JP2010156607A - X-ray tomographic imaging apparatus and x-ray tomographic imaging method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a clear and accurate reconstituted image by using a tomographic imaging method employing X-rays or the like, in a simple configuration. <P>SOLUTION: An inspection object 3 is set on a setting section in the state of being in an approximately perpendicular contact, in its through-thickness direction, with a slant height line of a hypothetical cone (contact cone) 50 having a prescribed vertex angle θ with a rotation axis R1 as the center line. Then, an X-ray focus position F is moved by using a moving mechanism into a space sandwiched by two hypothetical cone faces of a first hypothetical cone 51 which is symmetrical about a point with respect to the vertex of the contact cone 50 in contact with the inspection object 3, and a second hypothetical circumscribing cone 52 which includes/circumscribes the outermost portion of an interested part desired to obtain a tomographic image of the inspection object 3 set on the contact cone 50 and has the vertex angle identical to the contact cone and the center axis on the same axis. Next, a rotating mechanism is brought to rotate at a predetermined angular displacement, which is disposed between the X-ray focus position F and a two-dimensional detector 2 while setting the inspection object 3 thereon and rotates about the rotation axis R1 perpendicularly intersecting with a line segment connecting the X-ray focus position F and the center of the bottom plane of a conical beam formed by X rays emitted by an X-ray source. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an X-ray tomographic imaging apparatus and an X-ray tomographic imaging method for inspecting internal structure data of an object to be inspected using X-rays or the like.

  Conventionally, non-destructive three-dimensional analysis is required in the field of research and development of semiconductor elements and the like in order to inspect cracks, breaks, and the like that exist inside a micro-inspection object. As one of the methods, there is an inspection method using a computed tomography apparatus using X-rays (hereinafter referred to as “X-ray tomography apparatus”).

  The X-ray tomographic imaging apparatus is, for example, an X-ray source (an X-ray generator configured by an X-ray tube or the like) and an X-ray focal point from the X-ray source to form a cone (cone beam shape) on the object to be examined. Two-dimensional detection means for detecting irradiated and transmitted X-rays is provided. In addition, a rotating base that has a rotation axis that is perpendicular to a perpendicular line that is placed on the detection surface of the detection means from the X-ray focal point and that rotates with angular displacement based on the setting is placed between the two-dimensional detection means and the object to be inspected. Has a base.

  In such an X-ray tomographic imaging apparatus, an X-ray source irradiates an object to be inspected with X-rays, and a transmission X-ray projection image of the object to be inspected is picked up by a two-dimensional detection means and digitized for each angle phase. Process as multiple image data. Then, by reconstructing the internal structure data from each of these image data, it becomes easy to inspect and observe the inside of the inspection object. Such a tomographic imaging process is also referred to as a single scan cone beam CT (computerizing [computed] tomography) method.

FIG. 1 is a side view showing an arrangement of a general industrial inspection object, an X-ray tube, and a two-dimensional detector. FIG. 2 is a schematic view showing the positional relationship between the object to be inspected, the rotation base, and the X-ray tube in the X-ray tomographic imaging apparatus of FIG. 1, wherein A is a perspective view and B is a side view.
In this example, an object to be inspected 3 placed on a rotating base 4 as a rotating means is disposed between the X-ray focal point F of the X-ray tube 1 and the two-dimensional detector 2. The inspected object 3 is fixed to the rotating base 4. The inspection object 3 is assumed to have a flat and thin shape (flat plate or thin plate) such as an IC chip or a substrate (in the example of FIG. 1, it is a thin plate cylinder). For example, the thickness of a 15 mm square is about 2.5 mm. The size of the main surface (surface having a large area) and the thickness of the object to be inspected 3 are not limited to this example.

  Then, the rotation base 4 rotates with the set angular displacement about the rotation axis R1 perpendicular to the perpendicular line dropped from the X-ray focal point F of the X-ray tube 1 to the center O of the detection surface of the two-dimensional detector 2. Thus, the inspection object 3 is configured to rotate at a predetermined angular displacement (see, for example, Patent Document 1).

JP 2007-101247 A

  By the way, in an X-ray tomographic imaging apparatus for industrial use, a single scan cone beam CT method is usually applied. That is, an approximate three-dimensional CT image reconstruction algorithm proposed by Feldkamp and the like based on the idea that cone (conical) beam projection is composed of many fan beam projections is employed. In the X-ray tomographic imaging method to which this single scan cone beam CT method is applied, the thin plate-shaped inspected object 3 is placed in a plane parallel to the midplane MD and is set at predetermined angles around the rotation axis R1. Rotate (turn). The midplane MD is a plane that includes the X-ray focal point F and is orthogonal to the rotation axis R1 of the device under test 3.

  FIG. 3 shows imaging data obtained by the X-ray tomographic imaging method, where A is a projection image at a certain angle phase, and B is a reconstructed image. The object to be inspected used for the measurement has a laminated structure composed of three thin copper foils having a circular shape, and the distance between the copper foils is 3.2 mm and the total thickness is 6.4 mm. In FIG. 1, as the inspection object 3 moves away from the midplane MD, the obtained projection image (tomographic image) of the inspection object 3 gradually lacks accuracy. That is, when the layered (multilayer structure) inspection object 3 in the posture of FIGS. 1 and 2 is located at a position higher than the plane (midplane MD) passing through the X-ray focal point F and including the perpendicular to the two-dimensional detector 2. The tomographic image becomes ambiguous as shown in FIG. 3A.

  This phenomenon has also been explained by the shadow zone proposed by Pierre Grangeat et al. (Mathematical Framework of Cone Beam 3D Reconstruction Via The First Derivative of The Radon Transform). That is, this phenomenon occurs when the inspection object 4 is located in the shadow zone 6 as shown in FIG.

  In the single scan cone beam CT method in which the object to be inspected 3 rotates, a sphere of the same radius dOF / 2 that orbits a circular orbit on the midplane MD with the radius dOF / 2 around the rotation axis R1 of the object to be inspected 3 ( There are areas that are excluded by a collection of circles. This area is the shadow zone 6 and is conventionally considered to be an area in which strict reconstruction calculation of the inspection object 3 is theoretically impossible. Circles 5 </ b> A and 5 </ b> B represent a boundary area vertical cross section passing through the rotation axis R <b> 1 of the shadow zone 6.

  As described above, in the conventional methods, the object to be inspected is arranged so as not to enter the shadow zone, and then the object to be inspected is irradiated with X-rays to take a projected image. However, it is cumbersome to place an object to be inspected so as not to enter the shadow zone, and the effect is unclear.

  The present invention has been made in view of such circumstances, and an object thereof is to obtain a clear and accurate reconstructed image with a simple configuration by a computer tomography method using X-rays or the like.

In order to solve the above problems, the applicant considered that a clear reconstructed image could not be obtained because the photographing data was not good, not the reconstruction calculation method itself. And the means by which a clear reconstructed image is obtained by the following means even when the object 3 is arranged in the shadow zone has been invented.
That is, the present invention first places the object to be inspected on the mounting portion in a state where the plate thickness direction is substantially perpendicular to the generatrix of the cone (tangent cone) having the apex angle θ with the rotation axis as the center line. To do. Next, the first virtual cone that is point-symmetric with the apex of the contact cone with which the object to be inspected contacts, and the outermost part of the region of interest for obtaining a tomographic image of the object to be inspected placed on the contact cone The X-ray focal point of the X-ray source into the space between the two virtual conical surfaces of the second virtual circumscribed cone that is circumscribed and contained in the second cone and has the same apex angle as that of the cone and has a central axis on the same axis. The position is moved by a moving mechanism.
Subsequently, the conical beam formed by the X-rays emitted from the X-ray source is arranged by placing the object to be inspected between the X-ray focal point of the X-ray source and the two-dimensional detector. A rotation mechanism that rotates about a rotation axis orthogonal to a line segment that connects the center of the bottom surface and the X-ray focal point is rotated at a set angular displacement. And the internal structure data of the said to-be-inspected object are reconfigure | reconstructed from the projection image imaged for every angle phase.

  According to the above configuration, the main surface of the object to be inspected is gripped with a predetermined inclination so as not to be perpendicular to the rotation axis. At this time, the outermost part of the region of interest to obtain a tomographic image of the first virtual cone that is point-symmetric with the vertex of the virtual conical cone with which the object is in contact and the object to be inspected placed on the conical cone Assume a second virtual circumscribed cone circumscribing and included in the position, having the same apex angle as the tangent cone and having a central axis on the same axis. The position of the X-ray focal point of the X-ray source is moved into a space sandwiched between these two virtual conical surfaces, and a projected image having a set angular phase is captured.

  Thereby, the geometrical coordinates of the X-ray focal point with respect to the region of interest of the object to be inspected are in a positional relationship in which the front and back both-side directions of the region of interest of the object to be inspected 3 can be alternately seen in perspective. Further, when the thin object to be inspected is a part of a non-source plane, the plane intersects the X-ray focal point one or more times while the object to be inspected is rotated 360 degrees. Accordingly, it is possible to collect good projection data of the region of interest, which is suitable for obtaining reconstruction data.

  As described above, according to the present invention, a clear and accurate reconstructed image can be obtained with a simple configuration by a computed tomography method using X-rays or the like.

Hereinafter, an example of the best mode for carrying out the present invention will be described with reference to the accompanying drawings. The description will be given in the following order.
1. First embodiment (first and second virtual cones and positions of X-ray focal points)
2. Second embodiment (mounting unit: swivel stage)

<1. First Embodiment>
[Configuration of X-ray tomographic imaging apparatus]
First, an X-ray tomographic imaging apparatus according to the first embodiment of the present invention will be described.
FIG. 5 is a schematic side view showing the configuration of the X-ray tomographic imaging apparatus. FIG. 6 is a schematic diagram showing the positional relationship between the object to be inspected, the rotation base, and the X-ray tube. FIG. 7 is a schematic diagram showing the positional relationship between the shadow zone and the object to be inspected. 5-7, the same code | symbol is attached | subjected to the part corresponding to FIGS. 1-4, and detailed description is omitted.

  The X-ray tomographic imaging apparatus includes an X-ray tube 1, a two-dimensional detector 2, a linear motion mechanism 31 in the Z-axis direction (L31) of the two-dimensional detector 2, and a rotation mechanism. 32 etc. are mounted. The X-ray tube 1 and the rotation mechanism 32 can move on the main surface of the surface plate 40 along the rail 41, and the X-ray tube 1 can be positioned more precisely in the Y-axis direction by the linear motion mechanism 42. ing. Further, the two-dimensional detector 2 is movable on the main surface of the surface plate 40 along the rail 43. Note that the X-ray tomographic imaging apparatus used in the present embodiment may be a single scan cone beam CT, and is not limited to this example.

  The X-ray tube 1 uses, for example, an open X-ray tube 1, and an X-ray focal point F on a target (not shown) is the apex, and its central axis (the central axis of a cone formed by X-rays) is illustrated. The irradiation field is formed in a conical shape substantially coaxial (in the direction of the optical axis main line) with the electron flow emitted from the cathode. The main body of the X-ray tube 1 is configured such that a front housing 1 b and a rear housing 1 c are connected by a hinge 15. It is supported on the surface plate 40 by an L-shaped bracket 17 near the X-ray focal point F and a V block 18 provided immediately below the weight center of gravity 1d of the X-ray tube 1 and on the horizontal surface of the bracket 17. The horizontal portion of the L-shaped bracket 17 and the lower plate 16 disposed below the L-shaped bracket 17 constitute a double plate mechanism.

  The V block 18 is mounted on an V block cradle 20 that can rotate about a rotation fulcrum 21 on the bracket 17 via an elastic body 19. By placing the V block 18 immediately below the bracket 17 and the center of gravity 1d in this way, the cathode (not shown) of the X-ray tube 1 can be easily positioned. In addition, when the cathode 15 is replaced by releasing the connection by the hinge 15 and releasing the vacuum, the rear housing 1c of the X-ray tube 1 is supported by elastic force, so that precise mechanical work such as cathode coordinate adjustment is performed. It becomes easy even in a horizontal position. The rotation axis of the hinge 15 of the X-ray tube 1 main body connecting portion and the rotation fulcrum 21 of the V block cradle 20 are arranged substantially coaxially.

  X-rays are radiated from the X-ray tube 1 to the entire inspection object 3, and the transmitted X-rays of the inspection object 3 are detected by the two-dimensional detector 2 functioning as a two-dimensional detection means to obtain a projection image. Regarding the target and the cathode, refer to Japanese Patent Application Laid-Open No. 2006-258668 (FIG. 10 etc.) filed earlier by the present applicant as an example.

  The X-rays irradiated from the X-ray tube 1 are configured to form a minimal X-ray focal point F (microfocus) having a focal size of about 1 μm or less, for example. Since the X-ray focal spot size is a large factor that determines the resolution of the X-ray tomographic imaging apparatus, it is preferable that the smaller the numerical value, the smaller the size of damage in the inspection object can be observed.

  The two-dimensional detector 2 is composed of, for example, a flat panel detector (FPD), and two-dimensional detection is performed so that a line segment drawn from the X-ray focal point F of the X-ray tube 1 is irradiated to the approximate center of the two-dimensional detector 2. The left / right / up / down (XYZ direction) positions can be adjusted by the linear motion mechanism of the device 2.

  An example of the FPD is disclosed in Japanese Patent Laid-Open No. 6-342098 (hereinafter referred to as “Document 1”). In this FPD, X-rays transmitted through a subject are absorbed by a photoconductive layer to generate charges corresponding to the X-ray intensity, and the amount of charges is detected for each pixel. In the FPD of the method disclosed in Document 1, since the X-ray dose is directly converted into the charge amount for each pixel, the sharpness degradation in the FPD is small and an image with excellent sharpness can be obtained. As an example of other types of FPDs, as disclosed in, for example, JP-A-9-90048, X-rays are absorbed in a phosphor layer such as an intensifying screen to generate fluorescence, and the intensity of the fluorescence Is detected by a photoelectric conversion element. As other fluorescence detection means, there is a method using a CCD (Charge Coupled Devices) or a C-MOS (Complementary Metal Oxide Semiconductor) sensor. The two-dimensional detector 2 of this example is not limited as long as it can detect the transmitted X-rays of the inspection object 3 and process it for each pixel to obtain an image signal.

  The rotation base 4 is a component of the rotation mechanism 32, and rotates the device under test 3 by the driving force of the rotation means directly below the rotation mechanism 32 (not shown) while placing the device under test 3 thereon. The center line (optical axis main line) of the conical beam 10 emitted from the X-ray focal point F passes through the center of rotation and is substantially orthogonal to the detection surface of the two-dimensional detector 2.

  In the present embodiment, the inspection object 3 is placed on the rotating base 4 so that the rotation axis R1 (Z axis) of the thin inspection object 3 and the main surface of the inspection object 3 are not perpendicular to each other. It is gripped on the part 4a. That is, the inspection object 3 is placed (gripped) at an angle so as to be in contact with a conical surface of a virtual cone (see FIGS. 8 and 9 to be described later) having the same center line as the rotation axis R1 of the inspection object 3. The inspected object 3 is placed in a state where the conical surface of a virtual cone having a predetermined apex angle with the rotation axis R1 as the center line is a contact surface. The vertex of the virtual cone is the intersection of the conical surface that is the tangent surface of the main surface and the rotation axis R1.

  As shown in FIG. 7, the shadow zone 6 has a center on the midplane MD intersecting with the X-ray focal point F and orthogonal to the rotation axis R1 of the object 3 to be inspected. Circles 5A and 5B having the same diameter as the distance (dOF) and intersecting with the X-ray focal point F in the same manner as the midplane MD correspond to regions excluded by a solid drawn by turning around the rotation axis R1.

[Attitude and trajectory of the object to be inspected]
Next, with reference to FIG. 8 and FIG. 9, the posture of the inspection object 3 and the trajectory based on the posture will be described.
FIG. 8 shows the posture of the object to be inspected, A is a schematic perspective view when the two-dimensional detector 2 is viewed from the X-ray tube 1 side, and B is the X-ray tube 1 from the two-dimensional detector 2 side. It is a schematic perspective view when seen. FIG. 9 is a cross-sectional view of the first virtual cone and the second virtual cone of FIGS. 8A and 8B cut along a plane passing through the rotation axis R1.

  As described above, the X-ray tomographic imaging apparatus includes the rotation base 4 (see, for example, FIG. 6) disposed between the X-ray focal point F of the X-ray tube 1 and the two-dimensional detector 2. The rotation base 4 has a rotation axis R 1 that is substantially perpendicular to a line segment that connects the center of the bottom of the cone formed by the X-rays emitted from the X-ray tube 1 and the X-ray focal point F on which the object 3 is placed. And a function of rotating at a set angular displacement. Further, if necessary, the inspection object 3 placed in the storage case 7 is placed on the conical surface of a cone (tangent cone 50) having an apex angle of a predetermined angle with the rotation axis R1 as a center line with respect to the rotation axis R1. The thin plate-like surface (main surface 3A) in the thickness direction is provided with a mounting portion 4a (see FIG. 6) that is mounted in a state of being in contact with a substantially right angle.

  8B and FIG. 9, a first virtual cone 51 that is point-symmetric with the vertex 53 of the contact cone 50 with which the test object 3 contacts, and the test object that is virtually placed on the contact cone 50. 3, a second virtual cone 52 circumscribing and including the outermost position (main surface 3 </ b> B) of a region of interest (ROI) where a tomographic image is to be obtained is assumed. The conical surface of the contact cone 50 is a locus generated by the main surface 3A on the lower side of the inspection object 3 mounted on the mounting table 4a when the inspection object 3 rotates.

  The second virtual cone 52 has the same apex angle θ as that of the conical cone 50 and has a central axis substantially coaxially (rotation axis R1). In the present invention, the space between the first virtual cone 51 and the second virtual cone 52 (assuming that the conical surfaces of the two virtual cones are infinite size) is sandwiched between the positions of the X-ray focal point F. It is intended to be limited to the inside. That is, in FIG. 9 of the two-dimensional display, a region (shaded portion) sandwiched between the extension line of the bus 51A of the first virtual cone 51 and the extension line of the bus 52A of the second virtual cone 52 corresponds.

  For example, an air bearing is mounted on the rotation mechanism 32, and the X-ray tube 1 moves in two directions of the Z-axis direction (L 33) and the Y-axis direction (L 42) to move the X-ray focal point F, 42. Since the moving mechanisms of the linear motion mechanism 31, the rotation mechanism 32, and the linear motion mechanisms 33 and 42 are well-known techniques, a detailed description thereof will be omitted. As will be described later, the apex angle θ of the first virtual cone 51 is preferably 100 ° ≦ θ ≦ 150 °.

  If the object 3 is gripped so as to contact the conical surface of the first virtual cone 51 with respect to the rotation axis R1, the geometric coordinates of the X-ray focal point with respect to the region of interest (ROI) are The positional relationship is such that the front and back both sides of the region of interest can be alternately seen through in accordance with the rotation. Therefore, it is possible to collect projection data suitable for obtaining an accurate non-destructive tomographic image. Further, it is also possible to acquire projection data of many angular phases for separately and individually seeing a specific portion having a high absorption coefficient that aggregates on the plane of interest (such as a ball-shaped electrical joint of a BGA (Ball Grid Array)). Thereby, there is an effect of reducing inaccurate projection data caused by a decrease in X-ray transmittance. In other words, a large amount of projection data with good contrast can be collected even with relatively weak intensity X-rays.

  In this way, the inspection object 3 is placed at an angle so as to contact the conical surface of the first virtual cone 51, and the positional relationship between the inspection object 3 and the X-ray focal point F (see FIG. 9) should be noted. Thus, an accurate non-destructive inspection of the region of interest (ROI) of the inspection object 3 away from the midplane MD becomes possible. A plurality of objects to be inspected 3 may be provided.

[Variation 1: Position of inspection object and two-dimensional detector]
Here, with reference to FIG. 10, the positional relationship between the inspection object 3 and the X-ray focal point F will be described. FIG. 10 is a diagram for explaining a more preferable positional relationship between the object to be inspected 3 and the X-ray focal point F. FIG. “Φ” is a vertical axis L1 dropped from the X-ray focal point F to the two-dimensional detector 2, and the X-ray focal point F and the central pixel column in the direction of the rotational axis R1 (Z-axis) of the two-dimensional detector 2. It is an angle formed by a line segment connecting in a plane including.

  In FIG. 10, the object to be inspected 3 has a thin plate shape, and its longitudinal direction coincides with the direction of the generatrix of the tangent cone. Here, the object to be inspected 3 is placed at a position (on the conical cone 50-1) that intersects the midplane MD including the perpendicular L that is lowered from the X-ray focal point F to the rotation axis R1. It is assumed that an image is obtained by turning to a position where a straight line drawn on the conical surface of the cone 50-1 contacts. In this case, when the inspection object 3 is in contact with the conical surface at this height, the X-rays that pass through the inspection object 3 so that main surfaces 3A and 3B, which will be described later, are projected in a straight line are covered. 24 of the inspection object 3 having a high absorption coefficient shown in FIG. 24, for example, a large number of materials such as solder balls 91 (six in this example) must pass through, and the attenuation of X-rays is too large. In some cases, sufficient contrast cannot be obtained in the projection image acquired by the dimension detector 2.

  For such an inspected object 3, the inspected object 3 is raised to the position of the contact cone 50-2 by a linear motion mechanism (not shown) attached to the rotation mechanism 32, and the projected image of the inspected object 3 after the movement. The two-dimensional detector 2 is also raised from the position P1 to the position P2 so as to enter the detection surface. By doing so, X-rays when the main surface 3A or the main surface 3B as will be described later of the object 3 to be inspected are transmitted in a straight line are in the longitudinal direction (three) shown in FIG. Yes, it is possible to obtain a projected image of the inspection object 3 with sufficient contrast.

  When raising the to-be-inspected object 3 as mentioned above, there exists a suitable condition for the raising distance. A plane (midplane MD) that is substantially orthogonal to the perpendicular line L drawn from the X-ray focal point F to the rotation axis R1 and that is perpendicular to the rotation axis R1 and includes the perpendicular line L, and a vertical pixel row on the detection surface of the two-dimensional detector 2 The position (center O) at which the horizontal pixel rows passing through the center overlap is the origin of the detection surface. When the object to be inspected 3 can be moved vertically and linearly with respect to the midplane MD by a drive system (not shown), the plane A and the midplane MD including the X-ray focal point F and the lateral pixel column of the detection surface after the movement. It is necessary that the formed angle Φ has a moving distance that is in the vicinity of (π−θ) / 2. That is, in order for the X-ray focal point F to be in the region shown in FIG. 9, the angle Φ needs to be less than the inclination angle α of the tangent cone, but due to the reason of paragraph 0038, the material configuration of the object to be inspected is considered. Then, the movement distance may be effectively used. δ represents the dimension of the upper half of the longitudinal length of the detection surface of the two-dimensional detector 2.

  Note that since the transmission X-ray image of the object 3 to be inspected is projected onto the detection surface of the two-dimensional detector 2, the horizontal pixel row may not be the center of the vertical pixel row (including the center O). It does not have to be) Further, in order to assist in increasing the angle Φ, the X-ray tube 1 is moved in a direction perpendicular to the midplane MD (parallel to the rotation axis R1), and the X-ray focal point F in the hatched area shown in FIG. The coordinates may be changed by a minute amount.

[Measurement result]
Next, a measurement result when the subject is imaged by the X-ray tomographic imaging apparatus according to the present embodiment will be shown. For measurement, the same laminated structure of copper foil and resin plate as that to be inspected used for obtaining the measurement data of FIG. 3 is used, and the imaging specifications at the time of measurement are as follows.
Object to be inspected: Copper foil 3 layer structure Diameter 8mm
c: 660 mm
d: 44 mm
e: 11.5mm
Φ = 15 degrees

  FIG. 11 shows coordinate spaces (three-dimensional views) of the X-ray tube 1 (X-ray focal point F), the two-dimensional detector 2 and the tangent cone 50 (vertex 53) during imaging. “Β” is a turning angle when the object 3 is turned around the rotation axis R1 (Z axis) in the XY plane to change from the XYZ coordinate space to the X′Y′Z coordinate space. “Α” is a turning angle when the object 3 is turned around the X ′ axis in the Y′Z plane and moved from the X′Y′Z coordinate space to the X′Y ″ Z ′ coordinate space. Yes, the inclination angle of the object to be inspected 3. “D” is a distance (dOF) between the X-ray focal point F and the rotation axis R1 (Z axis). “E” is the distance between the perpendicular drawn from the X-ray focal point F to the two-dimensional detector 2 and the vertex 53 of the tangent cone 50.

  FIG. 12 is a projected image of the inspected object actually imaged according to the imaging method shown in FIGS. 9 and 10, and each when the inclination angle α of the tangent cone is 15 degrees (vertical angle θ = 150 degrees). An angle phase projection image is shown. Of these projected images, the swivel angle β is 0 °, 6 °, 12 °, 18 °, 45 °, 90 out of the projected images obtained by turning the object to be inspected as shown in FIG. Excerpts from °, 180 °, 240 °, 315 °, and 346 °. The initial position (β = 0 °) is when the generatrix where the object 3 is in contact with the tangent cone is roughly included in the YZ plane. FIG. 13 shows internal structure data (reconstructed image) obtained by reconstructing the projection image of each angular phase under the conditions shown in FIG. 12, where A is the X-axis cross section and B is the Y-axis cross section. Yes. If the reconstruction screen is tilted by 15 degrees, the Y-axis cross section becomes horizontal in the same manner as the X-axis cross section image.

  The object to be inspected used for the measurement has a laminated structure in which the distance between the copper foils is 3.2 mm and the total thickness is 6.4 mm (FIG. 13A). According to the present invention, when the internal structure of the object to be inspected used for measurement is a simple laminated structure with a significant difference in absorption coefficient, a clear reconstruction result as shown in FIG. 13 can be obtained. The inspected object 3 has little change in the projected image even after one rotation of 360 degrees. Unlike the projected image like the ellipse shown in FIG. 3B, the reconstruction calculation result is not blurred.

Among the projection images extracted for each angle phase of the object 3 to be inspected, which rotates around the rotation axis R1,
The copper foil in the middle of the three copper foils projected at β = 12 ° and the copper foil in the middle of the projection of β = 346 ° are visible as the thinnest lines. Assuming that the thin foil-like copper foils were a part of an infinitely large plane, the infinitely large plane is the same as the inspected object 3 rotates 360 degrees. This means that it intersects with the X-ray focal point F twice (β = 12 °, 346 °) each time. In this state, X-rays are transmitted so that the central (middle) surface sandwiched between the main surface 3A and the main surface 3B of the inspection object 3 is a straight line projection. FIG. 8 shows an almost linear projection image Im and an angle γ between the X axis and the projection image Im when it is assumed that an infinitely large plane including the inspection object 3 (copper foil) is projected at this time. Is shown.

  Furthermore, the measurement is performed also in the case where the inclination angle of the contact cone 50 with which the inspected object 3 contacts is another value.

  FIG. 14 shows a projected image of each angle phase when the inclination angle α of the tangent cone 50 is 30 degrees (vertical angle θ = 120 degrees). Of these projected images, the swivel angle β is 0 °, 30 °, 62 °, 90 °, 180 °, 270 °, 295.5 °, 315 among the projected images obtained by rotating the object to be inspected 360 °. Excerpts of stuff. FIG. 15 shows internal structure data (reconstructed image) obtained by reconstructing the projection image of each angle phase under the conditions shown in FIG. 14, where A is the X-axis cross section and B is the Y-axis cross section. Yes. If the reconstruction screen is tilted by 30 degrees, the Y-axis cross section becomes horizontal in the same manner as the X-axis cross-sectional image.

  In the example shown in FIG. 14 and FIG. 15, among the projection images extracted for each angular phase of the inspected object 3 turning around the rotation axis R 1, in three copper foils projected at β = 62 °. The copper foil in the middle and the copper foil in the middle of the projection of β = 295.5 ° are visible as the thinnest lines.

  FIG. 16 shows a projection image of each angle phase when the inclination angle α of the tangent cone 50 is 40 degrees (vertical angle θ = 100 degrees). Of these projected images, the swivel angles β are 0 °, 45 °, 72 °, 90 °, 180 °, 270 °, 288 °, and 315 ° among the projected images obtained by rotating the object to be inspected 360 °. Excerpts FIG. 17 shows internal structure data (reconstructed image) obtained by reconstructing the projection image of each angular phase under the conditions shown in FIG. 16, where A is the X-axis cross section and B is the Y-axis cross section. Yes. If the reconstruction screen is tilted by 40 degrees, the Y-axis cross section becomes horizontal as in the X-axis cross-section image.

  In the examples shown in FIGS. 16 and 17, among the projection images extracted for each angle phase of the inspection object 3 that rotates around the rotation axis R 1, the three copper foils projected at β = 72 °. The copper foil in the middle and the copper foil in the middle of the projection of β = 288 ° are visible as the thinnest lines.

  In the experiment, as the apex angle θ was smaller than 100 degrees or larger than 150 degrees, the degree of clarity of the reconstructed image was decreased.

[Consideration of measurement results]
When a thin inspection object is imaged by applying the imaging method of the present invention, it intersects the X-ray focal point F twice, but if it intersects at least once, a reconstruction calculation result sufficient for inspection can be obtained. On the other hand, when the X-ray focal point F exists outside the space between the first virtual cone 51 and the second virtual cone 52 (hatched portion in FIG. 9), the infinity including the copper foil is included. The plane of the size does not intersect with the X-ray focal point F even if it turns 360 degrees.

  Pierre Grangeat et al.'S paper "Mathematical Framework of Cone Beam 3D Reconstruction Via The First Derivative of Radon Transform" proposed a structure unique to an industrial X-ray CT system as shown in Fig. 5, called an orthogonal CT system. It was not a thing. In other words, the trajectory drawn by the X-ray focal point is a condition that allows reconstruction calculation, keeping in mind the structure in which the X-ray focal point and the two-dimensional detector circulate around the object to be inspected, such as a medical X-ray CT apparatus. It is argued that reconstruction calculation along the cross section is possible if it crosses a cross section of the object to be inspected. Applying this requirement to an industrial X-ray CT apparatus in which the object to be inspected as described above is equivalent to the infinitely large plane intersecting the X-ray focal point at least once.

  However, in the above paper, in the industrial X-ray CT apparatus, when the inspection target is a thin plate laminated structure such as the object to be inspected 3, what geometrical conditions are satisfied satisfies the conditions for reconstruction calculation. It was not discussed until it was done. Examples of inspected objects having a laminated structure of thin plates include various devices such as BGA substrates and multi-layer substrates that require non-destructive inspection, or those specialized for substrates that are inserted into mobile phone terminals. Examples are given.

  The projection images obtained with the imaging posture shown in FIG. 6 are shown in FIGS. 12, 14, and 16 described above, but the posture of the object 3 is different from the conventional case (see FIG. 4). Yes. That is, the posture of the conventional inspection object 3 is perpendicular to the rotation axis R1, but is inclined with respect to the rotation axis R1 in the imaging method of the present invention. As shown in the imaging specifications measured this time, even when the thin plate-like object 3 is in the shadow zone 6 (see FIG. 7), clear reconstruction results are obtained as shown in FIGS. .

  This means that it is inappropriate to discuss a thin plate-like object under the same standard as an object to be inspected, such as a human body, where spheres and egg-shaped examination parts are scattered independently. Is lacking. In addition, the difference in absorption coefficient is not always significant as in the inspected object 3, but a large number of spherical substances having a high absorption coefficient such as a BGA substrate are arranged in a single plane. The imaging concept of the present invention can also be applied to an inspecting body that forms the shape. The measurement result obtained by imaging on the BGA substrate will be described later.

  The reconstruction calculation can be realized by applying a filtered back projection method or a convolution back projection method (convolution & back projection method) according to a reconstruction algorithm such as Feldkamp. In the case where the back projection method is used for an object to be inspected whose aspect ratio of a rectangular cross-section with respect to a region of interest of the object to be inspected, which corresponds to almost all industrial devices, is thinner than 1: 5, Japanese Patent Application Laid-Open No. 2006-214879 ( The technique described in the prior application 1) may be used. In the technique described in the prior application 1, when an object to be inspected with a significant change in the projected image with respect to the angular displacement is imaged due to the angular phase, the rotational displacement of the rotating means is changed only with the angular phase in which the projected image changes with respect to the angular displacement. The projection image is collected with a small setting. Obtaining projection data of an angular phase characterizing the internal tomographic image of the object to be inspected is important for performing detailed observation. In the case of the imaging concept of the present invention, this corresponds to the projection data in which the turning angle of the object to be inspected shown in FIG. 12 is around 12 degrees and around 346 degrees. From these projected images, it can be easily understood that the inside of the object to be inspected is composed of three planes having a high absorption coefficient by human vision or recognition.

  Further, the projection images near 12 degrees and 346 degrees will be compared with projection data near 180 degrees which is an intermediate between them. Then, it is confirmed that the change rate of the projected image per step in the former (near 12 degrees and near 346 degrees) of the imaging method of the present invention in which the angular pitches of the imaging are equally spaced is slower than the latter (near 175 degrees). it can. That is, if the imaging concept of the present invention is applied, the same effect as the concept of the prior application 1 can be obtained while imaging with an equiangular pitch.

[Proof of effect]
The orthogonal coordinate space XYZ having the X-ray focal point F in FIG. 11 as the origin is moved by the distances d and e in FIG. 10 and then rotated by the angle β around the Z axis and then rotated by the angle α around the X ′ axis. The X′Y ″ plane of the coordinate space X′Y ″ Z ′ touches the tangent cone 50-2. The above coordinate conversion is expressed by the following equation (1).

  Unit vectors corresponding to the X ′ axis and the Y ″ axis after coordinate conversion are shown in the first and second columns ignoring the fourth row of the calculation result of Expression (1). Since the matter to be numerically analyzed is limited from the X′Y ″ plane and FIG. 11, using formula (1), formula (2) considering the moving distance (position), and numerical analysis considering only the angle Equation (4) can be derived.

The condition that the X′Y ″ plane of the new coordinate space is a plane passing through the origin (X-ray focal point F) of the original coordinate space, that is, the condition under which the X′Y ″ plane can be reconstructed is calculated by the following formula ( In other words, when x = y = z = 0 in 2), this is a range where the solution of the angle β can be obtained.

Expression (2) is a point on the X′Y ″ plane viewed from the original coordinate space after coordinate transformation, regardless of what numerical values are entered in the parametric variables t and s. The turning angle β when the X′Y ″ plane represented by the parametric variables t and s in Expression (2) passes through the X-ray focal point F is x = y = z = 0 on the left side of Expression (2). And substituting t and s
It is represented by
If the value in {} in equation (3) is 1 or less, there is a solution for β.

  Also, from the following equation (4), t and s are eliminated to obtain equation (5). That is, as described above, in the equation (4) in which only the angle is discussed, it is necessary to examine the angle formed by the XZ plane and the X′Y ″ plane in the original coordinate space parallel to the detection surface of the two-dimensional detector 2. . Therefore, if the equation (1) is not a calculation of 4 rows and 4 columns but a calculation of 3 rows and 3 columns that does not include movement, and in order to derive a straight line equation on the XZ plane, only y on the left side in the equation (4) is zero. As a result of eliminating the parametric variables t and s, a relational expression of x and z, that is, an expression (5) for calculating the angle γ is derived.

Then, by substituting the angle β obtained by the equation (3) into the equation (5), at the moment of passing through the X-ray focal point F, a straight line is formed to the two-dimensional detector 2 (a plane parallel to the original coordinate space XZ plane). An angle γ of the X′Y ″ plane (projected image Im) projected onto the image is calculated.

  For example, when the image data obtained when the measurement data of FIG. 12 is obtained is substituted into Expression (5), the instantaneous change rate at which the angle γ changes when the turning angle β is around 12 degrees and around 346 is 180 degrees. It is proved that the rate of change in the vicinity is about 20%, which is sufficiently gentle.

[Variation 2: When inspection object is large]
Next, the imaging concept of the present invention when the object to be inspected 3 is large and placed so as to intersect the central axis R1 of the contact cone 50 will be described.

  FIG. 18 is a schematic diagram for explaining the imaging concept when the object to be inspected is large. As shown in the side view of FIG. 18, the second virtual cone 52 assumes that the inspected object 3 is cut by a plane including the central axis (rotation axis R1) of each cone and there is no portion that does not contact the conical cone 50. Shown in Moreover, although the partial cross section of the to-be-inspected object 3 is shown in the side view, this partial cross section is a region of interest in the field of nondestructive inspection.

  FIG. 18 shows a state when the large object to be inspected 3 is placed so as to intersect the central axis of the contact cone 50. The remainder obtained by deleting the portion included in the first virtual cone 51 side, which is orthogonal to the plane including both the generatrix and the central axis of the connecting cone 50 with which the inspected object 3 is in contact, and is cut by the plane including the central axis The X-ray focal point F is arranged in a space between the second virtual cone 52 and the first virtual cone 51 that include and circumscribe the region of interest.

In FIG. 18, the second virtual cone 52 is plotted on the assumption that there is almost no rectangular depth (length t ₁ in the short direction in FIG. 10) of the region of interest of the inspection object 3. The thin plate-like inspected object 3 occupies the cylindrical reconstruction calculation area 61, which is a space where reconstruction calculation is possible, with the highest ratio, and a space-efficient placement format is the cylindrical reconstruction calculation area 61. It is to be placed so as to substantially follow a diagonal line of a rectangular cross section (side view) including the central axis. In this way, almost 100% of the projection image acquired by the two-dimensional detector 2 including the data separated by ± 5 degrees or more from the midplane MD is effectively used for the reconstruction calculation.

  The means for calculating the reconstruction calculation center is inaccurate with a known sinogram method or the like, and an error range of less than 10% of the pixel size of the two-dimensional detector 2 is preferable. The concept described in Request 2) should be used. In the technique described in the prior application 2, first, while changing the rotation center axis deviation amount as a parameter at the time of reconstruction calculation from pixel data for one line of a projection image of the object to be inspected imaged for each angular displacement, A cross-sectional image is calculated by performing configuration calculation. Then, the rotation center axis deviation amount when the sharpness of the cross-sectional image is maximized is identified as the optimum rotation center axis deviation amount, and correction is performed based on the value of the optimum rotation center axis deviation amount. Reconstruction calculation of internal structure data is possible.

  In the imaging of FIG. 12 and the like, a CMOS type two-dimensional detector having a pixel size of 50 μm is used, and the magnification of the projected image is about 15 times. Therefore, when calculating the center axis calculation accuracy of less than 10% of the pixel size, at least the rotation accuracy of the object to be inspected should be less than 0.3 μm, and the rotation mechanism 32 in FIG. Is essential.

  The two-dimensional detector 2 shown in FIG. 18 drawn by the trigonometric method is drawn in such a manner that the width of the detection surface is “a”, the length is “b”, and the end surface in the vertical direction is in contact with the midplane MD. However, Expression (6) described below is established up to a range where the two-dimensional detector 2 and the midplane MD are in contact with each other.

Assuming that the distance between the X-ray focal point F and the two-dimensional detector 2 is “c”, the space represented by the cylindrical reconstruction calculation area 61 is an area where reconstruction calculation can be performed after the projection image is captured. Assuming that the radius of the cylindrical reconstruction calculation region 61 around the rotation axis R1 around which the object to be inspected 3 turns is “r” and the height is “h”, the angle of the diagonal line is expressed by Equation (6). Is done.

In [] in the above formula (6)
It can be considered as a function represented by In this case, when x> 0, the maximum value of f (x) is 1, and as a / (2 * c) increases, it can be considered as a function that approaches zero as much as possible.

If the detection surface of the two-dimensional detector 2 is a square of 120 mm × 120 mm and the length c is 400 mm into Formula (6), a general numerical value of an industrial X-ray CT apparatus is input.
h / 2r = 0.872773891
It becomes.

  From the equation (8), the result is that the apex angle θ of the contact cone 50 on which the inspection object 3 is placed is about 100 degrees. When a horizontally long virtual detector shown in Japanese Patent Application Laid-Open No. 2005-37158 (prior application 3) is obtained, the apex angle θ is increased and the height h is decreased. Although it is thin, since the inspected object 3 also has a thickness, if the height h is extremely reduced, the cylindrical reconstruction calculation area 61 itself is reduced. Therefore, it is preferable to use the means described in the prior application 3. There is a limit. In a region where the apex angle θ of the contact cone 50 exceeds 150 degrees, the solder balls (BGA) and the like in FIG. 24 are not projected separately, and there is a concern that unnecessary artifacts may increase. In the region where the apex angle θ of the tangent cone 50 is smaller than 100 °, the rate of change for each angle pitch near the turning angle corresponding to the projection of 72 ° or 288 ° in FIG. 16 is high, and the effect of the imaging concept of the present invention is Cut in half.

  In FIG. 18, the end face of the two-dimensional detector 2 is in contact with the midplane MD. In this case, when the end surface is away from the midplane MD and goes to the position of the two-dimensional detector 2 in FIG. 10, the height h of the cylindrical reconstruction region 61 in FIG. In this case, the apex angle of the tangent cone 50 tends to increase.

As shown in FIG. 10, when the end face of the two-dimensional detector 2 is separated from the midplane MD by a distance δ in the Z-axis direction, the apex angle θ is calculated by the following equation (9).

  Both formulas (6) and (9) are independent of the distance d from the X-ray focal point F to the rotation axis R1. Here, it is assumed that the apex angle of the tangent cone of the inspection object 3 is 0 degree, that is, the case where the inspection object 3 is placed in a posture in which the longitudinal direction is vertical. In this case, X-rays are transmitted along the short direction of the object 3 shown in FIG. 24 in the vicinity of the turning angle corresponding to the projection of 72 ° or 288 ° in FIG. As a result, the part of the projection data whose contrast is extremely dark and the extremely bright part are polarized. In addition, unless the concept described in the prior application 1 is used, important angular phase information must be imaged at a rough angular pitch.

  If the tip of the open X-ray tube 1 is sufficiently thin and does not interfere with the turning of the inspection object 3, the position of the X-ray focal point F is the position of the X-ray focal point F ′ closer to the inspection object 3. It is possible to improve the magnification of the projection of the inspection object 3.

<Second Embodiment>
In the present embodiment, a swivel stage is used for a placement portion that is a gripping means for an object to be inspected.
FIG. 19 shows a resin swivel stage 70 that is directly connected to the tip of the rotating shaft portion 74 of the rotating mechanism 32 (see FIG. 5) as an example of a mechanism for placing an object to be inspected. The swivel stage (also called gonio stage) has a virtual center of rotation, and is a mechanism that moves by drawing an arc around the center. In this example, the rotation center of the swivel stage 70 is on the rotation axis R1. The angle θ is the angle of the apex angle of the tangent cone.

  In the swivel stage 70 of this example, for example, a first stage 71 that is directly connected and fixed to the rotation shaft portion 74 and an arc (convex portion) similar to the arc (concave portion) formed on the upper surface of the first stage 71 are formed. It consists of a second stage 72 that can freely turn on the first stage 71 and a gripping portion 73 that grips the device under test 3. It is desirable that the rotating shaft portion 74 is also made of resin.

  In FIG. 19, a bus line 75 of the first virtual cone is a virtual cone (first virtual cone) that is symmetrical with the vertex of the tangent cone (corresponding to the tangent cone 50) in contact with the lower surface (main surface 3 </ b> A) of the thin plate-shaped object 3. No. 51). The generatrix 76 of the second virtual cone has the same apex angle and central axis as the tangent cone, and the conical surface circumscribes and includes the highest portion (outermost position) of the upper surface of the inspection object 3 (second imaginary virtual). (Corresponding to a cone). The bus line 77 of the third virtual cone is a bus bar of the virtual cone that circumscribes the rotation trajectory of the cross section including the rotation axis R1 of the inspection object 3. M is the length in the short direction of the object 3 to be inspected, and m is the length protruding from the generatrix that is in contact with the tangent cone 50 in the short direction of the object 3 to be inspected. N is the length in the longitudinal direction of the device under test 3, and n is the length protruding from the apex along the generatrix in contact with the tangent cone 50 in the longitudinal direction of the device under test 3.

  By using the swivel stage 70 as a gripping means for the object 3 to be inspected, a fine angle adjustment is possible by turning a knob portion (not shown) and using the virtual rotation center as a rotation axis. Further, by forming the swivel stage with a resin material, the amount of X-ray transmission increases, and a good projection image can be obtained in the two-dimensional detector 2. The object 3 to be inspected gripped by the swivel stage 70 is in contact with a virtual cone (tangent cone 50) having a vertex angle θ of about 100 degrees to 150 degrees with a predetermined angle around the rotation axis R1 of the object 3 to be inspected. Rotates (turns) at every angle.

When the object to be inspected 3 has a size that intersects the rotation axis R1 as shown in FIG. 19, the gap ε in the direction of the central axis of the tangent cone that contacts the lower surface of the object to be inspected 3 and the second virtual cone is expressed by the equation (10 ). However, the inclination angle α of the object to be inspected 3 = (π−θ) / 2.

  When the object to be inspected 3 is placed, the gap ε may be calculated from the imaging parameters shown in FIG. 19 measured in advance, and the coordinates of the X-ray focal point F may be determined. If the flatness of the inspection object 3 is sufficiently high, the conical surface of the second virtual cone, the trapezoidal trajectory TL drawn by the end surface (side surface portion) of the inspection object 3 along with the turning, and the conical surface of the third virtual cone (bus 77 The space indicated by the oblique lines between () can also be used as an X-ray focal point region capable of reconstruction calculation.

[Block configuration of X-ray tomography system]
FIG. 20 shows an example of a block configuration of the X-ray tomographic imaging apparatus of the present invention.
As described above, the X-ray tube 1 irradiates the inspection object 3 placed on the rotating base 4 with X-rays. The intensity, quality, etc. of the X-rays irradiated at this time are controlled by the control unit 95 through the X-ray control unit 94 which is X-ray control means. Further, the position of the X-ray tube 1 is determined by driving the linear motion mechanisms 33 and 42 based on the drive signal supplied from the mechanism drive unit 91 under the control of the control unit 95.

  A mechanism that functions as a mechanism control means for controlling the position and movement of the rotation base 4 such as the position (including the height direction), rotation angle displacement, initial angle phase, and the like of the rotation base 4 on which the inspection object 3 is placed. It is controlled by the control unit 95 through the drive unit 92. The object to be inspected 3 is rotated at an angular displacement designated by the rotation of the rotation base 4 by a control signal from the control unit 95, and the projection image is taken by the two-dimensional detector 2.

  The two-dimensional detector 2 is controlled to move in the XYZ axis directions by driving the linear motion mechanism 31 based on the drive signal supplied from the mechanism drive unit 93 under the control of the control unit 95. The two-dimensional detector 2 may be provided with a rotation mechanism and rotated (tilted) about a rotation axis parallel to the Z-axis direction.

  The control unit 95 is an example of a control unit. For example, a computer to which an input unit such as a keyboard and a display unit for displaying a GUI (Graphical User Interface) screen, a reconstruction result of a subject image, and the like is applied. . A processor (arithmetic processing unit) mounted on a computer performs calculation / control of an X-ray tomographic imaging process, which will be described later, according to a program stored in a non-volatile memory (not shown) such as a ROM (Read Only Memory). In addition, the computer displays information such as the X-ray intensity of the X-rays emitted from the X-ray tube 1 on the display means, or instructs the X-ray control unit 94 based on an operation signal input via the input means. Output a control command. An appropriate positioning command for reconfiguring the object to be inspected 3 is output to the linear motion mechanism 31 of the two-dimensional detector 2 and the linear motion mechanisms 33 and 42 of the X-ray tube 1.

  X-rays that have passed through the inspection object 3 are captured and detected by the two-dimensional detector 2. The two-dimensional detector 2 supplies a projection image, which is detected X-ray information, to a projection image storage unit 96 as a projection image storage unit. This projection image is stored in the projection image storage unit 96 as digitized projection data in association with the introduction and angle phase at the time of imaging in accordance with an instruction from the control unit 95. The projection image storage unit 96 only needs to have a capacity capable of recording projection data, and various types can be applied, including a large-capacity magnetic recording device, a removable recording medium such as an optical recording medium and a semiconductor memory, and the like. can do. Further, the projection data supplied from the two-dimensional detector 2 may be stored in the projection image storage unit 96 in association with information such as the angle phase, angle displacement, initial angle phase, and X-ray intensity at the time of imaging.

  Then, the projection data stored in the projection image storage unit 96 is supplied to a reconstruction calculation unit 97 that functions as reconstruction means connected thereto. The reconstruction calculation unit 97 is a computer for executing the reconstruction calculation, and reconstructs the internal structure data of the inspected object 3 from the input projection data. The reconstructed internal structure data (reconstructed data) is stored in the projection image storage unit 96 or another recording medium, and is input to the display device 98 as display means via a display memory (not shown). Displayed on the screen.

  The reconstruction calculation unit 97 only needs to have an arithmetic processing capability capable of collecting input projection data and reconstructing internal structure data, and may be shared with the control means shown by the control unit 95. Further, the display device 98 may be shared with display means connected to the control unit 95.

  With the above configuration, the internal structure data of the device under test 3 is obtained on the display device 98, and the internal structure of the device under test 3 is displayed. The operator (operator) visually confirms the presence and state of defects such as cracks and breaks inside the object to be inspected, such as multilayer film plates and minute electronic component elements, by the internal structure displayed on the display device 98. can do.

[Imaging process flow]
Next, imaging processing by the X-ray tomographic imaging apparatus described above will be described with reference to the flowchart of FIG.
First, at the start of the imaging process, the device under test 3 is placed on a conical surface of a tangent cone having a predetermined apex angle. Specifically, as shown in FIG. 6, it mounts on the mounting base 4a with a predetermined inclination (step S1). At this time, an operation signal is sent to the control unit 95, and the drive mechanism units 91 to 93 are controlled to adjust the positions of the X-ray tube 1, the rotation base 4, and the two-dimensional detector 2.

  Subsequently, it is determined from the positional relationship between the inspection object 3 and the X-ray focal point F (see FIG. 10) whether or not good X-ray transmission to the region of interest of the inspection object 3 is difficult (step S2). If it is determined that satisfactory X-ray transmission is difficult, the process proceeds to step S3. If good X-ray transmission is possible, the process proceeds to step S4.

  If it is determined in step S2 that good X-ray transmission is difficult, the control mechanism 95 controls the drive mechanism units 92, 93 and the like to move the rotation base 4 and rotate the rotation axis of the object 3 to be inspected. And the height of the two-dimensional detector 2 is adjusted (step S3). This process is preferably performed, but is not necessarily performed, and may be skipped.

  On the other hand, when it is determined in step S2 that the X-ray transmission is good, the control unit 95 moves the space between the first virtual cone 51 and the second virtual cone 52 (shaded portion in FIG. 9). Calculate by calculation. Alternatively, if the swivel stage shown in FIG. 19 is used, the gap ε in equation (10) is calculated, and the space between the second virtual cone, the third virtual cone, and the trapezoidal locus TL can be calculated. Obtained (step S4).

  Based on the calculation result of step S4, the control unit 95 sends a control signal to the mechanism driving unit 93 to move the position of the X-ray focal point F to a space sandwiched between the two types of virtual conical surfaces (step S5).

  The control unit 95 determines whether or not the plane group having a high absorption coefficient in the inspection object 3 can be observed from the front and back on the projection as the inspection object 3 is turned (step S6).

  If the thin inspected object 3 can be observed through both the front and back surfaces, the inspected object 3 is configured twice (at least once), for example, while rotating 360 degrees on the assumption that the angular pitch of the rotation is sufficiently fine. This proves that the plane group was individually observable linearly. If it is determined that observation from the front and back sides is impossible, the process returns to step S5, and the X-ray focal point F is moved to the area indicated by the oblique lines in FIG. 9 or FIG.

  After the X-ray focal point F is moved to an appropriate position, the control unit 95 controls the mechanism control unit 92 to rotate the object 3 to be inspected by the two-dimensional detector 2 at every predetermined angle pitch. Is projected and stored in the projection image storage unit 96 (step S7).

  The control unit 95 transfers the obtained projection data from the projection image storage unit 96 to the reconstruction calculation unit 97 (step S8).

  Here, the reconstruction calculation unit 97 calculates the central axis by the method disclosed in the above-mentioned prior application 2 (step S9). However, using the method disclosed in the prior application 2 is preferable for obtaining reconstructed data that can be observed in detail, but does not constitute an essential part of the present invention.

  The reconstruction calculation unit 97 reconstructs and calculates all projection data with the obtained one central axis (step S10). After the process of step S10 is complete | finished, a series of imaging processes are complete | finished.

  The processing of these steps S1 to S10 is executed in parallel or in a different order even if processing is not necessarily performed in time series, as well as processing performed in time series in the described order. Also good.

[Comparison of Conventional Image and Reconstructed Image According to the Present Invention]
FIG. 22 is a diagram showing a BGA tomographic image taken by a general X-ray tomographic imaging method. On the other hand, FIG. 23 is a diagram showing a BGA tomographic image taken by the X-ray tomographic imaging method of the present invention.

  The tomographic image 81 of the conventional example shown in FIG. 22 is an artifact (a two-dimensional obstacle shadow other than the objective information appearing in the image, a pseudo image generated due to the backlash of the apparatus) as a BGA ball-shaped electrical joint (bump). The influence of is great. For this reason, the upper and lower portions 81A and 81B of the BGA are blurred, and the details of the substrate and the bumps cannot be observed. As already described, the projection image and the reconstructed image of the thin test object shown in FIGS. 3A and 3B cannot be observed in the same manner.

  On the other hand, the tomographic image 82 according to the present invention shown in FIG. 23 is a BGA tomographic image of the same part as that in FIG. 22, but is not affected by artifacts, and cracks are formed in the upper right 82A and lower left 82B of the electrical joint (bump). It can be recognized in detail that there is. As described above, when the orthogonal X-ray CT apparatus for industrial use is applied with the filter-corrected back projection method or the multiplicative integration method (convolution back projection method) according to a reconstruction algorithm such as Feldkamp. It is shown that it is only necessary to provide constraints on the mounting posture and the X-ray focal position of the thin test object. A clear non-destructive tomographic image can be obtained with respect to the region of interest of the object to be examined only by providing this constraint condition.

  In addition, as shown in FIG. 18, the diagonal cross section of the rectangular cross section passing through the central axis of the cylinder of the reconstruction calculation region (Region of Image Support) is determined by the detection surface size of the two-dimensional detector and the X-ray focal point position. It is preferable to select the apex angle of the conical cone on which the object to be inspected is placed so as to be substantially along. As a result, the region of interest of the thin object to be inspected efficiently occupies the reconfigurable area, and the projection data obtained by the two-dimensional detector is almost 100%, which is effectively used for reconfiguration calculation with one reconfiguration calculation center. It becomes possible to do.

  Furthermore, according to the imaging method of the present invention, projection data is collected at a fine change rate while imaging at an equal angular pitch for the vicinity of the projection of an important angular phase that characterizes the region of interest of the thin inspected object. It is possible. Furthermore, from the positional relationship between the two-dimensional detector and the inspection object shown in FIG. 10, projection data of the inspection object can be acquired from an advantageous direction with respect to the X-ray transmittance. If the interlayer distance of the copper foil of the object to be inspected shown in FIG. 13 or the like is extremely small, it is possible to obtain sufficient resolution while shortening the imaging time by using the method disclosed in the prior application 1 together. is there.

  In an industrial orthogonal X-ray CT apparatus, when the irradiation angle of a cone-shaped X-ray beam is large, projections that are more than ± 5 degrees away from the midplane may be discarded. However, in the imaging method of the present invention, unlike the conventional theory in orthogonal (single scan cone beam) CT, the detection surface of the two-dimensional detector may deviate from the midplane (see FIG. 10). Even if the detection surface of the two-dimensional detector deviates from the midplane, even if it is a substance that is difficult to transmit X-rays, the projection data of the flat object to be inspected will be varied, and it will be wrapped in a cylindrical jig etc. There is an effect that an image can be captured with an ideal contrast without reducing the change in contrast.

  The embodiment described above is a specific example of a preferred embodiment for carrying out the present invention, and therefore various technically preferable limitations are given. However, the present invention is not limited to these embodiments unless otherwise specified in the above description of the embodiments. For example, the materials used in the above description, the amount used, the processing time, the processing order, the numerical conditions of each parameter, etc. are only suitable examples, and the dimensions, shapes, and arrangement relationships in the drawings used for the description Etc. are also schematic drawings showing an example of the embodiment. Therefore, the present invention is not limited to the above-described embodiments, and various modifications and changes can be made without departing from the scope of the present invention.

It is a side view which shows arrangement | positioning of a general to-be-inspected object, an X-ray tube, and a two-dimensional detector. The positional relationship of a general to-be-inspected object, a rotation base, and an X-ray tube is shown, A is a perspective view and B is a side view. FIGS. 3A and 3B are diagrams illustrating examples of imaging results obtained by the X-ray tomographic imaging method in the positional relationship illustrated in FIGS. It is explanatory drawing which showed the shadow zone. 1 is a schematic side view of an X-ray tomographic imaging apparatus according to a first embodiment of the present invention. It is a schematic diagram which shows the positional relationship of the to-be-inspected object based on the 1st Embodiment of this invention, a rotation base, and an X-ray tube. It is a schematic diagram which shows the positional relationship of the shadow zone and to-be-inspected object which concerns on the 1st Embodiment of this invention. The posture of the to-be-inspected object which concerns on the 1st Embodiment of this invention is shown, A is a schematic perspective view at the time of seeing the two-dimensional detector side from the X-ray tube side, B is from the two-dimensional detector side. It is a schematic perspective view at the time of seeing the X-ray tube side. It is explanatory drawing of the space pinched | interposed into the 1st virtual cone and 2nd virtual cone of FIG. 8A and 8B. It is a figure which shows the preferable positional relationship of the to-be-inspected object 3 and X-ray focus F which concern on the 1st Embodiment of this invention. It is a figure which shows the coordinate space of each of the X-ray tube (X-ray focus), two-dimensional detector, and tangent cone (vertex) which were shown in FIG.8 and FIG.9. It is a figure which shows the projection image in each angle phase in case of the inclination angle 15 degree | times (vertical angle 150 degree | times) which concerns on the 1st Embodiment of this invention. FIG. 12 shows internal structure data obtained by reconstructing the projection image under the conditions shown in FIG. 12, where A indicates the X-axis cross section and B indicates the Y-axis cross section. It is a figure which shows the projection image in each angle phase in case of the inclination angle 30 degree | times (vertical angle 120 degree | times) which concerns on the 1st Embodiment of this invention. 14 is internal structure data obtained by reconstructing a projection image under the conditions shown in FIG. 14, wherein A indicates an X-axis cross section and B indicates a Y-axis cross section. It is a figure which shows the projection image in each angle phase in case of the inclination | tilt angle of 40 degree | times (vertical angle 100 degree | times) which concerns on the 1st Embodiment of this invention. FIG. 16 shows internal structure data obtained by reconstructing the projection image under the conditions shown in FIG. 16, where A indicates the X-axis cross section and B indicates the Y-axis cross section. It is a figure for demonstrating the example in case the to-be-inspected object is large in the 1st Embodiment of this invention. It is the schematic diagram which showed the mounting part using the swivel stage which concerns on the 2nd Embodiment of this invention. It is a functional block diagram of the X-ray tomographic imaging apparatus shown in FIG. It is a flowchart which shows the process by the X-ray tomographic imaging apparatus which concerns on each embodiment of this invention. It is a figure which shows the tomographic image of BGA image | photographed with the general X-ray tomography method. It is a figure which shows the tomographic image of BGA image | photographed with the X-ray tomography method of this invention. It is sectional drawing of an example of to-be-inspected object 3 which can apply this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... X-ray tube, F ... X-ray focus, 2 ... Two-dimensional detector, 3 ... Test object, 3A, 3B ... Main surface, 4 ... Rotation base, 4a ... Mounting part, 6 ... Shadow zone, 7 , Storage case, 31 ... linear motion mechanism, 32 ... rotation mechanism, 33 ... linear motion mechanism, 40 ... surface plate, 41 ... rail, 42 ... linear motion mechanism, 43 ... rail, 50, 50-1, 50-2 ... Tangent cone, 50A, 51A, 52A ... busbar, 51 ... first virtual cone, 52 ... second virtual cone, 53 ... vertex, 61 ... cylindrical reconstruction calculation area, 70 ... swivel stage, 71 ... first stage, 72 ... second stage 73 ... grip part 74 ... rotating shaft part 75 ... first virtual cone bus 76 ... second virtual cone bus 77 ... third virtual cone bus 81 ... reconstructed image 82 ... Reconstructed image

Claims (5)

An X-ray tomographic imaging apparatus for reconstructing the internal structure data of the inspection object from projection data of a plurality of inspection objects,
An X-ray source;
A two-dimensional detector for imaging transmitted X-rays of the object to be inspected;
The object to be inspected is placed between the X-ray focal point of the X-ray source and the two-dimensional detector, and the bottom surface of the conical beam formed by the X-rays emitted from the X-ray source. A rotation mechanism that rotates at a set angular displacement around a rotation axis orthogonal to a line segment that connects the center and the X-ray focal point;
A mounting portion for mounting the object to be tested in a state in which the thickness direction is substantially perpendicular to a generatrix of a virtual cone (tangent cone) having a vertex angle θ of a predetermined angle with the rotation axis as a center line;
A first virtual cone that is point-symmetric with respect to the vertex of the tangent cone with which the object to be inspected contacts, and the outermost position of the region of interest for obtaining a tomographic image of the object to be inspected placed on the tangent cone And a moving mechanism for moving the position of the X-ray focal point into a space sandwiched between two virtual conical surfaces of a second virtual circumscribed cone having the same apex angle as the tangent cone and having a central axis on the same axis, An X-ray tomographic imaging apparatus. The X-ray tomographic imaging apparatus according to claim 1, wherein an apex angle θ of the tangent cone is 100 ° ≦ θ ≦ 150 °. The detection surface of the two-dimensional detector is substantially orthogonal to a perpendicular drawn from the X-ray focal point to the rotation axis, and the detection surface is
A straight line that is perpendicular to the midplane along the rotation axis with a position where a plane (midplane) that includes the perpendicular line and is orthogonal to the rotation axis and a pixel row at the center of the detection surface in the rotation axis direction overlap. A movable distance that is disposed at a position where an angle formed by a plane connecting the X-ray focal point and the pixel row passing through the center and the midplane is equal to or greater than (π−θ) / 2 The X-ray tomographic imaging apparatus according to claim 2. The object to be inspected is placed so as to intersect the central axis of the tangent cone, orthogonal to a plane including both the generatrix and the central axis of the tangent cone in contact with the object to be inspected, and including the central axis A space that is cut by a plane and that includes the remaining region of interest from which the portion included on the first virtual cone side is deleted, and is sandwiched by the conical surfaces of both the second virtual cone and the first virtual cone that circumscribe When the X-ray focal point is located,
The detection surface of the two-dimensional detector is rectangular, the length of the detection surface in the direction perpendicular to the rotation axis is a, the length in the parallel direction is b, and the distance from the X-ray focal point to the detection surface is c
When the midplane and the detection surface intersect, the apex angle of the tangent cone is represented by the following equation:
tan {(π−θ) / 2} = b / a * [√ {1- (a / (2 * c)) ² } −a / (2 * c)]
When the end face of the detection surface is separated from the midplane, the distance is represented by the following formula with δ.
tan {(π−θ) / 2} = b / a * [√ {1- (a / (2 * c)) ² } −a / (2 * c)] − δ / c
The X-ray tomographic imaging apparatus according to claim 3. A first step of placing an object to be inspected on a placement portion in a state in which the plate thickness direction is substantially perpendicular to a generatrix of a virtual cone (tangent cone) having an apex angle θ with a rotation axis as a center line. When,
A first virtual cone that is point-symmetric with respect to the vertex of the tangent cone with which the object to be inspected contacts, and the outermost part of the region of interest to obtain a tomographic image of the object to be inspected placed on the tangent cone In addition, the position of the X-ray focal point of the X-ray source is moved into the space between the two virtual conical surfaces of the second virtual circumscribed cone having the same apex angle as the tangent cone and having a central axis on the same axis. A second step of moving by a mechanism;
The center of the bottom surface of the conical beam formed by the X-rays emitted from the X-ray source, which is arranged by placing the object to be inspected between the X-ray focal point of the X-ray source and the two-dimensional detector. And a third step of rotating a rotation mechanism that rotates about a rotation axis that is orthogonal to a line segment that connects the X-ray focal point with a set angular displacement;
A fourth step of reconstructing the internal structure data of the object to be inspected from projection images imaged for each angle phase.