GB2473137A

GB2473137A - An X-ray generator

Info

Publication number: GB2473137A
Application number: GB1014389A
Authority: GB
Inventors: Motohiro Suyama; Kinji Takase; Atsushi Ishii
Original assignee: Hamamatsu Photonics KK
Current assignee: Hamamatsu Photonics KK
Priority date: 2009-08-31
Filing date: 2010-08-27
Publication date: 2011-03-02
Anticipated expiration: 2030-08-27
Also published as: JP2011071101A; DE102010039926A1; JP5687001B2; GB201014389D0; GB2473137B

Abstract

An X-ray generator 1 has: an electron gun 3 to emit an electron beam; a target unit T; a coil part 9 capable of deflecting the travelling path of the electron beam; a detector 31 to detect electrons reflected from the target unit T; and a controller 33 to control the coil part 9 on the basis of a detection signal from the reflected electron detector 31. The target unit T comprises a substrate (21, Figure 2), and a target body (23) buried in the substrate (21). The diameter of an irradiation area of the electron beam on the target unit T is larger than the diameter of the target body 23. The controller 33 controls the coil part 9 to two-dimensionally scan the irradiation area on the target unit T so that the target body 23 is always included in the irradiation area of the electron beam on the target unit T. Alternatively, the detection signal used to control deflection of the electron beam may be obtained from an X-ray detector (41, Figure 26) or a target current detector (51, Figure 27).

Description

TITLE OF THE INVENTION

X-RAY GENERATOR

BACKGROUND OF THE INVENTION

Field of the Invention

[00011 The present invention relates to an X-ray generator.

Related Background Art

[0002] There is a known X-ray generator provided with an electron gun to emit an electron beam, and a target unit having a substrate and a target body buried in the substrate and comprised of a material to generate X-rays with incidence of the electron beam (e.g. cf. Japanese Patent Application Laid-open No. 2004-028845). There is also a known target unit provided with a substrate comprised of diamond and a target body of tungsten or the like buried in a non-penetrating state in the substrate (e.g., cf. U.S. Pat. No. 5,148,462).

SUMMARY OF THE INVENTION

[0003] The X-ray generator is configured to project the electron beam from the electron gun into the target body to radiate X-rays from the target body. In the X-ray generator, the positional relation between an irradiation area of the electron beam and the target body can vary from that at a start of driving of the generator because of a factor such as thermal expansion of components of the generator. If the positional relation between the irradiation area, of the electron beam and the target body varies, it becomes difficult to obtain a desired X-ray amount because the electron beam becomes applied to only a part of the target body.

[0004] It is an object of the present invention to provide an X-ray generator capable of achieving a desired X-ray amount on a stable basis while suppressing the variation in the positional relation between the irradiation area of the electron beam and the target body.

[0005] An X-ray generator according to the present invention is an X-ray generator comprising: an electron gun to emit an electron beam; a target unit having a substrate, and a target body buried in the substrate and comprised of a material to generate X-rays with incidence of the electron beam; an electron beam deflecting unit capable of changing a traveling path of the electron beam emitted from the electron gun; a detecting unit to detect reflected electrons from the target body, or X-rays generated from the target body, or a target current; and a control unit to control the electron beam deflecting unit on the basis of a detection signal from the detecting unit, wherein an irradiation area of the electron beam on the target unit includes the target body, and wherein the control unit controls the electron beam deflecting unit to two-dimensionally scan the irradiation area on the target unit so that the target body is always included in the irradiation area of the electron beam on the target unit.

[0006] In the X-ray generator according to the present invention, the control unit controls the electron beam deflecting unit to two-dimensionally scan the irradiation area on the target unit so that the target body is always included in the irradiation area of the electron beam on the target unit; therefore, variation is suppressed in the positional relation between the irradiation area of the electron beam and the target body. As a result of this, the present invention allows the generator to achieve a desired X-ray amount on a stable basis.

[0007] The X-ray generator may be configured as follows: the control unit controls the electron beam deflecting unit so as to rotationally scan the irradiation area on the target unit in a state in which the target body is included in the irradiation area of the electron beam on the target unit.

In this case, the X-ray amount obtained by irradiation with the electron beam becomes virtually constant and the control can be performed so as to stabilize the X-ray generation state.

[0008] The X-ray generator may be configured as follows: the detecting unit detects the reflected electrons from the target body or the X-rays generated from the target body; the control unit determines whether a detected amount by the detecting unit is constant; when the control unit determines that the detected amount is not constant, the control unit controls the electron beam deflecting unit so as to move a center of a rotational scan in a direction to increase the detected amount. In this case, it is feasible to certainly and readily perform the control for stabilizing the X-ray generation state.

[0009] The X-ray generator may be configured as follows: the detecting unit detects the target current; the control unit determines whether a detected amount by the detecting unit is constant; when the control unit determines that the detected amount is not constant, the control unit controls the electron beam deflecting unit so as to move a center of a rotational scan in a direction to decrease the detected amount. In this case, it is feasible to certainly and readily perform the control for stabilizing the X-ray generation state.

[0010] The X-ray generator may be configured as follows: the control unit controls the electron beam deflecting unit so as to scan the irradiation area in two intersecting directions on the target unit in a state in which the target body is included in the irradiation area of the electron beam on the target unit. In this case, it becomes feasible to perform the control so as to maximize the X-ray amount obtained by the irradiation with the electron beam.

[0011] The X-ray generator may be configured as follows: the detecting unit detects the reflected electrons from the target body or the X-rays generated from the target body, and the control unit controls the electron beam deflecting unit so as to maximize a detected amount by the detecting unit in each of the aforementioned two directions. In this case, it becomes feasible to certainly and readily perform the control for obtaining the maximum X-ray amount.

[0012] The X-ray generator may be configured as follows: the detecting unit detects the target current, and the control unit controls the electron beam deflecting unit so as to minimize a detected amount by the detecting unit in each of the aforementioned two directions. In this case, it is feasible to certainly and readily perform the control for obtaining the maximum X-ray amount.

[0013] The X-ray generator may be configured as follows: when the target body is not included in the irradiation area of the electron beam on the target unit, the control unit two-dimensionally scans the electron beam until the target body becomes included in the irradiation area, to specif' a position of the target body. In this case, it is feasible to certainly and readily specify the position of the target body even if the target body is not included in the irradiation area of the electron beam on the target unit.

[0014] The X-ray generator may be configured as follows: when the target body is not included in the irradiation area of the electron beam on the target unit, the control unit defocuses the electron beam until the target body becomes included in the irradiation area, to specify a position of the target body. In this case, it is feasible to certainly and readily specify the position of the target body even if the target body is not included in the irradiation area of the electron beam on the target unit.

[0015] The X-ray generator may be configured as follows: the control unit controls the electron beam deflecting unit so as to scan the focused electron beam along a circular orbit corresponding to a profile of the irradiation area of the defocused electron beam including the target body. In this case, it is feasible to certainly and readily scan the electron beam so that the target body becomes included in the irradiation area of the electron beam, after the position of the target body is specified.

[0016] The present invention successfully provides the X-ray generator capable of achieving the desired X-ray amount on a stable basis while suppressing the variation in the positional relation between the irradiation area of the electron beam and the target body.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Fig. 1 is a schematic configuration diagram showing an X-ray generator according to an embodiment of the present invention.

Fig. 2 is a drawing showing a configuration of a target unit.

Fig. 3 is a flowchart for explaining control on a coil part by a controller.

Fig. 4 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 5 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 6 is a graph showing an example of relationship between positions on a circular orbit in a rotational scan, and reflected electron intensity.

Fig. 7 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 8 is a graph showing an example of relationship between positions on a circular orbit in a rotational scan, and reflected electron intensity.

Fig. 9 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 10 is a graph showing an example of relationship between positions on a circular orbit in a rotational scan, and reflected electron intensity.

Fig. 11 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 12 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 13 is a flowchart for explaining another control on the coil part by the controller.

Fig. 14 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 15 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 16 is a graph showing an example of relationship between positions in a scan in the Y-axis direction, and reflected electron intensity.

Fig. 17 is a schematic diagram for explaining the control on the coil part by the controller.

Fig. 18 is a graph showing an example of relationship between positions in a scan in the X-axis direction, and reflected electron intensity.

Fig. 19 is a schematic diagram for explaining detection of a position of a target body.

Fig. 20 is a schematic diagram for explaining detection of the position of the target body.

Fig. 21 is a schematic diagram for explaining detection of the position of the target body.

Fig. 22 is a graph showing an example of relationship between positions on a circular orbit in a rotational scan, and reflected electron intensity.

Fig. 23 is a schematic diagram for explaining detection of the position of the target body.

Fig. 24 is a schematic diagram for explaining detection of the position of the target body.

Fig. 25 is a schematic diagram for explaining detection of the position of the target body.

Fig. 26 is a schematic configuration diagram showing a modification example of the X-ray generator according to the embodiment.

Fig. 27 is a schematic configuration diagram showing another modification example of the X-ray generator according to the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

In the description, identical elements or elements with identical functionality will be denoted by the same reference symbols, without

redundant description.

[0019] First, a configuration of an X-ray generator according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a schematic configuration diagram showing the X-ray generator according to the present embodiment.

[0020] The X-ray generator 1 is an open type and can optionally create a vacuum state, different from a closed type which is discarded after use.

The X-ray generator 1 allows replacement of a target unit T, a cathode of an electron gun 3, and so on. The X-ray generator 1 has a cylindrical portion 5 of stainless steel having a cylindrical shape. The interior space of the cylindrical portion 5 is brought into a vacuum state during operation. The cylindrical portion 5 is composed of a fixed section 5a located down and a detachable section 5b located up. The detachable section 5b is attached to the fixed section 5a through a hinge (not shown). Therefore, when the detachable section 5b is rotated into a horizontal posture through the hinge, the upper part of the fixed section 5a becomes open. This makes it feasible to access the electron gun 3 (cathode) housed in the fixed section 5a.

[0021] A tubular coil part 7 functioning as a focusing lens and a tubular coil part 9 functioning as a deflector coil (electron beam deflecting unit) are provided in the detachable section 5b. An electron passage 11 extends in the longitudinal direction of the cylindrical portion 5 so as to pass the center of the coil parts 7, 9, in the detachable section 5b. The electron passage 11 is surrounded by the coil parts 7, 9. A disk plate 13 is fixed to the lower end of the detachable section 5b so as to close it.

An electron inlet hole 13a is formed in a center of the disk plate 13 so as to be aligned with the lower end of the electron passage 11.

[0022] The upper end of the detachable section 5b is formed in a truncated circular cone shape. The target unit T which is located at the upper end of the electron passage 11 and which forms a transmission type X-ray exit window is mounted on the top of the detachable section 5b. The target unit T is housed in an earthed state in a detachable rotary cap part (not shown). Therefore, when the cap part is removed, it becomes feasible to replace the target unit T of a consumable part.

[0023] A vacuum pump 17 is fixed to the fixed section 5a. The vacuum pump 17 brings the entire interior of the cylindrical portion 5 into a high vacuum state. Since the X-ray generator 1 is equipped with the vacuum pump 17, it becomes feasible to replace the target unit T, the cathode, and so on.

[0024] A mold power supply unit 19 integrated with the electron gun 3 is fixed on the base end side of the cylindrical portion 5. The mold power supply unit 19 is one molded from an electrically insulating resin (e.g., epoxy resin) and housed in a metal case.

[0025] A high voltage generation unit (not shown) is sealed in the mold power supply unit 19. The high voltage generation unit has a transformer to generate a high voltage (e.g., up to -160 kV in the case where the target unit T is earthed). The mold power supply unit 19 is composed of a power supply main body part 19a of a block form of a rectangular parallelepiped shape located on the lower side, and a neck part 19b of a cylindrical column shape projecting upward from the power supply main body part 1 9a into the fixed section 5a. The high voltage generation unit is sealed in the power supply main body part 1 9a. The electron gun 3 arranged to face the target unit T with the electron passage 11 in between is mounted at the distal end of the neck part 19b. An electron emission control unit (not shown) electrically connected to the high voltage generation unit is sealed in the power supply main body part 1 9a of the mold power supply unit 19. The electron emission control unit is connected to the electron gun 3 to control the timing of emission of electrons, a tube current, and so on.

[0026] The target unit T, as also shown in Fig. 2, has a substrate 21 and a target body 23. The substrate 21 is made of a material with excellent X-ray transmission and heat dissipation, e.g., diamond and in a plate shape. The substrate 21 has first and second principal surfaces 2 la, 21 b opposed to each other. The thickness of the substrate 21 is set, for example, to about 100 m. The target body 23 is located on the first principal surface 21 a side of the substrate 21. The target body 23 is made of a metal (e.g., tungsten, gold, platinum, or the like), which is a material different from the substrate 21, and in a cylindrical column shape and is nanosized (e.g., in the outside diameter of about 100 nm).

In the present embodiment tungsten (W) is adopted as the metal of the target body 23.

[0027] Reference is made again to Fig. 1. The X-ray generator 1 is provided with a reflected electron detector 31 as a reflected electron detecting unit and a controller 33 as a control unit. The reflected electron detector 31 is arranged on the upper end side of the detachable section 5b so as to look out on the target body 23, via an unrepresented path or at a position where it does not affect the electron beam EB traveling toward the target unit T, and vice versa, in the electron passage 11. The reflected electron detector 31 detects electrons (reflected electrons) reflected by the target unit T (target body 23).

[0028] The controller 33 controls the high voltage generation unit and the electron emission control unit of the mold power supply unit 19.

Through this control, a predetermined current or voltage is applied between the electron gun 3 and the target unit T (target body 23), whereby an electron beam EB is emitted from the electron gun 3. The electron beam EB emitted from the electron gun 3 is appropriately converged by the coil part 7 controlled by the controller 33, to impinge upon the target body 23. At this time, for example, the diameter of an irradiation area of the electron beam EB on the target unit T is set larger than the diameter of the target body 23 so that the target body 23 is included in the irradiation area of the electron beam EB on the target unit T, when viewed from a direction perpendicular to the target unit I (i.e., an electron incidence direction). When the electron beam EB is incident to the target body 23, the target body 23 radiates X-rays XR and the X-rays XR travel through the substrate 21 to be radiated to the outside.

[0029] The controller 33 monitors the intensity of reflected electrons detected by the reflected electron detector 31, in real time and controls the coil part 9, based on the intensity of reflected electrons from the target unit T (target body 23) and position information set on the target unit T (target body 23). At this time, the coil part 9 deflects the electron beam EB from the electron gun 3 so as to two-dimensionally scan the irradiation area of the electron beam EB on the target unit T. [0030] When a substance is irradiated with the electron beam EB, it emits reflected electrons in an amount depending upon the atomic number of the substance (it emits more reflected electrons with increasing atomic number). In the present embodiment, since the target body 23 of tungsten is buried in the substrate 21 of diamond, the target body 23 can be determined to be located at a place where more reflected electrons are detected. Then the controller 33 controls the deflection of the electron beam EB so as to obtain more reflected electrons.

[0031] The below will describe an example of the control on the coil part 9 by the controller 33, with reference to Figs. 3 to 12. Fig. 3 is a flowchart for explaining the control on the coil part by the controller.

Figs. 4, 5, 7, 9, 11, and 12 are schematic diagrams for explaining the control on the coil part by the controller. Figs. 6, 8, and 10 are graphs each showing an example of relationship between positions on a circular orbit in a rotational scan and reflected electron intensity. The target body 23 is hatched.

[0032] First, the controller 33 detects the position of the target body 23 (S 101) and controls the coil part 9 to rotationally scan the irradiation area F of the electron beam on the target unit T (S 103). For detecting the position of the target body 23, an applicable method is to preliminarily acquire coordinate data of the target body 23 and to detect position data of the target body 23, based thereon; or the position data may be detected by a technique described later.

[0033] In the aforementioned rotational scan, as shown in Fig. 4, the irradiation area F of the electron beam scans along a circular orbit with a predetermined radius around a predetermined center C in a state in which at least a part of the target body 23 is included in the irradiation area F of the electron beam. At this time, if the irradiation area F of the electron beam rotationally scans in the state in which the whole target body 23 is included under the irradiation area F of the electron beam (cf. Fig. 5), an amount of reflected electrons or the intensity of reflected electrons detected by the reflected electron detector 31 becomes constant at a predetermined value (maximum), as shown in Fig. 6. For this reason, a desired X-ray state can be obtained when the electron beam spot position can be set so that the reflected electron intensity becomes the predetermined maximum.

[0034] Incidentally, in a state in which the center C of the circular orbit in the rotational scan is coincident with the center of the target body 23 (ef. Fig. 7), the whole target body 23 is not included under the irradiation area F of the electron beam, but the reflected electron intensity can be constant at a value smaller than the maximum (a characteristic indicated by a broken line in Fig. 8), as shown in Fig. 8.

[0035] Therefore, the controller 33 determines whether the reflected electron intensity detected by the reflected electron detector 31 is approximately constant at the predetermined value (S 105). When the reflected electron intensity is approximately constant at the predetermined value, the rotational scan is continued so as to maintain the state in which the whole target body 23 is included under the irradiation area F of the electron beam, as shown in Fig. 5.

[0036] If the size of the region of the target body 23 included in the irradiation area F of the electron beam varies depending upon positions on the circular orbit (cf. Fig. 9), the reflected electron intensity varies as shown in Fig. 10. Therefore, if the reflected electron intensity is not virtually constant at the predetermined value, the varying data of reflected electron intensity is compared with the position information on the circular orbit and the center C of the circular orbit is moved in a direction to increase the value (S 107). Then the controller returns to S 103 to scan the irradiation area F of the electron beam along a new circular orbit after the movement of the center C, and then the control is continued.

[0037] When at least a part of the target body 23 is included in the irradiation area F of the electron beam, there is no need for setting the radius of the circular orbit in the rotational scan to a larger value than necessary. Therefore, the center C of the circular orbit CO (orbit of the center of the irradiation area F) in the rotational scan is preferably set in the irradiation area F of the electron beam (also including a boundary line of the irradiation area F), as shown in Fig. 11. The radius of the circular orbit in the rotational scan (distance between the center C of the circular orbit and the center of the irradiation area F) is more preferably smaller than the radius of the irradiation area F. [0038] The radius of the circular orbit in the rotational scan may be a constant value preliminarily determined or may be gradually decreased based on the value of reflected electron intensity. By decreasing the radius of the circular orbit in the rotational scan, it becomes feasible to perform more appropriate irradiation with the electron beam.

Particularly, in the case where the center of the circular orbit in the rotational scan is coincident with the center of the target body 23, the reflected electron intensity becomes constant as described above, but the reflected electron intensity increases with decrease in the radius of the circular orbit, as shown in Fig. 12.

[0039] The aforementioned rotational scan does not have to be constantly continued. For example, when the reflected electron intensity is approximately constant at the desired value, the rotational scan may be suspended and the position of the irradiation area F of the electron beam may be fixed. Then the rotational scan may be restarted after a lapse of a predetermined time (e.g., 5 minutes or the like).

Another applicable method is to monitor the reflected electron intensity in the state in which the position of the irradiation area F of the electron beam is fixed, by the reflected electron detector 31, and to restart the rotational scan with decrease in the reflected electron intensity.

[0040] In the X-ray generator, the positional relation between the irradiation area F of the electron beam and the target body can change, when compared with that at a start of driving of the generator, because of the following factors.

[00411 For example, the cathode of the electron gun expands to be deformed by heat generated by itself, whereby the position of emission of the electron beam moves. This changes the arriving position of the electron beam on the target unit (target body).

[0042] For example, the electron lens shape changes because of heat generation and expansion of the coil parts controlling the electron orbit and focusing of the electron beam. This changes the trajectory of the electron beam.

[0043] For example, carbide (originating in vacuum grease or the like used at sealing portions of the X-ray generator) is decomposed by irradiation with the electron beam and the decomposed carbide attaches to the inside of the housing, e.g., the electron passage, to cause charge-up. This charge-up makes the electric field nonuniform, so as to change the trajectory of the electron beam.

[0044] In the X-ray generator 1 of the present embodiment, however, the controller 33 controls the coil part 9 to two-dimensionally scan the irradiation area F of the electron beam on the target unit T to specify the position of the target body 23, and performs such control as to keep the target body 23 always included in the irradiation area F of the electron beam on the target unit T. This control suppresses the change in the positional relation between the irradiation area F of the electron beam and the target body 23. As a result, the present embodiment allows a desired X-ray amount and X-ray focal-spot diameter to be obtained on a

stable basis.

[0045] Particularly, in the present embodiment, the controller 33 controls the coil part 9 to rotationally scan the irradiation area F of the electron beam on the target unit I in the state in which the target body 23 is included in the irradiation area F of the electron beam on the target unit T. This allows the X-ray generator 1 to generate X-rays in a virtually constant amount by the irradiation with the electron beam and to perform the control to stabilize the X-ray generation state.

[0046} In the present embodiment, the controller 33 determines whether the amount of reflected electrons (reflected electron intensity) detected by the reflected electron detector 31 is constant, and when it is determined not to be constant, the controller 33 controls the coil part 9 so as to move the center of the circular orbit in the rotational scan in a direction to increase the reflected electron intensity. This allows the X-ray generator 1 to certainly and readily perform the control for stabilizing the X-ray generation state.

[0047] The X-ray generator 1 of the present embodiment achieves the resolution determined by the size of the target body 23. Since the target body 23 is nanosized, the deterioration of resolution is suppressed without increase in the X-ray focal-spot diameter. Accordingly, the X-ray generator 1 is able to achieve the resolution of nanometer order (several ten to several hundred nm) while increasing the X-ray amount.

[0048] The below will describe another example of the control on the coil part 9 by the controller 33, with reference to Figs. 13 to 18. Fig. 13 is a flowchart for explaining the control on the coil part by the controller. Figs. 14, 15, and 17 are schematic diagrams for explaining the control on the coil part by the controller. Figs. 16 and 18 are graphs each showing an example of relationship between positions on a straight orbit in an X-or Y-directional scan, and reflected electron intensity.

[0049] First, the controller 33 detects the position of the target body 23 (S201) and controls the coil part 9 to scan the irradiation area F of the electron beam in one of two intersecting directions on the target unit T (S203). For detecting the position of the target body 23, as described above, an applicable method is to preliminarily acquire the coordinate data of the target body 23 and detect the position data of the target body 23, based thereon; or the position data may be detected by the technique described later. In the present example, it is assumed hereinafter that the two intersecting directions are mutually orthogonal directions, one direction is Y-direction, and the other direction is X-direction.

[0050] In the Y-directional scan, as shown in Fig. 14, the irradiation area F of the electron beam scans back and forth on a line along the Y-direction in a state in which at least a part of the target body 23 is included in the irradiation area F of the electron beam. For example, as the irradiation area F of the electron beam scans from bottom to top in the Y-axis direction via a state in which the entire target body 23 is included under the irradiation area F of the electron beam as shown in Fig. 15, the amount of reflected electrons detected by the reflected electron detector 31 (the reflected electron intensity) increases from the position where the irradiation area F of the electron beam is located at the lowermost position, to the position where the center of the irradiation area F of the electron beam coincides with the position in the Yaxis direction of the center of the target body 23, as shown in Fig. 16.

As the irradiation area F of the electron beam further scans upward from the position where the center of the irradiation area F of the electron beam coincides with the position in the Y-axis direction of the center of the target body 23, the reflected electron intensity decreases. When the reflected electron intensity starts decreasing, the scan direction is reversed to scan the irradiation area F of the electron beam from top to bottom in the Y-axis direction. By repeating this operation, it becomes feasible to specify the position in the Y-axis direction where the reflected electron intensity becomes maximum.

[00511 Namely, in S203 the controller 33 controls the coil part 9 to move the irradiation area F of the electron beam by a predetermined length in the plus direction of the Y-axis. Then the controller 33 determines whether the reflected electron intensity detected by the reflected electron detector 31 is decreased (S205). When the reflected electron intensity is determined not to be decreased, the controller returns to S203 to control the coil part 9 so as to further move the irradiation area F of the electron beam by the predetermined length in the plus direction of the Y-axis, thereby repeating the processing.

[0052] On the other hand, when the controller 33 determines in S205 that the reflected electron intensity is decreased, it controls the coil part 9 to move the irradiation area F of the electron beam by the predetermined length in the minus direction of the Y-axis (S207). Then the controller 33 determines whether the reflected electron intensity detected by the reflected electron detector 31 is decreased (S209).

When the reflected electron intensity is determined not to be decreased, the controller 33 returns to S207 to control the coil part 9 so as to further move the irradiation area F of the electron beam by the predetermined length in the minus direction of the Y-axis, thereby repeating the processing.

[0053] When the controller 33 determines in S209 that the reflected electron intensity is decreased, it returns to S203 to control the coil part 9 so as to further move the irradiation area F of the electron beam by the predetermined length in the plus direction of the Y-axis, thereby repeating the processing.

[0054] In this manner, the controller specifies the position where the reflected electron intensity becomes maximum in the Y-axis direction, in the irradiation area F of the electron beam, and determines the electron beam fixing position in the Y-axis direction. Furthermore, if the controller 33 is able to associate the position of the irradiation area F of the electron beam in the Y-axis direction with the reflected electron intensity, the controller 33 may be configured to control the coil part 9 so as to acquire the position of the maximum reflected electron intensity by a single scan of the irradiation area F of the electron beam in the Y-axis direction, and make the irradiation area F of the electron beam coincident with the acquired position.

[0055] After the position of the electron beam in the Y-axis direction is fixed, the scan in the X-axis direction is started. As the irradiation area F of the electron beam scans from left to right in the X-axis direction as shown in Fig. 17, the amount of reflected electrons detected by the reflected electron detector 31 (the reflected electron intensity) increases from the position where the irradiation area F of the electron beam is located at a left extreme, to the position where the center of the irradiation area F of the electron beam coincides with the position in the X-axis direction of the center of the target body 23, as shown in Fig. 18.

As the irradiation area F of the electron beam further scans rightward from the position where the center of the irradiation area F of the electron beam coincides with the position in the X-axis direction of the center of the target body 23, the reflected electron intensity decreases.

When the reflected electron intensity starts decreasing, the scan direction is reversed to scan the irradiation area F of the electron beam from right to left in the X-axis direction. By repeating this operation, it becomes feasible to specify the position where the reflected electron intensity becomes maximum in the X-axis direction.

[0056] Therefore, the controller 33 controls the coil part 9 to also scan the irradiation area F of the electron beam in the X-axis direction in the same manner as the scan in the Y-axis direction described above. This enables the controller to specify the position of the maximum reflected electron intensity in the X-axis direction as well. The detailed description about the scan in the X-axis direction is omitted herein by adding "X-axis" in parentheses to the corresponding steps in Fig. 13.

In this way, the controller is able to specify the position of the maximum reflected electron intensity both on the X-axis and on the Y-axis. The order of the scans in the X-axis direction and in the Y-axis direction may be reverse.

[0057] It is unnecessary to always continuously carry out the above-described X-and Y-directional scans. For example, when the reflected electron intensity is virtually constant at a desired value, the scanning operation may be suspended and the position of the irradiation area F of the electron beam may be fixed. The scanning operation may be restarted after a lapse of a predetermined time (e.g., 5 minutes or the like). Another applicable method is to monitor the reflected electron intensity in the state in which the position of the irradiation area F of the electron beam is fixed, by the reflected electron detector 31, and to restart the scanning operation with decrease in the reflected electron intensity.

[0058] In the present example, as described above, the controller 33 controls the coil part 9 to two-dimensionally scan the irradiation area F of the electron beam on the target unit T to specify the position of the target body 23, and performs such control as to keep the target body 23 always included in the irradiation area F of the electron beam on the target unit T. This control allows the generator to suppress the change in the positional relation between the irradiation area F of the electron beam and the target body 23 and thus to obtain the desired X-ray amount and X-ray focal-spot diameter on a stable basis.

[0059] Particularly, in the present example, the controller 33 controls the coil part 9 to scan the irradiation area F of the electron beam in the two intersecting directions (X-axis and Y-axis directions) on the target unit T in the state in which the target body 23 is included in the irradiation area F of the electron beam on the target unit T. For this reason, the present example allows the control to maximize the X-ray amount obtained by the irradiation with the electron beam.

[0060] In the present example, the controller 33 controls the coil part 9 to maximize the detected amount by the reflected electron detector 31 in each of the two directions. This permits the controller to certainly and readily perform the control to obtain the maximum X-ray amount.

[0061] In the case where the irradiation area F of the electron beam is completely off the target body 23 and where the target body 23 is not included in the irradiation area F of the electron beam, the position of the target body 23 may be detected by two-dimensionally scanning the area on the target unit T with the size of the irradiation area F being maintained, and determining the position of the target body 23, based on the reflected electron intensity detected by the reflected electron detector 31. The two-dimensional scanning operation on that occasion may be the rotational scan, or the scans in two intersecting directions of the irradiation area F on the target unit T. Namely, it is sufficient to detect a position where even a part of the target body 23 is included in the irradiation area F. In these cases, even if the target body 23 is not included in the irradiation area F of the electron beam on the target unit T, the position of the target body can be certainly and readily specified.

[0062] Incidentally, the position of the target body 23 can also be detected by the technique as described below.

[0063] In the case where the irradiation area F of the electron beam is completely off the target body 23 and where the target body 23 is not included in the irradiation area F of the electron beam as shown in Fig. 19 (a), the electron beam is defocused up to a region where a part of the target body 23 is included in the irradiation area F of the electron beam and where the reflected electrons are detected by the reflected electron detector 31 (region indicated by a solid line in Fig. 19 (b)) as shown in Fig. 19 (b). This allows the generator to specify at least the region including the target body 23 and thus to specify the region to be scanned. At this time, it is obvious that the target body 23 is located at an edge part of the region, and it is thus preferable to perform such a rotational scan as to make the area inscribed in the defocused region (indicated by a chain line in the drawing) as shown in Fig. 20. The position of the target body 23 can be detected by this scan.

[0064] It is also possible to defocus the electron beam until the whole target body 23 becomes included in the irradiation area F of the electron beam, as shown in Fig. 21. As the electron beam is defocused until the whole target body 23 becomes included in the irradiation area F of the electron beam, the reflected electron intensity becomes virtually constant over a radius at a predetermined value because the whole target body 23 becomes included in the area, as shown in Fig. 22.

[0065] For this reason, the position where almost the whole target body 23 is included can be specified by performing such a rotational scan, as shown in Fig. 23, as to make the area inscribed in a circle (indicated by a chain line in the drawing) where the reflected electron intensity begins to become virtually constant. In this case, a position indicating maximum data is the position of the target body 23.

[0066] In fact, the electron amount per unit area decreases because of the defocusing and thus the intensity does not reach the constant value as in the aforementioned graph, with a constant tube current. However, it is possible to capture a tendency of change in the reflected electron intensity and thus to detect an approximate position. It is also possible to perform such control as to increase the tube current value with defocusing so as to keep the electron amount equal.

[0067] If the rotational scan is performed along a circle with a radius (indicated by a chain line in the drawing) slightly larger than the radius of the circle where the reflected electron intensity starts to become virtually constant, as shown in Fig. 24, it is feasible to obtain an electron beam in a state in which the target is perfectly included inside. By controlling the tube current, it is also possible to define the position indicative of the maximum as a final target position as long as the data is accurate. An extent of aforementioned "slightly larger" is a value not exceeding a value obtained by subtracting the diameter of the target body 23 from the diameter of the irradiation area F of the electron beam as shown in Fig. 25.

[0068] As described above, the controller 33 in the present embodiment may be configured to specify the position of the target body 23 by defocusing the electron beam until the target body 23 becomes included in the irradiation area F, in the case where the target body 23 is not included in the irradiation area F of the electron beam on the target unit T. This allows the controller to certainly and readily specify the position of the target body even in the case where the target body 23 is not included in the irradiation area F of the electron beam on the target unit T. [0069] Particularly, the controller 33 controls the coil part 9 to scan the focused electron beam along the circular orbit corresponding to the profile of the irradiation area F of the defocused electron beam including the target body 23. This allows the controller to certainly and readily perform the scan so as to make the target body 23 included in the irradiation area F of the electron beam, after specifying the position of the target body 23.

[0070] The above described the preferred embodiments of the present invention, but it is noted that the present invention is by no means limited to the above-described embodiments but can be modified in various ways without departing from the spirit and scope of the invention.

[0071] In the embodiment the controller 33 controlled the coil part 9 on the basis of the reflected electron intensity, but, without having to be limited to this, the coil part 9 may be controlled on the basis of a characteristic X-ray amount. In this case, it is preferable to use an X-ray detector 41, instead of the reflected electron detector 31, as shown in Fig. 26. When an electron beam is applied to a substance, X-rays are generated. X-rays are classified into bremsstrahlung X-rays of continuous spectrum and characteristic X-rays of line spectrum, and the characteristic X-rays have energies peculiar to each element. The energy of K-series characteristic X-rays of W making up the target body 23 is approximately 59.3 keY and the energies of L-series characteristic X-rays thereof are approximately 8.4 keV and approximately 9.7 keV.

Therefore, the controller 33 controls the deflection of the electron beam so as to make the characteristic X-ray amount detected by the X-ray detector 41, constant at a predetermined value, or maximum, in the same manner as in the case of the aforementioned reflected electron intensity.

[0072] In the embodiment the substrate 21 is comprised of diamond and the target body 23 of tungsten. In this case, an amount of X-rays generated from the substrate 21 by irradiation with the electron beam is significantly different from an amount of X-rays generated from the target body 23 by irradiation with the electron beam. In the case where the amount of X-rays generated from the substrate 21 is significantly different from the amount of X-rays generated from the target body 23 as in this case, the deflection of the electron beam may be controlled so as to detect the overall X-ray amount by the X-ray detector 41, instead of only the characteristic X-ray amount, and to make the overall X-ray amount detected by the X-ray detector 41, constant at a predetermined value, or maximum.

[0073] The controller 33 may control the coil part 9 on the basis of a target current value detected from the target unit T. In this case, a current detector 51 (current detecting unit) for detecting the target current is provided as shown in Fig. 27 and a detection signal from the target unit T (target current value) is fed to the controller 33. The controller 33 itself may be provided with the current detecting unit.

[0074] When a substance is irradiated with an electron beam, it absorbs electrons in an amount depending upon the atomic number of the substance. Namely, the larger the atomic number, the smaller the target current value; the smaller the atomic number, the larger the target current value. Since in the embodiment the target body 23 of tungsten is buried in the substrate 21 of diamond, it can be determined that the target body 23 is located at a place where the target current value is small. Then the controller 33 controls the deflection of the electron beam EB so as to make the target current value smaller. For example, the controller 33 determines whether the target current value is constant and, when it determines that the target current value is not constant, it may control the deflection of the electron beam EB so as to move the center of the rotational scan in a direction to decrease the target current value. Furthermore, the controller 33 may be configured to control the deflection of the electron beam EB so as to minimize the target current value, in each of the Y-axis direction and the X-axis direction.

[0075] The number of target body 23 does not have to be limited to one, but may be two or more. It is possible to use a plurality of target bodies 23 of different materials and to generate X-rays of different energies. The shape of the irradiation area F of the electron beam on the target unit T is not limited to the circular shape, but may be an elliptical shape.

Claims

WHAT IS CLAIMED IS: 1. An X-ray generator comprising: an electron gun to emit an electron beam; a target unit having a substrate, and a target body buried in the substrate and comprised of a material to generate X-rays with incidence of the electron beam; an electron beam deflecting unit capable of changing a traveling path of the electron beam emitted from the electron gun; a detecting unit to detect reflected electrons from the target body, or X-rays generated from the target body, or a target current; and a control unit to control the electron beam deflecting unit on the basis of a detection signal from the detecting unit, wherein an irradiation area of the electron beam on the target unit includes the target body, and wherein the control unit controls the electron beam deflecting unit to two-dimensionally scan the irradiation area on the target unit so that the target body is always included in the irradiation area of the electron beam on the target unit.
2. The X-ray generator according to claim 1, wherein the control unit controls the electron beam deflecting unit so as to rotationally scan the irradiation area on the target unit in a state in which the target body is included in the irradiation area of the electron beam on the target unit.
3. The X-ray generator according to claim 2, wherein the detecting unit detects the reflected electrons from the target body or the X-rays generated from the target body, and wherein the control unit determines whether a detected amount by the detecting unit is constant, and when the control unit determines that the detected amount is not constant, the control unit controls the electron beam deflecting unit so as to move a center of a rotational scan in a direction to increase the detected amount.
4. The X-ray generator according to claim 2, wherein the detecting unit detects the target current, and wherein the control unit determines whether a detected amount by the detecting unit is constant, and when the control unit determines that the detected amount is not constant, the control unit controls the electron beam deflecting unit so as to move a center of a rotational scan in a direction to decrease the detected amount.
5. The X-ray generator according to claim 1, wherein the control unit controls the electron beam deflecting unit so as to scan the irradiation area in two intersecting directions on the target unit in a state in which the target body is included in the irradiation area of the electron beam on the target unit.
6. The X-ray generator according to claim 5, wherein the detecting unit detects the reflected electrons from the target body or the X-rays generated from the target body, and wherein the control unit controls the electron beam deflecting unit so as to maximize a detected amount by the detecting unit in each of said two directions.
7. The X-ray generator according to claim 5, wherein the detecting unit detects the target current, and wherein the control unit controls the electron beam deflecting unit so as to minimize a detected amount by the detecting unit in each of said two directions.
8. The X-ray generator according to any one of claims 1 to 7, wherein when the target body is not included in the irradiation area of the electron beam on the target unit, the control unit two-dimensionally scans the electron beam until the target body becomes included in the irradiation area, to specify a position of the target body.
9. The X-ray generator according to any one of claims 1 to 7, wherein when the target body is not included in the irradiation area of the electron beam on the target unit, the control unit defocuses the electron beam until the target body becomes included in the irradiation area, to specify a position of the target body.
10. The X-ray generator according to claim 9, wherein the control unit controls the electron beam deflecting unit so as to scan the focused electron beam along a circular orbit corresponding to a profile of the irradiation area of the defocused electron beam including the target body.