Patent Publication Number: US-2023136198-A1

Title: Wien filter and multiple electron beam inspection apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a Wien filter and a multiple electron beam inspection apparatus. 
     BACKGROUND ART 
     As LSI circuits are increasing in density, the line width of circuits of semiconductor devices is becoming finer. To form a desired circuit pattern onto a semiconductor device, a method of reducing and transferring, by using a reduction-projection exposure apparatus, onto a wafer a highly precise original image pattern formed on a quartz is employed. 
     An improvement in yield is indispensable for the fabrication of LSI, which takes a massive fabrication cost. With miniaturization of the dimensions of the LSI pattern formed on a semiconductor wafer, the dimensions of pattern defects to be detected are also extremely small. Thus, high precision of a pattern inspection apparatus that inspects a hyperfine pattern transferred onto a semiconductor wafer for defects is needed. 
     As an inspection method for pattern defects, there is known a method of comparing a measurement image obtained by capturing a pattern formed on a substrate, such as a semiconductor wafer and a lithography mask, with design data or a measurement image obtained by capturing the same pattern on the substrate. Examples of the inspection method include “die-to-die inspection” that compares pieces of measurement image data obtained by capturing the same patterns at different locations on the same substrate and “die-to-database inspection” that generates design image data (reference image) based on pattern-designed design data and that compares the design image data with a measurement image that is measurement data obtained by capturing a pattern. When the compared images do not match, it is determined that there are pattern defects. 
     There has been developing an inspection apparatus that acquires a pattern image by scanning on a substrate to be inspected with electron beams and detecting secondary electrons emitted from the substrate with application of electron beams. Development of an apparatus using multiple beams as an inspection apparatus using electron beams has been proceeding. 
     When a substrate to be inspected is irradiated with multiple beams (multiple primary electron beams), a flux of secondary electrons (multiple secondary electron beams) including reflected electrons corresponding to respective ones of the multiple beams is emitted from the substrate to be inspected. The multiple beam inspection apparatus includes a Wien filter for separating multiple secondary electron beams from multiple primary electron beams. 
     In a plane orthogonal to a beam traveling direction (or path central axis), the Wien filter generates an electric field and a magnetic field in directions orthogonal to each other. The multiple primary electron beams that enter the Wien filter from above travel straight downward, because forces of the electric and magnetic fields acting on the multiple primary electron beams cancel each other out. On the other hand, the multiple secondary electron beams that enter the Wien filter from below are bent obliquely upward and separated from the multiple primary electron beams, because forces of the electric and magnetic fields act in the same direction on the multiple secondary electron beams. 
     In a conventional Wien filter, a plurality of electromagnetic poles are arranged at regular intervals on the same inner circumference of a cylindrical yoke, and the electromagnetic poles each have a coil wound thereon. A voltage applied to each electromagnetic pole and the amount of current passing through the coil are controlled, so that the electric and magnetic fields are superimposed. 
     The cylindrical yoke has a ground potential. Each electromagnetic pole is joined to the inner periphery of the cylindrical yoke, with an insulator therebetween. The insulator has a resistance (magnetic resistance) against a magnetic flux generated in the coil. To provide an efficient Wien filter that uses less coil current, the insulator is required to be reduced in thickness. With a thin insulator, however, there is an increased risk of discharge between the cylindrical yoke and the electromagnetic pole (high-voltage portion) to which a voltage is applied.
     Patent Literature 1: JP 11-233062 A   Patent Literature 2: JP 2007-27136 A   Patent Literature 3: JP 2018-10714 A   Patent Literature 4: JP 2006-277996 A   

     SUMMARY OF INVENTION 
     An object of the present invention is to provide a Wien filter that has a low risk of discharge and operates efficiently and stably, and to also provide a multiple electron beam inspection apparatus that includes the Wien filter. 
     According to one aspect of the present invention, a Wien filter includes a cylindrical yoke, a plurality of magnetic poles arranged at intervals along an inner periphery of the yoke, the magnetic poles each joined at one end thereof to the yoke, a coil wound on each of the plurality of magnetic poles, and an electrode disposed at the other end of each of the plurality of magnetic poles, with an insulator between the electrode and the magnetic pole. 
     According to one aspect of the present invention, a multiple electron beam inspection apparatus includes an optical system irradiating a substrate with multiple primary electron beams, a beam separator separating, from the multiple primary electron beams, multiple secondary electron beams emitted as a result of irradiating the substrate with the multiple primary electron beams, and a detector detecting the multiple secondary electron beams separated. The above Wien filter is used as the beam separator. 
     Advantageous Effects of Invention 
     The present invention can reduce the risk of discharge in the Wien filter and allow the Wien filter to operate efficiently and stably. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic cross-sectional diagram of a Wien filter according to an embodiment of the present invention. 
         FIG.  2    is a perspective view of a magnetic pole. 
         FIG.  3    is a perspective view of a magnetic pole according to another embodiment. 
         FIG.  4    is a schematic diagram of the magnetic pole and an electrode according to the embodiment. 
         FIG.  5    is a schematic diagram of a magnetic pole and an electrode according to another embodiment. 
         FIG.  6 A  is a schematic diagram of a Wien filter according to another embodiment, and  FIG.  6 B  is an enlarged view of part of the Wien filter. 
         FIG.  7 A  and  FIG.  7 B  are schematic diagrams each illustrating a magnetic pole according to another embodiment. 
         FIG.  8    is a schematic diagram of a Wien filter according to another embodiment. 
         FIG.  9    is a diagram illustrating a general configuration of a pattern inspection apparatus according to an embodiment. 
         FIG.  10    is a plan view of a shaping aperture array substrate. 
         FIG.  11    is a schematic diagram illustrating electromagnetic poles of a Wien filter according to a comparative example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described on the basis of the drawings. 
       FIG.  1    is a schematic cross-sectional diagram of a Wien filter  1  according to an embodiment of the present invention. The Wien filter  1  includes a cylindrical yoke  2  and a plurality of magnetic poles  3  arranged along the inner periphery of the yoke  2 . The plurality of magnetic poles  3  are arranged at regular intervals on the same circumference about the cylindrical axis of the yoke  2 . Eight magnetic poles  3  are arranged in the example illustrated in  FIG.  1   . 
     Each magnetic pole  3  of the Wien filter  1  has a coil  4  wound thereon. The magnetic pole  3  extends in the radial direction of the yoke  2 . The magnetic pole  3  is joined to the yoke  2  at one end thereof, and has an electrode  5  at the other end thereof (or at an end portion thereof adjacent to the yoke center), with an insulator  6  between the magnetic pole  3  and the electrode  5 . A space in the yoke center surrounded by a plurality of electrodes  5  is a beam passage region. 
     Each coil  4  is connected to a current source (not shown) and configured to allow the amount of current to be independently controlled. Each electrode  5  is connected to a voltage source (not shown) outside the yoke and configured to allow a voltage applied thereto to be independently controlled. The yoke  2  has a ground potential. 
     The yoke  2  and the magnetic pole  3  may be made of a magnetic material, such as permalloy. The electrode  5  may be a conductive member, such as a copper plate. The insulator  6  may be made of, for example, a ceramic material. 
     As illustrated in  FIG.  2   , the magnetic pole  3  includes a first plate portion  31  and a second plate portion  32  coupled to the first plate portion  31 . 
     The first plate portion  31  has six surfaces: a first principal plate surface  31   a , a second principal plate surface  31   d  opposite the first principal plate surface  31   a , a proximal end  31   b , a distal end  31   e  opposite the proximal end  31   b , an upper surface  31   c , and a lower surface  31   f  opposite the upper surface  31   c . The first principal plate surface  31   a  and the second principal plate surface  31   d  are substantially parallel to the radial direction of the yoke  2 . 
     The first plate portion  31  is joined at the proximal end  31   b  thereof to the inner periphery of the yoke  2 . The distal end  31   e  of the first plate portion  31  is smaller than a first principal plate surface  32   a  of the second plate portion  32 . The distal end  31   e  is joined to a center of the first principal plate surface  32   a . The first plate portion  31  is joined to the second plate portion  32  in such a way as to be substantially perpendicular to the first principal plate surface  32   a . The first plate portion  31  and the second plate portion  32  may be formed as an integral unit having the structure described above. 
     A second principal plate surface  32   d  opposite the first principal plate surface  32   a  of the second plate portion  32  is slightly curved toward the first principal plate surface  32   a.    
     The coil  4  described above is wound around the first principal plate surface  31   a , the upper surface  31   c , the second principal plate surface  31   d , and the lower surface  31   f  of the first plate portion  31 . The electrode  5  is disposed on the second principal plate surface  32   d  of the second plate portion  32 , with the insulator  6  therebetween. 
     An electric field is generated by controlling a voltage applied to each electrode  5 . A magnetic field orthogonal to the electric field is generated by controlling current in each coil  4 . For example, an electric field is generated by applying predetermined voltages (e.g., +5 kV for one electrode  5  and −5 kV for the other electrode) from the voltage source to the electrodes  5  at the 6 o&#39;clock and 12 o&#39;clock positions in  FIG.  1   . Also, when a magnetic flux is generated by controlling the amount of current passing through the coils  4  at the 3 o&#39;clock and 9 o&#39;clock positions using the current source, the magnetic flux flows from the magnetic pole  3  at the 3 o&#39;clock position through the yoke  2  to the magnetic pole  3  at the 9 o&#39;clock position to generate a magnetic field orthogonal to the electric field. 
     Referring to  FIG.  11   , a conventional Wien filter applies a voltage to a magnetic pole  70  (electromagnetic pole) having the coil  4  thereon to generate an electric field. For example, an electric field is generated by applying a voltage of +5 kV and a voltage of −5 kV to one and the other of the magnetic poles  70  at the 6 o&#39;clock and 12 o&#39;clock positions. Also, when a magnetic flux is generated by controlling the amount of current passing through the coils  4  at the 3 o&#39;clock and 9 o&#39;clock positions, the magnetic flux flows from the magnetic pole  70  at the 3 o&#39;clock position through the yoke  2  to the magnetic pole  70  at the 9 o&#39;clock position to generate a magnetic field orthogonal to the electric field. Since the yoke  2  has a ground potential, the magnetic pole  70  and the yoke  2  need to be provided with an insulator  72  therebetween. When the insulator  72  is thick (i.e., there is a large insulation gap), the resulting large magnetic resistance makes passage of the magnetic flux difficult and increases the amount of coil current required. When the insulator  72  is made thinner to reduce the increase in coil current, there is an increased risk of discharge between the yoke  2  and the magnetic pole  70  to which a predetermined voltage for generating an electric field is applied. 
     In the present embodiment, on the other hand, a voltage for generating an electric field is applied to the electrode  5  separate from the magnetic pole  3  constituting a magnetic circuit. Since the insulator  6  interposed between the magnetic pole  3  and the electrode  5  has little impact on the magnetic resistance, it is possible to leave a sufficient insulation gap and reduce the risk of discharge. Also, since the yoke  2  and the magnetic pole  3  do not require an insulator therebetween, there is no need to increase coil current and the Wien filter can operate efficiently and stably. 
     As illustrated in  FIG.  3    and  FIG.  4   , a magnetic pole  3 A may have, in the center of the second principal plate surface  32   d  of the second plate portion  32 , a recess  33  toward the first principal plate surface  32   a . The magnetic pole  3 A may have an electrode  5 A at the bottom (or on the innermost surface) of the recess  33 , with the insulator  6  between the magnetic pole  3 A and the electrode  5 A. It is preferable that the electrode  5 A and the insulator  6  be accommodated in the recess  33 , and that a surface  5   a  of the electrode  5 A and the second principal plate surface  32   d  of the second plate portion  32  be curved surfaces with the same radius of curvature. Referring to  FIG.  4   , which is a cross-sectional plan view, the magnetic pole  3 A may be regarded as having a magnetic pole structure divided into two parts, with the recess  33  therebetween. 
     As illustrated in  FIG.  5   , a plate-like magnetic pole  3 B may include the second plate portion  32  having a width equal to the plate thickness of the first plate portion  31 , and an electrode  5 B and the insulator  6  may be disposed on each of both sides of the second plate portion  32 . The insulator  6  is formed into a plate shape and secured in place, with the electrode  5 B attached to the insulator  6  in such a way that the electrode  5 B can be disposed at a distance of about 2 mm from a side  3   s  of the magnetic pole  3 B. The insulator  6  may be secured to the side  3   s  of the magnetic pole  3 B, or may be secured to a component separate from the Wien filter  1  so that the insulator  6  is separated from the magnetic pole  3 B. It is preferable that surfaces  5   b  of the electrodes  5 B and the second principal plate surface  32   d  of the second plate portion  32  (i.e., an end face of the magnetic pole  3 B adjacent to the beam passage region) be curved surfaces with the same radius of curvature. In this structure, two separate electrodes  5 B are arranged, with the magnetic pole  3 B therebetween. 
     As illustrated in  FIG.  6 A , the Wien filter may include both the magnetic poles  3 A and  3 B. The magnetic poles  3 A and  3 B are arranged opposite each other, with the center of the yoke  2  therebetween. When orthogonal electric and magnetic fields are generated as illustrated, an electrode structure that generates the electric field is the same as a magnetic pole structure that generates the magnetic field. That is, the electrodes  5 B (two separate electrodes) on the respective sides of the magnetic pole  3 B and the electrode  5 A (single electrode) in the recess  33  of the magnetic pole  3 A generate an electric field. Also, the magnetic pole  3 A having a structure divided into two parts, with the recess  33  therebetween, and the plate-like single magnetic pole  3 B generate a magnetic field. With this configuration, when a plurality of electrons (multiple beams) pass through the beam passage region, the electric and magnetic fields on the axis of deflection control of the plurality of electrons are uniform and this enables deflection with high accuracy. 
     For example, by applying a voltage to the electrodes  5 B (two separate electrodes) at the 12 o&#39;clock position in  FIG.  6 A , an electric field is generated toward the electrode  5 A (single electrode) at the 6 o&#39;clock position opposite the 12 o&#39;clock position. Also, by exciting the coil  4 , a magnetic field emerges from the magnetic pole surface of the magnetic pole  3 A (with a magnetic pole structure divided into two parts) at the 9 o&#39;clock position, toward the magnetic pole surface of the magnetic pole  3 B at the 3 o&#39;clock position opposite the 9 o&#39;clock position. The electric and magnetic fields thus generated are orthogonal to each other and enable the function of the Wien filter. This relation produces similar effects on opposite electrodes and magnetic poles. The electric and magnetic fields on the axis of deflection control can thus be made uniform. 
     In the Wien filter that includes both the magnetic poles  3 A and  3 B, as illustrated in  FIG.  6 B , a width W 1  of the magnetic pole  3 B in a yoke circumferential direction (i.e., a circumferential direction of a circle concentric with the inner periphery of the yoke) may be equal to a width W 2  of the electrode  5 A in the circumferential direction, and a width W 3  of a magnetic pole surface of the magnetic pole  3 A divided into two parts (i.e., a portion adjacent to the recess  33  in the yoke circumferential direction) may be equal to a width W 4  of the electrode  5 B in the circumferential direction. This improves symmetry of the structure and enables deflection control with higher accuracy. 
     The first plate portion  31  and the second plate portion  32  of any of the magnetic poles  3 ,  3 A, and  3 B may be formed as an integral unit or may be separate components coupled together. Also, the yoke  2  and any of the magnetic poles  3 ,  3 A, and  3 B may be formed as an integral unit or may be separate components coupled together. 
     In the embodiments described above, the electrode  5  is provided separately from the magnetic pole  3 , and a voltage for generating an electric field is not applied to the magnetic pole  3 . As illustrated in  FIG.  7 A , however, a high-resistance permanent magnet  7  may be provided between the yoke  2  and the first plate portion  31  of the magnetic pole  3  (electromagnetic pole) and a voltage may be applied to the magnetic pole  3 . A high-resistance material, such as ferrite, may be used to form the permanent magnet  7 . 
     With the configuration illustrated in  FIG.  7 A , the permanent magnet  7  can reduce the occurrence of discharge between the magnetic pole  3  and the yoke  2 . Using both the permanent magnet  7  and an electromagnet (the magnetic pole  3  and the coil  4 ) can facilitate controlling a magnetic field. 
     With the configuration Illustrated in  FIG.  7 A , the high-resistance permanent magnet  7  disposed between the first plate portion  31  and the yoke  2  allows a voltage drop between a high-voltage portion (magnetic pole  3 ) and a ground portion (yoke  2 ) and this can reduce the risk of discharge. Also, since an electrode structure that generates an electric field and a magnetic pole structure that generates a magnetic field can be formed as a common structure, orthogonal electric and magnetic fields are similarly distributed and it is possible to simplify the structure and improve accuracy of beam control. 
     When the permanent magnet  7  is a permanent magnet member, such as a rubber magnet, that can be roughly regarded as an insulator, the high-voltage portion can be insulated from the ground portion and this can reduce the risk of discharge. Also, since the permanent magnet  7  provides a magnetomotive force for generating a magnetic field, it is possible to reduce magnetic-field control current flowing through the coil  4  and reduce the risk of heat generation. 
     Since the permanent magnet  7  allows a voltage drop and reduces the risk of discharge, the permanent magnet  7  and the yoke  2  may be provided with an insulator  8  therebetween, as illustrated in  FIG.  7 B , so as to ensure isolation from the ground portion (yoke  2 ). The insulator  8  may be made of the same material as the insulator  6 . 
     Although the Wien filter includes eight magnetic poles  3  in the embodiments described above, the number of magnetic poles  3  is not limited, as long as orthogonal electric and magnetic fields can be generated. For example, the Wien filter may include four magnetic poles  3  as illustrated in  FIG.  8   , or may include sixteen magnetic poles  3  (not shown). 
     Next, a pattern inspection apparatus  100  including the Wien filter will be described with reference to  FIG.  9   . The pattern inspection apparatus  100  is configured to obtain a secondary electron image by irradiating a substrate to be inspected, with multiple beams composed of electron beams. 
     As illustrated in  FIG.  9   , the pattern inspection apparatus  100  includes an image acquiring mechanism  150  and a control system circuit  160 . The image acquiring mechanism  150  includes an electron beam column  102  (electron beam optics) and an inspection chamber  103 . The electron beam column  102  includes therein an electron gun  201 , an electromagnetic lens  202 , a shaping aperture array substrate  203 , an electromagnetic lens  205 , an electrostatic lens  210 , a collective blanking deflector  212 , a limiting aperture substrate  213 , an electromagnetic lens  206 , an electromagnetic lens  207  (objective lens), a main deflector  208 , a sub-deflector  209 , a beam separator  214 , a deflector  218 , an electromagnetic lens  224 , and a multi-detector  222 . 
     A stage  105  movable in the horizontal direction, the rotational direction and the height direction is disposed in the inspection chamber  103 . A substrate  101  (sample) to be inspected is placed on the stage  105 . Examples of the substrate  101  include an exposure mask substrate and a semiconductor substrate, such as a silicon wafer. When the substrate  101  is a semiconductor substrate, a plurality of chip patterns (wafer dies) are formed on the semiconductor substrate. When the substrate  101  is an exposure mask substrate, a chip pattern is formed on the exposure mask substrate. The chip pattern is composed of a plurality of figure patterns. The chip pattern formed on the exposure mask substrate is exposed and transferred onto a semiconductor substrate multiple times, so that a plurality of chip patterns (wafer dies) are formed on the semiconductor substrate. 
     The substrate  101  is placed on the stage  105 , with a pattern side thereof facing upward. The stage  105  has a mirror  216  disposed thereon. The mirror  216  reflects laser light for laser measurement emitted from a laser measurement system  111  disposed outside the inspection chamber  103 . 
     The multi-detector  222  is connected to a detecting circuit  106  outside the electron beam column  102 . The detecting circuit  106  is connected to a chip pattern memory  123 . 
     In the control system circuit  160 , a control computer  110  that controls the overall operation of the inspection apparatus  100  is connected through a bus  120  to a position circuit  107 , a comparing circuit  108 , a reference image generating circuit  112 , a stage control circuit  114 , a lens control circuit  124 , a blanking control circuit  126 , a deflection control circuit  128 , a storage device  109  such as a magnetic disk device, a monitor  117 , a memory  118 , and a printer  119 . 
     The deflection control circuit  128  is connected through a digital-to-analog converter (DAC) amplifier (not shown) to the main deflector  208 , the sub-deflector  209 , and the deflector  218 . 
     The chip pattern memory  123  is connected to the comparing circuit  108 . 
     The stage  105  is driven by a driving mechanism  142  under the control of the stage control circuit  114 . The stage  105  is movable in the horizontal direction and the rotational direction. The stage  105  is also movable in the height direction. 
     The laser measurement system  111  measures the position of the stage  105  by receiving light reflected off the mirror  216  using the principle of laser interferometry. The position of the stage  105  measured by the laser measurement system  111  is sent to the position circuit  107 . 
     The lens control circuit  124  controls the electromagnetic lens  202 , the electromagnetic lens  205 , the electromagnetic lens  206 , the electromagnetic lens  207  (objective lens), the electrostatic lens  210 , the electromagnetic lens  224 , and the beam separator  214 . 
     The electrostatic lens  210  is composed of, for example, three or more electrode substrates that are open in the center thereof. An electrode substrate in the middle of the electrostatic lens  210  is controlled by the lens control circuit  124  through a DAC amplifier (not shown), and upper and lower electrode substrates of the electrostatic lens  210  are supplied with a ground potential. 
     The collective blanking deflector  212  is composed of two or more electrodes, each of which is controlled by the blanking control circuit  126  through a DAC amplifier (not shown). 
     The sub-deflector  209  is composed of four or more electrodes, each of which is controlled by the deflection control circuit  128  through a DAC amplifier. The main deflector  208  is composed of four or more electrodes, each of which is controlled by the deflection control circuit  128  through a DAC amplifier. The deflector  218  is composed of four or more electrodes, each of which is controlled by the deflection control circuit  128  through a DAC amplifier. 
     A high-voltage power supply circuit (not shown) is connected to the electron gun  201 . By applying an acceleration voltage from the high-voltage power supply circuit between a filament (cathode) and an extraction electrode (anode) (not shown) in the electron gun  201 , applying a voltage to another extraction electrode (Wehnelt), and heating the cathode to a predetermined temperature, a group of electrons emitted from the cathode is accelerated and emitted as an electron beam  200 . 
       FIG.  10    is a conceptual diagram illustrating a configuration of the shaping aperture array substrate  203 . The shaping aperture array substrate  203  has a two-dimensional array of apertures  203   a  arranged at a predetermined pitch in the x and y directions. The apertures  203   a  are rectangular or circular apertures with the same shape and size. Part of the electron beam  200  passes through the plurality of apertures  203   a  to form multiple beams MB. 
     An operation of the image acquiring mechanism  150  of the inspection apparatus  100  will now be described. 
     The electron beam  200  emitted from the electron gun  201  (emission source) is refracted by the electromagnetic lens  202  and illuminates the entire shaping aperture array substrate  203 . The shaping aperture array substrate  203  has the plurality of apertures  203   a  as illustrated in  FIG.  10   . A region of the shaping aperture array substrate  203  including the plurality of apertures  203   a  is illuminated with the electron beam  200 . The electron beam  200  with which the apertures  203   a  are irradiated passes through the apertures  203   a  to form multiple beams MB (multiple primary electron beams). 
     The multiple beams MB are refracted by the electromagnetic lens  205  and the electromagnetic lens  206  to repeatedly form an image and a crossover, and pass through the beam separator  214  disposed at a crossover of the multiple beams MB to reach the electromagnetic lens  207  (objective lens). The electromagnetic lens  207  focuses the multiple beams MB onto the substrate  101 . The multiple beams MB brought into focus on the surface of the substrate  101  (sample) by the electromagnetic lens  207  are deflected together by the main deflector  208  and the sub-deflector  209  to the respective irradiation positions on the substrate  101 . 
     When all the multiple beams MB are deflected together by the collective blanking deflector  212 , the multiple beams MB are displaced from a center hole of the limiting aperture substrate  213  and blocked by the limiting aperture substrate  213 . On the other hand, the multiple beams MB not deflected by the collective blanking deflector  212  pass through the center hole of the limiting aperture substrate  213 , as illustrated in  FIG.  9   . Turning on and off the collective blanking deflector  212  enables blanking control that collectively controls the on and off of the beams. 
     When the substrate  101  is irradiated with the multiple beams MB at desired positions, a flux of secondary electrons (multiple secondary electron beams  300 ) including reflected electrons corresponding to respective ones of the multiple beams MB (multiple primary electron beams) is emitted from the substrate  101 . 
     The multiple secondary electron beams  300  emitted from the substrate  101  pass through the electromagnetic lens  207  to reach the beam separator  214 . 
     The Wien filter according to any of the embodiments described above is used as the beam separator  214 . In a plane orthogonal to the direction in which a central beam of the multiple beams MB travels (i.e., in a plane orthogonal to the central axis of the path), the beam separator  214  generates an electric field and a magnetic field in directions orthogonal to each other. The electric field exerts force in the same direction regardless of the direction of travel of electrons. On the other hand, the magnetic field exerts force in accordance with the Fleming&#39;s left-hand rule. The direction of force acting on electrons can thus be changed by the direction of travel of the electrons. 
     The multiple beams MB that enter the beam separator  214  from above travel straight downward, because the forces exerted by the electric and magnetic fields and acting on the multiple beams MB cancel each other out. On the other hand, the multiple secondary electron beams  300  that enter the beam separator  214  from below are bent obliquely upward and separated from the multiple beams MB, because the forces exerted by the electric and magnetic fields act in the same direction on the multiple secondary electron beams  300 . 
     The multiple secondary electron beams  300  bent obliquely upward and separated from the multiple beams MB are deflected by the deflector  218 , refracted by the electromagnetic lens  224 , and projected onto the multi-detector  222 . Note that  FIG.  9    gives a simplified view, which does not depict the refraction of the path of the multiple secondary electron beams  300 . 
     The multi-detector  222  detects the multiple secondary electron beams  300  projected thereon. The multi-detector  222  includes, for example, a diode-type two-dimensional sensor (not shown). The secondary electrons of the multiple secondary electron beams  300  collide with the diode-type two-dimensional sensor at positions corresponding to respective beams of the multiple beams MB. This multiplies the electrons inside the sensor, and generates secondary electron image data for each pixel from an amplified signal. 
     Detection data of secondary electrons detected by the multi-detector  222  (i.e., measured image, secondary electron image, or image to be inspected) is output to the detecting circuit  106  in order of measurement. In the detecting circuit  106 , analog detection data is converted to digital data by an analog-to-digital (A/D) converter (not shown) and stored in the chip pattern memory  123 . The image acquiring mechanism  150  thus acquires a measured image of a pattern formed on the substrate  101 . 
     The reference image generating circuit  112  generates a reference image for each mask die, on the basis of design data serving as a basis for forming a pattern on the substrate  101 , or design pattern data defined by exposure image data of a pattern formed on the substrate  101 . For example, design pattern data is read from the storage device  109  through the control computer  110 , and each figure pattern defined by the read design pattern data is converted to binary or multilevel image data. 
     Figures defined by the design pattern data are composed of basic elements, such as a rectangle and a triangle. Figure data is stored, which defines the shape, size, position, and others of each pattern figure by using information, such as coordinates (x, y) of a reference position of the figure, lengths of sides of the figure, and a figure code serving as an identifier for identifying the figure type, such as rectangle or triangle. 
     When design pattern data used as figure data is received by the reference image generating circuit  112 , the data is developed into data of each figure, and the figure code indicating the figure shape of the figure data and the figure dimensions are interpreted. Then, the figure data is developed into binary or multilevel image data of the design pattern as a pattern to be arranged within squares in units of grids of predetermined quantization dimensions, and output. 
     In other words, the design data is read, the occupancy of a figure in the design pattern is calculated for each of squares into which an inspection region is virtually divided in units of predetermined dimensions, and n-bit occupancy data is output. For example, it is preferable to set one square as one pixel. When one pixel is given a resolution of 1/2 8  (=1/256), small regions with a resolution of 1/256 are allocated to the region of a figure in the pixel to calculate the occupancy in the pixel. The calculated occupancy is output as 8-bit occupancy data to the reference image generating circuit  112 . The square (inspection pixel) is simply sized to match the pixel of measured data. 
     The reference image generating circuit  112  then performs appropriate filter processing on design image data of the design pattern which is image data of the figure. Optical image data (measured image) is under the action of filtering performed thereon by the optical system or, in other words, in an analog state that continuously changes. Therefore, by also performing filter processing on image data of the design pattern which is design-side image data whose image intensity (gray value) is a digital value, it is possible to adjust the image data to the measured data. The generated image data of a reference image is output to the comparing circuit  108 . 
     The comparing circuit  108  compares the measured image (image to be inspected) obtained by measuring the substrate  101  with the reference image corresponding thereto. Specifically, the image to be inspected and the reference image, which are positioned with respect to each other, are compared pixel-by-pixel. The comparing circuit  108  compares them pixel-by-pixel by using a predetermined determination threshold, in accordance with predetermined determination conditions, and determines whether there is a defect, such as a shape defect. For example, if a difference in pixel-by-pixel gray level is greater than a determination threshold Th, the comparing circuit  108  determines the pixel as a defect candidate, and outputs the result of the comparison. The result of the comparison may be stored in the storage device  109  or the memory  118 , displayed on the monitor  117 , or may be printed out from the printer  119 . 
     Besides the die-to-database inspection described above, the die-to-die inspection may be performed. The die-to-die inspection compares data of measured images obtained by imaging the same patterns at different points on the same substrate  101 . Accordingly, from the substrate  101  on which the same figure patterns (first and second figure patterns) are formed at different positions by the multiple beams MB (electron beams), the image acquiring mechanism  150  acquires measured images that are secondary electron images of one figure pattern (first figure pattern) and the other figure pattern (second figure pattern). In this case, the acquired measured image of the one figure pattern serves as a reference image, and the acquired measured image of the other figure pattern serves as an image to be inspected. The acquired images of the one figure pattern (first figure pattern) and the other figure pattern (second figure pattern) may be within the same chip pattern data, or may be separate in different pieces of chip pattern data. The inspection may be carried out in the same manner as the die-to-database inspection. 
     The Wien filter  1  according to any of the embodiments described above is used as the beam separator  214 . This can reduce the risk of discharge in the image acquiring mechanism  150  and enables efficient and stable operation. 
     Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications can be made without departing from the intent and scope of the present invention. 
     This application is based on Japanese Patent Application 2020-180675 filed on Oct. 28, 2020, which is incorporated by reference in its entirety. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Wien filter 
               2 : yoke 
               3 ,  3 A,  3 B: magnetic pole 
               4 : coil 
               5 ,  5 A,  5 B: electrode 
               6 ,  8 : insulator 
               100 : pattern inspection apparatus