Patent Publication Number: US-2023144201-A1

Title: Image pickup device and image generation method

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-181351, filed Nov. 5, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments relate to an image pickup device and an image generation method. 
     BACKGROUND 
     A transmission X-ray microscope is known as a device for observing a structure of a subject with high resolution and non-destructively. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view showing an example of a configuration of an image pickup device according to at least one embodiment. 
         FIG.  2    is a perspective view of a vacuum suction ring. 
         FIG.  3    is a schematic view showing a positional relationship between a disk with an aperture held in a vacuum suction ring and a subject. 
         FIG.  4    is a schematic view showing a configuration of a one-dimensional detector. 
         FIG.  5    is a principle circuit configuration diagram of a one-dimensional detector. 
         FIG.  6    is a view showing a detection principle of X-ray photons in a superconducting strip. 
         FIG.  7    is a schematic view showing another configuration example of the image pickup device of the embodiment. 
         FIGS.  8 A to  8 D  are explanatory views of image reconstruction. 
         FIGS.  9 A and  9 B  are views showing a position of a subject (observation region) at each rotation angle. 
         FIGS.  10 A and  10 B  are views showing a position of a subject (observation region) at each rotation angle. 
         FIG.  11    is a view showing a positional deviation of a center of the aperture due to a rotation axis deviation. 
         FIG.  12    is a flowchart showing an example of an image generation method according to a first embodiment. 
         FIG.  13    is a diagram showing a position correction method of an image intensity profile in the first embodiment. 
         FIG.  14    is a flowchart showing an example of an image generation method according to a second embodiment. 
         FIGS.  15 A to  15 C  are views showing changes in a reconstruction image in the second embodiment. 
         FIG.  16    is a diagram showing a position correction method of an image intensity profile according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An object of at least one embodiment is to provide an image pickup device and an image generation method capable of acquiring a reconstruction image with high accuracy. 
     In general, according to at least one embodiment, an image pickup device of at least one present embodiment includes a sample holding plate having an aperture through which imaging light applied to a subject is capable of being transmitted. Further, the image pickup device also includes a fixing member that has a first surface to which the subject is capable of being fixed and a second surface to which the sample holding plate is capable of being fixed and which is formed in parallel with the first surface at a height different from that of the first surface, and fixes a position of the aperture with respect to the subject. Further, the image pickup device also includes a rotation member that holds the subject and the sample holding plate fixed by the fixing member, and is capable of rotating at a desired angle about a rotation axis along a direction perpendicular to a surface of the subject, and a detector in which a plurality of line-shaped pixels having a line-shaped light receiving surface extending in a first direction are disposed side by side in a second direction orthogonal to the first direction. Further, the image pickup device includes an imaging optical member that forms an image of the imaging light transmitting through the subject and the aperture on a detection surface of the detector and an image processing unit that corrects coordinates of an image intensity profile detected by the detector and reconstructs an image of the subject from the image intensity profile after correction. 
     First Embodiment 
     Hereinafter, embodiments will be described with reference to the drawings. The image pickup device of the embodiment is, for example, a transmission X-ray microscope. The transmission X-ray microscope is an imaging optical system using electromagnetic waves having a short wavelength, and has a high resolution of about several tens nm. Further, since X-rays have a high transmittance, it is possible to observe an internal structure of a relatively thick subject such as a silicon wafer where a semiconductor device or the like is formed on a surface thereof. 
       FIG.  1    is a schematic view showing an example of the configuration of the image pickup device of at least one embodiment. The image pickup device includes a light source  11 , an illumination mirror  12 , an imaging mirror  13 , and a one-dimensional detector  14 . Further, the image pickup device includes a disk  21  with an aperture, a rotation stage  22 , a stage drive unit  23 , a vacuum suction ring  24 , and a control analysis unit  31 . 
     The light source  11  is an X-ray source that generates X-rays by irradiating a target made of molybdenum or the like with an electron beam. The illumination mirror  12  is used to collect X-rays emitted from the light source  11  toward an aperture  211  of the disk  21  with an aperture. For the illumination mirror  12 , for example, a Montel mirror is used. 
     The disk  21  with an aperture as the sample holding plate is a thin plate-shaped disk having an aperture  211  in the center. It is desirable that the aperture  211  has a point-symmetrical shape such as a circle or a regular polygon having an even number of vertices. The disk  21  with an aperture is disposed parallel to the XY plane and a peripheral edge thereof is held by the rotation stage  22 . The rotation stage  22  as the rotation member is a member that rotatably supports the disk  21  with an aperture at a desired angle with a Z direction as a rotation axis  221 . The rotation stage  22  has a chuck that is a hollow ring-shaped member. The disk  21  with an aperture is placed on the chuck of the rotation stage  22  so that a center of the aperture  211  coincides with the rotation axis  221  as much as possible. 
     The subject  41  is, for example, a silicon wafer on which a semiconductor device is formed. The subject  41  is held by the vacuum suction ring  24  and is placed under the disk  21  with an aperture so that a surface thereof is parallel to the disk  21  with an aperture.  FIG.  2    is a perspective view of the vacuum suction ring. As shown in  FIG.  2   , the vacuum suction ring  24  as the fixing member is a ring-shaped member having an opening at a center. There is a step  240  on an upper surface, and an upper surface (first surface) of an inner peripheral ring  242  located inside the step  240  is formed lower than an upper surface (second surface) of an outer peripheral ring  241  located outside the step  240 . The inner peripheral ring  242  is a holding portion of the subject  41 , and the outer peripheral ring  241  is a connecting portion with the disk  21  with an aperture. That is, the subject  41  is placed on the upper surface of the inner peripheral ring  242  in a state where the surface (surface on which a pattern is formed) faces upward. Further, the disk  21  with an aperture is placed on the upper surface of the outer peripheral ring. 
       FIG.  3    is a schematic view showing a positional relationship between the disk  21  with an aperture and the subject  41  held by the vacuum suction ring  24 .  FIG.  3    is an enlarged view of a rectangular region A shown by a dotted line in  FIG.  1   . Assuming that a pixel pitch of the one-dimensional detector  14  described later is P, a wavelength of X-rays emitted from the light source  11  is A, and an imaging magnification by the imaging mirror  13  described later is M, it is desirable that a distance L between an upper surface of the subject  41  and a lower surface of the disk  21  with an aperture is set to P 2 /{(M 2 )×λ} or less. By setting in this way, it is possible to prevent the influence of Fresnel interference fringes due to the aperture  211 . That is, a height of the step  240  of the vacuum suction ring  24  is set such that the distance L between the upper surface of the subject  41  and the lower surface of the disk  21  with an aperture is P 2 /{(M 2 )×λ} or less when the subject  41  and the disk  21  with an aperture are placed on the vacuum suction ring  24 . 
     The vacuum suction ring  24  has a hollow structure. The inner peripheral ring  242  is provided with three vacuum suction ports  244 . The upper surface of the vacuum suction port  244  is formed higher than the upper surface of the inner peripheral ring  242  by a predetermined height. Further, the outer peripheral ring  241  is also provided with three vacuum suction ports  243 . The upper surface of the vacuum suction port  243  is formed higher than the upper surface of the outer peripheral ring  241  by a predetermined height. All vacuum suction ports  243  and  244  are provided with suction holes  245 . Further, the outer peripheral ring  241  is provided with a connection hole  246  for connecting one end of the tube  26 . 
     As shown in  FIG.  1   , a vacuum pump  25  is connected to the other end of the tube  26 . By driving the vacuum pump  25  in a state where the subject  41  is placed on the upper surface of the inner peripheral ring  242  and the disk  21  with an aperture is placed on the upper surface of the outer peripheral ring, the upper surface of the vacuum suction port  244  and the lower surface of the subject  41  are fixed, and the upper surface of the vacuum suction port  243  and the lower surface of the disk  21  with an aperture are fixed by vacuum suction. That is, by the vacuum suction, the subject  41  and the disk  21  with an aperture are fixed to the vacuum suction ring  24  in a state of being supported by three points, respectively. At this time, after adjusting the position of the disk  21  with an aperture so that an observation target area of the subject  41  is exposed from the aperture  211  when viewed from above in the Z direction, the subject  41  and the disk  21  with an aperture are fixed to the vacuum suction ring  24  by the vacuum suction. 
     As described above, by fixing the subject  41  and the disk  21  with an aperture to the vacuum suction ring  24  by the vacuum suction, even when the disk  21  with an aperture is rotated during observation, it is possible to prevent a positional deviation between the disk  21  with an aperture and the subject  41 . The subject  41 , the vacuum suction ring  24 , and the disk  21  with an aperture that are suction-fixed to each other are placed such that the center of the aperture  211  coincides with the rotation axis  221  of the rotation stage  22  as much as possible. Three or more vacuum suction ports  244  and  243  may be provided, respectively, and the subject  41  and the disk  21  with an aperture may be held at multiple points. 
     The imaging mirror  13  as the imaging optical member collects the X-rays that have passed through the aperture  211  and transmitted through the observation region  411  of the subject  41 , and forms an image of the subject  41  on the detection surface  141  of the one-dimensional detector  14 . The size of the detection surface  141  is sufficiently larger than the size of the image to be formed.  FIG.  4    is a schematic view showing a configuration of the one-dimensional detector. As shown in  FIG.  4   , the one-dimensional detector  14  includes line-shaped pixels  142  extending in a D2 direction at equal intervals in a D1 direction in the detection surface  141 . The D1 direction and the D2 direction are orthogonal to each other. The image of the subject  41  disposed parallel to an XY plane is formed on a D1-D2 plane of the detection surface  141 . That is, the D1 direction of the detection surface  141  corresponds to the X direction of the subject  41 . Further, the D2 direction of the detection surface  141  corresponds to the Y direction of the subject  41 . As the one-dimensional detector  14 , for example, a superconducting strip detector in which a plurality of superconducting strips (superconducting single photon detectors) are disposed is used. In order to reduce a cross-sectional area of the superconducting strip  142 , which is the line-shaped pixels, to such an extent that division of the superconducting region occurs, a width and a thickness of the superconducting strip  142  are preferably 200 nm or less. In the following description, a sum of a width of one superconducting strip  142  and a distance between adjacent superconducting strips  142  is defined as the pixel pitch P. 
       FIG.  5    is a principle circuit configuration diagram of the one-dimensional detector. As shown in  FIG.  5   , the one-dimensional detector  14  includes a current source  143  that is connected to one end of the superconducting strip  142  of which the other end is grounded, and supplies a bias current Ib to the superconducting strip  142 , an amplifier  144  that amplifies an electric signal generated by the superconducting strip  142 , and a measuring instrument  145  that counts a pulsed electric signal detected when the X-ray photon is detected based on an output signal of the amplifier  144 . The current source  143 , the amplifier  144 , and the measuring instrument  145  may also be provided outside the one-dimensional detector  14 . For example, the current source  143 , the amplifier  144 , and the measuring instrument  145  may be provided in the control analysis unit  31 . 
       FIG.  6    is a view showing a detection principle of the X-ray photons in the superconducting strip. First, the superconducting strip  142  is cooled to a critical temperature or lower by a refrigerator (not shown) to be a superconducting state. Then, the bias current Ib slightly lower than a critical current for maintaining the superconducting state of the superconducting strip  142  is supplied from the current source  143 . In this state, the X-ray photons are incident on the superconducting strip  142 . 
     At this time, the width (length W in the D1 direction in  FIG.  4   ) and the thickness of the superconducting strip  142  are about 200 nm, and the cross-sectional area of the superconducting strip  142  is small. Therefore, when the X-ray photons are absorbed by the superconducting strip  142 , as shown in  FIG.  5   , a region (hotspot region)  51  that transfers to normal conduction called a hotspot is formed in the superconducting region  50  of the superconducting strip  142 . Since an electrical resistance of the hotspot region  51  increases, as shown in  FIG.  6   , the bias current Ib bypasses the hotspot region  51  and flows in a detour region  52 , which is another region. 
     When a current that is a critical current or more flows in the detour region  52 , the detour region  52  transfers to normal conduction, the electrical resistance increases, and finally the superconducting region  50  is divided. That is, a state where the superconducting region of the superconducting strip  142  described above is divided (divided state) occurs. After that, the hotspot region  51  and the detour region  52  that have transferred to normal conduction rapidly disappear by cooling, so that the pulsed electric signal is generated by a temporary electrical resistance generated by the division of the superconducting region  50 . The number of the X-ray photons can be measured by amplifying the pulsed electric signal by the amplifier  144  and counting it by the measuring instrument  145 . The number of the X-ray photons (photons) for each superconducting strip  142  counted by the measuring instrument  145 , that is, the detection result of the one-dimensional detector  14 , is output to the control analysis unit  31 . 
     The control analysis unit  31  as an image processing unit analyzes the detection result output from the one-dimensional detector  14  and reconstructs the image of the subject  41 . For the control analysis unit  31 , for example, a personal computer equipped with a central processing unit (CPU) and a memory (RAM) can be used. An operation of reconstructing the image of the subject  41  is performed by software, for example, by being stored in the memory as a program in advance and being executed in the CPU. Further, the operation of reconstructing the image of the subject  41  may be performed by one or more processors configured as hardware. For example, it may be a processor configured as an electronic circuit, or may be a processor implemented by an integrated circuit such as a Field Programmable Gate Array (FPGA). Further, the control analysis unit  31  outputs a control signal to the stage drive unit  23  that rotates the rotation stage  22 , and instructs rotation timing, rotation angle, and the like. 
     The image pickup device of the embodiment is not limited to the configuration in which the subject  41  is disposed below the disk  21  with an aperture as shown in  FIG.  1   , and may be a configuration in which the subject  41  is disposed above the disk  21  with an aperture as shown in  FIG.  7   .  FIG.  7    is a schematic view showing another configuration example of the image pickup device of the embodiment. In the case of the image pickup device having the configuration shown in  FIG.  7   , the vacuum suction port  243  of the vacuum suction ring  24  is disposed on the lower surface of the outer peripheral ring  241 . The lower surface of the vacuum suction port  243  is formed higher than the lower surface of the outer peripheral ring  241  by a predetermined height, and the lower surface of the vacuum suction port  243  and the upper surface of the disk  21  with an aperture are fixed by the suction holes  245  provided in the vacuum suction port  243 . After adjusting the position of the disk  21  with an aperture such that the observation region  411  of the subject  41  is exposed from the aperture  211  when viewed from below in the Z direction, the subject  41  and the disk  21  with an aperture are fixed to the vacuum suction ring  24  by the vacuum suction. That is, in the case of the configuration shown in  FIG.  7   , the subject  41  is placed on the inner peripheral ring  242  in a state where a back surface (surface on which the pattern is not formed) faces upward. 
     Next, an image generation method will be described. Prior to the image generation method of at least one embodiment, an image generation method of a comparative example will be described.  FIGS.  8 A to  8 D  are explanatory views of image reconstruction. A case of observing the subject  41  in which the pattern as shown in  FIG.  8 A  is formed will be described below as an example. First, as shown in  FIG.  8 B ( a ), the subject  41  is set on the rotation stage  22 , and the subject  41  on the rotation stage  22  is irradiated with the X-rays from the light source  11 . In each of  FIG.  8 B ( a ) to ( d ), the upper part is a view showing a positional relationship between the subject  41  and the pixels  142  of the one-dimensional detector  14 , and the lower part is an example of an image intensity profile. In the upper view of  FIG.  8 B ( a ) to ( d ), the subject  41  is shown by a thick line rectangle, and the line-shaped pixels  142  are shown by a diagonal hatched band-shaped rectangle. Further,  FIG.  8 B ( a ), ( b ), ( c ), and ( d ) show cases where the rotation angles are 0°, 30°, 60°, and 90°, respectively. 
     Then, the X-rays transmitted through the subject  41  are formed on the detection surface  141  of the one-dimensional detector  14 . In the one-dimensional detector  14 , the number of X-ray photons (#ph) is measured for each line-shaped pixel (superconducting strip)  142  by counting the pulsed electric signal generated by the division of the superconducting region  50  by the measuring instrument  145 . Then, by plotting the X-ray intensity (number of the X-ray photons) detected by each pixel  142  with respect to the coordinates of each pixel, an image intensity profile as shown in  FIG.  8 B ( a ) is acquired. That is, the image intensity profile when the rotation angle of the subject  41  is 0° is acquired. 
     Similarly, while rotating the rotation stage  22  by any rotation angle (AG), the image intensity profile is acquired at each rotation angle ( FIG.  8 B ( b ) to ( d )). Then, an image is reconstructed from all the obtained image intensity profiles using a projection-slice theorem, and an image of the subject  41  is acquired. Specifically, the image intensity profile acquired at each rotation angle is Fourier-transformed, and contour lines of the profile after the Fourier transform at all rotation angles are created to generate a Fourier-transformed image of the X-ray transmission image of the subject  41 .  FIG.  8 C  is a diagram schematically showing the generated Fourier transform image. The X-ray transmission image of the subject  41  is reconstructed by performing an inverse Fourier transform on the Fourier transform image of the X-ray transmission image of the subject  41 .  FIG.  8 D  shows an image of the reconstructed subject  41 . 
     When generating the image, the image intensity profile is acquired at each rotation angle while rotating the rotation stage  22  at any rotation angle (As) in a state where the rotation axis  221  of the rotation stage  22  and the center of the observation region  411  of the subject  41 , that is, the center  211 C of the aperture  211  coincide with each other. In the image generation method of the comparative example described above, there is a possibility that the position of the rotation axis  221  deviates from the center  211 C of the aperture  211  at the time of setting or during the rotation due to a mechanical error of the rotation stage  22  or the like.  FIGS.  9 A to  10 B  are views showing the position of the subject (observation region) at each rotation angle, in which  FIGS.  9 A and  9 B  show a case where the rotation axis and the center of the aperture coincide with each other, and  FIGS.  10 A and  10 B  show a case where the rotation axis and the center of the aperture do not coincide with each other. 
     A case will be described in which the observation region  411  having a dotted line pattern in the X direction and the Y direction and having a circular pattern on the dotted line extending in the Y direction is rotated through the center  211 C. As shown in  FIG.  9 A , when the position of the rotation axis  221  coincides with the center  211 C of the aperture  211 , the position of the center  211 C of the aperture does not change even if the rotation stage  22  is rotated.  FIG.  9 B  is a view showing the observation region  411  superimposed when the rotation stage  22  is rotated by 0°, 30°, 60°, and 90°. When the rotation axis  221  coincides with the center  211 C, as shown in  FIG.  9 B , the position of the aperture  211  does not change and the position of the center  211 C also does not change even if the rotation angle is advanced. 
     On the other hand, as shown in  FIG.  10 A , when the position of the rotation axis  221  does not coincide with the center  211 C of the aperture  211 , the position of the center  211 C of the aperture changes when the rotation stage  22  is rotated.  FIG.  10 B  is a view showing the observation region  411  superimposed when the rotation stage  22  is rotated by 0°, 30°, 60°, and 90°. When the rotation axis  221  does not coincide with the center  211 C, as shown in  FIG.  10 B , the position of the aperture  211  changes and the position of the center  211 C also changes in the X direction and the Y direction when the rotation angle advances. 
       FIG.  11    is a view showing a positional deviation of the center of the aperture due to a positional deviation of the rotation axis. In the drawings of 2 rows×4 columns=8 observation regions  411  shown in  FIG.  11   , the upper  4  regions show the case where the rotation axis  221  coincides with the center  211 C of the aperture (no positional deviation), and the lower  4  regions show the case where the rotation axis  221  and the center  211 C of the aperture deviate (there is the positional deviation). From the left end to the right end, the first column shows the position of the observation region  411  when the rotation angle is 0°, the second column shows the position of the observation region  411  when the rotation angle is 30°, the third column shows the position of the observation region  411  when the rotation angle is 60°, and the fourth column shows the position of the observation region  411  when the rotation angle is 90°. 
     As shown in the first column, let the X coordinate of the center of the aperture be C when the rotation angle is 0°. When the rotation angle is 30° (second column) and there is no positional deviation, the X coordinate of the center of the aperture remains C. On the other hand, when there is a positional deviation, the X coordinate of the center of the aperture is B1, that is, moves to the position different from C. When the rotation angle becomes 60° (third column), the X coordinate of the center of the aperture remains C if there is no positional deviation, and the X coordinate of the center of the aperture is B2 if there is a positional deviation, and a distance to C is large. When the rotation angle becomes 90° (fourth column), the X coordinate of the center of the aperture remains C if there is no positional deviation, and the X coordinate of the center of the aperture is B3 if there is a positional deviation, and the distance to C is further large. 
     In this way, when the image intensity profile is acquired while rotating the rotation stage  22  in a state where the position of the rotation axis  221  deviates from the center  211 C of the aperture  211 , the X-ray photons are detected in the pixels that are different from the pixels that should be originally detected. When the image is reconstructed by using the projection-slice theorem, since the image intensity profile is integrated based on the pixel position, if the position of the center  211 C of the aperture deviates by the rotation angle due to the positional deviation of the rotation axis  221 , the correct integration cannot be performed and the accuracy of the reconstruction image is lowered. 
     Therefore, in the image generation method of at least one embodiment, in the image intensity profile for each rotation angle, the image is reconstructed after calculating the amount of the positional deviation of the center  211 C of the aperture  211  and correcting the position of the image intensity profile. Hereinafter, the image generation method of the embodiment will be described with reference to  FIG.  12   .  FIG.  12    is a flowchart showing an example of an image generation method according to a first embodiment. 
     Prior to the observation, the subject  41  is placed on the rotation stage  22  as a preparation before the observation. That is, the subject  41  and the disk  21  with an aperture are positioned so that the region that is the observation target of the subject  41  is exposed from the aperture  211  of the disk  21  with an aperture when viewed from the Z direction, and both are fixed by the vacuum suction ring  24 . Then, the disk  21  with an aperture is fixed to the rotation stage  22 , and the placement of the subject  41  is completed. 
     When the preparation described above is completed, observation (image generation) is started. First, the step angle Δθ of the rotation stage  22  is set (S1). The step angle Δθ is an interval between angles for acquiring the image intensity profile. In normal observation, the image intensity profile is acquired while rotating the rotation stage  22  in a range of 0° or more and less than 180°. Next, the rotation angle θ of the rotation stage  22  is set to 0° (initial value) (S2). 
     The image intensity profile of the subject  41  is acquired by irradiating the X-rays from the light source  11  (S3). When θ+Δθ is less than 180° (maximum value of the observation angle) (S4, NO), the step angle Δθ is added to the current rotation angle θ to calculate the next rotation angle θ, and the rotation stage  22  is rotated to the next rotation angle θ (S5). Then, S3 is executed, and the image intensity profile at the set rotation angle θ is acquired. 
     On the other hand, when θ+Δθ is 180° (maximum value of the observation angle) or more (S4, YES), the acquisition of the image intensity profile in the set rotation angle range is completed, so that the process proceeds to S6. In S6, the position of the acquired image intensity profile is corrected for each rotation angle.  FIG.  13    is a diagram showing a method for correcting the position of the image intensity profile in the first embodiment. In 3 rows×2 columns=6 image intensity profiles shown in  FIG.  13   , the left column shows the profile before correction, and the right column shows the profile after correction. The upper row shows the profile when the rotation angle is θ1, the middle row shows the profile when the rotation angle is θ2, and the lower row shows the profile when the rotation angle is θ3. For example, the image intensity profile acquired at the rotation angle θ is the profile P11 shown on the left side of the upper row, and the profile obtained by correcting the position thereof is the profile P12 shown on the right side of the upper row. The position correction of the image intensity profile in S6 will be described with reference to  FIG.  13   . 
     First, the pixel position of the center  211 C of the aperture is estimated in the image intensity profile acquired for each rotation angle. When the aperture  211  is circular, assuming that at least a part of the X-rays is transmitted in the entire observation region  411 , it can be estimated that the pixel position where the intensity is maximum in the image intensity profile corresponds to the center  211 C of the aperture. Alternatively, it may be estimated that the midpoint of the range (L1 to R1) where the X-ray intensity is observed in the image intensity profile corresponds to the center  211 C of the aperture. 
     When the axis deviation of the rotation axis  221  does not occur, the position of the center  211 C of the aperture of the image intensity profile is the same at all rotation angles. Therefore, the position of each image intensity profile is corrected so that the estimated position of the center  211 C of the aperture is the same position in the image intensity profile at all rotation angles. For example, the coordinates are corrected so that the pixel position of the center  211 C of the aperture in all the image intensity profiles comes to the origin point (X=0). 
     As shown in  FIG.  13   , when the pixel position of the center  211 C of the aperture in the image intensity profile P11 of the rotation angle θ is estimated to be C1, the pixel position of the center  211 C of the aperture in the image intensity profile P21 of the rotation angle θ2 is estimated to be C2, and the pixel position of the center  211 C of the aperture in the image intensity profile P31 of the rotation angle θ3 is estimated to be C3, a correction method will be specifically described. The image intensity profile P11 of the rotation angle θ shown on the left side of the upper row shifts the image intensity profile to the negative side in the X direction only by C1 and is corrected to the image intensity profile P21 in which the pixel position of the center  211 C of the aperture is 0 as shown on the right side of the upper row. The image intensity profile P21 of the rotation angle θ2 shown on the left side of the middle row shifts the image intensity profile to the negative side in the X direction only by C2, and is corrected to the image intensity profile P22 in which the pixel position of the center  211 C of the aperture is 0 as shown on the right side of the middle row. The image intensity profile P31 of the rotation angle θ3 shown on the left side of the lower row shifts the image intensity profile to the negative side in the X direction only by C3, and is corrected to the image intensity profile P32 in which the pixel position of the center  211 C of the aperture is 0 as shown on the right side of the lower row. 
     Finally, the X-ray transmission image of the observation region  411  is reconstructed by using the image intensity profiles of all the rotation angles corrected in S6 (S7), and a series of procedures relating to the image generation method of the embodiment is completed. 
     Second Embodiment 
     As described above, according to the image pickup device of at least one embodiment, since the positional relationship between the subject  41  and the aperture  211  is fixed by the vacuum suction ring  24 , only the X-rays transmitting the observation region  411  are detected by the one-dimensional detector  14 . Therefore, even if the rotation axis  221  of the rotation stage  22  fluctuates, the coordinates can be corrected from the obtained image intensity profile, and a highly accurate reconstruction image can be acquired. Next, a second embodiment will be described. In the image generation method of the present embodiment, the coordinate correction method of the image intensity profile is different from that of the first embodiment described above. Since the configurations of the image pickup device and the image forming apparatus are the same as those of the first embodiment described above, the description thereof will be omitted, and only the differences from the first embodiment will be described below. 
     In the first embodiment described above, the coordinates are corrected by using the center of the image intensity profile. On the other hand, in the present embodiment, a reference image intensity profile is created from the reconstruction image and the coordinates of the image intensity profile are corrected. Hereinafter, the image generation method of the embodiment will be described with reference to  FIGS.  14  to  16   .  FIG.  14    is a flowchart showing an example of the image generation method in the second embodiment.  FIGS.  15 A to  15 C  are views showing changes in the reconstruction image in the second embodiment.  FIG.  16    is a diagram showing a position correction method of the image intensity profile in the second embodiment. 
     A preparation before the observation (placement of the subject  41  on the rotation stage  22 ) and each procedure of S1 to S5 are the same as the procedure of the first embodiment shown in  FIG.  12   , so the description thereof will be omitted. After the acquisition of the image intensity profile in the set rotation angle range is completed (S4, YES), the X-ray transmission image of the observation region  411  is reconstructed by using the obtained image intensity profile (S6). The image intensity profile used in S6 is the image intensity profile (without coordinate correction) acquired in S3. As shown in  FIG.  15 A , in a case of observing the observation region  411  in which the pattern is formed such that the X-rays are difficult to transmit through the left half region and the X-rays are easy to transmit through the right half region by using the circular aperture  211 , when the axis deviation of the rotation axis  221  occurs, for example, as shown in  FIG.  15 B , an image in which a boundary between the region where the X-rays are difficult to transmit and the region where X-rays are easy to transmit is blurred is reconstructed. 
     Next, the image intensity profile for each rotation angle is calculated from the generated reconstruction image (S7). Specifically, the image intensity of the obtained reconstruction image is integrated in the direction of the tilt angle θ. The reference image intensity profile is created by plotting the integrated values with the direction orthogonal to the angle θ as the X coordinate axis. The reconstruction image obtained at this time corresponds to the image of the observation region  411 . In this way, the reference image intensity profile is calculated for each of all the rotation angles for which the image intensity profile was acquired in S3. 
     The reference image intensity profile integrated in the direction of the tilt angle θ corresponds to the image intensity profile acquired at the rotation angle θ. At the same angle θ, the reference image intensity profile generated in S7 and the image intensity profile acquired in S3 are compared, and the coordinates of the image intensity profile are corrected so that the both coincide most with each other (S8). 
     In 3 rows×3 columns=9 image intensity profiles shown in  FIG.  16   , the left column shows the profile (measurement profile) before correction acquired in S3. The middle column shows the reference image intensity profile (reference profile) generated in S7. Further, the right column shows the image intensity profile (corrected profile) coordinate-corrected in S8. The upper row shows the profile when the rotation angle is θ1, the middle row shows the profile when the rotation angle is θ2, and the lower row shows the profile when the rotation angle is θ3. For example, the image intensity profile acquired at the rotation angle θ1 is the profile P11 shown on the left side of the upper row. The reference image intensity profile at the angle θ1 which is created from the image reconstructed by using the measurement profile shown in the left column is the profile Pr11 shown in the middle of the upper row. The profile P11 of which the position is corrected based on the profile Pr11 is the profile Pc11 shown on the right side of the upper row. 
     For example, the coordinate correction of S8 may be corrected so that the coordinates at which the X-ray intensity peaks coincide with each other, may be corrected so that an area of a difference region between the image intensity profile acquired in S3 and the reference image intensity profile is minimized, or may be corrected by using other logic.  FIG.  16    shows, as an example, when the coordinates at which the X-ray intensity peaks are corrected so as to coincide with each other. For example, at the rotation angle θ2, the position where the X-ray intensity peaks in the measurement profile P12 is defined as C12, and the position where the X-ray intensity peaks in the reference profile Pr12 is defined as Cr12. The measurement profile P12 is shifted in the X direction by (Cr12-C12) so that the position where the X-ray intensity peaks is Cr12, and the corrected profile Pc21 is generated. The image intensity profiles of other rotation angles are similarly corrected. 
     Subsequently, the X-ray transmission image of the observation region  411  is reconstructed by using the image intensity profile corrected in S8 (S9). When the corrected image intensity profile and the reference image intensity profile are compared, and the difference between the both exceeds a predetermined range (degree of coincidence is less than the predetermined range) (S10, NO), the process returns to S7, and the reference image intensity profile is calculated by using the reconstruction image generated in S9. As a parameter indicating the difference (degree of coincidence) between the both in S10, for example, the difference in image intensity between the both at each coordinate can be used. 
     When the difference between both is within the predetermined range (degree of coincidence reaches the predetermined range) (S10, YES), the reconstruction image generated in S9 is used as the X-ray transmission image of the observation region  411 , and the series of procedures relating to the image generation method of at least one embodiment is completed.  FIG.  15 C  shows a reconstruction image after repeating the procedure of S7 to S10 m times. As described above, by repeatedly executing the generation of the reference image intensity profile and the correction of the image intensity profile, the degree of blurring of the boundary between the region where the X-rays are difficult to transmit and the region where the X-rays are easily transmitted due to the rotation axis deviation can be reduced, and an image close to the observation region  411  shown in  FIG.  15 A  is obtained. 
     As described above, according to the image generation method of at least one embodiment, the coordinates of the image intensity profile can be accurately corrected even when it is difficult to determine the center  211 C of the aperture based on the image intensity profile or when the shape of the aperture  211  is not point-symmetrical. Therefore, a highly accurate reconstruction image can be acquired. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.