Abstract:
In order to provide a charged particle beam apparatus enabling reduction of deflecting coma aberration in cases such as where wide field-of-view scanning is carried out, a charged particle beam apparatus is provided with an electromagnetic objective lens and a stage on which a sample is placed, wherein the electromagnetic objective lens is provided with the following: a plurality of magnetic paths; an objective lens coil; an opening disposed so as to face the sample; an inner lens deflector disposed more on the objective lens coil side than the end of the opening.

Description:
TECHNICAL FIELD 
     The present invention relates to a charged particle beam apparatus that performs an inspection, a measurement, and the like using a charged particle beam such as an electron beam. 
     BACKGROUND ART 
     A scanning electron microscope (SEM) used to observe, inspect, or measure a sample using an electron beam, which is one of charged particle beams, accelerates electrons emitted from an electron source and converges onto a surface of the sample using an electrostatic lens or an electromagnetic lens for irradiation thereon. They are referred to as primary electrons. Incidence of the primary electrons generates secondary electrons (sometimes low-energy electrons may be referred to as the secondary electrons and high-energy electrons as reflection electrons, distinctively). By detecting the secondary electrons while deflecting and scanning the electron beam, it is possible to obtain a scanning image of a fine pattern and a composition distribution on the sample. It is also possible to form an absorption current image by detecting electrons absorbed by the sample. 
     Conventional methods of deflecting an electron beam are disclosed in, for example, Patent Documents 1 and 2. These Patent Documents aim to reduce a deflection aberration caused by the deflection of the electron beam by overlapping a field of an electromagnetic deflector and a field of an electromagnetic lens. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent document 1: Japanese Patent Laid-open No. 2004-134379 
     Patent document 2: Japanese Patent Laid-open No. 2003-521096 
     SUMMARY OF INVENTION 
     Technical Problem 
     One of the features considered to be desirable for a scanning electron microscope may be an ability to scan a wide visual field without substantially degrading the resolution of the electron beam. As miniaturization of semiconductor devices progresses, two-dimensional high-speed inspection of a resist pattern is required more and scanning a wider visual field is required for enlargement of an inspection area and reduction of shrinkage. 
     The inventors now reviewed problems in a case where the wide visual field is scanned using the conventional deflection method. According to Patent Document 1, although the magnetic fields of the electromagnetic lens and the electromagnetic deflector are overlapping, the electromagnetic deflector is larger than an aperture of the electromagnetic lens and it is located on the side of the electron beam (opposite side form a lens coil). Thus, it is found that there is a large distribution of a deflection field in vertical directions on an axis that the electron beam passes resulting in a smaller proportion of the overlapping field, and that, in Patent Document 2, the electromagnetic deflector is surrounded by a magnetic structure around the electromagnetic lens and overlaps the field of the electromagnetic lens but it does not align with the aperture position of the electromagnetic lens to limit the proportion of the overlap. 
     Thus, it is concerned that scanning the wide visual field using these conventional techniques may not attain an effect of sufficiently reducing the deflection coma aberration. Moreover, it is also found that a larger electrostatic lens field is required to improve the on-axis resolution, and the turbulence of the electrostatic lens field due to the location of the electromagnetic deflector overlapping the electrostatic lens field may lead to degradation of the lens performance. 
     An object of the present invention is to provide a charged particle beam apparatus capable of reducing the deflection coma aberration even when scanning a wide visual field. 
     Solution to Problem 
     One implementation to achieve the above object is a charged particle beam apparatus including a charged particle source, an upper stage deflector deflecting a charged particle beam emitted from the charged particle source, an electromagnetic objective lens irradiating the charged particle beam deflected by the upper stage deflector onto a sample, and a stage on which the sample is placed, wherein the electromagnetic objective lens has a plurality of magnetic paths and an objective lens coil surrounded by the plurality of magnetic paths, an aperture constituted by the plurality of magnetic paths and positioned to face the sample, and an in-lens deflector arranged closer to the objective lens coil than an end of the aperture. 
     There is also provided a charged particle beam apparatus including a charged particle source, an upper stage deflector, an electromagnetic objective lens, and a stage on which a sample is placed, all sequentially arranged along a travelling direction of a charged particle beam emitted from the charged particle source, wherein the electromagnetic objective lens has an upper magnetic path and a lower magnetic path, an objective lens coil surrounded by the upper magnetic path and the lower magnetic path, an aperture constituted by the upper magnetic path and the lower magnetic path and arranged to face the sample, and an in-lens deflector including a toroidal coil and arranged closer to the objective lens coil than an end of the aperture. 
     Advantageous Effects of the Invention 
     The present invention can provide a charged particle beam apparatus capable of reducing the deflection coma aberration even when scanning a wide visual field. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall schematic configuration diagram showing a charged particle beam apparatus (scanning electron microscope) according to a first embodiment of the present invention; 
         FIG. 2  is a top view of a third deflector in the scanning electron microscope according to the first embodiment; 
         FIG. 3  is an enlarged cross sectional view of a main part illustrating an objective optical system in the scanning electron microscope according to the first embodiment; 
         FIG. 4  is a flowchart illustrating an example of a procedure for adjusting the scanning electron microscope according to the first embodiment; 
         FIG. 5  is a chart showing an example of items displayed on a user interface screen of the scanning electron microscope according to the first embodiment; 
         FIG. 6  is an enlarged cross sectional view of a main part illustrating an objective optical system in the scanning electron microscope (scanning electron microscope) according to a second embodiment of the present invention; 
         FIG. 7  is a top view of a third deflector in the scanning electron microscope according to the second embodiment; 
         FIG. 8  is an enlarged cross sectional view of a main part illustrating an objective optical system in the scanning electron microscope (scanning electron microscope) according to a third embodiment of the present invention; 
         FIG. 9  is a top view of a third deflector in the scanning electron microscope according to the third embodiment; and 
         FIG. 10  is a comparison chart between the deflection coma aberration in the scanning electron microscope according to the first embodiment and that in the conventional device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As a result of reviewing technologies for achieving the above object, the inventors found that the deflection coma aberration can be reduced by providing a deflector (in-lens deflector) inside an objective lens having a space surrounded by an upper magnetic path and a lower magnetic path, though in which configuration the effectiveness was initially considered doubtful. It is particularly effective to provide the in-lens deflector closer to the objective lens coil than an aperture end facing a sample for the objective lens (in a magnetic path of the objective electromagnetic lens). It should be noted that the term “facing” means that the aperture end has only to be directed to the sample and may have a certain angle between a plane of the aperture end and a surface of the sample. It is desirable, however, that these planes are parallel to each other. Furthermore, it is also effective to provide another deflector on an upper state of the objective lens and coordinate a plurality of deflectors. This enables reduction of the deflection coma aberration effectively and with the least adverse effect (such as degradation of the lens performance). 
     Detailed description is given below using embodiments. 
     First Embodiment 
     A first embodiment of the present invention is described below with reference to  FIGS. 1 to 5 and 10 .  FIG. 1  shows an overall schematic configuration diagram of a scanning electron microscope according to a first embodiment. Primary electrons  116  emitted from an electron gun  101  are focused on the sample by a first condenser lens  103 , a second condenser lens  105 , and an objective lens constituted by an objective lens upper magnetic path  110 , an objective lens lower magnetic path  111 , and an objective lens coil  112 . An objective lens upper magnetic path is applied with positive voltage and a sample  114  is applied with negative voltage, which forms an electrostatic lens, and thus the objective lens is an electromagnetic field superimposed lens. An aperture  400  of the lens is directed to the sample, which forms a lens structure called semi-in-lens type. However, the aperture of the objective lens has only to be directed to the sample but it is not limited to the semi-in-lens type. Secondary electrons  117  emitted from the sample  114  (although there is a case where low-speed electrons are referred to as secondary electrons and high-speed electrons as reflection electrons, they are collectively expressed as the secondary electrons herein) are detected by a detector  121  located in the middle. The primary electrons on the sample are two-dimensionally scanned by a first scanning deflector  106  and a second scanning deflector  108 , resulting in a two-dimensional image. The center of the two-dimensional image is defined by a first deflector  107  and a second deflector  109 . In this embodiment, the first and second scanning deflectors are electrostatic deflectors, and the first and second deflectors are electromagnetic deflectors. The two-dimensional image is displayed on a display device  147 . 
     Denoted by  115  is a stage,  131  is an electron gun control unit,  133  is a first condenser lens control unit,  135  is a second condenser lens control unit,  137  is a first scanning deflector control unit,  139  is a second scanning deflection control unit,  138  is a first deflector control unit,  140  is a second deflector control unit,  141  is a booster voltage control unit,  142  is an objective lens control unit,  144  is a sample voltage control unit,  136  is a detection system control unit,  102  is a first aligner,  132  is a first aligner control unit,  104  is a second aligner,  134  is a second aligner control unit,  118  is a focal point corrector,  148  is a focal point corrector control unit,  119  is an astigmatism corrector,  149  is an astigmatism corrector control unit,  120  is a deflection chromatic aberration corrector,  150  is a deflection chromatic aberration corrector control unit,  145  is a recording device, and  146  is a control operation device for the whole apparatus. It should be noted that the same reference numerals generally refer to the same components. 
     Moreover, a third deflector (in-lens deflector)  113  is arranged in the magnetic path of the objective lens (closer to the objective lens coil than the aperture of the magnetic path). Denoted by  143  is a third deflector control unit. Shown in  FIG. 2  is a top view around the third deflector. The third deflector  113  is a toroidal deflector including a toroidal coil  205  wound around a ring-shaped ferrite core  202 . Denoted by  201  is an outer periphery of the objective lens lower magnetic path,  204  is an inner periphery of the objective lens lower magnetic path,  207  is an outer periphery of the objective lens upper magnetic path, and  203  is an inner periphery of the objective lens upper magnetic path. In this embodiment, the deflection coma aberration is reduced by adding the third deflector to the first and second deflectors  107 ,  109  and cooperatively controlling them. It should be noted that the number of the deflectors outside the objective lens is not limited to two. Because an effect of the third deflector on the convergence action of electrons on the axis can be ignored by arranging the third deflector (in-lens deflector) in the magnetic path of the objective lens, the on-axis resolution is not degraded by the third deflector. Moreover, by arranging the deflector in the magnetic path of the objective lens, distribution of the deflecting lens magnetic field concentrates on the magnetic path aperture  400  allowing for increasing an overlap of positions of the lens magnetic field and the deflecting magnetic field, and consequently it can be expected to increase an in-lens effect of the deflector. Furthermore, in this structure, the electrostatic lens can be used because the electrostatic lens field formed by a potential difference between the objective lens upper magnetic path  110  and the sample  114  is not disturbed. In this structure, the on-axis resolutions with and without the electrostatic lens are 1.8 nm and 5.2 nm, respectively. 
     Using a dense-core toroidal deflector is effective for the following two reasons. Firstly, using ferrite that is a magnetic body can increase deflection sensitivity, thereby compensating the reduction of the deflection sensitivity due to the shielding effect of the lens magnetic path. Moreover, because the magnetic body is flat without any irregular structure in its rotating direction to have a rotationally symmetrical shape, it will not give any asymmetrical effect on the lens magnetic field. Note that the term “flat” includes a structure having no irregularity formed intentionally. For these reasons, efficient deflection sensitivity can be obtained without adversely affecting the lens performance on the axis, and consequently the correction of the deflection coma aberration is possible. To arrange the third deflector in the magnetic path of the objective lens as shown in  FIG. 2 , a diameter of the third deflector is made larger than the diameters of the inner peripheries  203 ,  204  of the upper magnetic path and the lower magnetic path forming the lens magnetic field and smaller than the diameters of the outer peripheries  207 ,  201  of the upper magnetic path and the lower magnetic path. This enables an arrangement of the electromagnetic deflector closer to the objective lens coil than the aperture  400  of the lens magnetic path. 
       FIG. 3  shows a part of the objective lens  300  in detail, particularly illustrating upper magnetic path and lower magnetic path ferrite shields  302 ,  303  and a height-detection optical path  301 . One problem for arranging the electromagnetic deflector  113  in the space surrounded by the objective lens upper magnetic path  110  and the objective lens lower magnetic path  111  (in the objective lens magnetic path) is response speed of the deflector  113 . When the magnetic field of the electromagnetic deflector  113  interacts directly with the lens magnetic paths  110 ,  111 , the response speed is delayed by eddy current. The eddy current easily occurs in the lens magnetic paths  110 ,  111  because of using permalloy or pure iron having a high magnetic permeability. Therefore, in this embodiment, shields  302 ,  303  made of ferrite are provided between the lens magnetic paths  110 ,  111  and the deflector  113  in the magnetic path. High-speed response can be expected because ferrite has no electrical conduction and causes no eddy current. The ferrite shields  302 ,  303  are arranged so as not to affect the lens magnetic field and not to be external to the lens magnetic paths  110 ,  111 . Moreover, to avoid magnetic coupling with permalloy, the ferrite shields  302 ,  303  are spaced from permalloy. Although the ferrite shield is preferably arranged facing an entire face of the inner wall of the magnetic path, the effect of reducing the eddy current can be achieved even when it is provided only partially. 
     Another problem for arranging the electromagnetic deflector  113  in the objective lens magnetic path can be the height-detection optical path  301 . When the objective lens  300  is close to the sample  114 , the height-detection optical path  301  often passes through the aperture  400  of the objective lens  300 . Therefore, the optical path is provided between the objective lens coil  112  and the electromagnetic deflector  113  so that the in-lens deflector  113  in this embodiment may not interrupt the optical path. Thus, the in-lens deflector  113  is to be arranged in a region between the magnetic path and the optical path in the objective lens  300 . 
     A method of adjusting the scanning electron microscope according to this embodiment is described below with reference to  FIG. 4 . Adjustment is started (S 401 ) and initial values are set (S 402 ). Firstly, initial values of the strength of each deflector in X and Y directions are set respectively. The initial value set here may be a parameter corresponding to an amount of current per amount of deflection on the sample. Next, a condition for the electron beam (primary electrons)  116  to vertically enter the sample  114  when deflected by the first and second deflectors  107 ,  109  is determined (S 403 ). The deflection is performed on the sample using the previously set initial values to measure an incidence angle. If the incidence angle is larger than a specified value, the incidence angle is corrected by adjusting the parameters corresponding to the respective amounts of current per amount of deflection of the first deflector and the second deflector in X and Y directions. Then, a track under the condition of reducing the deflection coma aberration is determined using the third deflector  113  (S 404 ). That is, the coma aberration at the time of deflection is determined, the parameters corresponding to the respective amounts of current per amount of deflection of the third deflector in X and Y directions are adjusted, and thereby correcting the deflection coma aberration. Next, the incidence angle of the electron beam  116  is measured (S 405 ). If the track deviates from the normal incidence, the process returns to Step S 403  to achieve the normal incidence using the first and second deflectors  107 ,  109 . By repeating this loop (Step S 403  to Step S 405 ), the condition is found for limiting the values for satisfying both the normal incidence and the reduction of the deflection coma aberration. When using the device, the three deflectors can be cooperatively operated by applying current thereto simultaneously according to the respective parameters the three deflectors. 
     This is made possible by coordinating the three deflectors  113 ,  107 ,  109  including the deflector disposed in the magnetic path of the lens. This cannot be achieved by only two deflectors  107 ,  109  or by three deflectors in a region away from the objective lens  300 . 
     Furthermore, the device according to this embodiment is provided with the deflection chromatic aberration corrector  120 , the focal point corrector  118 , and the astigmatism corrector  119  (see  FIG. 1 ), and capable of correcting these three aberrations that may degrade the resolution other than the deflection coma aberration caused by deflection. In the flow chart shown in  FIG. 4 , to use these correctors, parameters (amount corresponding to drive current or drive voltage of the corrector per amount of deflection) for correcting a deflection chromatic aberration (S 406 ), correcting a field curvature (S 407 ), and for correcting an astigmatism aberration (S 408 ) are determined, thereby terminating the entire adjustment (S 409 ). 
     Next, an example of items displayed on a user interface screen of the scanning electron microscope is described with reference to  FIG. 5 .  FIG. 5  shows the user interface for using the device. Each correction item is selectable, and in this embodiment, the selection is made so as to correct all correctable aberrations including the focal point astigmatism, deflection chromatic, and deflection coma aberrations. 
     The deflection coma aberration is corrected using the scanning electron microscope described above.  FIG. 10  shows size comparison between the deflection coma aberrations with and without actuation of the third deflector (in-lens deflector). Consequently, this achieves substantial elimination of the resolution degradation in a visual field with an angular deflection as large as 30 μm and the normal incidence by cooperatively operating the three deflectors and the three correctors. By using the device according to this embodiment, more precise observation images can be obtained and inspections and measurements can be performed with higher accuracy. 
     As described above, by providing the in-lens deflector, this embodiment can provide a charged particle beam apparatus capable of reducing the deflection coma aberration even when scanning a wide visual field. Moreover, by correcting the deflection chromatic aberration, the field curvature, and the astigmatism aberration as well, the substantial elimination of the resolution degradation can be achieved. 
     Second Embodiment 
     A second embodiment of the present invention is described below with reference to  FIGS. 6 and 7 . It should be noted that what is described in the first embodiment but not in the second embodiment is also applicable to this embodiment unless otherwise specified. 
       FIG. 6  is an enlarged cross sectional view of a main part illustrating an objective optical system in the scanning electron microscope according to this embodiment. The general schematic configuration of the device is similar to that shown in  FIG. 1 . In this embodiment, an octapole electrostatic deflector  601  in the magnetic path of the objective lens  300  as the third deflector (in-lens deflector). An advantage of the electrostatic deflector  601  is to avoid a response delay due to the eddy current. 
       FIG. 7  shows a top view of the third deflector in the scanning electron microscope according to this embodiment. A diameter of the electrostatic deflector  601  is made larger than diameters of the inner peripheries  203 ,  204  of the upper magnetic path and the lower magnetic path forming the lens magnetic field, and smaller than diameters of the outer peripheries  207 ,  201  of the upper magnetic path and the lower magnetic path. In the case of the electrostatic deflector  601 , because the magnetic path of the lens is conductive permalloy, there is also an effect of shielding an electrostatic deflection field. This makes the deflection field overlap the lens magnetic field, as in the first embodiment. In this embodiment as well the upper magnetic path  110  is applied with positive voltage and the sample  114  is applied with negative voltage, which forms the electrostatic lens on the sample. Therefore, it is effective that an average voltage of the electrostatic deflector  601  should be adjusted so as not to affect the electrostatic lens, and it is adjusted to be no higher than the voltage in the upper magnetic path and no lower than the voltage in the lower magnetic path. It is set to a voltage close to that in the lower magnetic path  111  in this embodiment. 
     Consequently, this achieves the substantial elimination of the resolution degradation in a visual field with an angular deflection as large as 30 μm and the normal incidence by cooperatively operating the three deflectors including the two electromagnetic deflectors  107 ,  109  on the upper stage and the three correctors. By using the device according to this embodiment, more precise observation images can be obtained and inspections and measurements can be performed with higher accuracy. 
     As described above, this embodiment can provide a similar effect as the first embodiment. Moreover, by replacing the in-lens deflector with the electrostatic deflector, the ferrite shield can be eliminated to simplify the device configuration. 
     Third Embodiment 
     A third embodiment of the present invention is described below with reference to  FIGS. 8 and 9 . It should be noted that what is described in the first or second embodiment but not in the third embodiment is also applicable to this embodiment unless otherwise specified. 
       FIG. 8  is an enlarged cross sectional view of a main part illustrating an objective optical system in the scanning electron microscope according to this embodiment. The general schematic configuration of the device is similar to that shown in  FIG. 1 . In this embodiment, a third deflector (in-lens deflector)  801 , which is an electromagnetic deflector, is arranged outside the upper magnetic path  110  of the electromagnetic lens (objective lens). Denoted by  902  is a ferrite core of the third deflector, and  905  is a toroidal coil. In this embodiment, the in-lens deflector is also disposed closer to the objective lens coil than the aperture end facing the sample  114  for the objective lens  300 .  FIG. 9  is a top view of the third deflector. The third deflector  801  is a dense-core toroidal deflector, of which diameter is made larger than the diameter of the inner periphery of the upper magnetic path forming the lens magnetic field and smaller than the diameter of the inner periphery  204  of the lower magnetic path and the diameters of the outer peripheries  207 ,  201  of the upper magnetic path and the lower magnetic path. In this embodiment, because the diameter of the third deflector  801  is smaller than the inner periphery  294  of the lower magnetic path, the shielding effect of the lens magnetic path is lowered. However, in the region above the sample which is important for the primary electrons, it is possible to achieve a sufficient effect of overlapping the lens field and the deflection field even with the shielding effect by the upper magnetic path  110  alone. Moreover, though the magnetic body of the third deflector may affect the distribution formation of the lens magnetic field, the toroidal deflector would have a rotationally symmetrical magnetic body, which should not generate an asymmetrical field that reduces the on-axis resolution. As in the first embodiment, the reduction of the deflection coma aberration is made possible by coordinating with the two-stage deflectors  107 ,  109  on the upper stage in the objective lens, and the resolution degradation can be avoided in the visual field with an angular deflection as large as 30 μm by coordinating corrections of the focal point, astigmatism, and deflection chromatic aberrations as well. By using the device according to this embodiment, more precise observation images can be obtained and inspections and measurements can be performed with higher accuracy. 
     As described above, this embodiment can provide a similar effect as the first embodiment. Moreover, this embodiment can eliminate the ferrite shield. 
     Although the device using an electron beam has been described in the above embodiments, it is also possible to use an ion beam. In such a case, the electrostatic deflector is desirable as the third deflector. This can largely deflect even an ion beam. Moreover, the present invention is not limited to the embodiments described above but can include various modifications. For example, the above embodiments are described in detail for clearly explaining the invention, and are not necessarily limited to include all the configurations described. Moreover, it is also possible to replace a part of a configuration in one embodiment with a configuration in another embodiment, and also to add a configuration in one embodiment to a configuration in another embodiment. Furthermore, it is also possible to add another configuration to, eliminate, or replace a part of a configuration in each embodiment. 
     REFERENCE SIGNS LIST 
     
         
         
           
               101  Electron Gun 
               102  first aligner 
               103  first condenser lens 
               104  second aligner 
               105  second condenser lens 
               106  first scanning deflector 
               107  first deflector 
               108  second scanning deflector 
               109  second deflector 
               110  objective lens upper magnetic path 
               111  objective lens lower magnetic path 
               112  objective lens coil 
               113  third deflector 
               114  sample 
               115  stage 
               116  primary electron 
               117  secondary electron 
               118  focal point corrector 
               119  astigmatism corrector 
               120  deflection chromatic aberration corrector 
               121  detector 
               131  electron gun control unit 
               132  first aligner control unit 
               133  first condenser lens control unit 
               134  second aligner control unit 
               135  second condenser lens control unit 
               136  detection system control unit 
               137  first scanning deflector control unit 
               138  first deflector control unit 
               139  second scanning deflector control unit 
               140  second deflector control unit 
               141  booster voltage control unit 
               142  objective lens control unit 
               143  third deflector control unit 
               144  sample voltage control unit 
               145  recording device 
               146  control operation device for whole apparatus 
               147  display device 
               148  focal point corrector control unit 
               149  astigmatism corrector control unit 
               150  deflection chromatic aberration corrector control unit 
               201  outer periphery of objective lens lower magnetic path 
               202  ferrite core of third deflector 
               203  inner periphery of objective lens upper magnetic path 
               204  inner periphery of objective lens lower magnetic path 
               205  toroidal coil 
               207  outer periphery of objective lens upper magnetic path 
               300  objective lens 
               301  height-detection optical path 
               302  upper magnetic path ferrite shield 
               303  lower magnetic path ferrite shield 
               400  aperture of objective lens 
               601  third electrostatic deflector 
               801  third deflector 
               902  ferrite core of third deflector 
               905  toroidal coil