a spatially resolving detector onto which an extended area of the object is imaged, wherein the extended area is simultaneously and essentially uniformly illuminated by the primary electron beam. These types of microscopes are known in the art, among others, as LEEM (low energy electron microscope), SEEM (secondary electron emission microscope) or TEM (transmission electron microscope). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The forgoing as well as other advantageous features of the disclosure will be more apparent from the following detailed description of exemplary embodiments of the disclosure with reference to the accompanying drawings, wherein 
         FIG. 1  shows schematically a conventional electron microscope of the SEM type; 
         FIG. 2  shows schematically an arrangement of field regions of the beam splitter according to  FIG. 1 ; 
         FIG. 3  shows schematically a cross section through one field region of the beam splitter shown in  FIGS. 1 and 2  with an example of a current conductor arrangement; 
         FIG. 4  shows schematically two other possible magnet and coil arrangements; 
         FIG. 5   a  shows schematically a magnet arrangement for a particle optical device according to an embodiment; 
         FIG. 5   b  shows schematically a similar magnet arrangement as the one of  FIG. 5   a;    
         FIG. 5   c  shows schematically a similar magnet arrangement as the ones of  FIGS. 5   a  and  5   b;    
         FIG. 6   a  shows schematically a top view of a magnet arrangement for a particle optical device according to an embodiment; 
         FIG. 6   b  shows schematically a cross sectional view to the top view of  FIG. 6   a  along the line A-A; 
         FIG. 7  shows schematically a magnet arrangement for a particle optical device according to an embodiment; 
         FIG. 8  shows schematically a magnet arrangement for a particle optical device according to an embodiment; and 
         FIG. 9  shows schematically a magnet arrangement for a particle optical device according to an embodiment. 
     
    
    
     The modifications shown in  FIGS. 5 to 9  can be combined in some embodiments. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The operating principle of an electron microscope of the SEM type is explained in  FIG. 1 . The electron microscope  100  includes a particle source  1  with a beam accelerator  2  following the particle source  1  in the direction of the beam. After passing through the beam accelerator  2 , the particles are accelerated to the potential of the outer beam guide tube  3 . In this region, a magnetic condenser lens  4  and a stigmator  5  following same are envisaged. A first electrostatic immersion lens  6   a  follows the stigmator  5 , by which lens  6   a  the electrons are accelerated or decelerated, respectively, to another kinetic energy, namely that of the inner beam guide tube  7 . The beam splitter  8  with its magnet sectors  8   a  to  8   c  is arranged in the region of the inner beam guide tube  7 . The beam splitter  8  is followed by a second electrostatic immersion lens  6   b , by which the electrons are decelerated or accelerated, respectively, to the energy of the objective beam tube  9 . Following this in the direction of the beam, a multipole system including 12-pole-elements  11 ,  13  and deflecting systems  10 ,  12 ,  14  are arranged, as well as, close to the object  15 , an objective lens  16  having a diffraction plane  17 . The objective lens  16  focuses the incident electron beam onto the focal plane  18  of the objective lens  16 . Herein, the objective lens  16  can be configured as a purely magnetic objective lens, or as a combination with an electrostatic immersion lens. In the latter case, the immersion lens is formed by letting the objective beam tube  9  end within the objective lens  16  at a position of the pole shoe gap or behind, and by decelerating the electrons to the potential of the object  15  positioned in the vicinity of the focal plane  18  after exiting from the objective beam tube  9 . 
     The pair of immersion lenses  6   a ,  6   b  further allows for compensating variations of the electron energy, such that the electron beam passing through the beam splitter  8  has a substantially constant energy. 
     The secondary electrons generated through interaction of the primary electron beam with the object  15  are accelerated back by the higher potential of the objective beam tube  9  and pass through the beam path between the objective lens  16  and the beam splitter  8  in opposite direction. Due to their reversed direction of motion, the electrons are deflected in the magnet sector  8   c  in the opposite direction, so that they are spatially separated from the primary electron beam. The secondary electrons can be detected by a detector  20  following the deflected branch of the beam splitter. A discrimination regarding the energies of the electrons coming from the sample, in particular according to mirror electrons, back scattered electrons, and several kinds of secondary electrons, is possible by an electrostatic lens  19  placed in front, to which different potentials are applied. 
       FIG. 2  elucidates the arrangement of beam regions and field regions in the beam splitter  8 . This exemplary beam splitter  8  consists of overall three magnet sectors  8   a ,  8   b ,  8   c  with surrounding current conductors (not shown) which are uniformly bent as regards the direction of the bends, and are placed in according conductor guides  21   a ,  21   b ,  21   c . The two outer sectors  8   a ,  8   c  may have identical structure, but it is sufficient if this symmetry holds for those beam regions transsected by the primary electron beam. The directions of the magnetic fields B a  and B c , in embodiments also their strengths, are the same in these outer sectors  8   a ,  8   c , while the magnetic field B b  of the inner sector  8   b  has the opposite direction. The inner magnetic sector  8   b  is symmetrical in itself, and arranged symmetrically to the plane  23  indicated by the dashed line in  FIG. 2 . Accordingly, the field regions including the field-free regions are symmetrically arranged, and therefore likewise the beam regions transsected by the beam. In addition, all the magnetic fields are parallel or anti-parallel, respectively, so that the primary electron beam passes coplanarly through the beam splitter, namely in the beam plane  24  (see  FIG. 3 ).  FIG. 2  thus represents a projection onto the beam plane  24 . 
     The beam deflection within the beam splitter  8  results in a beam deflection of about 90° in this example, between the first and second beam axes BA 1 , BA 2 . 
       FIG. 3  illustrates the principal design of the magnet sectors  81 ,  8   b ,  8   c . This drawing corresponds to a cross sectional view of the magnet sector  8   a  shown in  FIG. 2 , approximately along the line (III-III). The pole shoes  25  and the pair of coils  26  arranged in the conductor guide  21  are shown, as well as pairs of trim coils  27  serving for fine adjustment. In the space enclosed by the coils  26 , the magnetic field B a  is generated. Yokes magnetically connecting the pole shoes  25  outside of the region surrounded by the current conductor, and involved for completing the magnetic field flow, are not shown. Further, the yokes define the distance between the pole shoes. Both pole shoes  25  and the yokes may be made of soft iron, ferrite, an iron-nickel-alloy or another magnetic material. The coil wires may be made of a high conductance copper alloy or the like. 
       FIG. 4  illustrates two alternative structures of the pole shoes, wherein functionally corresponding components bear like numerals as above, but with subscript small letters: 
     In the upper part of this drawing, a bowl-shaped pole plate  25   a ″ with an island pole  25   a ′ is shown, between which the coil  26   a  is arranged. The surface of the pole plate is labeled  34 . The pole plate  25   a ″ and the island pole  25   a ′ are assembled with one another such that between them, a sufficient magnetic connection is provided, and together they form the pole shoe. Similarly, in the lower part of  FIG. 4  a variant is shown, in which the pole shoe is formed by an island pole  25   a ′, a ring-shaped pole plate  25   b ″ and a back plate  28 , between which the coil  26  is located, as before. 
     In  FIG. 5   a , the pole plate  25   c  has a pole lid  30   c  in its central portion, the pole lid partly covering the coil trench  21   c  from the radially inward side, such that only the outer wall  32  of the trench is formed by a single piece of the pole plate  25   c . Thereby, the width B 1  of the trench at the surface  34   c  of the pole plate  30   c  is less than the width B 2  of the trench below the surface  34   c . The width B 1  as well as B 2  may vary along the circumference of the poles. 
     Analogously, in  FIG. 5   b  the pole plate  25   d  has, in its peripheral region, a ring-shaped pole lid  31   d  partly covering the coil trench  21   d  from the radially outward side, so that only the inner wall  33  of the trench is formed from a single piece of the pole plate  25   d . Thereby, the width B 1  of the trench at the surface  34   d  of the pole plate  30   d  is again less than the width B 2  of the trench below the surface  34   d . The width B 1  as well as B 2  may again vary along the circumference of the poles. 
     In  FIG. 5   c , the modifications according to  FIGS. 5   a  and  5   b  are combined: The pole plate  25   e  has a ring-shaped pole lid  31   e  in its peripheral region, as well as a pole lid  30   e  in its central portion, together partly covering the coil trench  21   d  from the radially inward and outward sides, so that neither the inner wall  33   e  nor the outer wall  32   e  of the trench is formed from a single piece of the pole plate  25   e . Thereby, the width B 1  of the trench at the surface  34   e  of the pole plate  30   e  is more reduced compared with the width B 2  of the trench below the surface  34   e . The width B 1  as well as B 2  may again vary along the circumference of the poles. 
     In the top view according to  FIG. 6   a , it is shown how, in addition to a lead-through  36  for the coil wire terminal, plural through holes  38  for pillars of a coil mount (see  FIG. 6   b ) are distributed in the bottom of the coil trench  21   f . The number of these through holes may e.g. vary between 3 and 9, or between 5 and 7. 
     It will become evident from the corresponding side view according to  FIG. 6   b , that the coil wires  26   f  are mounted on a ring-shaped coil mount  40 ′, or a mount forming a substantially closed ring, having e.g. an L-shaped profile. This ring  40 ′ is itself mounted on top of several pillars  40 ″ made of a material well conducting heat, e.g. copper or aluminum. A gap  41  is provided between the coil mount  40 ′ and the pillars  40 ″, on the one hand, and the pole plate  25   f  on the other hand. The gap  41  presents a bar to direct flow of heat to the pole plate  25   f . The heat is conducted via the pillars  40 ″ to a common, e.g. plate-like base  42 . It is also envisaged to provide active cooling devices (not shown), e.g. water or air cooling, or a Peltier element. The arrangement for mounting the coils  40 ′,  40 ″,  42  is held in place in its position relative to the pole plate  25   f  by a heat-isolating spacer  44 . 
     The modifications mentioned above relate to the plate or trench geometry in a wider sense. Independently from, or in combination with, the modifications already mentioned, the following embodiments relate to the electrical configuration: According to  FIG. 7 , a double layer  46  of an electrically conductive, but non-magnetic material, e.g. a thin copper or gold layer, is provided between the pole plates  25   g , exemplarily shown here in the basic version of  FIG. 3 . An upper limit for the tolerable magnetic susceptibility of this material is μ r &lt;1.01. The material layer shall further be sufficiently electrically conductive such as to not accumulate electric charges. In the example according to  FIG. 7 , the electrically conductive layers  46 , between which the beam of particles passes through, are each supported by an isolating layer  48  substantially or entirely filling the space between the layers  46  and the respective adjacent pole plate  25   g . The isolating layers  48  may be formed monolithically, with only the holes for the ingoing and outgoing particle beams are provided; such a configuration obviates protective measures otherwise involved due to the high voltage to be applied to the double layer  46  and the pole plates  25   g . Herein, the isolator may be formed by hollowing out a massive block to form a so-called beam box. Alternatively, the hollow block may be constructed directly in the desired shape e.g. by Rapid Prototyping. In both cases, surface pairs are avoided which would be unavoidable e.g. if several parts would be assembled by screwing together, and which might provide electrical break-through paths. The layers  46 , together with head-on apertures transmitting the particle beam, form the beam box  45 , within which the particles are at a defined electrical potential. 
     In  FIG. 8 , a variant is shown in which the coil winding  26   h  is surrounded as a whole by an electrically and thermally isolating layer  50 , such that the latter occupies substantially the entire space in the coil trench  21   h  left by the coil winding  26   h . In this example, the pole plate geometry according to  FIG. 5   c  is taken as basis, in order to have available as much space as possible for the isolation, and to at the same time provide an as large as possible current conductor cross section for the coil wires  46   h . Other pole plate types than those with a ring-shaped pole lid  31   h  and an island-like pole lid  30   h  are also conceivable. In this arrangement, the coil may be provided at a different potential than the pole plate; furthermore, the heat transfer is reduced. The fact that the pole plate does not serve as a heat sink in this embodiment, and that the coils heat up more as a consequence, is not critical in respect of the constancy and precision of the magnetic field at least as long as a tolerance gap  51  remains between the isolation  50  and the wall  33   h  of the coil trench  21   h , of the isolation  50  is compressible. In this embodiment, the coil itself dissipates the major part of the heat generated in operation, via the coil wires. 
     A further possibility is shown in  FIG. 9 : Herein, a permanent magnet  554  is arranged below an island pole  52 , namely a plate made of a magnetic material, which at its periphery defines the coil trench. The permanent magnet provides the larger part of the magnetic flow involved. The coil wires  46   h  in operation carry considerably less current than in the examples without a permanent magnet discussed above at the same magnetic field, because they provide only a minor part of the total magnetic field strength. The permanent magnet  54  cannot be provided with an exact magnetic field strength, and furthermore its field depends—albeit only a little—from the temperature. Therefore, in this example, a temperature sensor  55  is provided, the output signal of which is used to adjust the power supply voltage DC of the coils  26   i  by a controller  56  and a regulator  57 , such that field variations are compensated. A power supply for the coils is also provided in the embodiments described previously; in addition, a controller may likewise be provided in each of these embodiments. 
     To summarize, the disclosure provides systems with which the heat generation of the coils and/or the heat transfer from the coils into the pole plates may, in embodiments, be reduced and/or stabilized. The skilled person will conceive of variations to the embodiments presented above, which variations are nevertheless covered by the appended claims.