JP2015179632A - ion source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ion source which enables the enhancement of the detection accuracy of impurity ions.SOLUTION: An ion source 10 comprises: a target 12 housed in a vacuum container 11 for generating plasma 14; a through-hole 15 through which an ion beam derived from the plasma 14 travels, the through-hole 15 being provided in the vacuum container 11; a branch hole 21 through which ions emitted from the target 12 toward a direction forming an angle with respect to the main axis X of the ion beam pass, the branch hole 21 being provided in the vacuum container 11; a detector 23 disposed on a travelling way 22 of ions after going through the branch hole 21; and a slit 24 disposed on the travelling way 22 upstream of the detector 23 and serving to reduce an ion-passage cross section.

Description

  The present invention relates to an ion source that generates plasma and extracts an ion beam.

An ion source using a laser condenses the laser to irradiate a solid target, and vaporizes and ionizes the target element by the energy of the laser to generate plasma.
Then, the generated plasma is transported to the entrance of the accelerator as it is, and then only ions are drawn into the accelerator due to a potential difference, and an ion beam is output.

On the other hand, the acceleration of ions in the accelerator is more excellent as the ion mass is smaller and the valence is larger. And it is known that the ion source using a laser is advantageous for generation of multivalent ions.
Further, if impurity ions other than the target ions are mixed in the plasma generated by the laser, there is a concern that the beam quality may be deteriorated.
Therefore, an interlock that stops the operation of the accelerator when the purity of the ion beam taken out from the ion source is measured and a decrease in purity is observed has been studied (for example, Patent Document 1).

JP 2013-187057 A

Examples of the contamination source that decreases the purity of the ion beam include impurity elements contained in the target, moisture adhering to the target, residual gas components in the laser irradiation atmosphere, and the like.
These impurity element ions, water-derived hydrogen ions and oxygen ions, and residual gas component ions are mixed into the ion beam as impurity ions and transported downstream as they are.
For this reason, it is necessary to monitor the purity in real time during the output operation of the ion beam, and to immediately stop the operation when a decrease in purity is observed.

However, when the mass-to-charge ratio between the target element ion (target ion) and the impurity ion is approximate, the detection peak of both ions is close, so that there is a problem that the separation and quantification of the impurity ion is lowered.
Therefore, in order to separate the adjacent detection peaks, it is considered to extend the flight distance of ions. However, in this case, the detection peak becomes broad, and the resolution of the detection peak may not be sufficiently improved.
For this reason, the above-described interlock function has a problem of reducing the stability of the ion beam output operation due to erroneous measurement of impurity ions.

  The present invention has been made in consideration of such circumstances, and an object thereof is to provide an ion source capable of improving the detection accuracy of impurity ions.

  In the ion source according to the present invention, a target that is contained in a vacuum vessel and generates plasma, a passage hole that is provided in the vacuum vessel and through which an ion beam extracted from the plasma passes, and provided in the vacuum vessel, A branch hole that allows ions emitted in a direction that forms an angle with respect to the main axis to the main axis; a detection unit that is disposed on a flight trajectory of the ions that have passed through the branch hole; and the flight that is before the detection unit And a slit arranged on the orbit to narrow the cross section of the ions.

  According to the present invention, an ion source capable of improving the detection accuracy of impurity ions is provided.

1 is a schematic cross-sectional view showing a first embodiment of an ion source according to the present invention. (A) The top view which shows the Example of a slit, (B) The top view which shows the other Example of a slit. (A) Graph showing the detection result of element ions contained in the ion beam as Comparative Example 1, (B) Graph showing the detection result of element ions contained in the ion beam as Comparative Example 2, (C) Ion beam as an example The graph which shows the detection result of the element ion contained in. The block diagram which shows 2nd Embodiment of the ion source which concerns on this invention. 6 is a timing chart showing the relationship between laser oscillation and shutter operation. (A) The top view which shows the Example of a shutter, (B) The longitudinal cross-sectional view which shows the other Example of a shutter. The schematic sectional drawing which shows 3rd Embodiment of the ion source which concerns on this invention. The schematic sectional drawing which shows the other Example of the ion source which concerns on 3rd Embodiment. The schematic sectional drawing which shows 4th Embodiment of the ion source which concerns on this invention. The schematic sectional drawing which shows 5th Embodiment of the ion source which concerns on this invention.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
As shown in FIG. 1, an ion source 10 according to the first embodiment includes a target 12 that is accommodated in a vacuum vessel 11 and generates plasma 14, and a passage through which an ion beam that is provided in the vacuum vessel 11 and is extracted from the plasma 14 passes. A hole 15, a branch hole 21 that is provided in the vacuum vessel 11 and allows ions emitted in a direction that forms an angle from the target 12 to the main axis X of the ion beam, and a flight trajectory 22 of ions that have passed through the branch hole 21 And a slit 24 that is arranged on the flight trajectory 22 in front of the detector 23 and narrows the cross section of ions.

As shown in FIG. 1, in the ion source 10, a target 12 is accommodated in a vacuum vessel 11, and the potential of the vacuum vessel 11 is set at the same level as the target 12.
A laser oscillation unit (not shown) is disposed outside the ion source 10, and the laser 13 incident on the inside through the transparent window irradiates the surface of the target 12.

The element of the target 12 is evaporated and ionized by the energy of the irradiated laser to generate the plasma 14.
The plasma 14 is in a state where the evaporated element of the target 12 is ionized into cations and electrons, and is electrically neutral as a whole.
When a carbon-based plate-like member is used as the target 12, the plasma 14 includes not only the desired polyvalent carbon ions but also impurity element ions, water-derived hydrogen ions and oxygen ions, and the vacuum vessel 11. The residual gas component ions are mixed as impurity ions.

The plasma 14 radially diffuses inside the vacuum vessel 11 along the main axis X of the ion beam extending in the vertical direction from the incident point where the laser 13 is incident on the target 12.
A passage hole 15 for plasma 14 is provided on the wall of the vacuum vessel 11 where the main axis X of the ion beam intersects.

  As a method for generating the plasma 14, the embodiment discloses a method for irradiating the target 12 with the laser 13. However, the present invention is not limited to this. There is a method of causing discharge.

A plasma transport tube 19 having the passage hole 15 as an open end is provided so that the main axis X of the ion beam is a concentric axis and further has the same potential as the vacuum vessel 11.
The plasma 14 is guided to the inside of the linear accelerator 17 by the plasma transport tube 19 without being diffused.

The vacuum vessel 11 and the linear accelerator 17 are connected by a communication path 16 that is an insulator, and the plasma transport tube 19 is arranged so as to coincide with the central axis of the communication path 16.
In this way, the plasma 14 generated in the ion source 10 passes through the communication path 16 and is introduced into the linear accelerator 17.

Since the ion source 10 and the linear accelerator 17 have different potentials, the positive ions from which electrons are separated from the plasma 14 introduced into the linear accelerator 17 are introduced into an acceleration channel (not shown) and accelerated to form an ion beam.
The ion beam that has been increased in energy from the ion source 10 in this way is used, for example, in a heavy particle beam irradiation apparatus used for cancer treatment.

The vacuum vessel 11 is provided with a branch hole 21 through which ions emitted in a direction that forms an angle with the main axis X of the ion beam from the target 12 pass.
The branch hole 21 guides the plasma 14 diffused inside the vacuum vessel 11 to the outside. The ions introduced from the branch hole 21 to the outside of the vacuum vessel 11 are guided to the analysis unit 20 that performs qualitative / quantitative analysis of the ion species.

  Since the composition of the ion species of the ions extracted from the branch hole 21 is the same as the composition of the ion species of the ion beam extracted from the passage hole 15, the analysis unit 20 monitors the ion species included in the ion beam in real time. Is possible.

The plasma 14 that diffuses radially decreases in ion current density as the angle of the ion beam with respect to the principal axis X increases.
For this reason, in order to ensure the accuracy of qualitative and quantitative analysis of ion species, it is desirable to provide the branch hole 21 at a position close to the passage hole 15.

However, the potential of the analysis unit 20 is the same as that of the vacuum vessel 11 and is set to a high potential, whereas the potential of peripheral devices such as the linear accelerator 17 is set to the ground level.
For this reason, when the positions of the passage hole 15 and the branch hole 21 are close, the analysis unit 20 and peripheral devices (such as the linear accelerator 17) may be electrically short-circuited.
Under such various constraints, the position of the branch hole 21 and the layout of the analysis unit 20 are determined by optimization.

  The analysis unit 20 secures the port 25 integrally formed with the vacuum vessel 11 so that the ion flight trajectory 22 coincides with the central axis with the branch hole 21 as an open end, and the flight trajectory 22 from the port 25 to obtain the ion. A guide tube 26 that guides the detection unit 23, a cartridge 27 that supports the detection unit 23 and accommodates an electrical system (not shown), and the guide tube 26 and the cartridge 27 are detachably connected and each internal space is evacuated. A blocking unit 28 for blocking and a slit 24 arranged on the flight trajectory 22 in front of the detection unit 23 and narrowing the cross section of ions are configured.

A structure in which the guide tube 26 and the blocking portion 28 are omitted and the cartridge 27 is directly connected to the port 25 of the vacuum vessel 11 may be employed.
Further, the potential levels of the guide tube 26, the blocking unit 28, and the cartridge 27 are set to the same potential level as that of the vacuum vessel 11 (and the port 25).
Alternatively, at least a part of the guide pipe 26 and the blocking part 28 may be made of an insulating material, and the downstream side may be set to the ground level.

Examples of the detection unit 23 include a Faraday cup, Q-Mass, a Wien filter, q / m analysis using an electric field or a magnetic field, and the like.
Alternatively, a detector having a signal amplification function such as a microchannel plate, a secondary electron multiplier, a photomultiplier, or the like can be used as the detection unit 23.

In the microchannel plate and the secondary electron multiplier, the ion current is amplified as it is and output as a signal. In the case of using a photomultiplier tube, ions are made incident on the scintillator, and the fluorescence or phosphorescence is converted into an electric signal by the photomultiplier tube, amplified and output.
By using the detection unit 23 having such a signal amplification function, detection sensitivity can be improved even when the number of ions that have reached is small.

  As shown in the plan view of FIG. 2A, the slit 24 is provided with a circular opening 29a (29) having a smaller area than the cross-section of ions in the branch hole 21 (or port 25). The shape of the opening 29 of the slit 24 is not particularly limited, and may be a rectangular opening 29b (29) as shown in FIG.

When ions are extracted from the plasma 14 (FIG. 1), the same charged particle collective motion is generated, so that ions are diffused by the space charge effect.
By arranging the slit 24 so that the opening 29 penetrates the ion flight trajectory 22 (FIG. 1), the total amount of charge of the ion population that partially passes the ion population and is directed to the detection unit 23 (FIG. 1). By reducing, the space charge effect can be suppressed and the divergence of ions can be suppressed.
Thereby, ions flying off the flight trajectory 22 are suppressed by the slit 24 and the probability of reaching the detection unit 23 is reduced.

Based on FIG. 3, the effect of the slit 24 (FIG. 1) will be described.
The analysis unit 20 (FIG. 1) performs qualitative / quantitative determination of the composition of ion species constituting the plasma 14 by measuring the flight time of ions flying along the flight trajectory 22.
Specifically, the ion species having a larger mass-to-charge ratio (q / m) has a shorter flight time, and a signal is output from the detection unit 23 at an earlier timing.

Therefore, as shown in Comparative Example 1 in FIG. 3A, detection peaks of ion species having mass-to-charge ratios (q / m) close to each other may be close to each other in time and may be difficult to separate. .
In such a case, as shown in Comparative Example 2 in FIG. 3B, the detection peak can be separated in time by extending the distance of the flight trajectory 22.

However, even if the distance of the flight trajectory 22 increases, the number of ions detected by flying off the center of the flight trajectory 22 increases and the peak shape becomes broad, so the resolution of the detected peak does not improve as expected. .
Ions flying off the center of the flight trajectory 22 have a different flight time. When the distance of the flight trajectory 22 increases, the distribution of such ions in flight time widens, so that the shape of the detection peak becomes broad.

  Therefore, as shown in the embodiment of FIG. 3C, by introducing the slit 24, the number of ions detected by flying off the flight trajectory 22 can be reduced, and the peak shape can be sharpened. . Thereby, the resolution of the detection peak can be improved.

(Second Embodiment)
Next, a second embodiment of the present invention will be described based on FIG.
The ion source 10 of the second embodiment includes a shutter 30 on the flight trajectory 22 that blocks light emission that reaches before the ions fly by an opening / closing operation.
In FIG. 4, the shutter 30 is shown as being provided close to the slit 24, but the position where the shutter 30 is provided is not particularly limited, and if it is on the flight path 22, the port 25, the guide tube 26 and the cartridge 27 can be provided.
4, parts having the same configuration or function as those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

The timing chart of FIG. 5 shows the relationship between laser oscillation and shutter operation.
As shown in the upper part of FIG. 5, the laser 13 is composed of laser pulses 13a, 13b, and 13c that are intermittently oscillated at regular intervals.
When the target 12 is irradiated with one laser pulse 13a, ions emitted from the target 12 fly along the flight trajectory 22 until the next laser pulse 13b is irradiated, and enter the detection unit 23. Detected.
Therefore, the graph shown in FIG. 3 is updated at the same time interval as the oscillation period T of the laser oscillation.

Incidentally, when the target 12 is irradiated with the laser pulse 13a, not only ion emission but also ablation light is emitted.
When this ablation light enters the detection unit 23, noise is generated in the waveform of the graph of FIG. 3, and the S / N ratio of the detection peak is lowered.

  Therefore, as shown in the lower part of FIG. 5, the shutter 30 is closed to block the ablation light during a period from when the laser pulses 13a, 13b, and 13c are irradiated until the ions fly to the detection unit 23. Thereafter, the shutter 30 is opened, and when all the ions having different mass-to-charge ratios (q / m) have finished flying, the shutter 30 is returned to the closed state again.

The opening / closing operation of the shutter 30 can be performed in synchronization with the oscillation period T of the laser pulses 13a, 13b, 13c, or can be performed based on a threshold value provided for the detection value of the ablation light.
As a result, the S / N ratio of the peak can be improved, and the ion species can be detected with high sensitivity.

As shown in the plan view of FIG. 6 (A), the shutter 30 according to the embodiment has windows 31 (31a, 31b) that coincide with the opening 29 of the slit 24 in order by the rotational movement about the rotation shaft 32. , 31c).
By appropriately adjusting the rotation speed and phase of the rotating shaft 32, the shutter 30 can be opened and closed in synchronization with the oscillation period T of the laser pulses 13a, 13b, and 13c.

As shown in the longitudinal sectional view of FIG. 6B, the shutter 30 according to another embodiment includes a cylindrical body 33 that shields diffusion of ablation light that has passed through the opening 29 of the slit 24, and the interior of the cylindrical body 33. And a valve body 34 that rotates on a rotation axis substantially orthogonal to the flight trajectory 22.
By appropriately adjusting the rotational speed and phase of the valve body 34, the shutter 30 can be opened and closed in synchronization with the oscillation period T of the laser 13.

In addition, the drive system of the shutter 30 is not limited to the form shown as an Example, For example, it is also realizable by reciprocating a shield (not shown) linearly.
In order to eliminate the influence of the ablation light described above, a paint that absorbs the ablation light is applied to at least some of the inner surfaces of the vacuum vessel 11, the port 25, the guide tube 26, the cartridge 27, and the blocking portion 28. May be. Or you may use the material which has the absorption characteristic of ablation light for the constituent material of the inner surface of these members.
Thereby, since the ratio of the ablation light which repeats reflection and reaches | attains the detection part 23 among the ablation light generate | occur | produced in the target 12 can be reduced, the noise superimposed on a signal waveform can be reduced.

(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIG.
In the ion source 10 of the third embodiment, a plurality of slits 24 (24 a, 24 b) are arranged on the flight path 22.
In FIG. 7, each of the slits 24 (24 a, 24 b) shows a mode of being provided inside the guide tube 26, but is not particularly limited. 28 and the cartridge 27 can be provided.

In FIG. 7, two slits 24 (24a, 24b) are arranged on the downstream side and the upstream side, respectively, but the number is not particularly limited.
7 that have the same configuration or function as those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

As a result, ions flying off the flight trajectory 22 are suppressed by the slit 24, and the probability of reaching the detection unit 23 is further reduced.
Therefore, the peak shape can be sharpened, and the resolution of the detected peak can be further improved.

FIG. 8 is a block diagram showing another example of the ion source according to the third embodiment.
In this embodiment, the first detector 23a is composed of a scintillator 41 that emits scintillation light when ions enter between the two slits 24a and 24b, and a photomultiplier tube 42 that detects the scintillation light. Is provided, and a second detection unit 23b is provided further downstream.

When the thickness of the scintillator 41 is thinner than the range of ions, the ions pass through the scintillator 41 and are transported further downstream.
The ions that have passed through the scintillator 41 diverge and travel toward the second detection unit 23b downstream. However, the presence of the slit 24b in the subsequent stage restricts the passage of ions and suppresses divergence.
Thereby, both the 1st detection part 23a and the 2nd detection part 23b can output the detection peak which is excellent in resolution | decomposability.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described based on FIG.
The ion source 10 of the fourth embodiment further includes an electrostatic deflector 43 that imparts a curvature to the flight trajectory 22 of ions that have passed through the branch hole 21.
9, parts having the same configuration or function as those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

The electrostatic deflector 43 deflects the flight trajectory 22 by applying an electric field to the flight trajectory 22 of ions extracted from the branch hole 21.
The ions flying along the flight trajectory 22 having a curvature enter the detection unit 23.

With this configuration, the layout of the analysis unit 20 is more flexible, and the analysis unit 20 is less likely to be electrically short-circuited with peripheral devices (such as the linear accelerator 17) set at a low potential. be able to.
Further, by providing the flight trajectory 22 with a curvature, the flight trajectory 22 can be made longer without increasing the installation area of the ion source 10.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described with reference to FIG.
The ion source 10 of the fifth embodiment further includes an extension mechanism 44 that extends the flight trajectory 22 of ions that have passed through the branch hole 21.
10, parts having the same configuration or function as in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

By adopting a bellows as the extension mechanism 44, it is possible to make the length variable while maintaining the vacuum state.
Thus, by extending the ion flight trajectory 22, the space charge effect of ions can be suppressed, and the detection peaks can be separated in time by extending the flight distance of ions.

  According to the ion source of at least one embodiment described above, it is possible to improve the detection accuracy of impurity ions by disposing the slit for narrowing the passage cross section on the flight trajectory of ions.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, changes, and combinations can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 10 ... Ion source, 11 ... Vacuum container, 12 ... Target, 13 ... Laser, 13a, 13b, 13c ... Laser pulse, 14 ... Plasma, 15 ... Pass-through hole, 16 ... Communication path, 17 ... Linear accelerator, 19 ... Plasma transport Pipes, 20 ... analyzing part, 21 ... branch hole, 22 ... flight trajectory, 23 (23a, 23b) ... detecting part, 24 (24a, 24b) ... slit, 25 ... port, 26 ... guide pipe, 27 ... cartridge, 28 ... Blocking part, 29 (29a, 29b) ... Opening, 30 ... Shutter, 31 (31a, 31b, 31c) ... Window, 32 ... Rotating shaft, 33 ... Cylinder, 34 ... Valve, 41 ... Scintillator, 42 ... Photoelectron Multiplier tube, 43 ... Electrostatic deflector, 44 ... Extension mechanism, X ... Main axis of ion beam.

Claims (5)

A target that is contained in a vacuum vessel and generates plasma;
A through hole provided in the vacuum vessel and through which an ion beam extracted from the plasma passes;
A branch hole that is provided in the vacuum vessel and allows ions emitted in a direction that forms an angle from the target to the main axis of the ion beam;
A detector disposed on a flight trajectory of ions that have passed through the branch hole;
An ion source comprising: a slit that is arranged on the flight trajectory before the detection unit and narrows a passage cross section of the ions.   2. The ion source according to claim 1, further comprising: a shutter that interrupts light emission that reaches before the ions fly on the flight orbit by an opening / closing operation.   The ion source according to claim 1, wherein a plurality of the slits are arranged on the flight trajectory.   The ion source according to claim 1, further comprising an electrostatic deflector that imparts a curvature to a flight trajectory of ions that have passed through the branch hole.   The ion source according to any one of claims 1 to 4, further comprising a mechanism for extending a flight trajectory of ions that have passed through the branch hole.

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