Abstract:
An improved objective lens for a charged particle beam device is constituted by, among other things, a magnetic lens that creates a first magnetic field for focussing the charged particle beam onto the specimen. Furthermore, a deflector is integrated into the magnetic lens by providing at least one additional coil arrangement that creates a second magnetic field used to deflect the charged particle beam. Thereby, the second magnetic field is guided through at least one of the pole pieces of the magnetic lens. The present invention also provides an improved column for a charged particle beam device including the improved objective lens.

Description:
This is a National stage entry under 35 U.S.C. § 371 of Application No. PCT/EP01/00857 filed Jan. 26, 2001; the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an apparatus for the examination of specimen with a beam of charged particles. In particular, this invention relates to an objective lens for a charged particle beam device. 
     BACKGROUND OF THE INVENTION 
     The resolution of the conventional optical microscopy is limited by the wavelength of the visible light. Furthermore, at the highest resolution the conventional optical microscopy has a very shallow depth of field. These two limitations have led to the increased popularity of charged particle devices for the examination of specimen. Compared to optical light, accelerated charged particles, for example electrons, exhibit a shorter wavelength, which leads to an increased resolution power. Furthermore, even at the highest resolution charged particle devices usually exhibit a large depth of field. Accordingly, charged particle beams, especially electron beams, are used in a variety of ways in biology, medicine, the materials sciences, and lithography. Examples include the diagnosis of human, animal, and plant diseases, visualization of sub cellular components and structures such as DNA, determination of the structure of composite materials, thin films, and ceramics, or the inspection of masks and wafers used in semiconductor technology. In addition to that, charged particle beam devices, may also be used for the modification of organic and inorganic materials and their surfaces. 
     In these instruments, the area to be examined and/or modified is irradiated with a charged particle beam, which may be static or swept in a raster across the surface of the specimen. Depending on the specific application, the charged particle beam is more or less focused and the kinetic energy of the particles can vary considerably. The types of signals produced when the charged particles impinge on a specimen surface include e.g. secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies. These signals are obtained from specific emission volumes within the sample and can be used to examine many characteristics of the sample such as composition, surface topography, crystallography, etc. 
     In charged particle beam devices, such as a scanning electron microscope (SEM), the charged particle beam exhibits a typical aperture angle as well as a typical angle of incidence in the order of several millirads. However, for many applications it is desirable that the charged particle beam hits the sample surface under a much larger angle of typically 5° to 10°, corresponding to 90 to 180 millirads. Stereoscopic visualization is an example for such an application. Some applications even require tilt angles in excess of 15° or even 20°. Thereby, a number of tilting mechanism can be used. In early solutions, an oblique angle of incidence was achieved by mechanically tilting the specimen. However, due to mechanical imperfections, a lateral movement of the specimen is inevitable, which often results in misregistrations between two pictures having two different viewing angles. 
     An oblique angle of incidence may also be achieved by electrically tilting the charged particle beam. This is usually done by deflecting the beam so that either by the deflection alone or in combination with the focussing of the beam an oblique angle of incidence results. Thereby, the specimen can remain horizontal, which is a significant advantage as far as the lateral coordinate registration is concerned. Electrical tilting is also much faster than its mechanical counterpart. The electrical method, however, has also certain drawbacks. Especially, in low energy electron microscopy the magnetic or compound objective lens has to be very strong with very short focal length (1-20 mm), in order to achieve high resolution. In the presence of such a lens it is difficult to influence the landing position or landing angle of the charged particle beam at the specimen surface. In general, the strength of the deflector field has to be comparable to the field of the objective lens. To achieve such strong deflection fields the deflector usually has to employ pole pieces made from magnetically soft material (e.g. mumetal or permenorm). Furthermore, it is usually necessary to concentrate the deflection field to an area close to the optical axis. However, in conventional systems the corresponding pole pieces can not be placed close to the objective lens gap because they would negatively influence the field distribution of the lens. If, however, the deflector is placed in sufficient distance after the objective lens, usually a resolution degradation due to the increased working distance will result. Furthermore, if the deflector is placed before the objective lens, usually high offaxis aberrations will result. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved objective lens for a charged particle beam device. According to one aspect of the present invention, there is provided a objective lens for a charged particle beam device as specified in independent claim  1 . According to a further aspect of the present invention there is provided a column for a charged particle beam device as specified in independent claims  16  or  17 . Further advantages, features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms. 
     The present invention provides an improved objective lens for a charged particle beam device. The objective lens comprises a magnetic lens that creates a first magnetic field for focussing the charged particle beam onto the specimen. Furthermore, a deflector is integrated into the magnetic lens by providing at least one additional coil arrangement that creates a second magnetic field used to deflect the charged particle beam. Thereby, the second magnetic field is guided through at least one of the pole pieces of the magnetic lens. The present invention also provides an improved column for a charged particle beam device including the improved objective lens. 
     By integrating the deflector into the objective lens, large angles of incidence can be achieved without causing large lateral movements of the charged particle beam on the specimen. Furthermore, due to the integration of the deflector into the objective lens, the working distance of the system can be kept small, so that the resolution of the system is not negatively influenced. The objective lens can be used to produce stereo images of a specimen in a fast and reliable manner. Accordingly, the additional information which is contained in stereo images and which is extremely valuable in many cases, can be accessed without causing any additional costs. 
     The present invention also provides an improved column for a charged particle beam device. The column comprises a magnetic deflector having at least one excitation coil arrangement for generating a magnetic field to deflect the charged particle beam and being arranged between the objective lens and specimen whereby the objective lens concentrates the magnetic field of the deflector in a region close to the specimen Due to field termination effect of the objective lens, the magnetic deflector can be placed very close to objective lens without interfering with the focussing properties of the objective lens. Accordingly, the working distance of the system can be kept small, so that the resolution of the system is not negatively influenced 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the above indicated and other more detailed aspects of the invention will be described in the following description and partially illustrated with reference to the figures. Therein: 
     FIG. 1 shows schematically a column according to a first embodiment according to the present invention, 
     FIG. 2 shows schematically an objective lens according to a embodiment according to the present invention, 
     FIG. 3 shows schematically a top view of the lower pole piece of the objective lens shown in FIG. 2, 
     FIG. 4 shows schematically an objective lens according to a further embodiment according to the present invention, 
     FIG. 5 shows schematically an objective lens according to a further embodiment according to the present invention, 
     FIG. 6 shows schematically a top view of the field termination structure and the lower pole piece of the objective lens shown in FIG. 5, 
     FIG. 7 shows schematically an objective lens according to a further embodiment according to the present invention, 
     FIG. 8 shows schematically an objective lens according to a further embodiment according to the present invention, 
     FIG. 9 shows schematically an objective lens according to a further embodiment according to the present invention, 
     FIG. 10 shows schematically an objective lens according to a further embodiment according to the present invention, 
     FIG. 11 shows schematically a column according to a further embodiment according to the present invention. 
     FIG. 12 shows schematically an enlarged view of the objective lens and the deflector shown in FIG.  11 . 
     FIG. 13 shows schematically an enlarged view of the deflector shown in FIG.  12 . 
     FIG. 14 shows schematically a deflector for a column as shown in FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment according to the invention is shown schematically in FIG.  1 . The column  1  for a charged particle beam device comprises a charged particle source  2  which emits a beam  4  of charged particles. In electron beam devices, electron sources such as Tungsten-Hairpin guns, Lanthanum-Hexaboride Guns, Field emission guns etc. can be used. The invention, however, is not limited to electron sources; it can be used together with all kinds of charged particle sources. The electrons are accelerated by an accelerating voltage supplied to the electron source  2 . Since the beam diameter produced directly by the electron source usually is too large to generate a sharp image at high magnification, the electron beam  4  is guided through the condenser lens  5 , which demagnifies the beam and leads the electron beam  4  towards a specimen  8 . 
     The electron beam  4  then passes the detector  9  which is used to detect those particles that come from the specimen  8  in order to produce an image of the specimen  8 . The detector  9  is followed by the scanning coils  6 , which are used to move the electron beam  4  in a television-like raster over the surface of the specimen  8 . After the scanning coils  12  the electron beam  4  enters the objective lens  10  that focuses the electron beam  4  onto the specimen  8 . The objective lens  10  not only focuses the electron beam  4  but also rotates the electron beam  4 . However, this effect is not shown, because it is difficult to depict in a two-dimensional drawing and because the skilled person is well aware of this additional effect. 
     When the particles of beam  4  strike the surface of specimen  8 , they undergo a series of complex interactions with the nuclei and electrons of the atoms of the specimen. The interactions produce a variety of secondary products, such as electrons of different energy, X rays, heat, and light. Many of these secondary products are used to produce the images of the sample and to collect additional data from it. A secondary product of major importance to examination or the image formation of specimens are secondary electrons that escape from the specimen  8  at a variety of angles with relatively low energy (3 to 50 eV). The secondary and the back scattered electrons reach the detector  16  and are measured. By scanning the electron beam over the specimen and displaying/recording the output of the detector  16  an image of the surface of the specimen  8  is formed. The specimen  8  is supported on a stage  7  (specimen support) which is moveable horizontally in all directions, in order to allow the electron beam  4  to reach the target areas on the specimen which are to be examined. 
     Due to the deflector  11  that is integrated into the objective lens  10  (FIG. 2, FIG. 3) the electron beam  4  hits the specimen under a predetermined angle of incidence θ, preferably in the range between 1° and 20° degrees. By providing an oblique angle of incidence stereo images of a specimen can be produced in a fast and reliable manner. Such stereo images of a specimen may for example be used in order to perform an accurate height measurements on features that can be found on the surface of the specimen. Once the depth or the height of a feature is known, this information can be used in order to determine further interesting parameters. For example, in the semiconductor industry during the processing of a semiconductor wafer, it is very helpful to know the true width of a contact hole at its bottom. By knowing the depth of a contact hole, stereo images of the contact hole can be used in order to determine this parameter. 
     FIG. 2 shows schematically an objective lens  10  according to a embodiment according to the present invention. The objective lens  10  includes a first excitation coil  12  which is used to generate a magnetic field which, in turn, is used to focus the electron beam onto the surface of the specimen  8 . To achieve a short focal lengths, the magnetic field generated by the first excitation coil  12  is guided through the upper pole piece  13  and the lower pole piece  14  to the pole piece gap  17 . Accordingly, the magnetic field is concentrated into a small spatial region around an axis of symmetry  16  of the objective lens  10 . The magnetic field is rotational symmetrically around the axis of symmetry  16  and reaches its maximum strength in a pole piece gap  17 . The electrons basically move along the axis of symmetry  16  of the objective lens  10 , which, accordingly, also represents the path of the electrons. 
     In order to achieve a predetermined angle of incidence θ on the surface of the specimen the objective lens includes a deflector  11 . The deflector  11  comprises four additional (second) excitation coils  15  located on the lower pole piece  14 . As can be seen from FIG. 3 the lower pole piece  14  is divided into four segments  18 A to  18 D thereby forming a four-pole. Thereby, each segment  18 A to  18 D has its corresponding second excitation coil  15 A to  15 D. The second excitation coils  15 A to  15 D are wrapped around the segments  18 A to  18 D, so that by exciting one the second excitation coils  15 A to  15 D a magnetic field is generated in the corresponding segment  18 A to  18 D of the lower pole piece  14 . FIG. 3 shows a situation in which the second excitation coils  15 B and  15 D are excited so that the magnetic field  20  (represented by the arrows in FIG. 3) is generated. As can be seen from FIG. 3 the magnetic field  20  is basically perpendicular to the path of the electron beam. Accordingly, a magnetic field across the path of the electron beam is generated which leads to a deflection of the electron beam. 
     Due to the segments  18 A to  18 D of the lower pole piece  14 , the magnetic field is guided to an area close above the specimen  8  and generates the required strong deflection field. It should be kept in mind that the segments  18 A to  18 D of the lower pole piece  14  at the same time also guide the magnetic field generated by the first excitation coil  12 . On the symmetry axis  16  the magnetic field generated the excitation coil  12  and magnetic field generated by the second excitation coils  15 A to  15 D partially overlap. Thereby, the maximum of the deflection field is closer to the specimen  8  than the maximum of the focussing field. In that manner, large angles of incidence θ in the range of 5-20° degrees can be achieved without creating large lateral movements of the beam on the specimen. 
     FIG. 4 shows schematically an objective lens according to a further embodiment according to the present invention. In addition to the magnetic lens the objective lens shown in FIG. 3 contains an electrostatic retarding lens which is situated close to magnetic lens. The electrostatic retarding lens has two electrodes  22 ,  23  held at different potentials. In the illustrated embodiment one of the two electrodes  22  is formed by a cylindrical beam tube which is arranged within the upper pole piece  13  of the magnetic lens along the path of the electron beam  4 . The second electrode  23  of the electrostatic retarding lens is a metallic cup provided below the magnetic lens. During the operation of the system the first electrode  22  is usually held at high positive potential, for example 8 kV, where as the second electrode  23  is held at lower positive potential, for example 3 kV, so that the electrons are decelerated in the corresponding electrostatic field from a first energy to lower second energy. 
     In the example shown in FIG. 4, the specimen  8  is held at ground potential. Accordingly, there is a further electrostatic retarding field between the metallic cup  23  and the specimen  8 . Due to the electrostatic retarding field between the metallic cup  23  and the specimen  8 , an initial deflection of the electron beam  4  caused by the magnetic field  20  is enhanced leading to an increased angle of incidence θ. Accordingly, in order to achieve a predetermined angle of incidence θ only a relative small deflection caused by the deflector  11  is necessary. However, the surface of the specimen  8  need not be grounded. The electric potential on the surface of the specimen  8  may also be adjusted by applying a voltage to the specimen  8 . A voltage can be applied to a wafer, for example, in order to obtain voltage contrast imaging which is used to detect shorts in a circuit. As long as the potential of the metallic cup  23  is higher than the potential on the surface of the specimen  8 , an electrostatic retarding field is produced. 
     FIG. 5 shows schematically an objective lens according to a further embodiment according to the present invention. The objective lens shown in FIG. 5 exhibits four trenches  24  arranged on the bottom surface of each segment  18 A to  18 D of the lower pole piece  14 . Thereby, each trench  24  is used to house the corresponding second excitation coils  15 A to  15 D. Accordingly, an increase in the working distance between the objective lens  10  and the specimen  8  due to the second excitation coils  15 A to  15 D can be avoided. Furthermore, the objective lens shown in FIG. 5 exhibits a field termination structure  25 , which is used to terminate the magnetic field of the deflector  11  close above the specimen  8 . In the example shown in FIG. 5 the field termination structure  25  is a ring made of magnetic material which is placed in the pole piece gap between the upper pole piece  13  and the lower pole piece  14 . There, the field termination structure  25  is used to concentrate the magnetic field of the deflector  11  in the region close to the specimen  8 . 
     As can be seen from FIG. 6 the lower pole piece  14  is divided into four segments  18 A to  18 D thereby forming a four-pole. Thereby, each segment  18 A to  18 D has its corresponding second excitation coil  15 A to  15 D. The second excitation coils  15 A to  15 D are wrapped around the segments  18 A to  18 D, so that by exciting one the second excitation coils  15 A to  15 D a magnetic field is generated in the corresponding segment  18 A to  18 D of the lower pole piece  14 . Again, FIG. 6 shows a situation in which the second excitation coils  15 B and  15 D are excited so that the magnetic field  20  (represented by the arrows in FIG. 6) is generated. As can be seen from FIG. 6 the magnetic field  20  is basically perpendicular to the path of the electron beam. Accordingly, a magnetic flux across the path of the electron beam is generated which leads to a deflection of the electron beam. Furthermore, that portion of the magnetic deflection field  20  that would extend upward within the objective lens  10  (FIG.  5 ), enters the field termination structure  25  and is guided therein from a position close to segment  18 B to a position close to segment  18 D where the magnetic deflection field  20  leaves the field termination structure  25  and enters the lower pole piece  14  again. Thereby, the field termination structure  25  does not significantly influence the focussing field of the objective lens  10 , but concentrates the deflection field  20  effectively. 
     FIG. 7 shows schematically an objective lens according to a further embodiment according to the present invention. Similar to the objective lens shown in FIG. 4 the objective lens shown in FIG. 7 contains, in addition to the magnetic lens, an electrostatic retarding lens. The electrostatic retarding lens has one electrode  22  which is formed by a cylindrical beam tube arranged within the upper pole piece  13  of the magnetic lens along the path of the electron beam  4 . The second electrode of the electrostatic retarding lens is the specimen  8  itself. During the operation of the system the electrode  22  is usually held at high positive potential, for example 8 kV, where as the specimen is held at lower positive potential, for example ground potential, so that the electrons are decelerated in the corresponding electrostatic field from a first energy to lower second energy. 
     Similar to the objective lens shown in FIG. 5 the objective lens shown in FIG. 7 exhibits four trenches  24  arranged on the bottom surface of each segment  18 A to  18 D of the lower pole piece  14 . Again, each trench  24  is used to house the corresponding second excitation coils  15 A to  15 D. Accordingly, an increase in the working distance between the objective lens  10  and the specimen  8  due to the second excitation coils  15 A to  15 D can be avoided. Furthermore, the tip of the upper pole piece  13  of the magnetic lens  10  is positioned inside the magnetic field of the deflector  11 . Thus, the tip of the upper pole piece  13  functions as a field termination structure for the magnetic field of the deflector  11 . This eliminates the expansion of the magnetic field of the deflector  11  higher in the objective lens  10 . The upper pole piece  13  leads to a termination of the magnetic field, which helps to concentrate the deflecting field in the area close to the specimen  8 . 
     FIG. 8 shows schematically an objective lens according to a still further embodiment according to the present invention. Again, the deflector  11  comprises four additional (second) excitation coils  15 A to  15 D. However, in contrast to the previous examples, the excitation coils  15 A to  15 D are not located on the lower pole piece  14  but on the outer pole piece  26  which connects the lower pole piece  14  with the upper pole piece  13 . Thereby, the lower pole piece  14  and the outer pole piece  26  are divided into four segments  18 A to  18 D thereby forming a four-pole. Again, each segment  18 A to  18 D has its corresponding second excitation coil  15 A to  15 D. The second excitation coils  15 A to  15 D are wrapped around the segments  18 A to  18 D on the outer pole piece, so that by exciting one the second excitation coils  15 A to  15 D a magnetic field is generated in the corresponding segment  18 A to  18 D of the outer pole piece and the lower pole piece  14 . Accordingly, there is no principle difference to the previous arrangement. However, by arranging the second excitation coils  15 A to  15 D at the outer pole piece  26 , usually more space is available for the second excitation coils  15 A to  15 D. 
     In the embodiment shown in FIG. 8 the lower pole piece  14  and the outer pole piece  26  are divided into four segments  18 A to  18 D. However, any other number of segments might also be used. The lower pole piece  14  and the outer pole piece  26  may, for example, be divided into eight segments  18 A to  18 D thus forming an eight-pole. It is not necessary that the complete outer pole piece  26  is divided into segments. It is sufficient that only that portion which located below the second excitation coils  15 A to  15 D is divided into the segments  18 A to  18 D. 
     FIG. 9 shows schematically an objective lens according to a still further embodiment according to the present invention. In contrast to the previous examples of the present invention, the embodiment shown in FIG. 9 exhibits two independent magnetic lenses  30  and  31  for focussing the beam  4  of charged particles onto the specimen  8 . The upper magnetic lens  30  comprises the first excitation coil arrangement  32  to produce a magnetic field within the pole piece gap between upper pole piece  34  and the middle pole piece  35 . Thereby, the middle pole piece  35  serves as a “lower” pole piece for the upper magnetic lens  30 . The lower magnetic lens  31  comprises the first excitation coil arrangement  33  to produce a magnetic field within the pole piece gap between middle pole piece  35  and the lower pole piece  36 . Thereby, the middle pole piece  35  serves as a “upper” pole piece for the lower magnetic lens  31 . By selecting the currents through the two coil arrangements  32  and  33  the magnetic fields in the two pole piece gaps can be adjusted which determines the overall focal length the objective lens  10 . 
     In order to achieve a predetermined angle of incidence θ on the surface of the specimen the objective lens shown in FIG. 9 also includes a deflector  11 . The deflector  11  comprises four additional (second) excitation coils  15 A to  15 D located on the lower pole piece  36 . Again the lower pole piece  36  is divided into four segments thereby forming a four-pole. Furthermore, each segment has its corresponding second excitation coil  15 A to  15 D. The second excitation coils  15 A to  15 D are wrapped around the segments, so that by exciting one the second excitation coils  15 A to  15 D a magnetic field is generated in the corresponding segment of the lower pole piece  36 . Furthermore, the middle pole piece  35  of the objective lens  10  is positioned inside the field of the deflector  11 . Thus part of the magnetic field of the deflector  11  closes through the tip of the middle pole piece  35 . Again, this eliminates the expansion of the magnetic field of the deflector  11  higher in the objective lens. Accordingly, the middle pole piece  35  leads to a termination of the magnetic field, which helps to concentrate the deflecting field in the area close to the specimen  8 . 
     The embodiment shown in FIG. 9 does not exhibit an electrostatic lens in addition to the two magnetic lenses  30  and  31 . However, an electrostatic lens can be generated by applying a potential difference between the specimen  8  and the objective lens  10 . Furthermore, additional electrodes like those shown in FIGS. 4 and 5 can also be provided. 
     FIG. 10 shows schematically an objective lens according to a still further embodiment according to the present invention. In contrast to the embodiment shown in FIG. 9, the embodiment shown in FIG. 10 exhibits only one first excitation coil arrangement  32  to produce a magnetic field within the pole piece gap between upper pole piece  34  and the middle pole piece  35 . In order to further focus the beam  4  of charged particles, the second excitation coils  15 A to  15 D arranged on the lower pole piece  36  can be used. An identical excitation (symmetric excitation) of the four second excitation coils  15 A to  15 D creates an axially symmetric field between the middle pole piece  35  and the lower pole piece that can be used to focus the beam  4  of charged particles onto the specimen  8 . An asymmetric excitation of the second excitation coils  15 A to  15  creates an asymmetric filed which to a deflection of the beam  4  of charged particles. The middle pole piece  35  again terminates the magnetic field produced by the second excitation coils  15 A to  15 D. 
     A further embodiment according to the invention is shown schematically in FIG.  11 . This embodiment is similar to that of FIG. 1, except for the following. The column  40  shown in FIG. 11 comprises a magnetic objective lens  40  for focusing the charged particle beam  4  onto the specimen  8 . Close to the specimen  8  the objective lens  40  has a lower pole piece  44 . However, in contrast to the previous example, the objective lens  44  used in this embodiment of the invention does not comprises a deflector integrated into the objective lens, which is used to tilt the beam  4  of charged particles. 
     As can be seen from FIG. 12, the actual deflector  50  is provided as a separate entity, even though it might be fixed to the objective lens. The deflector  50  is arranged below the lower pole piece  44  of the objective lens  40 . The deflector  50  is divided into four segments thereby forming a four-pole. Furthermore, each segment has its corresponding excitation coil  55 A to  55 D. The excitation coils  55 A to  55 D are wrapped around the segments, so that by exciting one the excitation coils  55 A to  55 D a magnetic field is generated in the corresponding segment of the deflector. A top view on the deflector  50  is similar to the top views on the lower pole piece  14  of the objective lens  10  as shown in FIG. 3 or FIG.  7 . 
     The magnetic deflector  50  is arranged between the lower pole piece  44  and the specimen  8  whereby the lower pole piece  44  is located within the magnetic filed of the deflector  50  in order to concentrate the magnetic field of the deflector in a region close to the specimen  8 . Thus, the lower pole piece  44  of the objective lens  40  functions as a field termination structure for the magnetic filed of the deflector  50 . Due to this field termination effect of the lower pole piece  44  of the objective lens  40 , the magnetic field of the deflector  50  does not negatively interfere with the focussing field provided in the pole piece gap between the upper pole piece  43  and the lower pole piece  44 . Due to the fact that the magnetic deflector  50  is arranged very close to the specimen  8 , a large angle of incidence can be provided on the surface of the specimen without producing large lateral movements of the charged particle beam on the specimen. 
     Similar to the objective lens shown in FIG. 5 the objective lens shown in FIG. 12 contains, in addition to the magnetic lens, an electrostatic retarding lens. The electrostatic retarding lens has one electrodes  22  which is formed by a cylindrical beam tube arranged within the upper pole piece  43  of the magnetic lens along the path of the electron beam  4 . The second electrode of the electrostatic retarding lens is the specimen  8  itself. During the operation of the system the electrode  22  is usually held at high positive potential, for example 8 kV, where as the specimen  8  is held at lower positive potential, for example ground potential, so that the electrons are decelerated in the corresponding electrostatic field from a first energy to lower second energy. In the region between the lower pole piece  44  and the specimen  8 , the electrostatic retarding field and the magnetic deflection overlap. Due to the electrostatic retarding field between the electrode  22  and the specimen  8 , an initial deflection of the electron beam  4  caused by the magnetic deflection field is enhanced leading to an increased angle of incidence. Accordingly, in order to achieve a predetermined angle of incidence only a relative small deflections caused by the deflector  50  are necessary. 
     FIG. 13 shows schematically an enlarged view of the deflector shown in FIG.  12 . As can be seen from FIG. 13, part of the magnetic flux of the deflector  50  closes through the tip of the lower pole piece  44  of the objective lens  40  so that the extension of the deflection field  51  far from the specimen is eliminated. The deflection field  51  thus is effective mainly in the area of the retarding electrostatic lens between the lower pole piece  44  and the specimen  8 . Thereby, the inner diameter “D” of the deflector pole piece  52  is significantly bigger than the distance “d” of its lower face from the lower face of the lower pole piece  44  of the objective lens  40 . The ratio Diameter/distance is preferably bigger than 2, more preferably bigger than 4. Furthermore, the deflector pole piece  52  can be electrically insulated from the lower pole piece  44 . This allows to select the electric potential on the deflector pole piece  52  arbitrarily, which can be used in order to produce a predetermined potential distribution adapted to specific measurement conditions. 
     FIG. 14 shows schematically an deflector arrangement  50  according to a still further embodiment according to the present invention. In contrast to the examples of the present invention shown in FIGS. 11 to  13 , the deflector shown in FIG. 14 exhibits four excitation coils  55 A to  55 D that are not located close to the specimen  8  but parallel to the excitation coil  12  of the objective lens  40 . Accordingly, more space is available for the four excitation coils  55 A to  55 D. Furthermore, the deflector pole piece  52  is not parallel to specimen  8 , as in the previous example, but parallel to lower pole piece  44  of the objective lens  40 . However, these differences do not affect the principle operation of the deflector  50 .