Collimator for x-ray inspection system, x-ray laminography system with such collimator, and use of such collimator

Abstract

To provide an X-ray laminography system, and to reduce a radiation exposure dose to a test object 2 targeted for inspection.SOLUTION: A collimator 1 for an X-ray inspection system is plate-shaped with a bottom side 14 and an top side 17, and is made of a material excellently absorbing X-rays. A collimator aperture 11 has: an inlet surface 15 on the bottom side 14 having a first area centroid 16; and an outlet surface 18 on the top side 17 having a second area centroid 19. When installed in an X-ray laminography system, the collimator aperture is not struck by a central beam 7 of a cone beam 6 emitted by an X-ray tube 4 of the X-ray laminography system, or the first area centroid 16 and the second area centroid 19 form a collimation axis 20, and the collimation axis 20 forms, with a normal line of a surface of the collimator 1, a collimation angle γ not equal to zero.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a plate-shaped collimator for an X-ray inspection system, which is made of a material that absorbs X-rays well and has a collimator opening. Further, the present invention relates to an X-ray laminography system provided with such a collimator, in which an effective beam is generated from a cone beam generated at the focal point of an X-ray tube by such a collimator. Further, the present invention relates to the use of such a collimator in an X-ray laminography system.

Background Art

[0002] X-ray laminography scans are typically used to inspect flat and large-area test objects such as circuit boards and wafers. Since there are usually assemblies with various different components on the circuit board, scans are usually continuously performed multiple times in close proximity to various inspection positions on the circuit board. In many cases, a transmission X-ray tube is used in the X-ray laminography system. According to this type of X-ray tube, the test object can be brought as close as possible to the focal point, so that a high magnification can be achieved. In a transmission X-ray tube, a target that generates radiation usually forms the end of the X-ray tube. In this type of X-ray tube, a cone beam with a large opening angle, for example 170°, is generated, and usually most of this cone beam collides with the circuit board. The combination of many inspection positions, a very small distance to the focal point, and a large opening angle of the cone beam that collides with the circuit board leads to high radiation exposure to the components to be inspected on the circuit board. Also, areas of the printed circuit board that cannot be visually recognized in the X-ray image can also be exposed to a high level of radiation throughout the inspection process. Depending on the components located in such areas, this can lead to radiation damage to the components, especially in the area located in the direction of the central beam of the cone beam.

Summary of the Invention

Problems to be Solved by the Invention

[0003] The object of the present invention is to provide a method for significantly reducing the radiation exposure to a component during X-ray inspection, particularly during laminography inspection, which occurs during the inspection of other components.

[0004] According to the prior art, such a solution is known in the form of a filter that reduces the total exposure level. However, such a filter reduces the dose of the entire cone beam, which includes the part of the beam necessary for inspecting a partial area of the target substrate. When using a filter, the low-energy component of the spectrum can be preferentially filtered out. In most cases, the low-energy radiation component only contributes to the exposure dose to the object of interest and has little impact on the image quality. Therefore, this filtering reduces the exposure dose to the test object. However, when filtering is performed, a part of the spectrum that is always beneficial for the image quality is filtered out. Therefore, when filtering is performed, it is always necessary to increase the integration time or power in order to obtain the same image quality as when no filtering is performed.

Means for Solving the Problem

[0005] According to the present invention, the above object is achieved by a collimator having the features of claim 1 or 2, a laminography X-ray device comprising such a collimator having the features of claim 10, and the use of such a collimator in an X-ray laminography system according to the features of claim 13. Advantageous embodiments are described in the dependent claims.

[0006] According to the present invention, the above problems are achieved by a collimator that is plate-shaped with a lower side and an upper side and is made of a material that absorbs X-rays well. The collimator has a collimator aperture, and the collimator aperture has a lower entrance surface having a first surface centroid and an upper exit surface having a second surface centroid. There are two embodiments that can be used to solve the above problems. In the first embodiment, when the collimator aperture is installed in an X-ray laminography system, it does not collide with the central beam of the cone beam emitted by the X-ray tube of the X-ray laminography system. In the second embodiment, the first surface centroid and the second surface centroid form a collimation axis, and the collimation axis forms a collimation angle that is not equal to zero with the normal of the surface of the collimator. In any design, only a very small part of all the X-ray beams emitted from the focal point of the X-ray tube onto the collimator passes through, and all parts of the test object that are not aligned with the focal point and the collimator aperture (inspection area) are not exposed to X-rays, so those parts cannot be damaged by the high dose of X-ray irradiation. The collimator aperture provides an X-ray beam with a very limited effective beam opening angle behind the collimator, and during scanning, it collides only with the inspection area of the test object.

[0007] According to an advantageous improvement of the present invention, it is provided that the entrance surface and the exit surface are each circular, elliptical, trapezoidal, square, pentagonal or hexagonal. In a preferred variant having two symmetric trapezoids as the entrance surface and the exit surface, when these surfaces are selected in a geometrically correct way, the spatial spread of the collimator aperture can be a frustum of a pyramid. This results in a rectangular beam as the effective beam behind the collimator that can illuminate a rectangular detector in the best possible way. In a variant having two ellipses as the incident surface and the exit surface, when these surfaces are selected geometrically correctly, the spatial spread of the collimator aperture can be a frustum of a cone. This results in a conical beam as the effective beam behind the collimator, which is preferable when there is a circular detector. Other variants represent collimator apertures that are easy to manufacture and provide a good effective beam although with inferior efficiency.

[0008] According to another advantageous improvement of the present invention, it is provided that the collimator opening does not include the center of gravity of the lower surface of the collimator, and the second center of gravity of the exit surface is located further outside than the first center of gravity of the entrance surface on the collimator. This makes it possible to create a collimator having a mass in the central region, so that the central beam of the X-ray tube is absorbed and does not collide with the test object. Further, the collimation axis has a direction component in the beam direction and extends outward within the collimator, so that when the collimator is installed, the central beam can collide with the center of the collimator. Further, the collimation axis of the collimator opening extends along an effective beam that forms a non-zero angle with respect to the central beam, so that the effective beam is incident on the inspection region arranged in alignment with the focal point and the collimator opening, and thus is not partially absorbed unnecessarily at the edge of the collimator opening (this causes an interference effect).

[0009] According to another advantageous improvement of the present invention, it is provided that the entrance surface and the exit surface of the collimator opening each extend to the edge of the collimator. Such a shape is easy to manufacture because the collimator opening coincides with the edge of the collimator on one side.

[0010] Preferably, the collimator has a thickness of 0.1 to 5.0 mm.

[0011] According to another advantageous improvement of the present invention, it is provided that the opening angle of the collimator opening is 1° to 40°. Thereby, a partial beam sufficient to illuminate the inspection region of the test object to be inspected is generated from the entire X-ray beam generated at the focal point. Other non-inspection regions of the test object are not exposed to an unnecessary radiation dose.

[0012] According to another advantageous improvement of the present invention, it is provided that the collimation angle is between 10° and 70°, preferably between 40° and 60°. Thereby, when performing the X-ray laminography procedure, it becomes possible to inspect the inspection area obliquely, whereby it becomes possible to very well detect possible defects in the component and to better separate various different depths (Ebene) within the inspection area.

[0013] According to another advantageous improvement of the present invention, it is provided that the collimator is made of a material with a high average atomic number and high density, in particular tungsten or a tungsten alloy, and the material preferably has the maximum possible dimensional stability. Thereby, good shielding of all non-inspected areas of the test object is provided, and it also becomes possible to make the collimator thinner.

[0014] According to another advantageous improvement of the present invention, it is provided that the collimator has two or more collimator openings. Thereby, it becomes possible to simultaneously inspect a plurality of inspection areas of the test object located outside the central beam of the X-ray tube.

[0015] Furthermore, the above problem is also achieved by the X-ray laminography system according to the present invention. The X-ray laminography system includes an X-ray tube, in particular a transmission X-ray tube, and the X-ray tube has a focal point at which X-rays are generated in the form of a cone beam. The X-ray laminography system also has a detector that is collided by the effective beam of the cone beam. Between the focal point and the detector, the collimator according to the present invention is arranged. The collimator opening of the collimator is oriented such that only the portion of the X-rays of the cone beam generated at the focal point that forms the effective beam passes through, whereby a test object to be inspected that can be arranged between the collimator and the detector within the X-ray beam is exposed to X-ray irradiation only in one inspection area. Thereby, the advantages already described above regarding the collimator and its use are achieved.

[0016] According to an advantageous improvement of the X-ray laminography system according to the present invention, it is provided that the detector is fully illuminated. As described above, this does not limit the field of view. By accurate illumination (i.e., by accurately illuminating the active surface of the detector with the entire effective beam), an optimal ratio between the dose introduced into the examination area and the low loss of information in the detector is obtained. Preferably, the entire X-ray radiation of the effective beam impinges on the detector. Since this is substantially impossible to achieve, the design is such that the X-ray radiation of the effective beam that does not impinge on the detector is minimized.

[0017] According to another advantageous improvement of the X-ray laminography system according to the present invention, it is provided that the normal to the surface of the collimator is parallel to the central beam of the cone beam. This simplifies the arrangement of the collimator in the X-ray laminography system very much.

[0018] Finally, by using the collimator according to the present invention in an X-ray laminography system, the above problem is also achieved because the dose described above is strongly reduced for all areas that are not the examination area of the test object to be examined. In the application of laminography, the detector is usually arranged at the largest possible angle with respect to the central beam emitted by the focal point. According to the present invention, the effective beam is reduced by the collimator to the angular range where the detector is located. Thereby, the exposure dose of all components not within the effective beam is reduced. This can be used to achieve a particularly large reduction in the exposure dose for components located directly above the focal point during a laminography scan. Due to their position, these components are irradiated more strongly during the scan than the components within the image in applications without using the collimator according to the present invention. In particular, when testing sensitive test specimens, the present invention reduces damage to semiconductor components, especially components such as wafers or semiconductor memories.

[0019] According to an advantageous improvement form of the use according to the present invention, the collimator is arranged between the focal point of the X-ray tube, the test object to be inspected, and a detector located behind the test object in the beam direction, so that a part of the cone beam emitted from the X-ray tube that can pass through the collimator opening passes through the test object to be inspected within a partial region, and in particular, it is provided that the detector is completely illuminated. By completely illuminating the detector, the field of view is not restricted and there is no loss of information.

Brief Description of the Drawings

[0020] Hereinafter, further details and advantages of the present invention will be described by referring to the exemplary embodiments shown in the drawings.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0021] Figure 1 shows a schematic example of the structure of an X-ray laminography system known from the prior art. The X-ray laminography system has an X-ray tube 4 (here a transmission X-ray tube), which generates a cone beam 6 with a semi-aperture angle α of approximately 85° at the focal point 5. Inside the cone beam 6, a test object 2 in the shape of a circuit board is arranged. The central beam 7 of the cone beam 6 is substantially perpendicular to the surface of the test object 2. Instead of the entire test object 2 being inspected, only a partial area is inspected, and this partial area is referred to as the inspection area 12. The inspection area 12 is not in the direction of the central beam 7 but is inclined by the observation angle γ with respect to the direction of the central beam 7. In the example shown, the observation angle γ is approximately 60°. In X-ray laminography, the larger the observation angle γ, the better the separation of the layers within the test object 2. The inspection area 12 is located between the focal point 5 and the detector 3. The rays of the cone beam 6 are emitted from the focal point 5 and impinge on the detector 3 at the outermost side, forming a field of view 8 formed around the observation axis 13. Here, the observation axis 13 is at the observation angle γ with respect to the central beam 7 and impinges on the center of the detector 3. The field of view 8 has a field-of-view opening angle δ of approximately 20°. The inspection area 12 must be within the field of view 8 during the inspection, i.e., while the laminography scan is being performed. In the example shown, most of the test object 2 is located within the cone beam 6 and is exposed to radiation but is not within the field of view 8. Therefore, for most of the test object 2, there is no benefit in the exposure dose. If X-ray-sensitive components are on the test object 2, such a high exposure dose may cause damage. The implementation of the laminography scan is known from the prior art and is not the subject of the present invention, so further description is omitted.

[0022] The present invention suppresses damage caused by such high-level exposure. FIG. 2 shows a schematic example of an X-ray laminography system according to the present invention. The structure of the X-ray laminography system according to the present invention in FIG. 2 is the same except that a collimator 1 according to the present invention is disposed between a focal point 5 and a test object 2. Therefore, the same reference numerals are given to the same features and will not be described again below. Instead, only the differences will be described in more detail.

[0023] Since the collimator 1 is plate-shaped, its thickness is very small compared to its extent on a plane. The collimator 1 has a lower side 14 facing the X-ray tube 4 side and an upper side 17 facing the side opposite to the X-ray tube 4. In order to shield the X-ray irradiation of the cone beam 6 as well as possible in all regions except the test region 12, it is made of a material with a large atomic number. When the collimator 1 is made of an alloy or different materials, it is desirable that it has a large average atomic number. A large (average) atomic number is considered to be a value exceeding 26. Further, the collimator 1 is made of a high-density material, preferably, for example, 7,500 kg / m 3 or more. In order to ensure simple manufacturing and little influence on mechanical deformation, a material that is dimensionally stable as much as possible is used. In this case, such a material is a tungsten alloy, that is, Densimet (registered trademark).

[0024] The collimator 1 has a collimator aperture 11, which can also be clearly confirmed in two embodiments of FIGS. 3 and 4 in addition to FIG. 2.

[0025] In the first exemplary embodiment according to FIG. 3, the collimator aperture 11 is conical around the central axis. For this purpose, an entrance surface 15 is provided on the lower side 14 of the collimator 1, and this entrance surface 15 is elliptical and has a first surface centroid 16. On the upper side 17 of the collimator 1 there is an exit surface 18, and this exit surface 18 is also elliptical and has a second surface centroid 19. The entrance surface 15 and the exit surface 18 match each other in geometric features such that the spatial extent of the collimator aperture 11 forms a frustum of a cone (the specific configuration is readily determined by those skilled in the art). The collimation axis 20 defined by the first surface centroid 16 and the second surface centroid 19 corresponds to the central beam 10. Thereby, a conical effective beam 9 that is symmetric with respect to the central beam 10 is generated behind the collimator 1, and this effective beam 9 illuminates the detector 3. If the detector 3 is circular, optimal illumination can be achieved.

[0026] The central beam 10 corresponds to the observation axis 13 described above with respect to FIG. 1, and the opening angle (the effective beam semi-opening angle β is shown in FIG. 3) is approximately equal to the field of view opening angle δ of FIG. 1. Since the collimation angle γ depends on the position of the detector 3, it increases as the observation axis 13 moves farther from the central beam 7. The size of the field of view opening angle δ depends on the size of the detector 3, as well as the distance from the focal point 5 of the collimator 1 and the distance from the collimator 1 to the detector 3, when an ideal arrangement is desired. This means that different collimators 1 are required for scans at different laminography angles, and such collimators 1 need to be exchanged before performing the laminography scan. In order to obtain optimal results, different collimators 1 need to be used even when the distance between the collimator 1 and the focal point 5, and the distance between the collimator 1 and the detector 3 are different. The collimator aperture 11 is formed outside the center of the collimator 1. As a result, the collimator 1 can be centered above the central beam 7 and shields the X-rays that are not emitted in the region of the effective beam 9 in all directions of the cone beam 6, thereby protecting the unexamined region of the test object 2. Thereby, the exposure dose in the part of the test object 2 that is not within the field of view 8 and thus not within the effective beam 9 is minimized. By using the collimator 1 according to the present invention, when the radiation-sensitive components of the test object 2 to be inspected are not within the field of view 8, the exposure received by these components is significantly reduced. This makes it possible to approach a plurality of inspection positions on the test object 2 without irradiating the entire test object 2 with radiation and affecting all components. This is particularly advantageous for a circuit board having X-ray sensitive components.

[0027] In the second embodiment shown in FIG. 4, only the shapes of the entrance surface 15 and the exit surface 18 are different from those in the first embodiment example shown in FIG. 3. They are not elliptical but have the shape of a symmetric trapezoid respectively. The display of the centroid of the surfaces 16, 19 of the entrance surface 15 and the exit surface 18, and the collimation axis 20 defined by them is omitted because there is no difficulty for those skilled in the art starting from FIG. 3. Therefore, the spatial spread of the collimator aperture 11 has a frustum of a pyramid shape here instead of the frustum of a cone shape according to FIG. 3, and as a result, a rectangular beam is obtained as the effective beam 9. This is particularly good when using a rectangular detector 3 (in the normal case), and since the detector 3 can be optimally and completely illuminated by the effective beam, the maximum efficiency can be achieved.

[0028] FIG. 5 shows a third embodiment, which has a rectangular entrance surface 15, but the entrance surface 15 extends to the edge of the collimator 1 (different from the previous two exemplary embodiments), and one of its sides coincides with the edge of the collimator 1. This increases the exposure dose of the components in the outer region, but makes the manufacture of the collimator aperture 11 easier. The alignment between the exit surface 18 and the entrance surface 15 can be easily determined by those skilled in the art based on geometric conditions.

[0029] FIG. 6 shows a fourth embodiment, which is different from the third embodiment in FIG. 5 only in that in addition to the right collimator aperture (represented by the entrance surface 15), there is also a left collimator aperture (represented by the entrance surface 15′). This makes it possible to simultaneously inspect two different inspection regions 12 within the test object 2. As can be more clearly confirmed from the right cross-sectional view, the left entrance surface 15′ has different dimensions from the right entrance surface 15, so there are different collimation angles (γ on the right and γ′ on the left). In this exemplary embodiment, the right collimation angle γ is smaller than the left collimation angle γ′. This is only an example, and the actual design selected depends on the requirements in each application, especially on where the inspection region 12 is located in the test object 2.

[0030] Instead of providing two collimator openings 11 as shown in FIG. 6, more collimator openings 11 may be provided, and the collimator openings 11 may be formed at other positions of the collimator 1. However, it should be noted that the exposure dose in the outer region increases with each of these collimator openings 11 according to FIGS. 5 and 6.

Explanation of Symbols

[0031] 1 Collimator 2 Test Object 3 Detector 4 X-ray Tube 5 Focus 6 Cone Beam 7 Central Beam 8 Field of View 9 Effective Beam 10 Central Beam 11 Collimator Opening 12 Inspection Region 13 Observation Axis 14 Lower Side 15, 15′ Entrance Surface 16 First Surface Centroid 17 Upper Side 18 Exit Surface 19 Second Surface Centroid 20 Collimation Axis α Half Opening Angle β Effective Beam Half Opening Angle γ, γ′ Collimation Angle δ Field of View Opening Angle

Claims

1. A collimator (1) for an X-ray inspection system, comprising: a plate-like structure with a lower side (14) and an upper side (17) made of a material that is highly absorbing of X-rays; a collimator aperture (11), The collimator opening (11) has an entrance surface (15) of the lower side (14) having a first surface centroid (16) and an exit surface (18) of the upper side (17) having a second surface centroid (19), The collimator (1) has a collimator opening (11) that is not struck by a central beam (7) of a cone beam (6) emitted by an X-ray tube (4) of the X-ray laminography system when installed in the X-ray laminography system.

2. A collimator (1) for an X-ray inspection system, the collimator (1) comprising: a plate-like structure with a lower side (14) and an upper side (17) made of a material that is highly absorbing of X-rays; a collimator aperture (11), The collimator opening (11) has an entrance surface (15) of the lower side (14) having a first surface centroid (16) and an exit surface (18) of the upper side (17) having a second surface centroid (19), A collimator (1), wherein the first surface centroid (16) and the second surface centroid (19) form a collimation axis (20), which forms a collimation angle (γ) not equal to zero with a normal to a surface of the collimator (1).

3. 2. The collimator (1) of claim 1, wherein the entrance face (15) and the exit face (18) of the collimator (1) are each circular, elliptical, square, pentagonal or hexagonal.

4. 2. The collimator (1) according to claim 1, wherein the spatial extent of the collimator opening (11) is a truncated cone or a truncated pyramid, and the central axis of each of the truncated cone or truncated pyramid is the collimation axis (20).

5. 2. The collimator (1) of claim 1, wherein the collimator opening (11) does not include the surface centroid of the lower side (14) of the collimator (1), and the second surface centroid (19) of the outlet surface (18) is located further outward than the first surface centroid (16) of the inlet surface (15) on the collimator (1).

6. 2. The collimator (1) according to claim 1, wherein the entrance face (15) of the collimator opening (11) and the exit face (18) of the collimator opening (11) each extend to an edge of the collimator (1).

7. The collimator (1) according to claim 1, wherein the thickness of the collimator (1) is 0.1 to 5.0 mm, and / or the opening angle of the collimator opening (11) is 1° to 40°, and / or the collimation angle (γ) is 10° to 70°.

8. 2. The collimator (1) according to claim 1, wherein the collimator (1) is made of a material with a high mean atomic number and high density.

9. A collimator (1) as described in claim 8, wherein the collimator (1) is made of tungsten or a tungsten alloy and / or the material has the greatest possible dimensional stability.

10. A collimator (1) as described in claim 1, having a plurality of collimator openings (11).

11. An X-ray laminography system having an X-ray tube (4), The X-ray tube (4) has a focal point (5) from which X-rays are generated in the form of a cone beam (6); a detector (3) struck by an effective beam (9) of the cone beam (6); a collimator (1) arranged between the focal point (5) and the detector (3) and designed according to any one of claims 1 to 10, an X-ray laminography system, wherein the collimator opening (11) of the collimator (1) is oriented to transmit only a portion of the X-rays of the cone beam (6) generated at the focal point (5) that form the effective beam (9), thereby exposing a test object (2) to be inspected that can be introduced into the X-ray beam between the collimator (1) and the detector (3) to X-rays only in an inspection region (12).

12. 12. An X-ray laminography system according to claim 11, wherein the detector (3) is fully illuminated, but as little X-ray radiation as possible from the useful beam (9) does not impinge on the detector (3).

13. 12. The X-ray laminography system according to claim 11, wherein the normal to the surface of the collimator (1) is parallel to the central beam (7) of the cone beam (6).

14. Use of a collimator (1) described in any one of claims 1 to 10 in an X-ray laminography system to reduce the exposure dose to the test object (2) to be inspected.

15. The use of claim 14, wherein the test object (2) to be inspected is a semiconductor component, a wafer or a semiconductor memory.

16. 15. The use according to claim 14, wherein the collimator (1) is arranged between the focal point (5) of the X-ray tube (4) and the test object (2) to be examined and a detector (3) located behind the test object (2) in the beam direction, so that a part of the cone beam (6) emitted from the X-ray tube (4) that can pass through the collimator opening (11) is transmitted through the test object (2) to be examined within the examination area (12).

17. The use described in claim 16, wherein a portion of the cone beam (6) emitted from the X-ray tube (4) that can pass through the collimator opening (11) completely illuminates the detector (3).

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