JP2010205651A - Plasma generation method, and extreme ultraviolet light source device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To supply plasma generated by irradiation of an energy beam to a zone separated from a generating point of the plasma at a condition maintaining high density. <P>SOLUTION: A laser beam is concentrated and irradiated to a target 13 in a target chamber 11 in order to generate plasma from the target 13. The generated plasma is extended to a direction wherein the laser beam is irradiated, and the plasma is accelerated by supply to an acceleration cavity 21. The surface of the target 13 is arranged by shifting at an incident side rather than a light-concentrating point of the laser beam, and the plasma is transported to a zone separated from the generating point of the plasma while the plasma keeps high density at good directivity. Furthermore, a plasma generation method can be applied to an extreme ultraviolet light source device for generating extreme ultraviolet light (EUV) while the laser beam is irradiated to an extreme ultraviolet light emissive species arranged outside a discharge zone and the generated plasma is supplied to the discharge zone, and the plasma can be supplied to the discharge zone while it maintains the high density. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  In the present invention, plasma is generated by irradiating a target (raw material) with an energy beam, and the target material in a plasma state is supplied to a region to which an electric field is applied (for example, an acceleration region or a discharge region) in a high density state The present invention relates to a plasma generation method, and an extreme ultraviolet light source device that uses this plasma generation method to supply a source gas to a discharge region in a high-density state and discharge it to generate extreme ultraviolet light.

Conventionally, a technique for generating plasma by irradiating an energy beam to a target to generate plasma, sending the target material in a plasma state to a region to which an electric field is applied, and accelerating it has been studied. .
For example, in the field of cancer radiotherapy, a carbon plasma is generated by irradiating a solid target such as carbon placed in a vacuum with a laser beam, and this is supplied to a high-voltage stage to generate multivalent charges. It has been studied to extract ions, generate a strong ion beam, and irradiate it with cancer cells (see, for example, Non-Patent Documents 1 and 2).
Moreover, in patent document 1, it irradiates with an energy beam with respect to the raw material (target) supplied to the space except a discharge area | region, a plasma is generated, the target substance of a plasma state is sent to the discharge area | region to which an electric field is applied, An extreme ultraviolet light source device for generating extreme ultraviolet light has been proposed.

FIG. 1 and FIG. A conventional apparatus will be described with reference to FIG.
FIG. 6 shows FIG. 1 corresponds to FIG. FIG. 7 is a diagram showing a basic configuration of an apparatus for generating and accelerating plasma, which has been studied for cancer treatment, and FIG. 7 is an enlarged view of a target chamber portion of FIG. FIG.
As shown in FIG. 6, the plasma generating apparatus is composed of a target chamber 11 and a target accelerating radio frequency (RFQ) generator 20, and inside the target chamber 11, as shown in FIG. And a focusing mirror 14 that reflects and focuses the laser beam. The target chamber 11 is evacuated by an exhaust device (not shown).
A window 15 through which the laser beam passes is formed in the target chamber 11, and the laser beam is irradiated from outside the target chamber 11. The irradiated laser beam passes through the laser passing window 15, is reflected by the condensing mirror 14, and is condensed and irradiated onto the target 13 (carbon).

From the target 13 irradiated with the laser beam, plasma (carbon plasma in this example) is generated by the target material. The generated plasma has the property of spreading in the direction of irradiation with the laser beam.
An acceleration region (acceleration cavity) 21 having a radio frequency (RFQ) generator is formed in the direction in which the carbon plasma spreads. The generated carbon plasma is incident on the acceleration cavity 21 through the slit 23 of the isolator 22 formed on the incident side of the acceleration cavity 21. The carbon in the plasma state is accelerated by applying a high-frequency electric field from a high-frequency generator in the acceleration cavity 21, exits from the acceleration cavity 21, and is irradiated to, for example, cancer cells in the body.

JP 2008-53696 A

T.A. Takeuchi, et. al. Review of Instruments Vol. 73, no. 2, P764 (2002). T.A. Takeuchi, et. al. Review of Instruments Vol. 73, No, 2, P767 (2002).

As described above, the plasma generated by the irradiation of the laser beam which is an energy beam is supplied to the acceleration cavity 21 and accelerated. Since the target and the acceleration cavity are separated from each other, in order to efficiently use the carbon plasma generated from the target, that is, to make more carbon plasma enter the acceleration cavity 21, the carbon plasma is made as high as possible in the acceleration cavity 21. It must be supplied in a density state (localized).
However, since the carbon plasma generated by irradiating the target 13 with the laser beam expands freely, the density of the carbon plasma is 3 to 3 from the square of the distance from the plasma generation point, that is, the position where the target 13 is disposed. It decreases rapidly in proportion to the power.
Therefore, in order to allow more carbon to enter the acceleration cavity 21, the plasma generation point (position where the target 13 is disposed) and the acceleration cavity 21 must be brought close to each other.

However, a high voltage for generating a high frequency is applied to the high frequency generator 20 forming the acceleration cavity 21. Therefore, when the target 13 and the acceleration cavity 21 are brought close to each other, a dielectric breakdown may occur between the acceleration cavity 21 and the target 13 having the ground potential.
Therefore, the target 13 cannot approach the acceleration cavity 21 below a certain distance.
For this reason, in the carbon plasma generated by irradiating the laser beam, many components that do not enter the acceleration cavity 21 are generated, and the utilization efficiency of the carbon plasma cannot be increased.
The above description is a case where plasma is generated and supplied to an acceleration region (acceleration cavity) that is a region to which an electric field is applied. The generated plasma is supplied to a discharge region that is a region to which an electric field is applied. The same problem occurs in the extreme ultraviolet light source device that generates extreme ultraviolet light by heating.
The present invention has been made in view of the above circumstances, and in the present invention, the plasma generated by the irradiation of the energy beam is maintained in a desired region slightly away from the plasma generation point while maintaining a high density. An object is to enable supply to a certain region to which an electric field is applied.

As a result of intensive studies, the inventors generate an ablation plasma jet with good directivity when the surface of the target is arranged at a position shifted from the condensing point of the energy beam (for example, laser beam) to the incident side of the energy beam. I found that I can do it.
Thereby, it is possible to send the plasma to a position farther than in the past while maintaining a high density.
If the surface of the target is disposed at a position shifted to the incident side from the condensing point of the energy beam, it is considered that the plasma having good directivity is generated for the following reason.
This will be described with reference to FIG.
In general, when the target is irradiated with a laser beam (ii) having an intensity exceeding a certain threshold value, ablation plasma (iii) is ejected in the normal direction of the target surface (i).
At that time, a plasma (iii) having a high density gradient exists in the vicinity of the target surface 1 irradiated with the laser beam, and a critical density surface (electromagnetic wave is generated in the plasma (iii) with respect to the irradiated laser beam. Surface (iv) that cannot enter any more is formed.

As described above, when the surface (i) of the target is placed at a position shifted to the incident side from the condensing point and irradiated with the laser beam (ii), it has a concave-shaped (saddle-shaped) critical density surface (iv). Plasma (iii) is formed near the target surface (i).
Laser irradiation continues even after the concave critical density surface (iv) is formed, and the energy by the laser irradiation is absorbed by the plasma (iii) in the vicinity of the critical density surface (iv). Therefore, the plasma jet (v) is generated from the vicinity of the surface of the plasma (iii) forming the concave critical density surface (iv).
The plasma jet (v) is focused for generation in a direction perpendicular to the concave critical density surface.
Since the plasma moves while being focused by the concave surface, the plasma is transported far while maintaining high directivity and high density. The focusing distance depends on the radius of curvature of the saddle-shaped plasma.
When the target surface (i) is placed at the focal point of the laser beam (ii) or at a position shifted behind the focal point, the shape of the critical density surface of the plasma generated near the irradiation surface of the target is It will not be concave. For this reason, plasma having a higher directivity cannot be obtained as much as the case where the surface of the target is arranged at a position shifted to the incident side from the condensing point of the laser beam.

Based on the above, the present invention solves the above problems as follows.
(1) In a plasma generation method in which a plasma target is generated by irradiating a focused energy beam to a plasma target disposed outside a region to which an electric field is applied, and the generated plasma is sent to the region to which the electric field is applied. The surface of the plasma target is shifted to the incident side from the condensing point of the irradiated energy beam.
(2) A pair of discharge electrodes arranged opposite to each other, pulse power supply means for supplying pulse power to the discharge electrodes, extreme ultraviolet light radiation species arranged outside a discharge region by the discharge electrodes, and the extreme In an extreme ultraviolet light source device comprising an energy beam irradiation means for irradiating a focused energy beam to an ultraviolet light radiation species to generate plasma and starting discharge between the pair of discharge electrodes to which a voltage is applied The surface of the extreme ultraviolet light radiation species is disposed at a position shifted to the incident side from the condensing point of the energy beam.

In the present invention, the following effects can be obtained.
(1) Since the surface of the plasma target is shifted to the incident side from the focal point of the energy beam to be irradiated, the plasma generated by the energy beam (laser beam) is high up to a region farther than before. It can be sent while maintaining the density.
For this reason, the generated plasma can be sent to a region to which an electric field is applied and effectively used.
(2) In an extreme ultraviolet light source device that generates an extreme ultraviolet light by irradiating an extreme ultraviolet light radiation species arranged outside the discharge region to generate an energy beam and sending the plasma into the discharge region. Since the surface of the ultraviolet light radiation species is placed at a position shifted from the condensing point of the energy beam to the incident side, the generated plasma can be sent to the discharge region while maintaining a high density. In particular, extreme ultraviolet light can be generated.

It is a figure which shows the structural example of the target chamber of the plasma production apparatus of the Example of this invention. It is a figure explaining the reason why the plasma with good directivity is generated by setting the surface of the target to a position shifted to the incident side from the condensing point of the energy beam. It is a figure explaining the experiment example of this invention. It is a figure which shows the shape of the plasma which is an experimental result. It is a figure which shows the basic structural example of the EUV light source device of the Example of this invention. It is a figure which shows the basic composition of the conventional plasma generator. It is a figure which shows the target chamber of FIG.

FIG. 1 is a diagram showing a configuration example of a target chamber of a plasma generation apparatus according to an embodiment of the present invention, and the basic configuration is the same as that shown in FIG.
That is, the plasma generating apparatus is configured by the target chamber 11 and the high frequency (RFQ) generator 20 as shown in FIG.
As shown in FIG. 1, a target 13 (for example, carbon) supported by a target support 12 and a condensing mirror 14 for reflecting and condensing a laser beam are placed inside the target chamber 11. The target chamber 11 is evacuated by an exhaust device (not shown).
A window 15 through which the laser beam passes is formed in the target chamber 11, and the laser beam is irradiated from outside the target chamber 11.
The irradiated laser beam passes through the laser passing window 15, is reflected by the condensing mirror 14, and is condensed and irradiated onto the target 13. In this embodiment, compared with FIG. 7 shown in the conventional example, The surface of the target 13 (carbon) is arranged so as to be shifted to the incident side from the condensing point of the laser beam. Therefore, as described with reference to FIG. 2, the plasma is transported far away with good directivity and high density.
An acceleration region (acceleration cavity) 21 which is a region to which electrolysis is applied is formed in the direction in which the plasma spreads, and the generated carbon plasma is accelerated through the slit 23 of the isolator 22 while maintaining a high density. The light enters the cavity 21, is accelerated in the acceleration cavity 21, and exits from the acceleration cavity 21.

FIG. 3 is a diagram for explaining an experimental example of the present invention. In this experiment, as shown in FIG. 3, the wavelength (λ) emitted from the laser source 30 is 532 nm (twice the 1.06 μm YAG laser beam), the pulse energy is 60 mJ, the pulse width is 10 ns, and the diameter (D) is 10 mm. Was focused to 100 μm by a lens 31 having a focal length f of 600 mm.
Then, with respect to a tin (Sn) plate 33 having a thickness (t) of 3 mm, as shown in FIGS. 3 (a), (b) and (c), the surface thereof is arranged at the condensing point of the laser beam and at the front and rear thereof. The shape of the plasma generated by the CCD camera 32 was imaged.
4A shows a case where the surface of the tin plate 33 is arranged at a position 7 mm before the condensing point of the laser beam, FIG. 4B shows a case where the surface is arranged at the condensing point, and FIG. The shape of the plasma when placed 7 mm after the focal point.
The laser beam is irradiated from the upper side of each figure, and is indicated by a dotted line in the figure. Further, the hatched portion in the figure is a portion where the density of the generated plasma is high.
In the case of (a) in which the surface of the tin plate is disposed in front of the condensing point, the plasma generated by the laser beam irradiation converges in the direction of irradiation with the laser beam (upper side of the drawing), and the plasma density The high part is long. Therefore, the vaporized target material can be supplied to a far region while maintaining a high density.
On the other hand, in the case of (b) where the surface of the tin plate 33 is arranged at the condensing point, or in the case of (c) arranged after the condensing point, the high plasma density portion has a round shape. Therefore, the vaporized target material cannot be supplied far away while maintaining a high density.

When the target is irradiated with a laser beam to generate plasma (vaporize), the position where the power density of the laser beam is the highest is the focal point. For this reason, until now, it has been common knowledge to place the surface of the target at the condensing point of the laser beam, and it has been thought that plasma can be generated most efficiently if placed here.
However, in an apparatus using plasma, it is a means to generate plasma. For the final purpose, the generated plasma is applied to the next process (stage) in a high density state, that is, an electric field is applied. It is important to transport to an acceleration region or a discharge region, which is a region to be generated. The inventors focused on this point and reached the present invention.
By applying the present invention to the plasma generation apparatus shown in FIG. 1, carbon plasma can be supplied to the acceleration region in a high density state, and an ion beam can be generated effectively.

The plasma generation method can also be applied to an extreme ultraviolet light source device (hereinafter referred to as EUV light source device), and the case where the present invention is applied to an extreme ultraviolet light source device will be described below.
FIG. 5 shows a basic configuration example of the EUV light source apparatus according to the embodiment of the present invention.
In the figure, a first electrode 2a and a second electrode 2b are installed inside a chamber 1 which is a discharge vessel. The first electrode 2a is a cathode, the second electrode 2b is an anode, and the second electrode 2b is grounded. That is, a negative high voltage is applied between both electrodes.
A pulse power supply means 3 is connected to both the electrodes 2a and 2b. The pulse power supply means 3 employs, for example, a PFN (Pulse Forming Network) circuit system in order to flow a current having a long pulse width between both electrodes.
When a plasma raw material is fed between the electrodes 2a and 2b and a high voltage is applied between the electrodes 2a and 2b, a discharge is generated between the electrodes. A region where this discharge occurs is called a discharge region. The discharge is generated by applying an electric field, and the discharge region corresponds to the electric field application region.

Although near the pair of electrodes 2a and 2b, the high temperature plasma raw material 8 is installed outside the discharge region. For example, a metal such as tin (Sn) or lithium (Li) is used as the high temperature plasma raw material 8. These may be solid or liquid. FIG. 2 schematically shows an example in which the high-temperature plasma raw material 8 is a solid metal.
A laser source 7 is used to generate a low temperature plasma obtained by vaporizing the high temperature plasma raw material 8. The laser beam L1 emitted from the laser source 7 is condensed by the condenser lens 6, guided into the chamber 1, and irradiated onto the solid or liquid high-temperature plasma raw material 8.
The irradiation energy of the laser beam vaporizes the solid or liquid high-temperature plasma raw material but does not increase the electron temperature so much, for example, in the range of 10 ⁵ W / cm ² to 10 ¹⁶ W / cm ^2. .

When the high temperature plasma raw material 8 is irradiated with the laser beam, at least a part of the high temperature plasma raw material 8 is vaporized and ejected as a low temperature plasma gas 8 ′. By appropriately setting the conditions of the laser beam to be irradiated, for example, the high-temperature plasma raw material (low-temperature plasma gas 8 ′) continuously vaporized from the solid high-temperature plasma raw material 8 for a period of about 10 μs is ejected. It is sent to the discharge region between the electrode 2a and the second electrode 2b.
The electron temperature of the low-temperature plasma gas 8 ′ sent to the discharge region rises due to the discharge generated between the first electrode 2 a and the second electrode 2 b by the power supplied from the pulse power supply means 3 (20 ˜30 eV) to form high temperature plasma, and EUV light is emitted from this high temperature plasma. The emitted EUV light is reflected and collected by the EUV collector mirror 4 and emitted from the EUV extraction unit 5 to an irradiation unit (not shown).

If the ion density in the plasma is not within a predetermined range, for example, about 10 ¹⁷ to 10 ²⁰ cm ⁻³ , the high temperature plasma that efficiently emits EUV light is not easily generated. Therefore, the low temperature plasma generated by vaporizing the high temperature plasma raw material 8 must be supplied to the discharge region in a desired ion density range.
The reason why the high-temperature plasma raw material 8 is disposed outside the discharge region is that the following problems may occur when the high-temperature plasma raw material 8 is disposed within the discharge region.
(1) The temperature of the discharge electrode rises due to the drive current accompanying the progress of discharge. When the high temperature plasma raw material 8 is disposed at a position close to the electrode, the high temperature plasma raw material 8 is vaporized by the radiant heat from the electrode that has become high temperature, and the ion density of the plasma gas changes. It becomes difficult to control the plasma gas supplied to the discharge region within a desired ion density range.
(2) Furthermore, since the high temperature plasma raw material 8 is vaporized also by radiant heat from the high temperature plasma generated in the discharge region, it becomes more difficult to control the ion density of the plasma gas.

Therefore, in this embodiment, the high temperature plasma raw material 8 is arranged at a certain distance, for example, 1 mm or more from the discharge region (between the discharge electrodes). Therefore, the technology of the present invention is used so that low temperature plasma generated by the raw material 8 generated by the laser beam can be sent to the discharge region located at a distance of 1 mm or more while maintaining a high density.
That is, the surface of the high-temperature plasma raw material 8 is arranged so as to be shifted to the incident side of the laser beam from the position of the condensing point of the laser beam irradiated for vaporizing the high-temperature plasma raw material 8.
By configuring in this way, even if the high temperature plasma raw material 8 is separated from the discharge region by 1 mm or more, the low temperature plasma generated by the raw material 8 generated by laser irradiation reaches the discharge region (between the electrodes) while maintaining a high density. It is possible to obtain a high-temperature plasma that can be sent and efficiently emit EUV light.

DESCRIPTION OF SYMBOLS 1 Chamber 2a 1st electrode 2b 2nd electrode 3 Pulse power supply means 4 EUV condensing mirror 5 EUV light extraction part 6 Condensing lens 7 Laser source 8 High temperature plasma raw material 8 'Low temperature plasma gas 11 Target chamber 12 Target support body 13 Target 14 Condensing mirror 15 Window through which laser beam passes 20 High frequency (RFQ) generator 21 Acceleration cavity (acceleration region)
22 Isolator 23 Slit 30 Laser source 31 Lens 32 CCD camera 33 Tin (Sn) plate

Claims (2)

In a plasma generation method of generating plasma by irradiating a focused energy beam to a plasma target disposed outside a region to which an electric field is applied, and sending the generated plasma to a region to which the electric field is applied,
A plasma generation method, wherein the surface of the plasma target is at a position shifted from the condensing point of the irradiated energy beam toward the incident side. A pair of discharge electrodes arranged opposite to each other, pulse power supply means for supplying pulse power to the discharge electrodes, an extreme ultraviolet light radiation species disposed outside a discharge region by the discharge electrodes, and the extreme ultraviolet light radiation In an extreme ultraviolet light source device comprising energy beam irradiation means for irradiating a condensed energy beam to a seed to generate plasma and starting discharge between the pair of discharge electrodes to which a voltage is applied,
The extreme ultraviolet light source device, wherein the surface of the extreme ultraviolet light emitting species is disposed at a position shifted to the incident side from the condensing point of the energy beam.

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