JP4091217B2 - X-ray tube - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray tube that generates X-rays.
[0002]
[Prior art]
In an X-ray tube, an electron gun that emits electrons by heating a cathode in a high vacuum tube is used, and electrons emitted from the electron gun are incident on an X-ray generation surface of an anode target to which a high voltage is applied. To generate X-rays.
[0003]
An example of such an X-ray tube is disclosed in Japanese Patent Laid-Open No. 7-296751. In this X-ray tube, the central axis of the first cylindrical member in which the electron gun and the electron gun are accommodated is substantially orthogonal to the central axis of the second cylindrical member in which the target and the target are accommodated. Each part is configured. As a result, the electron incident axis from the electron gun to the target and the X-ray emission axis from the target to the outside are substantially orthogonal, and the hood electrode is attached to the target and the tip of the target for electron acceleration and trajectory control. An X-ray tube having a central axis substantially parallel to the X-ray emission axis is configured.
[0004]
[Problems to be solved by the invention]
X-rays are electromagnetic waves that are highly permeable to many substances and objects, and are often used for non-destructive and non-contact inspection of the internal structure of objects. In recent years, semiconductor electronic components, which are one of such inspection objects, have been reduced in size and increased in density, and accordingly, a microfocus X-ray tube (MFX) used for inspection is also capable of X-ray fluoroscopic images. An X-ray tube capable of increasing the resolution is desired.
[0005]
In order to make a fluoroscopic image clear in such applications, it is necessary to satisfy three conditions: (1) high magnification, (2) small focus, and (3) high contrast. That is, (1) a finer structure can be observed with high magnification, (2) an image with a clear outline can be obtained with a small focal point, and (3) an image with clear contrast can be obtained with high contrast. be able to.
[0006]
In the X-ray tube as described above, an electron incident point on the X-ray generation surface is an X-ray generation point. However, an electron incident from an electron gun is incident at an incident point / focusing point on the X-ray generation surface. Since the electron beam diameter is not sufficiently reduced, there has been a problem that (2) the small focal point and (3) the high contrast cannot be obtained sufficiently among the above-mentioned conditions necessary for high resolution. That is, the spread of the X-ray generation position increases corresponding to the size of the beam diameter at the focal point of electrons, so that the focal point is enlarged and the outline of the image becomes unclear. There is a problem that the noise component of the X-ray from the incident point of the spread electrons increases and the contrast decreases.
[0007]
The present invention has been made in view of the above problems. An X-ray tube in which the focused beam diameter of electrons incident on the X-ray generation surface is reduced and the spread of the obtained X-ray beam is reduced. The purpose is to provide.
[0008]
[Means for Solving the Problems]
In order to achieve such an object, the X-ray tube according to the present invention generates an X-ray by causing an electron gun to emit electrons from the cathode in the direction of the electron incident axis and causing the electrons to enter the X-ray generation surface. A target that emits in the direction of the X-ray emission axis, and an X-ray tube configured so that the center axis of the target is substantially parallel to the X-ray emission axis; Between the X-ray generation surface of the target, (1) a first grid electrode that limits a current due to electrons from the cathode, and (2) a second grid electrode that accelerates electrons that have passed through the first grid electrode, (3) an auxiliary focusing electrode system that controls the spread of the trajectory of electrons that have passed through the second grid electrode; and (4) a focus grid electrode that focuses the electrons to a predetermined electron incident point on the X-ray generation surface. We prepare sequentially and ( ) Electrons emitted from the cathode the distance from the virtual object point that once focused in the vicinity of the cathode to focus grid electrode, and greater than the distance from the focus grid electrode to the electron incident points.
[0009]
The focused beam diameter of electrons on the X-ray generation surface greatly depends on the enlargement ratio by the electron lens system from the cathode to the target, and the enlargement ratio depends on the distance a from the virtual object point formed near the cathode to the focus grid electrode. , Which is determined by the ratio to the distance b from the focus grid electrode to the electron incident point. The conventional X-ray tube is formed such that this distance is a <b. In this case, the enlargement ratio is increased.
[0010]
If the enlargement ratio is large, the focused beam diameter at the electron incident point where the virtual object point is imaged again on the X-ray generation surface cannot be made sufficiently small. As a result, the beam diameter (orbit variation) and noise component (straight spread) of the obtained X-ray are increased, and the conditions of small focus and high contrast in the X-ray fluoroscopic image cannot be achieved.
[0011]
On the other hand, the above-described X-ray tube is configured such that the distances a and b satisfy the condition a> b. As a result, the magnification of the electron lens system can be reduced and the beam diameter can be reduced.
[0012]
However, with such a configuration, the distance from the cathode to the focus grid electrode increases, and the electron trajectory spreads between them increases. At this time, there arises a problem that the spread of the electron trajectory tends to be affected by the aberration of the electron lens made of the focus grid electrode as the main lens. Therefore, by setting a> b, the beam diameter becomes smaller and the focal point becomes smaller, but the noise component cannot be reduced and the X-ray image cannot be made high in contrast.
[0013]
On the other hand, in the X-ray tube described above, an auxiliary focusing electrode having at least one auxiliary focusing electrode is further used to control the electron trajectory so as to suppress the spread of the electron trajectory from the cathode to the focus grid electrode. A system is in place. As a result, it is possible to suppress an increase in noise components due to the influence of the aberrations described above. Therefore, a small focus by reducing the beam diameter of the X-ray and a high contrast by reducing the noise components. Can be achieved together.
[0014]
The auxiliary focusing electrode system has at least a first auxiliary focusing electrode, and includes a potential V0 set for the cathode, a potential V1 set for the first grid electrode, a potential V2 set for the second grid electrode, The potential V3 set on the auxiliary focusing electrode is
V1 ≦ V0 ≦ V3 <V2
It is characterized by satisfying the relationship.
[0015]
By setting each potential in this manner, the current limiting function of the first grid electrode, the electron acceleration function of the second grid electrode, and the function of suppressing the orbital spread by the auxiliary focusing electrode system can be efficiently realized.
[0016]
Further, the auxiliary focusing electrode system further includes a second auxiliary focusing electrode provided between the first auxiliary focusing electrode and the focus grid electrode, and the potential V4 set for the second auxiliary focusing electrode is:
V2 = V4
It may be characterized by satisfying this relationship.
[0017]
As a result, an electron lens for control in the auxiliary focusing electrode system can be suitably formed. In addition to the above, the number of electrodes and the set voltage that constitute the auxiliary focusing electrode system can be variously set.
[0018]
Further, the focus grid electrode may be formed so as to protrude from the envelope of the electron gun toward the target.
[0019]
As a result, the distance from the focus grid electrode to the virtual object point near the cathode can be increased, and the condition a> b for the distance can be easily realized.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of an X-ray tube according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.
[0021]
FIG. 1 is a cross-sectional view showing a configuration of a first embodiment of an X-ray tube according to the present invention. FIG. 1 (a) is a cross-sectional view of the X-ray tube 1, and FIG. Show. Each of these shows a cross section on a plane including an electron incident axis and an electron incident trajectory le described later. As shown in FIGS. 1A and 1B, an X-ray tube 1 includes an electron gun unit 2 that generates and emits electrons, and X-ray generation that generates X-rays upon receiving electrons from the electron gun unit 2 Part 3.
[0022]
The electron gun unit 2 includes an outer container 21 that houses each component and whose central axis is set substantially parallel to the electron incident axis, and a heater 22 that generates heat by external power supply is provided in the container 21. Is provided. The electron gun unit 2 includes a cathode 23 that is heated by a heater 22 and emits electrons, a first grid electrode 26 and a second grid electrode 27 for extracting electrons emitted from the cathode 23 under predetermined conditions, an electron trajectory. An auxiliary focusing electrode system 6 comprising a first auxiliary focusing electrode 61 and a second auxiliary focusing electrode 62 for controlling and setting the angle, and a focus grid electrode 24 for focusing electrons are provided.
[0023]
Further, the container 21 is formed with an electron passage 25 for emitting electrons emitted from the cathode 23 and focused by the focus grid electrode 24. In the present embodiment, the edge portion of the electron passage opening 25 functions as the focus grid electrode 24. Electrons are emitted from the electron gun section 2 by the electron incident trajectory le in the electron incident axis direction and are incident on the X-ray generation section 3.
[0024]
On the other hand, the X-ray generation unit 3 includes an outer container 31 that houses each component and has a central axis that is installed substantially parallel to the X-ray emission axis. The container 31 is in communication with the container 21 of the electron gun unit 2 via the electron passage port 25, and thus has a structure in which electrons emitted from the cathode 23 can enter. These containers 21 and 31 are sealed, and the inside is kept in a substantially vacuum state.
[0025]
Inside the container 31, a target 4 that receives electrons from the electron gun unit 2 and generates X-rays is installed. The target 4 is a metal rod-like body, and the central axis is the X-ray emission axis direction, and the target 4 is arranged in a direction substantially orthogonal to the electron incident axis from the electron gun unit 2. The tip surface formed on the tip portion 40 of the target 4 is an X-ray generation surface 41 that receives electrons from the electron gun unit 2 and generates X-rays. The X-ray generation surface 41 is obliquely (orthogonal) at a predetermined angle with respect to the central axis of the target 4 and thus with respect to the electron incident axis and the X-ray emission axis at a position in front of the electron incident trajectory le where electrons are incident. And is not parallel and formed in a planar shape. Electrons from the electron gun unit 2 enter the X-ray generation unit 3, and X-rays are generated and emitted from the electron incident point Pi of the X-ray generation surface 41 by the X-ray emission trajectory lx in the X-ray emission axis direction. .
[0026]
Here, in the present embodiment, the distance a from a virtual object point Pc, which will be described later, to the passing point Pf of the electron passing port 25 that is the opening of the focus grid electrode 24 is the distance from the passing point Pf to the electron incident point Pi. It is set to be larger than b (a> b).
[0027]
Note that the electron incident trajectory le and the X-ray emission trajectory lx, which are substantially orthogonal to each other, are actually trajectories in a range having a predetermined spread, but in FIG. It is shown in line with the axis.
[0028]
The container 31 is provided with an X-ray emission window 32 at an opening formed at a predetermined position. The X-ray emission window 32 is a window for emitting X-rays emitted from the X-ray generation surface 41 of the target 4 to the outside of the container 31, for example, a plate material made of a Be material that is an X-ray transmission material, or the like. Consists of. The X-ray irradiation window 32 is provided at a position in front of the X-ray emission trajectory lx facing the X-ray generation surface 41 that is the tip surface of the target 4. The X-ray exit window 32 is disposed so that the center thereof substantially coincides with the extension of the center axis of the target 4 and is formed so that the X-ray exit axis passes through the inside thereof.
[0029]
A hood electrode 5 is attached to the tip portion 40 of the target 4. The hood electrode 5 has a function of accelerating electrons from the cathode 23 entering the X-ray generation surface 41 and controlling the trajectory thereof, and is made of a metal substantially parallel to the central axis / X-ray emission axis of the target 4. It is formed as a cylindrical body. The cylindrical shape of the hood electrode 5 has an inner diameter that is substantially the same as the outer diameter of the tip portion 40 of the target 4, and an axial length that is substantially the same as the length of the tip portion 40. Has been. A positive high voltage is applied to the target 4 and the hood electrode 5 with respect to the potential of the focus grid electrode 24 that is the edge portion of the electron passage port 25 of the electron gun unit 2.
[0030]
The hood electrode 5 is installed so as to cover the periphery of the X-ray generation surface 41, and the opening portion facing the X-ray emission window 32 on one end of the cylindrical shape is the X-ray generation surface 41 of the target 4. Are arranged so as to be an X-ray emission port 51 through which the X-ray from the X-ray emission path 1x passes. An electron incident port 52 through which electrons from the cathode 23 of the electron gun unit 2 pass along the electron incident trajectory le is formed at a predetermined portion facing the electron passing port 25 on the peripheral surface. In the present embodiment, the electron entrance 52 is formed in a shape that is an opening range that is substantially symmetrical with respect to the electron entrance axis in the axial direction of the target 4.
[0031]
FIG. 2 is an enlarged cross-sectional view of the X-ray tube 1 shown in FIG. In the drawing, the electron trajectory le controlled and formed by these electrodes is also shown by way of example of the central trajectory and the trajectories on both ends.
[0032]
In the electrode configuration shown in FIG. 2, electrons emitted from the cathode 23 set at the potential V0 are within the opening 26a of the first grid electrode 26 set at the potential V1 and the second set at the potential V2. The grid electrode 27 sequentially passes through the opening 27a.
[0033]
The first grid electrode potential V1 is preferably set to satisfy V1 ≦ V0 with respect to the cathode potential V0. Accordingly, the first grid electrode 26 functions as a current control electrode for electrons from the cathode 23. A virtual object point (cross) where electrons are once focused at a predetermined position in the vicinity of the cathode 23, such as in the vicinity of the opening 26a of the first grid electrode 26 or the area from the opening 26a of the second grid electrode 27 to the opening 27a. Overpoint) Pc is formed.
[0034]
The second grid electrode potential V2 is preferably set to satisfy V0 <V2 with respect to the cathode potential V0. Thus, the second grid electrode 27 functions as an electron acceleration electrode for electrons. The electrons that have passed through the opening 26 a of the first grid electrode 26 are once focused at the virtual object point Pc, and then accelerated by the second grid electrode 27 in the direction of the X-ray generator 3 and extracted.
[0035]
The accelerated electrons sequentially pass through the opening 61a of the first auxiliary focusing electrode 61 set to the potential V3 and the opening 62a of the second auxiliary focusing electrode 62 set to the potential V4. The first auxiliary focusing electrode 61 and the second auxiliary focusing electrode 62 constitute an auxiliary focusing electrode system 6 that controls and adjusts the orbital spread of electrons in the electron gun unit 2 of the present embodiment.
[0036]
The first auxiliary focusing electrode potential V3 is preferably set to satisfy V0 ≦ V3 <V2, and the second auxiliary focusing electrode potential V4 is preferably set to satisfy V2 = V4. As a result, an electric field serving as an electron lens is formed around the first auxiliary focusing electrode 61, and electron trajectory adjustment (prefocus) is performed so that the spread of the electron trajectory after passing through the auxiliary focusing electrode system 6 is reduced. Is called.
[0037]
Here, the above potential settings are summarized as follows:
V1 ≦ V0 ≦ V3 <V2 = V4
It becomes. However, the potential setting of each electrode is not necessarily limited to the setting that satisfies this condition, and may be set to a potential that does not satisfy this condition as long as the electron trajectory can be suitably controlled.
[0038]
The overall operation of the X-ray tube 1 including the electron gun section 2 formed and set as described above is as follows.
[0039]
When a positive high voltage is applied to the target 4 and the hood electrode 5, the target 4 and the hood electrode 5 have a positive high potential with respect to the focus grid electrode 24 that is the edge of the electron passage port 25 of the electron gun unit 2. Therefore, a predetermined electric field is formed in the space between the electron gun unit 2 and the target 4 and the hood electrode 5.
[0040]
As a result, the electrons emitted from the cathode 23 of the electron gun unit 2 are extracted by the first and second grid electrodes 26 and 27 described above, and the auxiliary focusing electrode system including the first and second auxiliary focusing electrodes 61 and 62. 6, the trajectory is adjusted so that the trajectory spread is reduced, and then focused by the focus grid electrode 24 and passes through the electron passage opening 25. Furthermore, the electrons formed on the X-ray generation surface 41 of the target 4 are accelerated by the electric field between the electron gun unit 2 and the target 4 and the hood electrode 5 and pass through the electron entrance 52 of the hood electrode 5. The light enters and enters a predetermined electron incident point Pi that is a converging point.
[0041]
Then, when electrons enter the X-ray generation surface 41, X-rays are generated at the electron incident point Pi. The generated X-rays pass through the X-ray exit 51 of the hood electrode 5 and the X-ray exit window 32 made of Be and exit in the direction of the X-ray exit trajectory lx. Here, the electron incident point (X-ray generation point) Pi on the X-ray generation surface 41 is configured to be substantially on the central axis of the target 4. Therefore, the X-ray emission axis substantially coincides with the center axis of the target 4. For X-rays generated on the X-ray generation surface 41, components emitted in a predetermined orbit range including this X-ray emission axis and limited or determined by the X-ray emission window 32 or the like are externally used as signal components. Is emitted.
[0042]
Next, effects of the X-ray tube 1 according to the present embodiment will be described.
[0043]
In the electron gun section applied to the X-ray tube, the trajectories of electrons from the cathode are once crossed and focused in the vicinity of the cathode to form a virtual object point (crossover point). Then, the electron beam whose trajectory has spread again from this virtual object point is focused on the X-ray generation surface of the target with a micro focus by the electron lens formed by the opening of the focus grid electrode.
[0044]
At this time, the diameter of the electron beam at the focal point / electron incident point on the X-ray generation surface greatly depends on the magnification of the electron lens system of the electron gun including the focus grid electrode and the like. In other words, the electron beam diameter can be reduced by reducing the enlargement factor for imaging the virtual object point near the cathode on the X-ray generation surface of the target, and conversely, the electron beam diameter can be reduced by increasing the enlargement factor. Will grow.
[0045]
This magnification is geometrically determined by the ratio b / a of the distance a from the object point to the main lens and the distance b from the main lens to the focusing point. In the X-ray tube 1 described above, the distance a is the distance from the virtual object point Pc near the cathode 23 to the passing point Pf of the opening of the focus grid electrode 24 (in the present embodiment, the electron passing port 25), The distance b corresponds to the distance from the passing point Pf to the electron incident point Pi (see FIG. 1).
[0046]
In a conventional X-ray tube, the distance a from the virtual object point to the focus grid electrode is the distance from the focus grid electrode to the electron incident point, for example, as in the X-ray tube disclosed in JP-A-7-296751. It is set smaller than b (a <b). For this reason, the magnification for imaging the virtual object point on the X-ray generation surface of the target is not sufficiently small, and it is difficult to reduce the electron beam diameter at the electron incident point.
[0047]
FIG. 3A and FIG. 3B show the electron trajectory in the conventional X-ray tube and the obtained X-ray output distribution. FIG. 3A is an end view showing a partially enlarged configuration of a conventional X-ray tube in which a <b. In the drawing, a solid line with an arrow indicates a target from a cathode (not shown). 4, the electron trajectory le to the X-ray generation surface 41, and the broken line indicates the equipotential surface of the electric field formed by each electrode or the like. Also, the potential set for each electrode etc.
Cathode 23: V0 = −600V
First grid electrode 26: V1 = −650V
Second grid electrode 27: V2 = 0V
Focus grid electrode 24: Vf = 0V
Target 4: Vt = 60 kV
It is.
[0048]
The electrons emitted from the cathode form a virtual object point Pc by a first grid electrode (not shown), then pass through the second grid electrode 27 and are focused by the focus grid electrode 24. Further, it is accelerated by an electric field applied to the target 4 and the hood electrode 5 and is incident on an electron incident point Pi that is a focusing point on the X-ray generation surface 41. At this time, since the distance a from the virtual object point Pc to the passing point Pf is smaller than the distance b from the passing point Pf to the electron incident point Pi, the enlargement ratio of the electron lens system is increased. The diameter of the electron beam focused on becomes larger.
[0049]
When the spread of the electron beam diameter at the focusing point on the target is increased, the range of the X-ray generation point is widened, resulting in variations in the X-ray generation position and the trajectory after the generation. In addition, of the electron beam spread at the focal point on the target, the skirt spread causes an increase in the noise component of the X-ray. That is, the X-ray two-dimensional output distribution (right graph) in FIG. 3B and the two-dimensional output distribution are also shown in FIG. As shown by a one-dimensional output distribution (graph on the left) projected in the x-axis direction, the output distribution range becomes large. Here, the range of the solid line in the two-dimensional output distribution mainly indicates the beam diameter (orbit variation) of the obtained X-ray beam, and the range of the broken line mainly indicates the noise component (side spread) of the X-ray beam. All of these X-ray tubes are large.
[0050]
When such X-rays from the X-ray tube are applied to the acquisition of an X-ray fluoroscopic image by nondestructive inspection, since the microfocus is not sufficient, the contour of the image is unclear due to an increase in the X-ray beam diameter. At the same time, the contrast decreases due to an increase in the X-ray noise component. Therefore, among (1) high magnification, (2) small focus, and (3) high contrast necessary for high resolution of the X-ray image, (2) small focus and (3) high contrast are sufficient. There is a problem that it cannot be obtained.
[0051]
On the other hand, in the X-ray tube according to the present embodiment, as shown in FIG. 1, each part of the electron gun unit 2 and the X-ray generation unit 3 is set so that the distances a and b satisfy the condition a> b. It is composed. Specifically, by increasing a or decreasing b, the enlargement ratio can be reduced and the electron beam diameter at the electron incident point Pi on the X-ray generation surface 41 can be reduced. If b is reduced, the distance between the focus grid electrode 24, the target 4 and the hood electrode 5 is reduced, and a high voltage between them causes discharge. For this reason, in the present embodiment, the distance a from the virtual object point Pc to the passing point Pf of the focus grid electrode 24 is increased by increasing the distance from the cathode 23 to the electron passing port 25, and the above-described conditions a> b is realized.
[0052]
4A and 4B show the electron trajectory in the X-ray tube configured as described above and the X-ray output distribution obtained. The configuration of this X-ray tube is almost the same as that shown in FIG. 3, except that the cathode, the first grid electrode, and the second grid electrode 27 are installed at positions away from the focus grid electrode 24. The above-described condition a> b is satisfied. Further, the potentials set for the respective electrodes and the like are all the same as the example shown in FIG.
[0053]
At this time, since the distance a from the virtual object point Pc to the passing point Pf is larger than the distance b from the passing point Pf to the electron incident point Pi, the enlargement ratio of the electron lens system is reduced, and therefore, on the electron incident point Pi. The electron beam diameter at is small. However, in this case, since the distance from the second grid electrode 27 to the focus grid electrode 24 is large as shown in FIG. 4A, the divergence of the electron beam at the virtual object point (crossover point) is achieved. Depending on the angle, the electron beam trajectory greatly spreads to the focus grid electrode 24 as the main lens.
[0054]
As a result, the electron trajectory is easily affected by the aberration of the electron lens formed by the focus grid electrode 24, and the spread of the incident electron distribution is increased. In addition, the X-ray obtained is an X-ray beam having a small beam diameter (solid line) but a large noise component (dashed line) as shown in FIG. 4B. . In this case, in the X-ray fluoroscopic image, the outline of the image becomes clear by reducing the X-ray beam diameter, but the contrast is lowered due to a large amount of X-ray noise components, and the high resolution of the X-ray image is obtained. Cannot be fully achieved.
[0055]
On the other hand, in the X-ray tube according to the present embodiment, as shown in the X-ray tube 1 of FIGS. 1 and 2, the cathode 23, the first and second grid electrodes 26 and 27, and the focus grid electrode 24. The auxiliary focusing electrode system 6 including the first and second auxiliary focusing electrodes 61 and 62 is installed at a predetermined position between the first and second electrodes, thereby controlling the electron trajectory and suppressing the spread of the electron trajectory. .
[0056]
FIG. 5A and FIG. 5B show the electron trajectory and the obtained X-ray output distribution in the X-ray tube in which the condition a> b is satisfied and the auxiliary focusing electrode system 6 is installed. The configuration of this X-ray tube is substantially the same as that shown in FIG. 4, but an auxiliary focusing electrode system comprising first and second auxiliary focusing electrodes 61 and 62 between the second grid electrode 27 and the focus grid electrode 24. 6 is installed. Also, the potential set for each electrode etc.
Cathode 23: V0 = −600V
First grid electrode 26: V1 = −650V
Second grid electrode 27: V2 = 0V
First auxiliary focusing electrode 61: V3 = −300V
Second auxiliary focusing electrode 62: V4 = 0V
Focus grid electrode 24: Vf = 0V
Target 4: Vt = 60 kV
It is. These potentials satisfy the condition V1 ≦ V0 ≦ V3 <V2 = V4.
[0057]
The first auxiliary focusing electrode 61, which is located between the second grid electrode 27 and the second auxiliary focusing electrode 62 set to V2 = V4 and set to V3 <V2 = V4, shows the first auxiliary with a broken line in the drawing. An electron lens is formed so as to show an electric field formed so as to surround the focusing electrode 61, thereby reducing the direction in which the electron trajectory converges, that is, the spread of the electron trajectory between the second grid electrode 27 and the focus grid electrode 24. In this direction, the electron trajectory is adjusted and controlled.
[0058]
At this time, since the distance a from the virtual object point Pc to the passing point Pf is larger than the distance b from the passing point Pf to the electron incident point Pi, the enlargement ratio of the electron lens system is reduced, and therefore, on the electron incident point Pi. The electron beam diameter at is small. Furthermore, since the spread of the electron trajectory is reduced as described above, it becomes difficult to be affected by the aberration of the electron lens formed by the focus grid electrode 24 functioning as the main lens, so that the spread of the electron distribution is also reduced. . Therefore, also in the obtained X-ray, the X-ray output in which both the beam diameter (solid line) and the noise component (dashed line) are reduced as shown in FIG. Distribution can be realized. In this case, in the X-ray fluoroscopic image, the outline of the image becomes clear by reducing the X-ray beam diameter, and the contrast is improved by reducing the noise component of the X-ray, thereby greatly increasing the X-ray image. High resolution is achieved.
[0059]
FIG. 6 shows an example of wire bonding observation / inspection using a microfocus X-ray tube, and the above-described X-ray fluoroscopic image is sharpened by reducing the beam spread (beam diameter and noise component) of the X-ray beam. It is a schematic diagram explaining an effect. 6 (a) and 6 (b) show (a) normal wire bonding and (b) broken wires, respectively, and these wire bonding portions are made using the X-ray tube of a <b shown in FIG. 6 (c) and (d) show images obtained when observed in FIG. 6, and FIG. 6 (e) and (f) show images obtained using the a> b X-ray tube shown in FIG. FIGS. 6 (g) and 6 (h) show images obtained by using an X-ray tube having a> b and an auxiliary focusing electrode system according to the present embodiment shown in FIGS. 1, 2, and 5, respectively.
[0060]
In the images shown in FIGS. 6C and 6D by the X-ray tube of a <b, the outline is unclear because the focal point and the beam diameter are large, and there are many noise components of the X-rays. Since the ratio is poor, the contrast between light and dark is not sufficient, and the disconnection of the wire cannot be grasped also in FIG.
[0061]
Further, in the images shown in FIGS. 6 (e) and 6 (f) using the X-ray tube of a> b, the outline is almost clear due to the small focal point, but the contrast between light and dark is high due to a large amount of X-ray noise components. It is not sufficient, and it is still difficult to grasp the disconnection of the wire in FIG.
[0062]
On the other hand, in the image by the X-ray tube of this embodiment shown in FIGS. 6G and 6H, in addition to the clear outline, the X-ray noise component is reduced and the S / N is reduced. As the ratio is improved, the contrast between light and dark becomes high. Therefore, as shown in FIG. 6 (h), the disconnection of the wire can be clearly confirmed.
[0063]
The X-ray tube according to the present invention is not limited to the above-described embodiment, and the configuration of each electrode can be variously changed.
[0064]
7A and 7B are cross-sectional views showing the configuration of the second embodiment of the X-ray tube according to the present invention. FIG. 7A is a cross-sectional view of the X-ray tube 1, and FIG. Show. The configuration of the X-ray tube 1 according to the present embodiment is substantially the same as that of the first embodiment, but the auxiliary focusing electrode system 6 is configured only by the first auxiliary focusing electrode 61. Even with such a configuration, by setting the potential V3 of the first auxiliary focusing electrode 61 so as to satisfy the condition V0 ≦ V3 <V2, it is possible to similarly perform trajectory adjustment that reduces the spread of the electron trajectory.
[0065]
As described above, regarding the auxiliary focusing electrodes constituting the auxiliary focusing electrode system 6, the number and setting voltage of the auxiliary focusing electrodes are appropriately set according to the arrangement relationship of the other electrodes in each X-ray tube and the electron trajectory. Is possible. For example, the number of installation may be composed of three or more electrodes, and the setting voltage may be selected according to the number of electrodes, their arrangement interval, opening shape, etc. it can.
[0066]
8A and 8B are cross-sectional views showing the configuration of the third embodiment of the X-ray tube according to the present invention. FIG. 8A is a cross-sectional view of the X-ray tube 1, and FIG. 8B is a vertical cross-sectional view. Show. The configuration of the X-ray tube 1 according to the present embodiment is substantially the same as that of the second embodiment, but the focus grid electrode 24 and the electron passage port 25 protrude from the surface of the outer container 21 of the electron gun unit 2 on the target 4 side. It is formed to do. Even with such a structure, the distance condition a> b can be realized.
[0067]
【The invention's effect】
As described in detail above, the X-ray tube according to the present invention has the following effects. That is, (1) The distance a from the virtual object point formed near the cathode to the passing point of the focus grid electrode is set larger than the distance b from the passing point to the electron incident point on the X-ray generation surface (a > B) This reduces the electron beam focused on the X-ray generation surface and the beam diameter (orbital variation) in the obtained X-ray beam. Further, (2) an auxiliary focusing electrode system is provided between the cathode and the first and second grid electrodes and the focus grid electrode, and the electron trajectory is controlled and adjusted so that the spread of the electron trajectory between them is reduced. By doing so, the influence of the aberration of the focus grid electrode and the like are suppressed, and the noise component (the spread) in the electron beam and the X-ray beam is reduced.
[0068]
When an X-ray fluoroscopic image is acquired with a microfocus X-ray tube having such a configuration, a small focus is achieved by reducing the beam diameter, and a high contrast is achieved by reducing noise components. High resolution is realized. As a result, it is possible to efficiently observe the structure of a semiconductor electronic component that has been reduced in size and increased in density, which is one of inspection targets by non-destructive inspection using, for example, X-rays. .
[Brief description of the drawings]
FIG. 1A is a cross-sectional view showing the configuration of a first embodiment of an X-ray tube, and FIG.
FIG. 2 is an end view showing a partially enlarged electron gun portion of the X-ray tube shown in FIG. 1;
3A is an end view showing an electron trajectory in an X-ray tube where a <b, and FIG. 3B is a graph showing an X-ray output distribution.
4A is an end view showing an electron trajectory in an X-ray tube where a> b, and FIG. 4B is a graph showing an X-ray output distribution.
5A is an end view showing an electron trajectory in the X-ray tube shown in FIG. 1, and FIG. 5B is a graph showing an X-ray output distribution.
6 is a schematic diagram for comparing an X-ray fluoroscopic image obtained using the X-ray tube shown in FIG. 1 with a case where a conventional X-ray tube or the like is used. FIG.
FIGS. 7A and 7B are a cross-sectional view and a vertical cross-sectional view, respectively, showing the configuration of the second embodiment of the X-ray tube. FIGS.
FIGS. 8A and 8B are a cross-sectional view and a vertical cross-sectional view, respectively, showing the configuration of a third embodiment of the X-ray tube. FIGS.
[Explanation of symbols]
1 ... X-ray tube,
DESCRIPTION OF SYMBOLS 2 ... Electron gun part, 21 ... Container, 22 ... Heater, 23 ... Cathode, 24 ... Focus grid electrode, 25 ... Electron passage port, 26 ... 1st grid electrode, 27 ... 2nd grid electrode,
DESCRIPTION OF SYMBOLS 3 ... X-ray generation part, 31 ... Container, 32 ... X-ray emission window, 4 ... Target, 40 ... Tip part, 41 ... X-ray generation surface, 5 ... Hood electrode, 51 ... X-ray emission port, 52 ... Electron incidence mouth,
6 ... auxiliary focusing electrode system, 61 ... first auxiliary focusing electrode, 62 ... second auxiliary focusing electrode.

Claims (4)

An electron gun that emits electrons in the direction of the electron incident axis from the cathode; and a target that emits X-rays by causing the electrons to enter the X-ray generation surface and emits the X-rays in the direction of the X-ray emission axis. In the X-ray tube configured so that the center axis of the target is substantially parallel to the X-ray emission axis,
Between the cathode of the electron gun and the X-ray generation surface of the target,
A first grid electrode for limiting current due to the electrons from the cathode;
A second grid electrode for accelerating the electrons that have passed through the first grid electrode;
An auxiliary focusing electrode system for controlling the spread of the trajectory of the electrons that have passed through the second grid electrode;
A focus grid electrode for sequentially focusing the electrons to a predetermined electron incident point on the X-ray generation surface;
The distance from the virtual object point where the electrons emitted from the cathode are once focused in the vicinity of the cathode to the focus grid electrode is larger than the distance from the focus grid electrode to the electron incident point. X-ray tube. The auxiliary focusing electrode system has at least a first auxiliary focusing electrode;
A potential V0 set for the cathode, a potential V1 set for the first grid electrode, a potential V2 set for the second grid electrode, and a potential V3 set for the first auxiliary focusing electrode are:
V1 ≦ V0 ≦ V3 <V2
The X-ray tube according to claim 1, wherein the relationship is satisfied. The auxiliary focusing electrode system further includes a second auxiliary focusing electrode provided between the first auxiliary focusing electrode and the focus grid electrode,
The potential V4 set on the second auxiliary focusing electrode is
V2 = V4
The X-ray tube according to claim 2, wherein the relationship is satisfied. 4. The X-ray tube according to claim 1, wherein the focus grid electrode is formed so as to protrude from an outer container of the electron gun toward the target. 5.

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