Patent Publication Number: US-8536773-B2

Title: Electron beam source and method of manufacturing the same

Description:
FIELD 
     The disclosure relates to an electron beam source and to a method of manufacturing and using an electron beam source. The electron beam source can be capable of providing an electron beam having a high brightness and/or a small energy spread. 
     BACKGROUND 
     Conventional electron beam sources include an electrode from which an electron beam may be extracted by exposing the electrode to an electrical potential, a thermal excitation, a photonic excitation or a combination of an electrical potential, thermal excitation and photonic excitation. 
     SUMMARY 
     In some embodiments, the disclosure provides an electron beam source including an electrode tip having a comparatively small radius. 
     In certain embodiments, the disclosure provides an electron beam source which can be easily manufactured or handled. 
     In some embodiments, the disclosure provides an electron beam source which has high durability. 
     In some embodiments, an electron beam source is provided having a tip which includes a core. The core is at least partially provided with a conductive coating. 
     In certain embodiments, the surface of the core is formed of a material having a lower electrical conductance than the material of the coating. Since the tip of the electron beam source may be very small, it may be difficult to measure the conductivities of the materials from which the tip is formed at the tip itself. In the context of the present disclosure, values of conductivities or values of a modulus of elasticity of materials which are employed for forming the tip therefore relate to values of resistance and modulus of elasticity, respectively, which may be measured in the bulk of the respective material. 
     In some embodiments, a further material having a comparatively lower electrical conductivity is provided below the coating of the comparatively conductive material which at least partially provides the surface of the tip. It is possible that the coating of the material having the greater conductivity is directly applied onto a surface formed by the further material having the comparatively lower conductivity and that both materials directly contact each other. However, it is also possible that a layer from still a further material is arranged between the two above-noted materials, for example to improve an adherence of the coating made from the material having the greater conductivity at the material having the lower conductivity. 
     In certain embodiments, the core whose surface is provided by the material having the lower conductivity and on which the coating made of the material having the higher conductivity is applied consists of an inner core and a layer of the material having the lower conductivity applied thereon. Thereby, the inner core is formed from a third material which may be different from the material having the lower conductivity and which in particular may have a higher electrical conductivity than the material having the lower conductivity. 
     In some embodiments, the third material of which the inner core is made has a modulus of elasticity of greater than 150 kN/mm 2  (e.g., greater than 300 kN/mm 2 , greater than 600 kN/mm 2 , greater than 700 kN/mm 2 ). 
     The elastic modulus of an object is defined as the slope of its stress-strain curve in the elastic deformation region, i.e. the ratio between the force causing a deformation divided by an area to which the force is applied and the ratio of a change caused by the stress to an original state of the object. The elastic modulus of a material may be measured by applying a pressure along an axis and determining a relative deformation of the material along that axis. 
     In particular the modulus of elasticity of the third material may be greater than that of plastic material. In particular the hardness of the third material is greater than that of plastic material. Thus, the tip is rigid to withstand forces acting on the tip when an electric field is generated. Therefore the shape of the tip and the shape of the entire electron source is maintained and thus the distribution of the electric field generated is substantially maintained to favourably emit electrons. 
     In certain embodiments, the third material of which the inner core is made has a modulus of elasticity of less than 1100 kN/mm 2  (e.g., less than 1000 kN/mm 2 , less than 800 kN/mm 2 ). The third material may be diamond like carbon which has a modulus of elasticity of about 770 kN/mm 2 . 
     In some embodiments, a center of the core is formed of the first material which may have a modulus of elasticity of greater than 150 kN/mm 2  (e.g., greater than 300 kN/mm 2 , greater than 600 kN/mm 2 , greater than 700 kN/mm 2 ). The first material may have a modulus of elasticity of less than 1100 kN/mm 2  (e.g., less than 1000 kN/mm 2 , less than 800 kN/mm 2 ). 
     In certain embodiments, the layer from the first material on the inner core has a thickness of more than a monolayer from the layer material. Exemplary thicknesses are in ranges from 0.1 nm to 100 nm, such as from 1 nm to 50 nm, or from 1 nm to 30 nm. 
     The coating from the material having the greater conductivity applied to the core may have a thickness of 2 nm to 50 nm (e.g., from 2 nm to 30 nm, from 20 nm to 50 nm). In some embodiments, the tip is fixed at the base and its end extending away from the base has a surface having a small radius of curvature. In some embodiments, the radius of curvature is less than 100 nm (e.g. less than 80 nm, less than 60 nm). In certain embodiments, the radius of curvature is greater than 5 nm, such as greater than 10 nm. 
     In some embodiments, a maximum dimension of the tip in its longitudinal direction may be greater than 50 nm (e.g., greater than 100 nm, or greater than 300 nm). In certain embodiments, a maximum dimension of the tip in its transversal direction may be less than 500 nm (e.g., less than 200 nm, less than 100 nm, or less than 50 nm). 
     In certain embodiments, a ratio between the extension of the tip in the longitudinal direction and the dimension of the tip in the transversal direction (aspect ratio) may be greater than 2:1 (e.g., greater than 5:1, greater than 10:1, or still greater). Further, the tip may taper towards its end. 
     In some embodiments, an electrical conductivity of a material at least partially providing the surface of the tip may be greater than 10 −7  S/m (e.g., greater than 10 −5  S/m, greater than 10 −3  S/m, greater than 10 −1  S/m, greater than 10 2  S/m, greater than 10 4  S/m, or greater than 10 6  S/m). 
     In certain embodiments, a material providing a surface of a core of the tip onto which core a coating from a material having a comparatively greater electrical conductivity is applied may have an electrical conductivity which is less than 10 5  S/m (e.g., less than 10 3  S/m, less than 10 S/m, less than 10 −1  S/m, less than 10 −3  S/m, less than 10 −5  S/m or less than 10 −7  S/m). 
     In some embodiments, a ratio between an electrical conductivity of a coating material applied onto a core of a tip of an electron beam source and an electrical conductivity of a material onto which the coating is applied may be greater than 10:1 (e.g., greater than 100:1, greater than 10 4 :1, greater than 10 6 :1, or greater than 10 8 :1). 
     In certain embodiments, a material onto which a coating of a tip of an electron beam source is applied is formed from diamond like carbon material. Diamond like carbon material is amorphous carbon material having some of the properties of natural diamond. In particular, diamond like carbon includes substantial amounts of sp 3 -hybridized carbon atoms. Today seven forms of diamond like carbon are known. Of these seven forms, there are three hydrogen free forms which are denoted as a-C, pa-C, and a-C:Me, where the latter includes a metal (Me) as dopant. Of the seven forms, there are four hydrogenated, hydrogen including forms, which are denoted as a-C:H, ta-C:H, a-C:H:Me, and a-C:H:X, wherein the two latter include a metal (Me) and other elements (X), respectively, as dopant. The metals Me may include tungsten, titanium, gold, molybdenum, iron and chromium, among others, and the other elements X may include silicon, oxygen, nitrogen, fluorine, and boron among others. 
     There are different methods known for manufacturing diamond like carbon. These methods have in common, that the material is deposited onto a substrate, where pressure, energy, catalysis, or a combination of the same is used to deposit atoms so that these atoms form a sp 3 -bond with already deposited carbon atoms to a significant percentage. 
     In some embodiments, a diamond like carbon material may be doped with different elements, such as Au, Mo, Fe, Cr and others. 
     In certain embodiments, a material which is applied as a coating onto a core of a tip of an electron source and which forms at least a portion of its surface is a metal. The metal also may have an oxide layer that decreases the work of emission for electrons in some situations. Examples of metals include Ti, Pt, Al, W, WTi, V, Hf, Zr and others as well as oxides of these, such as TiO X , VO X , HfO X , ZrO X  and others. 
     In some embodiments, the material which is applied as a coating onto a core of a tip is a semiconductor material. Examples of semiconductor materials include Ge and/or GaAs. 
     In certain embodiments, an electron beam source includes a tip fixed at a base and extending away from the base. The tip includes a core having a surface formed of a first material. The tip also includes a coating formed of a second material. The second material is applied to the core. The second material at least partially forms a surface of the tip. A first electrical terminal is electrically connected to the coating. An extraction electrode has an opening arranged opposite to the tip. A second electrical terminal is electrically connected with the extraction electrode. 
     In some embodiments, a surface of the base includes a convex surface portion extending around an end of the tip close to the base. The convex surface contributes to favourably shaping an electric field for emitting electrons from the tip. 
     In certain embodiments, a radius of curvature associated with the convex surface portion of the base is between 10 μm and 0.1 μm (e.g., between 3 μm and 0.3 μm, between 0.6 μm and 0.4 μm). 
     In some embodiments, the base has a surface portion having a surface shape of axial symmetry relative to an axis of symmetry. An angle between the axis of symmetry and a longitudinal direction of the tip is less than 10°. This further ensures that the electric field around the tip which generated when suitable electric potentials are applied to the base and to the extraction electrode is favourably shaped for emitting electrons from the electron source. 
     In certain embodiments, a tip including a coating on a core may be employed as tip of an electron beam source. The electron beam source is integrated in an electron beam system that includes among the source one or more beam shaping components, such as an aperture, an electron optical lens, a beam deflector or the like. The tip may be used to generate an image of an object using electrons. For example, the tip may be used to generate of one or more primary beams of an electron microscopic system. 
     In some embodiments, the tip may be used to generate one or more writing beams to expose a beam sensitive layer with a predetermined pattern. For example, the tip may be used in an electron lithography system. 
     In certain embodiments, the second material of the coating which forms a surface of the tip includes more than 30% by weight of at least one lanthanide element, and the source further includes a magnet configured to generate a magnetic field at the tip. With such configuration, it is possible to generate a beam of polarized electrons. In general, in a beam of polarized electrons, a number of electrons having spins oriented in one direction is higher than a number of electrons having spins oriented in a direction opposite to the one direction. According to exemplary embodiments herein, the second material includes more than 30% by weight of Gadolinium. Optionally, the source includes a cooler, such as a Peltier element, configured to cool the tip below a Curie temperature of the second material. 
     According to some embodiments, a source including a magnet is included in a system which also includes a lens configured to focus a beam of electrons generated by the source in a sample plan, and at least one pair of electron detectors symmetrical arranged relative to an axis of symmetry of the lens. 
     In certain embodiments, a method for manufacturing an electron beam source is provided. The method includes growing a core from a first material onto a base using deposition, and applying a coating from a second material onto the core. In some embodiments, the deposition may include electron beam induced deposition, ion beam induced deposition, growing the base from amorphous carbon, from a local catalyst, which was for example applied using a lithography method or ion deposition, and other methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure are subsequently explained in more detail referring to drawings, in which: 
         FIG. 1  is a schematic illustration of an electron beam system; 
         FIG. 2A  is a schematic sectional illustration of an electron beam source used in the electron beam system of  FIG. 1 ; 
         FIG. 2B  schematically illustrates geometric properties of the electron beam source shown in  FIG. 2A  and employed in the electron beam system of  FIG. 1 ; 
         FIG. 3  is a photograph of a tip of the electron beam source schematically illustrated in  FIG. 2A ; 
         FIG. 4  is a schematic sectional illustration of an electron beam source; 
         FIG. 5  is a flow diagram for a method of manufacturing an electron beam source; 
         FIG. 6  is a schematic illustration of an electron beam system; 
         FIG. 7  is an elevational view of a detector used in the system shown in  FIG. 6 ; and 
         FIG. 8  is a flow diagram of a method of inspecting a sample. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the disclosure are explained below in the context with the Figures. Thereby, components corresponding to each other with respect to their structure and function are denoted by the same numeral but are denoted for distinction with an additional letter. For explaining the components it is therefore also referred to the entire respective preceding or succeeding description. 
       FIG. 1  is a schematic illustration of an electron beam system. The electron beam system  3  includes an electron beam source  5  having a cathode  7  which is described in more detail below, and having two successively arranged anode apertures  6  and  8  each providing an opening  9 . The two electrodes  6  and  8  are arranged close to the cathode such that the openings  9  are arranged opposite to a tip of the cathode and such that the openings  9  are traversed by an electron beam  13 , when the source  5  is operated. The electron beam  13  generated by the source  5  traverses a condenser lens  11  and an opening  15  in a secondary electron detector  17  and is directed onto a location  21  at a surface  23  of an object  25  by further electron optics  19 . In the schematic illustration of  FIG. 1  electron optics  19  include a magnetic objective lens  27  and may further include magnetic or electrostatic lenses and electrodes which are not illustrated in  FIG. 1 . Optics  19  further include deflectors  29  to deflect the electron beam  13  from its straight propagation so that the electron beam may be directed to selectable locations  21  within an extended region  31  at the surface  23  of the object  25 . 
     The electrons of the electron beam  13  release secondary electrons at the location  21  at which they impinge onto the object. The secondary electrons are accelerated away from the surface  23  of the object  25  by an electrode  22  at which an appropriate voltage is applied and the secondary electrons enter into the objective lens  27 . Exemplary trajectories of such secondary electrons are denoted by reference sign  37  in  FIG. 1 . A portion of the secondary electrons impinges at the detector  17  and is detected there. 
     A controller  33  is configured to supply, via lines  35 ,  36 , a current through the cathode  7  of the electron source  5 , to heat the same if desired. In general the cathode may be operated without heating. The lines  35 ,  36  further serve to apply a predetermined electrical potential, for example referred to ground potential, at the tip of the cathode  7 . Lines  37  and  38  serve to apply a respective predetermined electrical potential with respect to the tip of the cathode  7  to the two electrodes  6  and  8 , so that an emission of electrons from the tip is assisted and so that a distribution of the electrical field in the region of the tip favourably causes forming the beam  13  from the electrons emitted from the tip. Via a line  39  a predetermined electrical potential with respect to the anode  7  is applied by the controller  33  to a holder  24  for the object  23  and thus to the object  23  itself so that the electrons of the beam  13  impinge at the object  25  with a predetermined kinetic energy. The secondary electrons thereby released and impinging onto the detector  17  generate signals in the detector which are read out from the controller  33  via line  41 . Via control line  42  the controller  33  controls the deflector  29  to shift the location  21  at which the electron beam  13  impinges onto the object  25 . By recording the signals supplied by the detector  17  depending on the deflection of the electron beam  13  it is possible to gain electron microscopic image information from the object  25 . 
     As far as described above the electron beam system  3  has the function of an electron microscope of type SEM (scanning electron microscope). However, some embodiments also include other types of electron beam systems, such as for example electron microscopy systems of type LEEM (low electron emission microscope) and of type TEM (transmission electron microscope). Certain embodiments include lithography systems in which one or more electron beams are employed to expose a beam sensitive layer (resist) with a predetermined pattern. Hereby, an electron beam may again systematically be scanned in the way described in the context of  FIG. 1  across the beam sensitive layer and may thereby be controlled in its intensity, to expose selective regions of the beam sensitive layer with the energy of the impinging electrons. The intensity of the beam impinging onto the layer may thereby be varied by controlling the source or a separate beam blanker may be provided to switch the beam on and off. The beam sensitive layer exposed in this way may subsequently be developed and may be subjected to further lithographical steps, such as for example ion implantation, sputtering or material deposition, to manufacture miniaturized components. 
       FIG. 2A  is an illustration of a detail of the electron beam source  5  in a cross section. The electron beam source  5  of the embodiment shown here includes a carrier plate  51  which may for example be formed from metal through which also a current may flow to heat the cathode. The carrier plate  51  has a terminal for a control line to apply a predetermined electrical potential relative to other components in an electron beam system to the cathode. 
     A base  53  is fixed at the plate  51  in which base a cylindrical recess  55  is provided. A material for the base  53  may for example be silicon, and the recess  55  may for example be realized by etching into the body of the base  53 . A core  57  of a tip  60  of the cathode  7  is fixed in the recess  55 . The core may be formed from an electrically substantially non-conductive material. For example its electrical conductivity may be less than 10 −6  S/m. In the embodiment illustrated here the core is made from diamond like carbon. One possibility to manufacture the core  57  made of diamond like carbon is the application of an electron beam induced deposition. In this method the base is placed into a vacuum chamber in which a predefined atmosphere of process gas prevails for which for example carbon compounds may be employed. In this method further an electron beam is directed to the bottom of the recess  55  to start the deposition of diamond like carbon at this location. By selectively directing the electron beam onto those locations at which at a given moment during the process carbon material should be deposited the process gas is exited to deposit carbon atoms at the surface such that these at least partially form an sp 3 -bond with carbon atoms already deposited at this location. Hereby it is also possible, to choose the process gas such that the depositing material contains (besides carbon) dopants, such as for example hydrogen. 
     In the example described here the deposition was controlled such that the core  57  has a dimension  1  in a direction extending away from the base  53  of about 560 nm. Further, the deposition was designed such that the core has a substantially round cross section, namely with a maximal dimension b orthogonal to the longitudinal direction of about 150 nm close to the base, wherein the cross section of the core  57  continuously tapers towards its end distant from the base  53 . At the end distant from the base  53  the core has a radius of curvature r 1  at the surface of about 13 nm. 
     Using the described method the tip may be grown on the base with a high angular velocity. A longitudinal axis of the tip may for example be oriented such that it includes an angle of less than 0.5° with a symmetry axis of the base and may be spaced apart from the symmetry axis of the base less than 0.1 mm. 
     A surface of the core  57  is coated with a conductive coating  59  from Ti and TiO X . This coating has a coating thickness c of 10 nm and was for example applied onto the core  57  by sputter coating or electron beam evaporation. This coating also extends across the surface of the base  53  up to the plate  51  so that a comparatively good electric connection is established between the plate  51  and the end of the tip  7  distant from the base  53 . In the state schematically illustrated in  FIG. 2A  also the end of the core  57  pointing away from the base  53  is covered with the coating  59  so that an outer surface of the cathode  7  has a radius r 2  of curvature of about 23 nm at this end. 
       FIG. 2B  illustrates further geometrical properties of the electron beam source  5  of  FIG. 2A . The base  53  has an exposed surface  53 ′, wherein the exposed surface  53 ′ is provided by the conductive coating  59 . Other surface regions of the base  53  may be exposed during manufacturing the electron source, such as for example the recess  55  at which the tip  60  is fixed. However, the recess  55  is not exposed in the completed electron source. A region  52  defines a portion of the exposed surface  53 ′ of the base  53 . This surface portion may amount to at least 50%, such as at least 80%, of an entire exposed surface  53 ′ of the base. The thus defined surface portion extends around an end of the tip  60  close to the base  53 . This surface portion has a convex shape. However, not the entire exposed surface  53 ′ of the base need to be convex. 
     The shape of the convex surface portion of the surface  53 ′ of the base  53  may be approximated by a spherical surface by fitting a sphere having a center C and having an appropriate radius R at the convex surface portion, as illustrated in  FIG. 2B . In the illustrated example the best fitting sphere has a radius R=0.5 μm which corresponds to a radius of curvature of the surface portion of the exposed surface  53 ′. In other embodiments the radius of curvature R may be between 0.1 μm and 10 μm. 
     In the illustrated example the shape of the surface portion of the base  53  around the tip  60  is axially symmetric having a symmetry axis  54 . The symmetry axis coincides with a longitudinal axis  61  of the tip  60 . In other embodiments the base may be rotationally symmetric having a symmetry axis. In other embodiments the longitudinal axis  61  of the tip  60  and the symmetry axis  54  of the base  53  include an angle of less than 10°. 
       FIG. 3  shows a photograph of the cathode  7  described above with reference to  FIGS. 2A and 2B . 
     A cathode of the constitution shown in  FIG. 3  is capable of generating an electron beam of comparatively large intensity over a relatively long time. In a performed experiment a beam current of 3 μA was achieved and could be maintained over a time span of 1500 hours. Thereby, the beam current stayed comparatively constant, wherein maximum current oscillations of 10% occurred, while current oscillation were less than 3% over a time span of some minutes. 
     An explanation of the capability of the construction shown in  FIG. 2  as electron beam source is still not entirely possible by the inventors. According to speculations an explanation may be seen in that the manufactured construction has a relatively large ratio between length and width and a small radius at the tip so that a favourable amplification of the electrical field results there. Another reason may be that the material of the core has a large durability and resists bombardment by ions of a residual gas in the vacuum chamber present around the tip during a long time, wherein the bombardment occurs during the operations. Even if the coating  59  at the end of the tip is degraded due to such a bombardment a good emission of electrons may nevertheless occur, since the tip conducts the electrons across the coating  59  extending cylindrically around the core  57  up to the end of the tip. According to further considerations the electrons emitting from the tip move within the conductive coating having the form of a face of a circular ring in which states of the electrons are quantized. The energy levels for electrons in the coating resulting thereby are occupied such that the work of emission from the coating is relatively low for electrons residing in one of the higher occupied energy levels or such that a tunnelling probability for electrons out of the tip is relatively high. 
       FIG. 4  shows a further embodiment of a tip  60   a  for an electron beam source  7   a . The tip  60   a  shown in  FIG. 4  has a similar construction and a similar function as the tip explained referring to  FIGS. 2A and 2B . In contrast thereto, however, the core  57   a  is however not integrally formed from a single material piece. Rather, the core  57   a  has an inner core  65  having a longitudinally extending shape and having a rounded surface at its end distant from a base  53   a . The inner core  65  is coated with a layer  63  from a non-conductive material. The layer  63  is further coated with a coating  59   a  from conductive material so that the coating  59   a  extends up to the end of the core  57   a  distant from the base  53   a  and further extends up to a plate  51   a  at which the base  53   a  is held to provide a conductive connection between the plate  51   a  and the end of the core  57   a  distant from the base  53   a.    
     Similar to  FIGS. 2A and 2B , the tip  60   a  thus includes a conductive coating  59   a  at least partially providing its surface, wherein the coating  59   a  is applied in a region of the tip onto a non-conductive material  63 . Therefore, it is possible that the inner core  65  is formed from a material which itself is conductive or non-conductive. 
     Exemplary materials for the inner core are diamond like carbon, exemplary materials for its layer  57   a  are SiO 2  and exemplary materials for the outer coating  59   a  are Ti and TiO X . 
     A method for manufacturing an electron beam source is explained below referring to  FIG. 5 . The method starts with a step  101  in which a base is provided at which an appropriate tip is fixed in the subsequent steps. Therefore, in a step  103 , a core of the tip is grown on the base. In  FIGS. 2A ,  2 B and  4 , the base exhibits a recess in which the growing core is fixed. However, it is also possible to grow the core directly on a smooth surface of the base, i.e. not in a recess of the same, such that the core is sufficiently fixed at the base. After growing the core of the tip such that it exhibits a desired length, a desired cross section and a desired tapering towards its end distant from the base, if desired, the core is coated with a conductive material in a step  105 . With step  105  a method for manufacturing a tip for an electron beam source according to an embodiment of the disclosure is completed. 
     The method for manufacturing the electron source according to the embodiment of the disclosure explained here includes a subsequent further step  107  in which the tip with the base is assembled to an electron source. Thereby, one or more apertures are arranged relative to the base such that the aperture is located opposite to the tip so that, upon applying an electrical voltage between the aperture and the base, a distribution of the electrical field between the aperture and the tip results such that electrons can be extracted from the tip and formed into a beam. With the assembly into the electron beam source in the step  107  an embodiment of the method for manufacturing an electron beam source is completed. 
     In contrast, manufacturing an electron beam system still includes a further step  109  in which the electron beam source is installed into such an electron beam system. The electron beam system may for example be an electron microscopy system or an electron lithography system. 
     The core may be deposited onto the base by electron beam induced deposition. However, it is also possible to employ other methods for deposition onto the base, such as for example catalytically growing amorphous carbon. Further, it is possible to also manufacture the core with a tip of a small tip radius in that an initially larger blank is reduced in its dimension by directed ablation. Examples of such methods are milling processes using gallium ions or argon ions. 
     It is further possible to use a tip such as one employed for a scanning tunnelling microscope or an atom force microscope as core for a tip of an electron beam source according to an embodiment of the disclosure. An example is an AFM probe which may be purchased from the company nano-tools GmbH, 80469 Munich, Germany. Then the step  105  of coating with a conductive material is still to be applied to the thus obtained core to obtain a tip for an electron beam source according to an embodiment of the disclosure. 
     In some embodiments, a tip of an electron beam source includes a core carrying a coating. The coating is formed from a material having a greater electrical conductivity than a material forming the surface of the core. 
       FIG. 6  is a schematic illustration of an electron beam system  3 , which has a configuration similar to the electron beam system illustrated with reference to  FIGS. 1 to 5  above. The electron beam system  3  shown in  FIG. 6  is configured to generate and use a polarized electron beam. For this purpose, an electron beam source  5  includes a cathode tip  7  having a core and a conductive coating as illustrated above with reference to  FIGS. 2 to 5 . The source further includes a magnet  67  configured to generate a magnetic field at the tip  7  of the electron beam source  5 . In the illustrated example, the magnet  67  is an electromagnet provided by a coil of insulated wires which is connected to a controller (not shown in  FIG. 6 ) of the system  3  via electrical connectors  68 . The tip  7  of the source  5  is located within a bore of the magnet  67 . The magnet may be integrated with pole pieces (not shown in  FIG. 6 ) focusing a magnetic field generated by the magnet at the tip  7 . For example, such pole pieces may provide a gap, similar to gaps provided by pole pieces of the condenser lens  11  and the objective lens  27  shown in  FIG. 6 . The magnet  67  can be energized by the controller such that a magnetic field strength provided at the tip  7  is greater than 0.5 Tesla (e.g., greater than 1 Tesla, greater than 1.5 Tesla). By reversing the current supplied to the coil, it is possible to change an orientation of the magnetic field applied to the tip. 
     As illustrated above, the tip  7  includes a core having a surface including a first material and a coating applied to the core, where the coating includes a second material having a higher electrical conductivity than the first material. In the illustrated example, the second material providing the coating of the core includes more than 30% by weight (e.g., more than 50% by weight, more than 75% by weight, more than 90% by weight, more than 95% by weight) of at least one lanthanide element. For example, the second material may include more than 30% by weight of Gadolinium. For example, the coating can be provided by pure Gadolinium. In some embodiments, the second material includes more than one lanthanide element. 
     The electron beam system  3  further includes a cooler, such as one or more Peltier elements  69  or a reservoir of liquid nitrogen, to cool the tip  7  such that its temperature is below a Curie temperature of the second material providing the coating of the core. For this purpose, the Peltier elements or the liquid nitrogen reservoir are thermally connected by a suitable heat transfer path (not shown in  FIG. 6 ), such as a metal rod or cord, to the tip  7 . The Peltier elements  69  shown in  FIG. 6  are driven by the controller of the system  3  via terminals  70 , such that the controller can selectively adjust the temperature of the tip to be above the Curie temperature or below the Curie temperature of the second material providing the coating of the tip  7 . 
     In order to generate a polarized beam of electrons, the tip  7  is heated such that its temperature is above the Curie temperature of the second material. Such heating can be achieved by reducing power supplied to the Peltier elements  69  and/or by supplying a heating current to the tip via terminals  35  and  36 . When the temperate of the tip  7  is above the Curie temperature of the coating, the magnet  67  is energized to generate a magnetic field at the tip and to orient the lanthanide element(s) included in the second material. Thereafter, the temperature of the tip is reduced to below the Curie temperature of the second material while the magnetic field is maintained. The cooling of the tip can be achieved by increasing the power supplied to the Peltier elements  69  and/or by reducing a heating current supplied to the tip via terminals  35 ,  36 . 
     When the tip is oriented in the magnetic field below the Curie temperature, the tip can emit a polarized electron beam  13  by applying suitable extraction fields to electrodes  6  and  8 . 
     The lanthanide element(s) included in the tip  7  remain oriented and can emit the polarized electron beam  13  even if the magnetic field is reduced or switched off after polarization of the tip, as long as the temperature of the tip is continuously maintained below the Curie temperature of the second material. 
     The polarized electron beam  13  can be advantageously used to inspect samples  23  having magnetic properties and to obtain information relating to the magnetic properties of the sample  23 . Such information is obtained by detecting secondary electrons and backscattered electrons released from the sample due to the incident polarized electron beam  13 . The detection can be performed by a single detector configured to determine a polarization of the detected electrons. The detection can also be performed by a pair of detectors arranged at opposite sides of the sample, wherein the detectors of the pair differ with respect to a polarization of the electrons they can detect. Still further, the information relating to the magnetic properties of the sample  23  can be obtained by directing the polarized electron beam to the sample and detecting backscattered electron and secondary electron intensities, reversing the polarization of the beam, directing the beam of reversed polarization onto the sample and detecting electron intensities generated from the beam of reversed polarization. By comparing the electron intensities detected with the two different polarizations, it is possible to determine information relating to the magnetic properties of the sample. 
       FIG. 6  shows a pair of detectors  72  and  73  mounted on a vacuum vessel  75  of the electron beam system  3  such that electron receiving surfaces  76  of the detectors  72 ,  73  are close to a region  31  of the sample  23  where the polarized electron beam  13  is incident on the sample. Lines  37 ′ in  FIG. 6  show exemplary trajectories of secondary electrons emanating from the sample  23  and incident on receiving surfaces  76  of the detectors  72  and  73 . When the polarized electron beam  13  is incident on a magnetic sample  23 , intensities of scattered electrons will show a directional dependency depending on a direction of orientation of the magnetic field within the sample (see e.g. Daniel T. Pierce, “Spin-Polarized Electron Microscopy”, Physica Scripta, Vol. 38, 1988, pages 291 to 296). The detectors  72  and  73  are configured such that they can detect electrons scattered in different directions at the sample  23 . 
       FIG. 6  illustrates a further alternative detection method for obtaining information relating to magnetic properties of the sample. This method uses a detector  17  having an aperture  15  provided upstream of an objective lens  27  of the system  3  in the beam path of the primary electron beam  13  such that the primary beam  13  traverses the aperture  15  provided in the detector  17 . As illustrated above with reference to  FIG. 1 , secondary electrons and backscattered electrons released from the sample  23  can traverse the objective lens  27  to be incident on the detector  17 . Reference numerals  37   b  and  37   d  in  FIG. 6  illustrate exemplary trajectories of secondary and backscattered electrons incident on the detector  17 . As it is apparent from  FIG. 6 , the electron travelling along trajectory  37   b  is emitted from the surface to an upward right direction and is incident on a left portion  17   b  of the detector, while the electron travelling along trajectory  37   d  is emitted to an upward left direction from the sample and is incident on a right portion  17   d  of the detector. The intensities of electrons emitted to the upward left direction and the upward right direction are different in a situation where the polarized primary electron beam  13  is incident on a magnetic sample. Therefore, a difference in electron intensities detected by detector portions  17   b  and  17   d  is indicative of magnetic properties of the sample. 
       FIG. 7  is a more detailed elevational view of detector  17  of the electron beam system  3  shown in  FIG. 6 . The detector  17  includes four sectors  17   a ,  17   b ,  17   c  and  17   d  of scintillator material which generates photons upon incidence of secondary or backscattered electrons. Photo multipliers  79  are attached to the sectors  17   a ,  17   b ,  17   c ,  17   d  of scintillator materials to produce amplified electrical signals at terminals  41   a ,  41   b ,  41   c  and  41   d  from photons generated by the sectors  17   a ,  17   b ,  17   c  and  17   d , respectively. The controller of the electron beam system  3  is connected to the terminals  41   a ,  41   b ,  41   c  and  41   d  in order to receive the electrical signals generated by the photo multipliers  79  and indicative of the amount of secondary and backscattered electrons incident on the sectors or portions  17   a ,  17   b ,  17   c ,  17   d  of the detector  17 . The controller may store the detected signals and compare the signals generated from different portions of the detector with each other for obtaining the desired information with respect to magnetic structures of the sample  23 . 
       FIG. 8  is a flow diagram of a method of inspecting a sample using an electron beam system similar to that shown in  FIG. 6 . In a step  112 , a temperature of a tip of an electron beam source is set higher than a Curie temperature of a material providing a surface of the tip. A current supplied to an electromagnet generating a magnetic field at the tip is set in a step  114  in order to magnetically orient the material providing the tip. Thereafter, in a step  116 , the tip is cooled down to a temperature below the Curie temperature of the material providing the surface of the tip. A polarized electron beam is extracted from the tip and directed to a sample in a step  118 , and electron intensities are detected using at least one detector. 
     Upon completion of inspection of the sample using the polarized electron beam in step  118 , the polarization of the beam is reversed and the inspection is repeated with the beam of reversed polarization. For this purpose, the temperature of the tip is again increased to above the Curie temperature in a step  120 , the electric current supplied to the magnet is inverted in a step  122 , in order to orient the material providing the surface of the tip in a direction opposite to the orientation generated in step  114 . Thereafter, the temperature of the tip is reduced to below the Curie temperature of the material providing the surface of the tip in a step  124 . A polarized electron beam is extracted from the tip in a step  125 , wherein a polarization direction of the beam extracted in step  125  is opposite to the polarization direction of the beam extracted in step  118 . The extracted polarized beam is directed to the sample and electrons released from the sample are detected in step  125 . Information relating to magnetic properties of the sample is obtained by comparing the intensities of electrons detected in step  127  with the intensities of electrons detected in step  118 . Such comparison may include calculating a difference between the detected electron intensities. 
     While the disclosure has been provided via certain exemplary embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the disclosure set forth herein are intended to be illustrative and not limiting in any way. Various changes may be made without departing from the spirit and scope of the present disclosure as defined in the following claims.