Collimator for an X-ray inspection system, X-ray laminography system with such a collimator and use of such a collimator

Abstract

Collimator (1) for an X-ray inspection system, which is plate-shaped with a bottom side (14) and a top side (17), which is made of a material that absorbs X-rays well, with a collimator opening (11), wherein the collimator opening (11) has an inlet surface (15) on the underside (14) which has a first center of gravity (16) and an outlet surface (18) on the upper side (17) which has a second center of gravity (19), wherein the collimator opening (11) is not struck by a central beam (7) of a cone beam (6) emitted by an X-ray tube (4) of said X-ray laminography system when installed in an X-ray laminography system and wherein the volume of the collimator opening (11) is a truncated cone or a truncated pyramid, the respective central axis of which is the collimation axis (20).

Description

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

[0002] X-ray laminography scans are typically used to inspect flat, large-area test objects, such as circuit boards or wafers. Since circuit boards typically contain various assemblies with different components, various test positions on the boards are usually approached and several scans are performed sequentially. X-ray laminography systems often have transmission X-ray tubes, as this type of tube allows the test object to be brought as close as possible to the focus, thus achieving high magnifications. In transmission X-ray tubes, the target that generates the radiation usually forms the end of the X-ray tube. This type of X-ray tube produces cone beams with a large aperture angle, for example, 170°, with a large portion of the cone beam typically hitting the circuit board.The combination of multiple inspection positions, very close distances to the focus, and large aperture angles of the cone beam hitting the board results in high radiation exposure for the components being inspected on the board. This can also expose areas of the board to high levels of radiation that are not visible in the X-ray image during the entire inspection process. Depending on the component located in such an area, this can lead to radiation damage to the component, particularly in the areas located in the direction of the central beam of the cone beam.

[0003] DE 10 2014 226 898 A1 discloses an X-ray system comprising an X-ray source and a shielding device arranged between the X-ray source and a target area for shielding the target area from at least a portion of the X-ray radiation. The shielding device has an adjustable collimator opening for transmitting the X-ray radiation to the target area onto a reference impact area. The reference impact area is imaged by a detector. The size of the collimator opening and its position can be adjusted to achieve precise positioning of the object—in this case, for example, a patient—without requiring the patient to assume an uncomfortable posture.

[0004] DE 10 2008 050 571 A1 discloses a tomosynthesis device having an X-ray source which generates an X-ray beam emanating from a focus and received by an area detector. To adjust the tomosynthesis angle, the position of the central axis of the X-ray beam can be changed. A collimator diaphragm is arranged in the beam path between the focus and the area detector, wherein the shape and size of the diaphragm opening can be changed depending on the tomosynthesis angle such that the extent of the X-ray beam at the location of the area detector essentially corresponds to its dimensions. For this purpose, a collimator diaphragm is used which has three blades arranged in a U-shape and fixed to one another. These blades are supplemented by a rotating blade which can be rotated perpendicular to the plane of the diaphragm opening and which delimits the diaphragm opening on the open side of the U.

[0005] The object of the invention is to provide a way to significantly reduce the radiation exposure for components during an X-ray inspection, which occurs during the inspection of other components, in particular during a laminography scan.

[0006] Such solutions are known from the state of the art in the form of filters that reduce the overall dose rate. However, such filters reduce the dose of the entire cone beam, including the portion required to examine the relevant part of the circuit board. A filter can disproportionately filter out the low-energy components of the spectrum. This reduces the dose exposure of the test object, since the low-energy radiation components largely only result in a dose exposure for the test object and add little value to the image quality. However, filtering always also results in parts of the spectrum being filtered out that are beneficial to the image quality. Filtering therefore always requires an increase in the integration time or power to achieve the same image quality as without filtering.

[0007] The object is achieved according to the invention by a collimator having the features of patent claim 1 or 2, a laminography X-ray device having such a collimator having the features of patent claim 10, and a use of such a collimator in an X-ray laminography system according to the features of patent claim 13. Advantageous embodiments are specified in the subclaims.

[0008] According to this, the problem is solved by a collimator that is plate-shaped with a bottom and a top side and is made of a material that absorbs X-rays well. The collimator has a collimator opening, which has an inlet surface on the bottom that has a first centroid, and an outlet surface on the top that has a second centroid. Two possible designs are available to solve the problem. In the first design, the collimator opening, when installed in an X-ray laminography system, is not hit by a central beam of a cone beam emitted by an X-ray tube of this X-ray laminography system, and the volume of the collimator opening is a truncated cone or a truncated pyramid, whose respective central axis is the collimation axis.In the second design, the first centroid and the second centroid form a collimation axis that encloses a non-zero collimation angle with the normal to the collimator's surface. Thus, in both designs, only a small portion of the total X-ray beam emitted onto the collimator from the focus of the X-ray tube is transmitted, and all components of a test object that are not aligned between the focus and the collimator aperture (the test area) are not exposed to X-ray radiation, thus preventing them from being damaged by a high dose of X-ray radiation. The collimator aperture provides an X-ray beam with a severely restricted effective beam aperture angle behind the collimator, which only hits the test area of ​​the test object during the scan.

[0009] An advantageous development of the invention provides that the inlet surface and the outlet surface are each a circle, an ellipse, a trapezoid, a quadrilateral, a pentagon, or a hexagon. In the second embodiment, with a preferred variant of two symmetrical trapezoids as inlet and outlet surfaces, and with the geometrically correct selection of these surfaces, a truncated pyramid-shaped volume of the collimator opening can be provided, the respective central axis of which is the collimation axis - as is the case with the first embodiment according to the invention - which leads to a rectangular beam as the useful beam behind the collimator, which can optimally illuminate a rectangular detector.In the second embodiment, in the variant with two ellipses as inlet and outlet surfaces, if these surfaces are geometrically correctly selected, a truncated cone-shaped volume of the collimator opening can be obtained, the respective central axis of which is the collimation axis—as is the case with the first embodiment according to the invention—leading to a conical beam as the useful beam behind the collimator. This is preferred when a circular detector is present. The other variants (circle, square, pentagon, hexagon) represent easily manufactured collimator openings that also lead to good, albeit less efficient, useful beams.

[0010] A further advantageous development of the invention provides that the collimator opening does not encompass the center of gravity of the underside of the collimator and the second center of gravity of the outlet surface is located further out than the first center of gravity of the inlet surface on the collimator.This makes it possible to create a collimator which has mass in its central region so that the central beam of the X-ray tube is absorbed and does not hit the test object, and the collimation axis extends outwards in the collimator with a directional component in the beam direction, whereby the central beam can hit the collimator centrally when the collimator is installed and the collimation axis of the collimator opening extends along the useful beam, which is at an angle other than zero to the central beam, so that the useful beam falls on the test area arranged in alignment with the focus and the collimator opening, so that it is not unnecessarily partially absorbed at the edges of the collimator opening, which would lead to interference effects.

[0011] A further advantageous development of the invention provides that the inlet surface of the collimator opening and the outlet 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.

[0012] Preferably, the collimator has a thickness of 0.1 - 5.0 mm.

[0013] A further advantageous development of the invention provides for the aperture angle of the collimator to be between 1° and 40°. This results in a partial beam from the entire X-ray beam generated at the focus, which is sufficient to illuminate a test area of ​​a test object. The other, unexamined areas of the test object are not exposed to an unnecessary radiation dose.

[0014] A further advantageous development of the invention provides for the collimation angle to be between 10° and 70°, preferably between 40° and 60°. This allows the inspection area to be examined at an angle during an X-ray laminography process that allows for very good detection of any defects in components and better separation of the different planes in the inspection area.

[0015] A further advantageous development of the invention provides that it is made of a material with a high average atomic number and high density, in particular tungsten or a tungsten alloy, wherein the material preferably has the greatest possible dimensional stability. This provides good shielding of all areas of the test object that are not to be examined, and the collimator can also be designed thinly.

[0016] A further advantageous development of the invention provides for the collimator to have more than one collimator aperture. This allows for the simultaneous examination of multiple test areas of the test object that lie outside the central beam of the X-ray tube.

[0017] Furthermore, the object is also achieved by an X-ray laminography system according to the invention. It comprises an X-ray tube, in particular a transmission X-ray tube, which has a focus at which X-rays are generated in the form of a cone beam. It also has a detector which is struck by a useful beam of the cone beam. A collimator according to the invention is arranged between the focus and the detector. The collimator opening of the collimator is aligned such that only the portion of the X-ray radiation of the cone beam generated at the focus that forms the useful beam is transmitted through it, so that a test object to be tested, which can be placed between the collimator and the detector in the X-ray beam, is only exposed to the X-rays in one test area. This achieves the advantages already mentioned above regarding the collimator and its use.

[0018] An advantageous development of the X-ray laminography system according to the invention provides for the detector to be fully illuminated. As already described above for use, this does not restrict the field of view. With precise illumination – i.e., the entire useful beam illuminates precisely the active surface of the detector – an optimal ratio is achieved between the dose introduced into the test area and minimal information loss in the detector. Preferably, the entire X-ray radiation of the useful beam strikes the detector. Since this is practically impossible to achieve, the design is such that as little of the useful beam as possible does not strike the detector.

[0019] A further advantageous development of the X-ray laminography system according to the invention provides that the normal of the surface of the collimator is parallel to the central ray of the cone beam, thereby enabling a very simple arrangement of the collimator in the X-ray laminography system.

[0020] Finally, the object is also achieved by the use of a collimator according to the invention in an X-ray laminography system, since this results in the above-described significant dose reduction for all areas of the test object that are not the test area. In laminography applications, the detector is usually positioned at the largest possible angle to the central beam emitted by the focus. With the invention, the useful beam is reduced by a collimator to the angular range in which the detector is located. This reduces the dose exposure for all components that are not located in the useful beam. The dose exposure for components that are directly above the focus during a laminography scan can be reduced particularly significantly. Due to their position, these components would be even more intensely irradiated in an application without the collimator according to the invention than the components that are in the image during the scan.Especially when testing sensitive test pieces, the invention reduces damage to components such as semiconductor components, in particular wafers or semiconductor memories.

[0021] An advantageous further development of the use according to the invention provides that the collimator is arranged between a focus of an X-ray tube and a test object to be examined, along with a detector located behind it in the beam direction, such that the portion of a conical beam emitted by the X-ray tube that passes through the collimator opening penetrates the test object in a partial area to be examined and, in particular, fully illuminates the detector. Due to the full illumination of the detector, the field of view is not restricted, and no information is lost.

[0022] Further details and advantages of the invention will now be explained in more detail with reference to exemplary embodiments illustrated in the drawings. They show: Fig. 1 a schematic representation of a known X-ray laminography system, Fig. 2 a schematic representation of an X-ray laminography system according to the invention, Fig. 3 a first embodiment of a collimator according to the invention in plan view and in cross section, Fig. 4 shows a second embodiment of a collimator according to the invention in plan view and in cross section, Fig. 5 a third embodiment of a collimator according to the invention in plan view and in cross section and Fig. 6 a fourth embodiment of a collimator according to the invention in plan view and in cross section.

[0023] In Fig. Figure 1 schematically illustrates the structure of an X-ray laminography system known from the prior art. It has an X-ray tube 4 – here: a transmission X-ray tube – which generates a conical beam 6 with a half aperture angle α of approximately 85° at its focus 5. A test object 2 in the form of a circuit board is located within the conical beam 6. The central beam 7 of the conical beam 6 is essentially perpendicular to the surface of the test object 2. Not the entire test object 2 is to be examined, but only a partial area – which is referred to below as the test area 12. The test area 12 is not aligned with the central beam 7, but is tilted relative to it by an observation angle γ. In the example shown, the observation angle γ is approximately 60°. In X-ray laminography, the separation of layers within the test object 2 is improved the larger the observation angle γ.The test area 12 lies between the focus 5 and a detector 3. The rays of the cone beam 6, which emanate from the focus 5 and hit the detector 3 at the outermost point, form a field of view 8. This field of view is formed around the observation axis 13, which is at an observation angle γ to the central beam 7 and hits the detector 3 centrally. The field of view 8 has a field of view opening angle δ of approximately 20°. The test area 12 must lie within the field of view 8 during its examination—i.e., during the laminography scan. In the example shown, large parts of the test object 2 are located within the cone beam 6 and are exposed to a dose, but are not within the field of view 8. Therefore, the dose exposure is of no benefit in large parts of the test object 2. Such a high dose exposure can lead to damage to X-ray-sensitive components on the test object 2.The implementation of the laminography scan is known from the prior art and is not the subject of the invention, so it will not be discussed further.

[0024] The invention prevents such damage due to high dose exposure. Fig. 2 schematically shows an example of a structure of an X-ray laminography system according to the invention. The structure of the X-ray laminography system according to the invention according to Fig. 2 is identical except for a collimator 1 according to the invention arranged between the focus 5 and the test object 2, so that identical features are provided with the same reference numerals and will not be discussed again below. Rather, only the differences will be discussed in more detail.

[0025] The collimator 1 is plate-shaped, thus having a very small thickness compared to its extension in a plane. It has a bottom side 14 facing the X-ray tube 4 and a top side 17 facing away from the X-ray tube 4. In order to shield the X-ray radiation of the cone beam 6 as effectively as possible in all areas except the test area 12, it is made of a material with a high atomic number – if the collimator 1 is made of an alloy or various materials, it should have a high average atomic number. A high (average) atomic number is considered to be a value above 26. In addition, the collimator 1 is made of a high-density material, preferably, for example, above 7,500 kg / m 3 To ensure simple manufacturing and low susceptibility to mechanical deformation, a material with the most dimensionally stable possible is used. In this case, this is a tungsten alloy, namely Densimet®.

[0026] The collimator 1 has a collimator opening 11, as shown in the Fig. 2 also good in the two examples of the Fig. 3 and Fig. 4 can be seen.

[0027] The collimator opening 11 is in the first embodiment according to Fig. 3 is frustoconical in shape around a central axis. For this purpose, an inlet surface 15 is provided on the underside 14 of the collimator 1, which is elliptical and has a first centroid 16. On the top side 17 of the collimator 1, an outlet surface 18 is provided, which is also elliptical and has a second centroid 19. The geometric features of the inlet surface 15 and the outlet surface 18 are coordinated such that the volume of the collimator opening 11 forms a truncated cone (the specific design is easy to determine for a person skilled in the art). The collimation axis 20 defined by the first centroid 16 and the second centroid 19 corresponds to the central beam 10. This produces a conical useful beam 9 behind the collimator 1 that is symmetrical about a central beam 10 and illuminates the detector 3. If the detector 3 is circular, optimal illumination can be achieved.

[0028] The central beam 10 corresponds to the Fig. 1 described observation axis 13 and the aperture angle - shown in Fig. 3 half the useful beam opening angle β - is approximately equal to the field of view opening angle δ from Fig. 1. The collimation angle γ depends on the position of detector 3 – it is therefore larger the further the observation axis 13 is from the central beam 7. The size of the field of view opening angle δ depends – if an ideal configuration is desired – on the size of detector 3 and the distances of collimator 1 from focus 5 and collimator 1 from detector 3. This means that scans at different laminography angles require different collimators 1, which must be exchanged before performing the laminography scan. For optimal results, different collimators 1 would also have to be used at different distances between collimator 1 and focus 5 and between collimator 1 and detector 3.The collimator opening 11 is formed outside the center of the collimator 1; this allows the collimator 1 to be positioned centrally above the central beam 7 and shields the X-rays emitted in all directions of the cone beam 6, not within the range of the useful beam 9, thus protecting the unexamined areas of the test object 2. This minimizes the dose exposure of the part of the test object 2 not located in the field of view 8 and thus in the useful beam 9. By using a collimator 1 according to the invention, radiation-sensitive components of the test object 2 are significantly less exposed when they are not in the field of view 8. This also allows multiple test positions on the test object 2 to be approached without irradiating the entire test object 2 and exposing all components to radiation. This is particularly advantageous for a circuit board with X-ray-sensitive components.

[0029] The second embodiment of the Fig. 4 differs from the first embodiment of the Fig. 3 only by the shape of the inlet surface 15 and the outlet surface 18. Instead of being elliptical, they each have the shape of a symmetrical trapezoid. The representation of the centroids 16, 19 of the inlet surface 15 and the outlet surface 18 as well as the collimation axis 20 defined thereby has been omitted, since this would be too obvious for the person skilled in the art, based on Fig. 3 is no problem. Thus, the volume of the collimator opening 11 instead of a truncated cone shape according to Fig. 3 now has a truncated pyramid shape, resulting in a rectangular beam as the useful beam 9. This is particularly advantageous when using a rectangular detector 3—which is the usual case—because it can be optimally and completely illuminated by the useful beam, thus achieving the greatest possible efficiency.

[0030] In Fig. Figure 5 shows a third embodiment, which has a rectangular inlet surface 15, which, however—unlike the two previous embodiments—extends to the edge of the collimator 1, thus causing one of its sides to coincide with the edge of the collimator 1. Although this increases the dose load of the component in the outer region, the collimator opening 11 is easier to manufacture. The alignment of the outlet surface 18 with the inlet surface 15 is easily determined by a person skilled in the art due to the geometric conditions.

[0031] In Fig. 6 shows a fourth embodiment which differs from the third embodiment of the Fig. 5 only in that, in addition to the collimator opening on the right-hand side (represented by the inlet surface 15), there is another collimator opening on the left-hand side (represented by the inlet surface 15'). This allows two different test areas 12 in the test object 2 to be examined simultaneously. The left inlet surface 15' has different dimensions than the right inlet surface 15, as can be clearly seen from the cross-sectional view on the right, so that different collimation angles (right γ and left γ') are also present - whereby in the exemplary embodiment the right collimation angle γ is smaller than the left collimation angle γ'. This is only an example, and the actual design to be selected depends on the requirements of the respective application, in particular where the test areas 12 are located in the test object 2.

[0032] Instead of providing two collimator openings 11 as in Fig. 6, there could also be more collimator openings 11 or the collimator openings 11 could be formed at other locations in the collimator 1. However, it should be noted that with each of these collimator openings 11 according to the Fig. 5 and Fig. 6 the dose exposure in outdoor areas is increased. List of reference symbols 1 collimator 2 Test object 3 Detector 4 X-ray tubes 5 Focus 6 cone beam 7 Central ray 8 Field of view 9 Useful beam 10 Central beam 11 Collimator aperture 12 Test area 13 Observation axis 14 Bottom 15, 15' inlet area 16 first centroid 17 Top 18 outlet area 19 second centroid 20 Collimation axis α half opening angle β half useful beam angle γ, γ' collimation angle δ field of view opening angle

Claims

[1] Collimator (1) for an X-ray inspection system, which is plate-shaped with a bottom (14) and a top (17), which is made of a material that absorbs X-rays well with a collimator aperture (11), wherein the collimator opening (11) has an inlet surface (15) on the underside (14) which has a first centroid (16) and an outlet surface (18) on the top side (17) which has a second centroid (19), wherein the collimator opening (11) in the state installed in an X-ray laminography system is not struck by a central beam (7) of a cone beam (6) emitted by an X-ray tube (4) of this X-ray laminography system and wherein the volume of the collimator opening (11) is a truncated cone or a truncated pyramid, the respective central axis of which is the collimation axis (20). [2] Collimator (1) for an X-ray inspection system, which is plate-shaped with a bottom (14) and a top (17), which is made of a material that absorbs X-rays well with a collimator aperture (11), wherein the collimator opening (11) has an inlet surface (15) on the underside (14) which has a first centroid (16) and an outlet surface (18) on the top side (17) which has a second centroid (19), wherein the first centroid (16) and the second centroid (19) form a collimation axis (20) which encloses a collimation angle (y) non-zero with the normal of the surface of the collimator (1). [3] Collimator (1) according to claim 2, wherein the volume of the collimator opening (11) is a truncated cone or a truncated pyramid, the respective central axis of which is the collimation axis (20). [4] Collimator (1) according to any of the preceding claims, wherein its inlet surface (15) and its outlet surface (18) are each a circle, an ellipse, a quadrilateral, a pentagon or a hexagon. [5] Collimator (1) according to one of the preceding claims, wherein the collimator opening (11) does not include the centroid of the underside (14) of the collimator (1) and the second centroid (19) of the outlet surface (18) is located further outwards than the first centroid (16) of the inlet surface (15) on the collimator (1). [6] Collimator (1) according to one of the preceding claims, wherein the inlet surface (15) of the collimator opening (11) and the outlet surface (18) of the collimator opening (11) each extend to the edge of the collimator (1). [7] Collimator (1) according to one of the preceding claims, wherein it has a thickness of 0.1-5.0 mm and / or the opening angle of the collimator opening (11) is between 1° and 40° and / or the collimation angle (y) is between 10° and 70°, preferably between 40° and 60°. [8] Collimator (1) according to one of the preceding claims, wherein it consists of a material with a high average atomic number and high density, in particular tungsten or a tungsten alloy, wherein the material preferably has the greatest possible dimensional stability. [9] Collimator (1) according to one of the preceding claims, wherein it has more than one collimator opening (11) according to one of the preceding claims. [10] X-ray laminography system with an X-ray tube (4), in particular a transmission X-ray tube, which has a focus (5) at which X-ray radiation in the form of a cone beam (6) is generated, with a detector (3) which is struck by a useful beam (9) of the cone beam (6), and with a collimator (1) arranged between the focus (5) and the detector (3), which is designed according to one of claims 1 to 9, wherein the collimator opening (11) of the collimator (1) is oriented such that only the part of the X-ray radiation of the cone beam (6) generated at the focus (5) which forms the useful beam (9) is transmitted through it, so that a test object (2) which can be placed in the X-ray beam between the collimator (1) and the detector (3) is exposed to the X-ray radiation only in a test area (12). [11] X-ray laminography system according to claim 10, wherein the detector (3) is fully illuminated, but as little as possible of the X-ray radiation of the useful beam (9) does not hit the detector (3). [12] X-ray laminography system according to claim 10 or 11, wherein the normal of the surface of the collimator (1) is parallel to the central beam (7) of the cone beam (6). [13] Use of a collimator (1) according to any one of claims 1 to 9 in an X-ray laminography system for reducing the dose exposure for a test object (2), in particular a semiconductor component, such as a wafer or semiconductor memory. [14] Use according to claim 13, wherein the collimator (1) is arranged between a focus (5) of an X-ray tube (4) and a test object (2) together with a detector (3) located behind it in the direction of the beam, such that the part of a cone beam (6) emitted by the X-ray tube (4) which passes through the collimator opening (11) penetrates the test object (2) in a test area (12) to be examined and in particular fully illuminates the detector (3).

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