Abstract:
A particle-optical apparatus is proposed as well as a method for operating the same. The particle-optical apparatus provides a magnetic field for deflecting charged particles of a beam of charged particles and comprises a body of a material with a permeability number around which a current conductor at least partially engages and a temperature-adjusting unit for adjusting a temperature of the magnetic-flux-carrying body substantially to a nominal temperature. A relative variation of the permeability number relative to a width of a temperature range is to be smaller than a limit value a, wherein a is preferably smaller than 3·10 −3 K −1 . In particular, the nominal temperature is at an extremum of a temperature dependence of the permeability number. Preferably, such a particle-optical apparatus can be employed in a microscopy or a lithography apparatus.

Description:
BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to a particle-optical apparatus for manipulating a beam of charged particles, a method for operating such a particle-optical apparatus, a microscopy system and a lithography system. 
   2) Brief Description of Related Art 
   The particle-optical apparatus provides a magnetic field which the beam to be manipulated traverses. This manipulation may include a focusing, a deflection, a conversion of the beam or the like. 
   Magnetic field configurations are known from the prior art which have a focusing, deflecting or converting effect on a beam of charged particles. 
   For example, a beam deflector is known from U.S. Pat. No. 6,188,071 B1 for use in a lithography system. Here, a beam traversing the apparatus is an electron beam which is used as writing beam of the lithography system. A resolution of the lithography method performed therewith is thus determined also by the accuracy with which the deflection or/and focusing of the writing beam is performed in the apparatus. The apparatus comprises ferrite bodies for carrying the magnetic fields produced by current conductor windings. It is a property of ferrite materials that their magnetic permeability is temperature-dependent. Accordingly, if the temperature of the ferrite body changes, its magnetic property will also change and, correspondingly, the effects which the apparatus exerts on the writing beam traversing the same will change with temperature variations. According to U.S. Pat. No. 6,188,071 B1, a temperature control is provided to stabilize the temperature of the ferrite body in order to reduce influences of temperature on the quality of the lithographic process. It is also recognized in the document that the temperature regulation might, under certain circumstances, be too slow to sufficiently suppress temperature changes in the ferrite body. Therefore, the conventional apparatus comprises an additional correcting coil with low inductivity to actively compensate for the influences of the temperature dependence of the permeability of the ferrite material on the beam which are not completely suppressed by the temperature control. 
   SUMMARY OF THE INVENTION 
   It has been found that it is not easy to suppress temperature influences on the ferrite material. Accordingly, it is an object of the present invention to provide a particle-optical apparatus with a magnetic-flux-carrying body, such as a ferrite, wherein temperature changes in the magnetic-flux-carrying body have less influence on a beam of charged particles to be manipulated by the apparatus. Moreover, it is an object of the invention to provide a method for operating such apparatus. A further object of the invention is to provide an electron microscopy system and/or a lithography system wherein relatively good imaging properties are achievable. 
   The invention proceeds from a particle-optical apparatus for providing a magnetic field for manipulating a beam of charged particles which comprises: a magnetic-flux-carrying body made of a material having a high permeability number, at least one current conductor engaging at least partially around the magnetic-flux-carrying body and a temperature-adjusting unit for adjusting a temperature of the magnetic-flux-carrying body substantially to a nominal or target temperature. 
   The invention takes into account the fact that the permeability number of the magnetic-flux-carrying body always depends on temperature. In order to reduce influences exerted by temperature changes in the magnetic-flux-carrying body on the manipulation of the beam, the temperature-adjusting unit is accordingly provided for stabilizing the magnetic-flux-carrying body substantially to the nominal or target temperature. Here, too, the invention takes further into account the fact that such a temperature-adjusting unit, be it a temperature-adjusting-unit with or without feed-back control, will not be able to perfectly prevent temperature variations in the magnetic flux-carrying body. At this point, the concept which the invention is based upon sets in, namely to select the nominal or target temperature of the magnetic-flux-carrying body such that it is in a temperature range in which the permeability number of the material of the magnetic-flux-carrying body has relatively low temperature dependent changes. 
   If temperature changes occur in such a range, they have thus a relatively small influence on the permeability number and thus on the effect exerted by the particle-optical apparatus on the beam traversing the same. 
   A temperature range in which the permeability number has relatively low changes is a range in which a graph which represents the dependence of the permeability number on the temperature exhibits a relatively flat slope. Accordingly, such a range can be characterized by the following formula: 
               μ   max     -   μmin           μ   max     ·   Δ     ⁢           ⁢   T       =   c     ,       
 
wherein c&lt;3·10 −3  K −1 .
 
   Here,
         μ max  is a maximum value of the permeability number in the temperature range,   μ min  is a minimum value of the permeability number in the temperature range and   ΔT is a width of the temperature range.       

   In order to achieve a particularly small temperature dependence of the permeability within the temperature range in which the nominal temperature is under practical conditions, c is preferably selected to be c&lt;9·10 −4  K −1 , preferably, c&lt;3·10 −4  K −1 , more preferred, c&lt;9·10 −5  K −1 , even more preferred, c&lt;3·10 −5  K −1 . Further preferred are even smaller values of c, namely c&lt;9·10 −6  K −1 , in particular, c&lt;3·10 −6  K −1  and, even more preferred, c&lt;1·10 −6  K −1 . 
   A particularly favorable independence of temperature variations is achieved if the nominal or target temperature is adjusted such that the temperature dependence of the permeability number of the material of the magnetic-flux-carrying body has an extremum at this nominal temperature. The extremum can be a maximum or a minimum. 
   According to the invention, the above-described particle-optical apparatus and the method for operating the same are preferably employed in a lithography system or/and a microscopy system. 
   Embodiments of the invention are described hereinbelow with reference to drawings, wherein: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an electron microscopy system according to an embodiment of the invention, 
       FIG. 2  is a perspective partial view of a beam deflector according to an embodiment of the invention which may be used in the microscopy system of  FIG. 1 , 
       FIG. 3  is a plan view on the beam deflector shown in  FIG. 3 , 
       FIG. 4  shows a graph showing the temperature dependence of a ferrite material which may be used in the beam deflector according to  FIGS. 2 and 3 , and 
       FIG. 5  shows a lithography system according to an embodiment of the invention in which the beam deflectors according to  FIGS. 2 and 3  may be used. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  schematically shows a microscopy system  1  for imaging a semiconductor wafer  5  positioned in an object plane  3  of the microscopy system  1  onto a position-sensitive detector  7 . To this end, the microscopy system  1  comprises a microscopy optics  11  which provides a beam path for secondary electrons to electron-optically image a region  13  of the object plane  3  onto the detector  7 . The beam path used for imaging the region  13  which is imaged onto the detector  7  is displaceable parallel to an optical axis  17  of the microscopy system  1  (in  FIG. 1  a displacement is designated by M). 
   The microscopy optics  11  comprises a plurality of components which are symmetrically disposed centrally in respect of the optical axis  17 , namely an objective lens  19 , a field lens  21  and a further magnification optics  23 . Between the objective lens  19  and the field lens  21 , there are provided two beam deflectors  25  and  27  spaced apart from each other along the optical axis  17 . The deflectors  25 ,  27  are controlled by a controller  29 . Each one of the beam deflectors  25 ,  27  provides for the secondary electron beam  14  an adjustable deflection angle β controllable by the controller  29 , the deflection angles provided by the two beam deflectors  25 ,  27  being, however, opposite in sign. Accordingly, the secondary electron beam  14  passes through the two lenses  19 ,  21  straightly, however, adjustably displaced parallel to the optical axis  17 . 
   The secondary electrons extracted from the semiconductor wafer  5  are accelerated by an extraction electrode  18  in a direction parallel to the optical axis  17 . 
   The objective lens  19  provides a focusing field for the secondary electron beam  14 , the optical axis  31  of which is displaceable relative to the optical axis  17  of the other components. The controller  29  controls the objective lens  19  such that the optical axis  31  of the objective lens  19  vertically intersects centrally the region  13  of the object plane  3  which is imaged on the detector  7 . An example of such an objective lens is described in the article “MOL”(“moving objective lens”), Optik 48 (1977), no. 2, pages 255-270, by E. Goto et al., or in U.S. Pat. No. 4,376,249. A further example of such an objective lens is described in U.S. Pat. No. 2003/0066961 A1 of the assignee of the present application. 
   The secondary electrons are extracted from the semiconductor wafer  5  by a primary electron beam  33  which is generated by an electron source  35 , which is collimated by a collimating lens  37  and shaped by an aperture stop  39  and supplied to a beam combiner/beam splitter  41 . The beam combiner  41  superimposes the primary electron beam  33  on the beam path of the secondary electron beam  14 . The primary electron beam  33  passes through the field lens  21 , the deflectors  25 ,  27  and the objective lens  19 . The primary electron beam  33  is also deflected by the deflectors  25 ,  27 , however, not necessarily exactly by the same angle as the secondary electron beam  14 . However, it is sufficient for the primary beam  33  to illuminate the field  13  imaged onto the detector  7  merely fairly homogenously. Accordingly, the demands put on the imaging properties of the optical system  11  are less for the primary electron beam  33  than for the secondary electron beam  14 . 
     FIG. 2  schematically shows the deflector  25  in perspective partial view. It comprises a plurality of rings  43  disposed concentrically in respect of the optical axis  17  and made of a material with a low permeability number, and a plurality of rings  45  which are made of a material with a high permeability number and are disposed between adjacent rings  43  made of the material with the low permeability number. The rings  43 ,  45  are thus alternately disposed on each other as a stack. Current conductors  47  engage around the rings  43 ,  45 , which current conductors extend substantially parallel to the optical axis  17  and radially penetrate the uppermost and lowermost rings  43 . 
     FIG. 3  shows the arrangement of the current conductor windings in circumferential direction around the optical axis  17 . The angles Θ 1  to Θ 7  indicated in  FIG. 3  have the following values: Θ 1 =21.6°, Θ 2 =41.6°, Θ 3 =47.6°, Θ 4 =62.4°, Θ 5 =68.4°, Θ 6 =78.5° and Θ 7 =84.5°. These angles are selected such that the magnetic field generated by the current conductor windings  47  is a substantially homogeneous magnetic field oriented in y-direction. 
   By exciting the current conductor windings  47  with a current adjusted by the controller  29 , it is thus possible to deflect the secondary electron beam in x-direction by adjustable angles β. 
   The rings  43  with the low permeability number can be made of a material called Macor® which is obtainable from Corning, Inc., New York, USA. 
   The rings  45  made of the material with the high permeability number are made of a manganese/zinc/ferrite material which is obtainable from Ceramic Magnetics, Inc., New Jersey, USA under the product name MN-60 for example. 
   The permeability number of this material is dependent upon the temperature.  FIG. 4  shows a graph of this dependency for a sample of this material. It is evident therefrom that the permeability number has a maximum at a temperature of about 20° C. and a minimum at a temperature of about 75° C. At a temperature in a range of from 25° to 40° C. in which operating temperatures of technical apparatuses normally lie the temperature dependence shows a relatively steep slope. Even if one tries to actively stabilize the temperature of the rings  45  in this range by means of a temperature-adjusting unit, inevitable temperature variations nevertheless result in changes in the permeability number of the rings  45 . According to the invention, the temperature of the rings  45  is thus adjusted to a nominal temperature which is in a temperature range in which there are only small temperature-dependent variations in the permeability number and which is preferably at an extremum of the temperature dependency of the permeability number, i.e., either to a temperature of about 20° or a temperature of about 75°. 
   To this end, a temperature-adjusting unit  49  schematically shown in  FIG. 2  is provided. It comprises plural windings  51  of a conduit  53  through which a liquid flows, for example, water. The liquid flows through the conduit  53  in a closed circuit  55  which passes through a heating/cooling unit  57  in which the liquid flowing through the conduit  53  is brought to a temperature which is adjustable by a controller  61 . The windings  51  of the conduit  53  abut with a heat contact  59  against a beam tube  63  which forms part of a vacuum chamber of the particle optics  11 . 
   The beam deflector  25  is disposed within the beam tube  63  in the region in which the windings  51  are wound around the outside of the beam tube  63 . This allows a heat exchange between the beam tube  63  and the deflector  25  to take place by thermal radiation transfer. By adjusting the temperature of the medium flowing through the windings  51  by means of the heating/cooling unit  57 , it is thus possible to adjust the temperature of the ferrite rings  45  of the beam deflector  25  in a range about the nominal temperature. This adjustment is effected through a feed-back control which comprises a sensor  65  fixed to the stack of rings  43 ,  45  and read out by the controller  61 . Accordingly, the controller  61  can compare an actual temperature of the rings  43 ,  45  with the nominal temperature thereof and accordingly influence the temperature of the medium in the windings  51  via the cooling/heating unit  57 . 
   In the temperature-adjusting unit shown in  FIG. 2 , the ferrite rings  45  are adjusted to a nominal temperature of 20° C., since at this temperature the material used for the rings  45  shows a maximum of the permeability number (see FIG.  4 ). As an alternative thereto, it is possible to select as the nominal temperature a value about 75° C. at which the permeability number of the material used for the rings  45  has a minimum. It is evident from  FIG. 4  that at this temperature the extremum (minimum) has a considerably flatter shape than the extremum (maximum) at the temperature of 20° C. The permeability number can thus be held more accurately around the extremum at the nominal temperature of 75° C. than at the nominal temperature of 20° C. On the other hand, at the minimum at the higher temperature of 75° C., the value of the permeability number is considerably lower than at the maximum at 20° C., so that the magnetic effect of the ferrite rings  45  is correspondingly reduced at this temperature. 
   The beam deflector  25  further comprises sector electrodes  67  which are disposed radially within the rings  43 ,  45 . They provide an additional electric deflecting field for the beam traversing the deflector  25 , which deflecting field is superimposed with the magnetic field provided by the beam conductors  47 . Accordingly, the deflectors  25 ,  27  can be operated as a Wien filter, and deflection angles β provided by the deflectors  25 ,  27  can thus be adjusted with additional degrees of freedom. In particular, the magnetic fields and the electric fields provided for the deflection can be adjusted such that substantially the same deflection angles β result for both the primary electrons and the secondary electrons. 
   The beam conductors  47  are energized such that the magnetic flux in the ferrite rings  45  is well below a saturation value, such that changes in the magnetic field have a substantially linear dependency from variations of an energizing current. For example, a maximum flux induced in the ferrite rings  45  may be below 25% of a saturation flux therein, or in particular below 15% or even below 10%. Further, the energizing current may be an AC current such that an orientation of the magnetic flux carried by the ferrite rings changes from time to time or periodically. 
   A variant of the embodiment described with reference to  FIGS. 1  to  4  will now be described. Components which correspond in function or structure to components of  FIGS. 1  to  4  are designated by the same reference numbers, however, supplemented by an additional letter for the purpose of distinction. Reference is made to the entire above description. 
     FIG. 5  schematically shows a lithography system  71  used for transferring a pattern stored in a memory  73  of a controller  29   a  onto a particle-sensitive layer or resist with which a surface  3   a  of a semiconductor wafer  5   a  is coated in a lithography process for manufacturing miniaturized semiconductor structures. 
   The transfer of the pattern is effected by means of a writing electron beam  33   a . It is generated by an electron source  35   a  which comprises a cathode plate  75  which has a tip  77  embossed therein. The tip  77  is disposed opposite to a bore of an aperture stop  39   a  which is biased in respect of the cathode plate  75  as anode. Furthermore, the tip  77  is disposed on an optical axis  17   a  of the lithography system  71  and is illuminated from above by a laser beam  78  generated by a semiconductor laser  79  and collimated by a collimating lens  81  into the tip  77 . By controlling the laser  79  via the controller  29   a , it is possible to rapidly switch the laser beam  78  on and off. The laser beam  78  supports a photon-assisted field emission in the region of the tip  77 , as a result of which electrons are extracted from the tip  77  and accelerated through the aperture stop  39   a  to form the writing electron beam  33   a  which, after having passed through the aperture stop  39   a , passes through a collimating lens  37   a.    
   The collimating lens  37   a  further forms the writing electron beam  33   a  to a substantially parallel beam travelling along the optical axis  17   a . This beam successively passes through two deflectors  25   a  and  27   a  concentrically disposed in respect of the optical axis  17   a  and then enters an objective  19   a  which finely focuses the same on the semiconductor surface  3   a  or object plane of the lithography system  71 . The structure of the deflectors  25   a  and  27   a  is similar to that of the beam deflectors described with reference to  FIGS. 2 and 3 . Furthermore, a coil  83  likewise controlled by the controller  29   a  is disposed within the objective lens  19   a , which itself is rotationally symmetric to the optical axis  17   a . The coil superimposes a dipole field with the focusing field of the objective lens  19   a  in order to displace the optical axis of a focussing effect of the objective  19   a  away from the optical axis  17   a  such that it coincides with the center of the writing electron beam  33   a  displaced from the optical axis by the deflectors  25   a ,  27   a . The deflectors  25   a ,  27   a , and the coil  83  within the lithography system  71  can also be configured as described with reference to  FIGS. 2 and 3 , wherein electrodes (reference number  67  in  FIG. 2 ) are not necessarily disposed within the magnetic deflectors and the coil, respectively. 
   In order for the pattern stored in the memory  73  to be transferred to the surface  3   a  of the wafer  5   a , the controller  29   a  thus controls the deflectors  25   a ,  27   a  and the coil  83  as well as the laser  79  such that the writing electron beam  33   a  is moved across the surface  3   a  as desired and is switched on and off as required. 
   The electron microscopy system which has been described with reference to  FIG. 1  images an extended region of the object surface onto a position-sensitive detector, for example, a CCD chip. Such an electron microscopy system is usually referred to as a LEEM (low energy electron microscope) or SEEM (secondary electron emission microscope). However, it is also possible to use the concept on which the invention is based, namely to stabilize a magnetic-flux-carrying material to such a temperature at which the temperature dependence of the permeability number thereof is small or has an extremum, to other types of electron microscopes. An example of this is a SEM (scanning electron microscope). 
   The lithography system described with reference to  FIG. 5  is a “mask-less” lithography system wherein the writing beam is switched on and off via a beam source. However, it is also possible to realize the concepts of the invention in a lithography system wherein a mask or reticle is used for the definition of the pattern to be transferred. 
   In the above-described embodiments, the magnetic-flux-carrying body which is stabilized to a correspondingly selected nominal temperature is disposed in a particle-optical apparatus which serves as beam deflector. However, it is also possible to adjust the magnetic-flux-carrying body to a selected temperature in other particle-optical apparatuses. Examples of this are particle-optical apparatuses which act as focusing lenses or correction members, such as a hexapole field generating members or the like. 
   On the other hand, if a operating temperature of a particle-optical apparatus is predetermined, a ferrite material can be suitably selected. The temperature dependence of a ferrite material is dependent upon a composition thereof. Therefore, it is preferred to use or design a ferrite material which exhibits only slight permeability variations in a temperature range about the operating temperature. 
   The electron microscopy system described with reference to FIG.  1  and the lithography system illustrated with reference to  FIG. 5  each operate with one primary electron beam and writing beam, respectively. However, it is also possible to use plural primary beams and writing beams, respectively, in parallel with to each other in such apparatuses. 
   The present invention has been described by way of exemplary embodiments to which it is not limited. Variations and modification will occur to these skilled in the art which do not depart from the scope of the present invention as recited in the claims appended hereto.