Abstract:
There is disclosed an energy filter for use in an electron microscope, the energy filter having an electron passage that can be evacuated more reliably than heretofore. The filter can be designed compactly without increasing the polepiece gaps or the spaces to accommodate coils. The energy filter has an electron-deflecting magnet assembly. This assembly comprises a pair of opposite polepiece bases, a pair of spacers interposed between the polepiece bases, and a yoke mounted to side surfaces of the polepiece bases. Magnetic polepieces and coil grooves of a given width are formed in the opposite surfaces of the polepiece bases. Bulges forming shunts are formed outside the coil grooves in the polepiece bases. O-ring grooves are formed in the spacers on the sides of the polepiece bases around the coil grooves. Electron passage grooves are formed in the opposite surfaces of the spacers to form the electron passage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an energy filter consisting of at least one electron-deflecting magnet assembly to pass incident electrons which have a certain energy. 
     2. Description of the Related Art 
     Electron microscopes having electron optics incorporating an energy filter have been developed. Such a conventional electron microscope is shown in FIG. 6, in which the microscope is indicated by numeral  1 . This microscope has an electron gun  2  emitting a beam of electrons e. The beam is directed to a specimen  5  via a condenser lens system  3 . The beam transmitted through the specimen  5  is projected onto a fluorescent screen  11  via an objective lens  4 , an intermediate lens  6 , an entrance aperture  7 , a spectrometer  8 , a slit  9 , and a projector lens  10 . Thus, a transmission image of the specimen is observed. The entrance aperture  7 , the spectrometer  8 , and the slit  9  constitute an energy filter  12 , known as an Ω-filter. 
     The spectrometer  8  incorporated in the energy filter  12  of the electron microscope  1  is equipped with at least one electron-deflecting magnet assembly. One example of the electron-deflecting magnet assembly is shown in FIGS.  7 ( a ) and  7 ( b ), where the magnet assembly, indicated by  13 , comprises a pair of opposed magnetic polepiece bases  100 ,  101 . Coil grooves  22  and  24  are formed adjacent to each other in one surface of the magnetic polepiece base  100 . Thus, those portions which are surrounded by the coil grooves  22  and  24  form magnetic polepieces  14  and  16 , respectively. Coils  18  and  20  are received in the coil grooves  22  and  24 , respectively. Similarly, the other magnetic polepiece base  101  is provided with coil grooves  23  and  25  formed adjacent to each other. Thus, those portions which are surrounded by the coil grooves  22  and  24  form polepieces  15  and  17 , respectively. Coils  19  and  21  are received in the coil grooves  23  and  25 , respectively. The polepiece bases  100  and  101  are so positioned that the formed polepieces  14  and  16  are located opposite to the polepieces  15  and  17 , respectively. Those portions of the polepieces  14 - 17  that are surrounded by the coil grooves  22 - 25  are recessed as viewed from the other portions. Gaps  26  and  27  are formed between them and in communication with each other via a passage  28 . These gaps  26 ,  27 , and passage  28  together form an electron passage  29 . 
     Electrical current is supplied from a current source (not shown) to the coils  18 - 21  to produce magnetic fields in the gaps  26  and  27  between the polepieces  14  and  15  and between the polepieces  16  and  17 , respectively. Shunts (not shown) are mounted at the entrance and exit surfaces of the gaps  26  and  27  to prevent ooze or spreading of the magnetic fields. Using these shunts, the distributions of the magnetic fields developed in the gaps  26  and  27  between the polepieces are tightly controlled. Electrons are caused to pass through these magnetic fields. This gives good electron optical characteristics to the electron-deflecting magnet assembly  13  acting to deflect electrons. 
     Electrons react with molecules within air and are lost rapidly. It is necessary to evacuate the coil grooves  22 - 25  and the electron passage  29  within the electron-deflection magnet assembly  13  to create a low-pressure condition. In the past, therefore, the electron-deflecting magnet assembly  13  itself has been accommodated within a vacuum chamber. With this method for evacuating the electron-deflecting magnet assembly  13 , however, it is very difficult to pump down the inside of the magnet assembly  13  because the components of the magnet assembly  13 , such as the coils  18 - 21 , have large surface areas. Where there is a large amount of residual gas, the electron microscope  1  fitted with the energy filter  12  suffers from various problems, such as instability of the accelerating voltage and specimen contamination due to electron irradiation. 
     In an attempt to solve these problems, the following two methods have been adopted. A first method consists of placing a tube  30  along an electron passage  29  as shown in FIG.  8  and evacuating only the inside of the tube  30 . With this first method, it can be expected that the aforementioned problems will be solved at the highest efficiency, since the volume evacuated is smallest. 
     A second method consists of covering the coils  18 - 21  with vacuum-resistant packs  31 - 34 , respectively, as shown in FIG. 9, to suppress degassing from the coils  18 - 21 . With this second method, intrusion of gas into the electron passage  29  is suppressed, the gas escaping from the coils  18 - 21 . Therefore, the aforementioned problems can be effectively solved. 
     With the first method, it is necessary to accurately shape the tube  30 . Since the tube  30  is very complex in shape, it is very difficult to shape the tube  30  accurately. Furthermore, it is necessary to clean the inside of the tube  30 . However, it is not easy to finish the interior of the tube  30  with a high degree of cleanliness. 
     To put the tube  30  in the electron passage  29 , the gaps  26  and  27  between the polepieces  14  and  15  and between the polepieces  16  and  17  are inevitably set large. If these gaps are made large, a larger power supply is necessary to produce a given magnitude of magnetic field. In addition, the aberrations of the deflecting magnetic field increase. Accordingly, limitations are imposed on increase of the gaps  26  and  27 . 
     In the second method described above, the coils  18 - 21  are separately covered with the vacuum-resistant packs  31 - 34 , respectively. Therefore, the coil grooves  22 - 25  in the coils  18 - 21  must have large space. This increases the size and complexity of the electron-deflecting magnet assembly  13 . Additionally, the gap between each shunt and the corresponding polepieces, such as  14 - 17 , is increased to secure spaces to accommodate the coils  18 - 21 . 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention has been made. 
     It is an object of the present invention to provide an energy filter that can be designed compactly without increasing the gaps between polepieces or spaces to accommodate coils and has an electron passage capable of being evacuated more reliably. 
     An energy filter built in accordance with a first embodiment of the present invention solves the foregoing problems and comprises at least one magnet assembly mounted in a vacuum created within an electron microscope, the magnet assembly being designed to pass only incident electrons which have a certain energy. The magnet assembly comprises a pair of polepiece bases located opposite to each other, polepieces and coil grooves formed in respective surfaces of the polepiece bases, coils inserted in the coil grooves, respectively, a pair of spacers interposed between the polepiece bases, and a yoke fixedly mounted to side surfaces of the polepiece bases. The coil grooves are located opposite to each other. The spacers are provided with sealing grooves to accommodate hermetic seals, respectively, for hermetically sealing the coils received in the coil grooves in the opposite polepiece bases, respectively. At least one electron passage gap is between the spacers to form an electron passage. Seal members are inserted in the sealing grooves, respectively, to permit the coils to be located outside the vacuum described above. 
     An energy filter in accordance with a second embodiment of the present invention is based on the energy filter in accordance with the first embodiment and further characterized in that the polepieces have bulges swelling outward from the coil grooves, respectively, to form shunts for preventing ooze of magnetic fields. 
     An energy filter in accordance with a third embodiment of the present invention is based on the energy filter in accordance with the first or second embodiment and further characterized in that at least one magnet assembly described above is plural and fixedly mounted to a platen. 
     An energy filter in accordance with a fourth embodiment of the invention is based on the energy filter in accordance with the first or second embodiment and has the following features. The aforementioned at least one magnet assembly is plural. The magnet assemblies include first magnet assemblies having a pair of polepiece bases which are integrally fabricated, respectively, and a pair of spacers which are integrally fabricated, respectively. The electron passage gap is formed between the integrally fabricated spacers. Sealing grooves are formed in the integrally fabricated spacers, respectively. Sealing members are received in the sealing grooves, respectively. 
     In the energy filter constructed as described above, the sealing members permit the coils of the magnets to be located outside the vacuum, and the coils are not brought within the vacuum. This prevents deterioration of the vacuum inside the electron microscope. Hence, the performance of the electron microscope is prevented from deteriorating. 
     The electron passage gap formed between the spacers form a single electron passage. This passage is much easier to machine and clean than the conventional tube described above. 
     Since no tube is accommodated within the electron passage, it is not necessary to secure a large space between the opposite polepieces. In consequence, the magnet assembly can be designed compactly. Furthermore, only a small-size power supply suffices. 
     The shunts for preventing ooze of the magnetic fields are formed integrally with the polepieces at the bulges of the polepieces. Therefore, it is not necessary to take account of the accuracy with which these components are assembled. The magnet assembly can be assembled easily. 
     Other objects and features of the invention will appear in the course of the description thereof, which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 ( a ) is a view of an electron-deflection magnet assembly in a spectrometer used in an energy filter in accordance with the present invention; 
     FIG.  1 ( b ) is an assembly drawing of an electron-deflection magnet assembly in a spectrometer used in an energy filter in accordance with the present invention; 
     FIG.  2 ( a ) is a plan view of one polepiece base of the electron-deflecting magnet assembly shown in FIG.  1 ( a ), showing a plane at which a coil groove is formed; 
     FIG.  2 ( b ) is a cross-sectional view taken on line IIB—IIB of FIG.  2 ( a ); 
     FIG.  3 ( a ) is a plan view of one spacer of the electron-deflecting magnet assembly shown in FIG.  1 ( a ), showing a plane at which an electron passage groove is formed; 
     FIG.  3 ( b ) is a plan view of the spacer shown in FIG.  3 ( a ), showing a plane at which an O-ring groove is formed; 
     FIG. 4 is a view of an energy filter composed of plural discrete electron-deflecting magnet subassemblies; 
     FIG. 5 is a view of an energy filter in which some of electron-deflecting magnet subassemblies are fabricated integrally, while the others are formed by a large common member; 
     FIG. 6 is a schematic diagram of the prior art electron microscope having electron optics incorporating an energy filter; 
     FIG.  7 ( a ) is a view of an electron-deflecting magnet assembly used in the prior art spectrometer; 
     FIG.  7 ( b ) is an assembly drawing of an electron-deflecting magnet assembly used in the prior art spectrometer; 
     FIG. 8 is a view illustrating a first method of solving the problems with the prior art electron-deflecting magnet assembly shown in FIG.  7 ( a ); 
     FIG. 9 is a view illustrating a second method of solving the problems with the prior art electron-deflecting magnet assembly shown in FIG.  7 ( a ); 
     FIG.  10 ( a ) is a view of a modification of the spacers shown in FIG.  1 ( a ); 
     FIG.  10 ( b ) is a view of another modification of the spacers shown in FIG.  1 ( a ); 
     FIG.  10 ( c ) is an assembly drawing of the spacers shown in FIG.  10 ( b ); 
     FIG.  11 ( a ) is a view of a modification of the spacer shown in FIG. 5; and 
     FIG.  11 ( b ) is a view of another modification of the spacer shown in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are hereinafter described in detail by referring to the drawings. FIGS.  1 ( a ) and  1 ( b ) show an electron-deflecting magnet assembly incorporated in a spectrometer that is used in an energy filter in accordance with the present invention. Note that like components are indicated by like reference numerals in various figures and that those components which have been already described will not be described in detail below. 
     Referring to FIGS.  1 ( a ) and  1 ( b ), the electron-deflecting magnet assembly, generally indicated by reference numeral  13 , has a pair of polepiece bases  100 ,  101 . As shown in FIGS.  2 ( a ) and  2 ( b ), the side of one polepiece base  100  that faces the other polepiece base  101  is provided with a coil groove  22  of a given width. This groove  22  forms a closed loop and accommodates a coil  18 . That portion which is surrounded by the coil groove  22  forms a magnetic polepiece  14 . The surface of the polepiece base  100  that faces the polepiece base  101  is flush except for the portion where the coil groove  22  is formed. 
     The other polepiece base  101  is identical in contour to the polepiece base  100 . The side of this polepiece base  101  facing the polepiece base  100  is similarly provided with a coil groove  23  (shown in FIGS.  3 ( a ) and  3 ( b )) of the same width as the coil groove  22 . The groove  23  forms a closed loop and accommodates a coil  19 . That portion which is surrounded by the coil groove  23  forms a magnetic polepiece  15 . The surface of the polepiece base  101  that faces the polepiece base  100  is flush except for the portion where the coil groove  23  is formed. Where the polepiece bases  100  and  101  are made to overlap each other, the coil grooves  22  and  23  are placed opposite to each other. Accordingly, the polepieces  14  and  15  are also located opposite to each other. 
     A pair of spacers  36 ,  37 , each consisting of a flat plate, are mounted between the polepiece bases  100  and  101 . As shown in FIGS.  3 ( a ) and  3 ( b ), one spacer  36  is identical in contour with the polepiece base  100 . The thickness of the spacer  36  is half the length of the gap  26  of the prior art electron-deflecting magnet assembly  13  (FIGS. 7-9) taken in the direction along which the polepiece bases  100  and  101  are spaced from each other. 
     The surface of one spacer  36  that faces the polepiece  14  is provided with an O-ring groove  39  forming a closed loop. An O-ring  38  (FIG.  1 ( a )) is inserted in this groove  39  (FIGS.  1 ( a ) and  1 ( b )), which is located around the coil groove  22 . The surface of one spacer  36  that faces the other spacer  37  is provided with an electron passage groove  40  forming a part of an electron passage  29 . 
     The other spacer  37  is identical in contour and thickness with spacer  36 . The surface of the other spacer  37  that faces the other polepiece base  101  is provided with an O-ring groove  42  (FIG.  1 ( a )) forming a closed loop. An O-ring  41  (FIG.  1 ( a )) is received in this groove  42 , which is located around the coil groove  23 . The surface of the spacer  37  facing the spacer  36  is formed with an electron passage groove  43  (FIGS.  1 ( a ) and  1 ( b )) forming the remaining part of the electron passage  29 . Where the spacers  36  and  37  are made to overlap each other as shown in FIGS.  1 ( a ) and  1 ( b ), the electron passage grooves  40  and  43  are placed opposite to each other, forming the single electron passage  29 . 
     As shown in FIG.  10 ( a ), the electron passage  29  may have a deeper groove portion  43  whose depth is equal to the sum of the depths of the electron passage grooves  40  and  43  of the structure shown in FIG.  1 ( a ). The deeper groove portion  43  is formed only in one (e.g., the spacer  37 ) of the two spacers  36  and  37 . In this case, the spacer  36  has no electron passage groove. 
     As shown in FIGS.  10 ( b ) and  10 ( c ), the electron passage  29  may made by inserting third spacers  50  between the spacers  36  and  37  which have no electron passage groove. The thickness of the third spacers  50  is equal to the depth of the deeper groove  43 . 
     After fitting the O-rings  38  and  41  in the O-ring grooves  39  and  42 , respectively, the spacers  36  and  37  are inserted between the polepiece bases  100  and  101  and made to overlap completely. Under this condition, a given number of screws  44  are inserted into one polepiece base  100  and into the other polepiece base  101  through the spacers  36 ,  37  outside the O-ring grooves  39  and  42 . Thus, the polepiece bases  100 ,  101  and the spacers  36 ,  37  are coupled together. 
     After the polepiece bases  100 ,  101  and the spacers  36 ,  37  have been coupled together in this way, the coils  18  and  19  are located in the O-rings  38  and  41 , respectively, and hermetically confined between the polepiece base  100  and the spacer  36  and between the polepiece base  101  and the spacer  37 , respectively, by the O-rings  38  and  41 , respectively. The polepiece bases  100  and  101  have bulges  45  and  46 , respectively, swelling outward from the coil grooves  22  and  23 , respectively. These bulges  45  and  46  act as shunts for suppressing ooze of magnetic fields. 
     A yoke  47  is mounted to side surfaces of the polepiece bases  100  and  101 . This yoke  47  completes a magnetic circuit and forms the electron-deflecting magnet assembly  13 . 
     The electron-deflecting magnet assembly  13  constructed in this manner is aligned within the vacuum vessel of the electron microscope and fixed. Since the coils  18  and  19  are confined between the polepiece bases  100 ,  101  and the spacers  36 ,  37 , respectively, by the closed-loop O-rings  38  and  41  as mentioned above, the electron passage  29  can be evacuated while leaving the vicinities of the coils surrounded by the O-rings  38  and  41  at atmospheric pressure. 
     In this example of energy filter  12 , the O-rings  38  and  41 , each forming a closed loop, prevent the coils  18  and  19  of the electron-deflecting magnet assembly  13  in the spectrometer  8  from entering a vacuum. Therefore, the vacuum inside the electron microscope  1  is prevented from being impaired. This prevents the performance of the microscope  1  from being deteriorated. 
     Furthermore, the coils  18  and  19  are directly mounted in the coil grooves  22  and  23 , respectively, in the polepiece bases  100  and  101  and so these coil grooves  22  and  23  can be reduced in size. Consequently, magnetic shielding can be accomplished efficiently. 
     The single electron passage  29  is formed by the electron passage grooves  40  and  43  formed in the spacers  36  and  37 , respectively. Therefore, neither a difficult machining operation nor a difficult cleaning operation is necessary, unlike the above-described conventional first method of machining the tube  30  of complex shape. Moreover, the spacer is composed of at least two parts, or  36  and  37 . This further facilitates machining the spacer. 
     In addition, the tube  30  is not accommodated in the electron passage  29 . This makes unnecessary to set large the distance between the polepieces  14  and  15 . Hence, the electron-deflecting magnet assembly  13  can be designed compactly. It is not necessary to use a large-sized power supply. 
     Further, the shunts for preventing ooze of magnetic fields are coupled to the polepieces  14  and  15  at their bulges  45  and  46 . Therefore, it is not necessary to take account of the assembly accuracy for them. This improves the efficiency of the operation for assembling the magnet assembly  13 . 
     In the above example, the spectrometer  8  is made of one electron-deflecting magnet assembly  13 . The spectrometer  8  may also be made of plural electron-deflecting magnet assemblies or subassemblies each of which has the same structure as the aforementioned electron-deflecting magnet assembly  13 . For example, where the spectrometer  8  is made up of two identical electron-deflecting magnet assemblies  13  and  13 ′, these magnet assemblies  13  and  13 ′ are mounted on a platen  48  as shown in FIG.  4 . The magnet assemblies  13  and  13 ′ have yokes  47  and  47 ′, respectively. The two magnet assemblies  13  and  13 ′ are mounted to the platen  48  such that unfixed side surfaces of the yokes  47  and  47 ′ are located opposite to each other. Those components of the electron-deflecting magnet assembly  13 ′ which correspond to their counterparts of the electron-deflecting magnet assembly  13  are indicated by like reference numerals with “′” attached, and these components indicated by numerals with prime “′” will not be described in detail below. 
     In the spectrometer  8  shown in FIG. 4, two discrete electron-deflecting magnet assemblies  13  and  13 ′ are used. As shown in FIG. 5, of the components of the two electron-deflecting magnet assemblies  13  and  13 ′, the polepieces  14  and  14 ′ may be integrally fabricated in the polepiece base  100 . Also, the polepieces  15  and  15 ′ may be integrally fabricated in the polepiece base  101 . Furthermore, the spacers  36  and  36 ′ may be formed into one larger spacer  36 . The spacers  37  and  37 ′ may be formed into one larger spacer  37 . The electron passage grooves  40  and  40 ′ may be formed into one larger electron passage groove  40 . The electron passage grooves  43  and  43 ′ may be formed into one larger electron passage groove  43 . The O-ring grooves  39  and  39 ′ may be formed into one larger O-ring groove  39 . The O-ring grooves  42  and  42 ′ may be formed into one larger O-ring groove  42 . The O-ring grooves  38  and  38 ′ may be formed into one larger O-ring groove  38 . The O-ring grooves  41  and  41 ′ may be formed into one larger O-ring groove  41 . In this case, the magnet assemblies may be fabricated without a platen  48  shown in FIG.  5 . 
     As shown in FIG.  11 ( a ), the electron passage  29  may have a deeper groove portion  43  whose depth is equal to the sum of the depths of the electron passage grooves  40  and  43  of the structure shown in FIG.  5 . The deeper groove portion  43  is formed only in one (e.g., the spacer  37 ) of the two spacers  36  and  37 . In this case, the spacer  36  has no electron passage groove. 
     As shown in FIG.  11 ( b ), the electron passage  29  may be made by inserting third spacers  50  between the spacers  36  and  37  which have no electron passage groove. The thickness of the third spacers  50  is equal to the depth of the deeper groove  43 . 
     As can be understood from the description provided thus far, in the energy filter in accordance with the present invention, sealing members permit coils of magnets to be located outside a vacuum. Therefore, deterioration of the vacuum inside the electron microscope can be prevented. This prevents deterioration of the performance of the microscope. 
     At least one electron passage groove is formed in one of a pair of spacers, or a third spacer is inserted between a pair of spacers, to form one electron passage. Consequently, this structure is easier to machine and cleanse than the prior art tube. 
     Furthermore, it is not necessary to set large the distance between a pair of polepieces, since no tube is accommodated in the electron passage. Hence, the magnet assembly can be designed compactly. In addition, a small-sized power supply suffices. 
     Additionally, shunts for preventing ooze of magnetic fields are coupled to these polepieces at their bulges. Therefore, it is not necessary to take account of the accuracy with which they are assembled. In consequence, the magnet assembly can be assembled with improved efficiency. 
     Having thus described my invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.