JP2017069126A - Electrode structure of plasma light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode structure of a plasma light source capable of achieving sufficient filling of a plasma medium.SOLUTION: An electrode structure of a plasma light source comprises: a pair of coaxial electrodes including a center electrode 12 extending on a single axis line and an external electrode and arranged so as to face each other across a symmetric surface 1; a reservoir 20 supporting the center electrode 12 and including a filling opening 22 communicating with a space 20a for reserving the plasma medium; and a sealing member 24 for sealing the filling opening 22. An inner surface of the filling opening 22 includes an annular step part 22c. A tip of the sealing member 24 abuts on the step part 22c over the whole region in a circumferential direction of the step part 22c. The reservoir 20 and the sealing member 24 are formed of metal or alloy with low wettability of the plasma medium.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an electrode structure of a plasma light source that generates extreme ultraviolet light.

  In order to further miniaturize semiconductor circuits, extreme ultraviolet light has attracted attention as next-generation irradiation light in photolithography. As a method for generating extreme ultraviolet light, a method using high energy density plasma is known. Specifically, a discharge generated plasma (DPP) method using pulse discharge and a laser generated plasma (LPP) method using pulse laser irradiation are known. In any of the above methods, the generated light is pulsed light.

  In photolithography, control of the exposure time is extremely important. Therefore, it is necessary not only to ensure sufficient intensity and luminance of extreme ultraviolet light, but also to obtain these stably. Further, since the emission time of extreme ultraviolet light is as short as several tens of μs, it is necessary to repeat the generation of plasma at a high speed.

  The extreme ultraviolet light source device of Patent Document 1 generates a discharge with a period of 10 kHz between a pair of disk-shaped discharge electrodes, and irradiates the discharge electrodes with laser light in synchronization with the discharge. Discharge occurs between the periphery of each discharge electrode. On the other hand, the laser light irradiates tin applied to the periphery, and vaporizes tin. That is, the laser beam generates tin ablation. Tin vaporized by ablation is taken into the discharge between the discharge electrodes and grows into plasma. The electrical energy applied to the discharge electrode heats the plasma. As a result, ionization of tin proceeds and extreme ultraviolet light is emitted from the high temperature plasma.

  The plasma light source of Patent Document 2 includes a pair of coaxial electrodes configured by a rod-shaped center electrode and an external electrode provided so as to surround the outer periphery thereof. The coaxial electrodes are arranged to face each other with a plane of symmetry. The plasma light source of Patent Document 2 also emits a plasma medium (that is, a raw material) between the center electrode and the external electrode by ablation, and generates a discharge between these electrodes to generate a medium plasma. While the plasma in each coaxial electrode grows by electric energy applied to each coaxial electrode, it proceeds to a symmetry plane by a self magnetic field or the like, and then coalesces on the symmetry plane. The combined plasma is further increased in density and temperature by the pinch effect. As a result, the ionization of the medium proceeds, and extreme ultraviolet light is emitted from the high temperature plasma.

  In the plasma light source of Patent Document 2, a low melting point metal such as lithium (Li) is used as a plasma medium (hereinafter referred to as plasma medium). The low melting point metal is pre-filled and stored in a reservoir that supports the center electrode. The low melting point metal is melted by the heat of the heater that heats the reservoir. The molten low melting point metal is supplied to the porous medium holding part via the supply path in the center electrode. The medium holding part is provided on the side surface of the center electrode, and is heated by the above heater together with the center electrode. Therefore, the low melting point metal is held by the medium holding part, and a part of the low melting point metal exudes from the medium holding part. The extruding low melting point metal is irradiated with laser light for ablation.

JP 2010-232150 A JP 2014-235887 A

  A plasma medium such as lithium (Li) is easily oxidized. For this reason, when filling the reservoir with a plasma medium that is easily oxidized, a container filled with an inert gas such as a rare gas is prepared, and the filling operation is performed therein.

  The form of the plasma medium during the filling operation is a particle or a small piece. Therefore, when the reservoir is filled with the plasma medium, a slight gap is generated in the reservoir. The gas remaining in the gap becomes bubbles when the metal is melted, and is irregularly discharged from the medium holding portion which is the only outlet of the supply path. The discharge of the bubbles causes the ejection or scattering of the plasma medium, and adheres to a part other than the medium holding part. As a result, unexpected phenomena such as abnormal discharge and excessive supply of the plasma medium occur. That is, the operation of the plasma light source becomes unstable.

  Residual gas emissions occur frequently, particularly in the early stages of melting of the plasma medium. That is, the plasma light source cannot be operated while this emission occurs frequently. That is, it takes a long time for the plasma light source to operate stably. Therefore, in order to suppress the discharge frequency of the residual gas as much as possible, it is necessary to sufficiently fill the plasma medium in the reservoir.

  Therefore, an object of the present invention is to provide an electrode structure of a plasma light source that can be sufficiently filled with a plasma medium.

  One aspect of the present invention is an electrode structure of a plasma light source, which includes a rod-shaped center electrode extending on a single axis and an external electrode provided so as to surround the outer periphery of the center electrode, and sandwiching a plane of symmetry. A pair of coaxial electrodes arranged opposite to each other to generate plasma that emits extreme ultraviolet light and confine the plasma, and are individually provided for each coaxial electrode, support the central electrode, and provide a medium for the plasma And a reservoir having a filling port communicating with the space, and an enclosing member for sealing the filling port of the reservoir, wherein the central electrode of each coaxial electrode communicates with the reservoir, and the center A supply path for the medium that opens as a supply port for the medium at a position irradiated with laser light for ablating the medium on the side surface of the electrode; The inner surface of the filling port in the reservoir has an annular stepped portion, and the tip of the sealing member abuts on the stepped portion over the entire circumferential direction of the stepped portion, and the reservoir and the sealing member are The gist is that the medium is formed of a metal or alloy having low wettability.

  The filling port may be provided on a back surface of the reservoir, and may be positioned on an extension line of the supply path along the central axis.

  The reservoir and the sealing member may be made of copper or stainless steel.

  ADVANTAGE OF THE INVENTION According to this invention, the electrode structure of the plasma light source which can fully be filled with a plasma medium can be provided.

It is a schematic block diagram (sectional drawing) of a plasma light source provided with the electrode structure which concerns on one Embodiment of this invention. It is a figure which shows the II-II cross section of FIG. It is a figure which shows the electric system of the plasma light source which concerns on one Embodiment of this invention. It is a schematic block diagram (sectional drawing) of the electrode structure which concerns on one Embodiment of this invention. It is an image which shows an example of the time-dependent change of initial stage discharge. It is an image which shows an example of the time-dependent change of initial stage discharge. It is a figure which shows a mode that a low melting metal flows through the center electrode which concerns on one Embodiment of this invention.

  Hereinafter, a plasma light source including an electrode structure according to an embodiment of the present invention will be described with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure, and the overlapping description is abbreviate | omitted.

  FIG. 1 is a schematic configuration diagram (cross-sectional view) of a plasma light source having an electrode structure according to the present embodiment, FIG. 2 is a diagram showing a II-II section of FIG. 1, and FIG. 3 is a diagram showing an electrical system of the plasma light source FIG. 4 is a schematic configuration diagram (cross-sectional view) of the electrode structure according to the present embodiment. As shown in these drawings, the plasma light source includes a pair of coaxial electrodes 10 and 10 as an electrode structure according to the present embodiment. The plasma light source further includes a reservoir 20, a voltage application device 30, and a laser device 40 that are individually provided for each coaxial electrode 10. In FIG. 1, the right coaxial electrode 10 has the same configuration as the left coaxial electrode 10, and thus detailed illustration is omitted.

  The pair of coaxial electrodes 10 and 10 are installed at positions symmetrical to each other with respect to the symmetry plane 1 in a vacuum chamber (not shown). Specifically, it is installed at a certain interval across the symmetry plane 1, and the tip side (side from which the planar discharge 2 b is emitted) is opposed to each other. The coaxial electrodes 10 and 10 generate a plasma 3 including a plasma medium (hereinafter referred to as a plasma medium) 6 and confine the plasma 3 therebetween. The plasma 3 confined between the coaxial electrodes 10 and 10 is heated and emits plasma light 8 including extreme ultraviolet light.

  The plasma medium 6 of this embodiment is a low-melting-point metal (low-melting-point alloy) that can be supplied from the reservoir 20 to a supply port 18a (described later) of the supply path 18, and its composition is selected according to the wavelength of the necessary ultraviolet rays. Is done. For example, when 13.5 nm ultraviolet light is required, lithium (Li) or tin (Sn) is included, and when 6.7 nm ultraviolet light is required, bismuth (Bi) is included.

  Each coaxial electrode 10 includes a center electrode 12, a plurality of external electrodes 14 provided so as to surround the outer periphery of the center electrode 12, and an insulator 16. As shown in FIGS. 1 and 2, the center electrode 12 has a single axis ZZ common to the coaxial electrodes 10 as a central axis (hereinafter, this axis is referred to as the central axis Z). It is a rod-shaped conductor extending on Z, and its diameter is, for example, 5 mm. For example, as shown in FIG. 1, the center electrode 12 is formed in a substantially conical shape having an apex angle on the symmetry plane 1. That is, the diameter of the center electrode 12 becomes smaller toward the symmetry plane 1. The center electrode 12 is formed using a material having heat resistance against high temperature plasma. Such a material is, for example, a refractory metal such as tungsten or molybdenum.

  The center electrode 12 includes a front end portion 12 a facing the symmetry plane 1, a side surface 12 b formed around the center axis Z, and a supply path 18 for the plasma medium 6 provided inside the center electrode 12. The end of the center electrode 12 opposite to the tip 12a is attached to the reservoir 20 and joined to the reservoir 20 by electron beam welding or the like. Thereby, thermal contact between the two is ensured, and leakage of the plasma medium 6 is prevented.

  The distal end portion 12 a has a hemispherical curved surface facing the symmetry plane 1. However, the shape of the surface facing the symmetry plane 1 is not limited to a curved surface, and may be a simple plane. Moreover, you may provide the recessed part (not shown) recessed along the central axis Z. As shown in FIG.

  As shown in FIGS. 1 and 2, the supply path 18 communicates with the reservoir 20 and opens on the side surface 12 b of the center electrode 12. Specifically, the supply path 18 extends along the central axis Z from the reservoir 20 side in the center electrode 12, and further branches outward in the radial direction according to the number of supply ports 18a. An opening is formed on the side surface 12 b of the center electrode 12. A plurality of supply ports 18 a are formed on the side surface 12 b of the center electrode 12 and function as supply points for supplying the plasma medium 6 between the center electrode 12 and the external electrode 14. Each supply port 18 a is located at a position that opposes the portion G (see FIG. 2) of the external electrode 14 and includes the irradiation position of the laser light 42. The supply port 18a of the present embodiment is formed at two locations across the central axis Z.

  As described above, the supply path 18 communicates with the reservoir 20. Accordingly, the plasma medium 6 is supplied to the supply port 18a through the supply path 18. At this time, the surface (exposed surface) of the plasma medium 6 is located at the same height as the side surface 12 b of the center electrode 12. That is, the supply port 18 a is filled with the plasma medium 6. Alternatively, the surface of the plasma medium 6 may be about 1 mm to several mm lower than the side surface 12 b of the center electrode 12. Note that the “same height” here means “substantially the same height”. That is, the “same height” includes a relative position error caused by the curvature due to the surface tension of the plasma medium 6, the fluctuation of the plasma medium 6 during discharge, the wettability of the plasma medium 6 with respect to the center electrode 12, and the like. .

  The inner diameter of the supply port 18a is set to a value at which the plasma medium 6 can remain in the supply port 18a by its surface tension and the laser beam 42 does not hit the side surface 12b around the supply port 18a. Such a value is several times the beam diameter (several hundred μm) at the irradiation position of the laser light 42, for example, 1.5 mm.

  As shown in FIG. 1, the center electrode 12 has at least one vent hole 19 penetrating from the inner surface of the supply path 18 to the side surface 12 b of the center electrode 12. The vent hole 19 is provided on the side opposite to the symmetry plane 1 across the supply port 18a (that is, the reservoir 20 side when viewed from the supply port 18a). For example, as shown in FIG. 1, the central electrode 12 is provided in a region between the supply port 18 a and the insulator 16 on the side surface 12 b. As will be described later, the vent hole 19 is provided to remove bubbles in the supply path 18. Foam tends to accumulate on the upper side in the supply path 18. In order to promote the discharge of the bubbles, the vent hole 19 opens upward with respect to the horizontal plane in a state where the coaxial electrode 10 is installed in the vacuum chamber. However, even if the vent hole 19 is opened in another direction, the bubble can be removed depending on the size or position of the bubble.

  The inner diameter of the vent hole 19 is smaller than the inner diameter of the supply path 18 (including the supply port 18a). The inner diameter of the vent hole 19 is set to a value that prevents the plasma medium 6 from entering the vent hole 19 due to its surface tension. For example, if the plasma medium 6 is lithium, the material of the center electrode 12 is tungsten, and the inner diameter of the supply port 18a is 1.5 mm, the inner diameter of the vent hole 19 is set to about 0.8 mm. However, this value is set in consideration of the difficulty of machining with respect to tungsten. Therefore, the inner diameter of the vent hole 19 may be smaller as long as there is no hindrance to machining and gas flow is possible. From the viewpoint described above, the inner diameter of the vent hole 19 is preferably 0.1 mm to 0.8 mm. Since the inner diameter of the supply path 18 is larger than the inner diameter of the vent hole 19 and the inner diameter of the vent hole 19 is set to a value that satisfies the above-described condition, the plasma medium 6 is preferentially sent from the reservoir 20 to the supply port 18a. As a result, the plasma medium 6 does not enter the vent hole 19.

  As shown in FIG. 1, the external electrode 14 is a rod-shaped conductor that extends in a direction inclined with respect to the central axis Z along the side surface 12b of the central electrode 12, and has a diameter of, for example, 3 mm. Further, as shown in FIG. 2, a plurality of the electrodes are arranged at each angle θ along the circumferential direction of the center electrode 12 while being separated from the center electrode 12 by a predetermined distance (for example, 2.5 mm). In other words, each external electrode 14 surrounds the center electrode 12. In the example shown in FIG. 2, six external electrodes 14 are arranged every 60 ° around the center electrode 12.

  As shown in FIG. 2, each external electrode 14 has a cross section including only one point G where the distance from the side surface 12 b of the center electrode 12 is the smallest in a plane perpendicular to the axial direction. The cross section having such a shape is, for example, a circle. However, the cross-sectional shape of the external electrode 14 is not limited to a circular shape, and it is sufficient that at least the surface facing the center electrode 12 has a curved surface protruding toward the center electrode 12. In any case, the part G is arranged around the center electrode 12 at every angle θ. Further, each external electrode 14 may be formed so that the distance between the portion G on the plane orthogonal to the central axis Z and the side surface 12b of the central electrode 12 is constant, and becomes smaller toward the symmetry plane 1. It may be formed as follows.

  It is desirable that the external electrodes 14 be installed at equal intervals around the center electrode 12. For example, from the viewpoint of processing and assembly, each external electrode 14 is preferably installed at a rotationally symmetric position with respect to the center electrode 12. However, the present invention is not limited to such an arrangement. Further, the number of external electrodes 14 is not limited to six, and is appropriately set according to the size and shape of the center electrode 12 and the external electrode 14, the distance between them, and the like. The external electrode 14 may be a cylindrical (tubular) electrode having the central electrode 12 as a central axis. However, considering the ease of formation of the planar discharge 2b (described later), it is desirable that the external electrode 14 is composed of a plurality of rod-shaped electrodes.

  The external electrode 14 is formed using a conductive material having heat resistance against high-temperature plasma, like the center electrode 12. Further, the end face of the external electrode 14 facing the symmetry plane 1 may be either a curved surface or a flat surface.

  By arranging the plurality of external electrodes 14 around the center electrode 12 in this way, an initial discharge 2a (see FIG. 1) reaching the planar discharge 2b is generated between each external electrode 14 and the center electrode 12. be able to. That is, it is possible to generate the initial discharge 2a over the entire circumference of the center electrode 12 by preferentially generating the initial discharge 2a including each part G in the discharge path. Formation becomes easy.

  The insulator 16 is formed using, for example, ceramic, supports the base portions of the center electrode 12 and the external electrode 14, defines the distance between them, and electrically insulates between them. The insulator 16 is formed in a disk shape, for example, and has a structure such as a hole or a groove that supports the center electrode 12 and the external electrode 14.

  The reservoir 20 is individually provided for each coaxial electrode 10. As shown in FIG. 1, the reservoir 20 supports the base of the center electrode 12 and stores the plasma medium 6 in a space 20a formed therein. This space 20 a communicates with the supply path 18. The reservoir 20 may be formed in a cylindrical shape symmetric with respect to the central axis Z or an axis parallel to the central axis Z, or may be formed as a hexahedron such as a rectangular parallelepiped. In any shape, the reservoir 20 (the outer surface of the reservoir 20) is at least perpendicular to the central axis Z or an axis parallel to the central axis Z, and a plane where the central electrode 12 is joined to the plane with the space 20a interposed therebetween. And a back surface 20c located on the opposite side. When lithium is assumed as the plasma medium 6, the material of the reservoir 20 is copper (Cu). However, the material of the reservoir 20 may be any metal (alloy) that has high durability (corrosion resistance) and low wettability of the plasma medium 6, and is not limited to copper. For example, when the plasma medium 6 is tin, stainless steel may be used as the material of the reservoir 20.

  The reservoir 20 is provided with a heater 23. The heater 23 is composed of, for example, a heat medium (oil) circulation heater or an electrothermal heater, and is fixed to the outer surface of the reservoir 20 so as to ensure thermal contact. The heater 23 melts the plasma medium 6 in the reservoir 20 and also melts the plasma medium 6 in the center electrode 12 (supply path 18). Accordingly, the plasma medium 6 flows in the liquid state through the supply path 18 and is supplied to the supply port 18a. Further, the plasma medium 6 in the supply port 18a is held in a molten state.

  As shown in FIGS. 1 and 4, the reservoir 20 has a pressurized gas inlet 21. The introduction port 21 communicates from the upper surface 20b of the reservoir 20 to the space 20a. The introduction port 21 is connected to the gas introduction pipe 52 from the pressurizing device 50 by connection parts using a metal O-ring or brazing. The pressurizing device 50 pressurizes the space 20a of the reservoir 20 with a pressure of about several tens of Torr using an inert gas such as a rare gas, and promotes the flow of the molten plasma medium 6 to the supply port 18a. The pressurizing device 50 includes, for example, an inert gas cylinder, a regulator attached to the cylinder, a valve whose inlet side is connected to the outlet of the regulator, and the pressure in the gas introduction pipe 52 connected to the outlet side of the valve. And a pressure gauge to measure. In addition, when the flow of the plasma medium 6 to the supply port 18a is ensured by the dead weight of the plasma medium 6 in the reservoir 20, the pressurizing device 50 is unnecessary, and the introduction port 21 opens upward with respect to the horizontal plane. It may be opened in a vacuum.

  The reservoir 20 has a filling port 22 for filling the plasma medium 6 into the space 20a. The filling port 22 communicates with the space 20 a from the outer surface of the reservoir 20 except for a plane joined to the center electrode 12. For example, as shown in FIG. 4, the filling port 22 is provided on the back surface 20 c of the reservoir 20. In this case, the filling port 22 is located on an extension line of the supply path 18 along the central axis Z. That is, when the inside of the reservoir 20 (that is, the space 20a) is viewed from the filling port 22, the inside of the supply path 18 along the central axis Z can be viewed.

  As shown in FIG. 4, the inner surface of the filling port 22 has a large-diameter portion 22 a and a small-diameter portion 22 b that are formed axially symmetrically and located concentrically. The large diameter portion 22a opens to the outer surface (for example, the back surface 20c) of the reservoir 20, and the small diameter portion 22b communicates between the large diameter portion 22a and the space 20a. When the enclosing member 24 described later is a male screw, a thread groove corresponding to the thread of the male screw is formed on the inner peripheral surface of the large diameter portion 22a.

  The inner diameter of the small diameter portion 22b is smaller than the inner diameter of the large diameter portion 22a. Accordingly, an annular step portion 22c is formed on the inner peripheral surface of the filling port 22 due to the difference in inner diameter between the large diameter portion 22a and the small diameter portion 22b. In other words, the large-diameter portion 22a and the small-diameter portion 22b are connected to the inner peripheral surface of the filling port 22 by the difference in inner diameter between the large-diameter portion 22a and the small-diameter portion 22b, and the annular surface facing the outside of the reservoir 20 is connected. A surface is formed. As will be described later, the contact surface 24c of the enclosing member 24 comes into contact with this surface (stepped portion 22c).

  An enclosure member 24 is attached (inserted) to the filling port 22. The enclosing member 24 seals the filling port 22 and prevents the molten plasma medium 6 from leaking from the filling port 22. That is, the enclosing member 24 is a so-called “plug”. The enclosing member 24 has a cylindrical portion 24a to be inserted into the filling port 22 and a head portion 24b to be gripped (held) during mounting (insertion). The diameter of the cylindrical portion 24a is the same as the diameter of the large diameter portion 22a. However, the “same” includes a tolerance that allows the cylindrical portion 24a to be inserted (slided) into the large-diameter portion 22a. When the enclosing member 24 is a male screw, a screw thread corresponding to the screw groove of the large diameter portion 22a is formed on the side surface of the cylindrical portion 24a.

  The enclosing member 24 has a head 24b provided at the rear end of the cylindrical portion 24a. The head portion 24b has a size larger than the diameter of the cylindrical portion 24a, and is formed in a shape that assists the screwing of the cylindrical portion 24a into the filling port 22. For example, the head 24b is formed in a hexagonal shape so that a tool such as a spanner can be fitted. As indicated by the dotted line in FIG. 4, the head 24b may be formed in a disk shape and fixed to the back surface 20c of the reservoir 20 with screws or bolts.

  A contact surface 24c is formed at the front end of the enclosing member 24 (that is, the front end or the end of the cylindrical portion 24a facing the space 20a). In a state where the sealing member 24 is completely attached to the filling port 22, the contact surface 24c contacts the stepped portion 22c over the entire circumferential direction of the stepped portion 22c. That is, the contact surface 24c seals the opening of the step portion 22c.

  The material of the enclosing member 24 is selected based on the same criteria as the selection of the material of the reservoir 20. That is, when lithium is assumed as the plasma medium 6, the material of the enclosing member 24 is copper (Cu). The material of the enclosing member 24 is not limited to copper, and may be any metal or alloy that has high durability (corrosion resistance) to the plasma medium 6 and low wettability of the plasma medium 6. The wettability of the metal or alloy is such that it cannot enter the gap between the contact surface 24c and the stepped portion 22c by its own repulsive force (surface tension). For example, when the plasma medium 6 is tin, stainless steel may be used as the material of the enclosing member 24. That is, the reservoir 20 and the sealing member 24 of the present embodiment are formed of a metal with low wettability of the plasma medium 6.

  Next, an electrical system in the plasma light source of this embodiment will be described. As shown in FIG. 3, the plasma light source includes a voltage applying device 30 connected to each coaxial electrode 10. The voltage application device 30 applies a discharge voltage having the same polarity to each coaxial electrode 10.

  The voltage application device 30 includes a high voltage power supply 32. The output side of the high voltage power supply 32 is connected to the center electrode 12 of the coaxial electrode 10 and applies a positive discharge voltage (for example, 5 kV) higher than the external electrode 14 corresponding to the center electrode 12. Therefore, when the external electrode 14 is grounded, the potential of the center electrode 12 is positive.

  As described above, a plurality of external electrodes 14 are provided around each center electrode 12. In order to obtain an ideal planar discharge 2b, it is necessary to generate a discharge between all the external electrodes 14 and the center electrode 12. Moreover, it is desirable that these discharges are spatially distributed at equal intervals around the center electrode 12. For this reason, each external electrode 14 has a surface facing the center electrode 12 as a curved surface to define a preferential discharge location. However, even if the discharge location is fixed and a laser beam 42 to be described later is applied to the supply port 18a of each center electrode 12 at the same time, the discharge between each external electrode 14 and the center electrode 12 can be generated strictly at the same time. In reality, there is a slight deviation in the timing of occurrence of each discharge. The discharge energy supplied from the high-voltage power supply 32 tends to be preferentially consumed with respect to the first generated discharge. In this case, it is difficult to generate a plurality of discharges substantially simultaneously.

  Therefore, the voltage application device 30 includes an energy storage circuit 34 that stores the discharge energy of the discharge voltage for each external electrode 14. For example, as shown in FIG. 2, the energy storage circuit 34 includes a plurality of capacitors C that individually connect the center electrode 12 and each external electrode 14. Each capacitor C has a capacitance that allows a discharge current of about 10 kA to flow at the peak of discharge, and is connected to each output side and each common side of the high-voltage power supply 32.

  Thus, by providing the capacitor C for storing discharge energy for each external electrode 14, it is possible to generate discharge in all the external electrodes 14. That is, it is possible to prevent a large amount of discharge energy from being consumed by the first generated discharge, and it is possible to obtain an ideal planar discharge 2b that occurs over the entire circumference of the center electrode 12.

  Furthermore, the voltage application device 30 may include a discharge current blocking circuit 36 that blocks the discharge current from returning. For example, as shown in FIG. 2, the discharge current blocking circuit 36 includes an inductor L that connects each external electrode 14 and the voltage application device 30 (specifically, the common side of the high-voltage power supply 32). Since the inductor L has a sufficiently high impedance with respect to the discharge current, the discharge current that has passed through the center electrode 12 and the external electrode 14 can be returned to the energy storage circuit 34 that is the generation source thereof. That is, in order to prevent the discharge energy accumulated in each capacitor C from being supplied to the external electrode 14 other than the external electrode 14 directly connected to the capacitor C, the distribution of discharge in the circumferential direction of the center electrode 12 is biased. Can be prevented.

  The plasma light source according to the present embodiment includes a laser device 40. The laser device 40 irradiates the surface of the center electrode 12 of each coaxial electrode 10 with a laser beam 42, thereby releasing the medium of the plasma 3 and generating the initial discharge 2a of the plasma 3. The laser device 40 is, for example, a YAG laser, and outputs a double wave of the fundamental wave as a short pulse laser beam 42 in order to perform ablation. The laser beam 42 is branched by an optical element such as a beam splitter (half mirror) and irradiated to the supply port 18 a of each center electrode 12. The beam diameter of the laser beam 42 at the supply port 18a is about several hundred μm. At the supply port 18a irradiated with the laser beam 42, the plasma medium 6 is discharged as neutral gas or ions by ablation of the laser beam 42.

  On the other hand, at the time of irradiation with the laser light 42, the discharge voltage by the voltage application device 30 has already been applied between the center electrode 12 and the external electrode 14 of each coaxial electrode 10. Therefore, when ablation occurs, an initial discharge 2a between the center electrode 12 and each external electrode 14 is induced, and a planar discharge 2b (see FIG. 1) is formed by this discharge.

  In addition, the generation | occurrence | production location of the initial discharge 2a may be restrict | limited to the irradiation area | region of the laser beam 42, and its vicinity. Therefore, it is preferable to irradiate a plurality of laser beams 42 at intervals along the circumferential direction of the central axis Z, and the number thereof is at least two.

This is based on an experimental result in which the generation region of the initial discharge 2a has an opening angle of 180 degrees or more with the axis of the center electrode 12 as a base point. An example of the experimental results is shown in FIGS. 5 (a) to 5 (c). FIG.
-(C) is measured with a high-speed camera having four CCDs. FIG. 5A to FIG. 5C are images showing changes over time in the discharge distribution between the center electrode and the external electrode to which a discharge voltage of 5 kV has been applied. FIG. 5A shows a state after 100 ns from the first discharge, FIG. 5B shows a state after 300 ns from the first discharge, and FIG. 5C shows a state after 500 ns from the first discharge. Is shown. The accumulation time (exposure time) in each image is 100 ns. White parts in the figure indicate that plasma is generated and light is emitted. In order to induce the first discharge, laser ablation is performed on one place of the insulator between the center electrode and the external electrode. However, such a change with time of discharge can be similarly obtained by laser ablation with respect to the holding portion 18 shown in FIG.

  As can be seen from the time changes shown in FIGS. 5 (a) to 5 (c), the discharge induced by ablation is generated despite the fact that the laser beam is irradiated only at one point. It can be confirmed that each of the light has been enlarged approximately 90 degrees clockwise and counterclockwise from the light irradiation point. As a result, it can be confirmed that a sufficient discharge is generated between each of the four external electrodes in the center and the right side of the photograph and the center electrode. That is, the discharge is not only between the external electrode and the center electrode closest to the place where the phenomenon that induces the discharge (laser ablation in FIGS. It also occurs between the external electrode located at the center and the center electrode. That is, by disposing a plurality of external electrodes to which discharge energy is applied around the center electrode, it is possible to expand the entire discharge without localizing the discharge in the circumferential direction of the center electrode.

  FIG. 6 shows the change over time of the discharge after laser ablation is performed on two places of the insulator sandwiching the center electrode, and corresponds to FIG. As shown in this figure, the initial discharge grows into a circularly distributed discharge (planar discharge 2b described later). In consideration of this result, it is desirable to irradiate the laser beam 42 at a rotationally symmetric position with respect to the center electrode 12 as the number of irradiated portions is smaller. Note that simultaneous irradiation with a plurality of laser beams can be easily achieved by forming a plurality of optical paths having optical path lengths using optical elements such as a beam splitter and a mirror.

  As described above, in the plasma light source of the present embodiment, a pair of coaxial electrodes 10 are provided in a vacuum chamber (not shown). The pair of coaxial electrodes 10 are disposed to face each other with the symmetry plane 1 in between. On the other hand, the inside of the vacuum chamber is maintained at a temperature and pressure suitable for generating the plasma 3. In addition, a discharge voltage having the same polarity is applied to each coaxial electrode 10 before discharge by the voltage application device 30.

  Next, preparation work for the plasma light source will be described. First, particles (small pieces) of the plasma medium 6 are prepared. For example, when the plasma medium 6 is lithium, a lump of lithium is cut into small pieces. The prepared particles or small pieces of the plasma medium 6 are put into the space 20 a of the reservoir 20 from the filling port 22 until the space 20 a is filled with the plasma medium 6. When the filling port 22 is provided in the back surface 20 c of the reservoir 20, a rod thinner than the inner diameter of the supply path 18 may be inserted from the filling port 22 and the plasma medium 6 may be pushed into the supply path 18. After the space 20 a is filled with the plasma medium 6, the sealing member 24 is attached to the filling port 22. At this time, the sealing member 24 is inserted until the contact surface 24 c of the sealing member 24 sufficiently contacts the stepped portion 22 c of the filling port 22. The filling operation of the plasma medium 6 is completed by the above procedure. When the plasma medium 6 is a material that is easily oxidized, such as lithium, the series of operations described above is performed in a container filled with an inert gas such as argon gas.

  Thereafter, the coaxial electrode 10 and the reservoir 20 are installed in a vacuum chamber (not shown), and the vacuum chamber is evacuated by a vacuum pump (not shown). After the pressure (vacuum degree) in the vacuum chamber reaches a desired value, the reservoir 20 is heated by the heater 23 and the plasma medium 6 in the reservoir 20 is melted.

  The enclosing member 24 and the reservoir 20 are formed of a metal (alloy) having high durability (corrosion resistance) and low wettability of the plasma medium 6, and the contact surface 24 c of the enclosing member 24 is a stepped portion 22 c of the filling port 22. Abut. Therefore, the melted plasma medium 6 flows (leaks) between the contact surface 24c of the sealing member 24 and the stepped portion 22c of the filling port 22 due to a repulsive force caused by its own surface tension and wettability. Can not. In order to prevent leakage of liquid, a sealing material such as a gasket or an O-ring is usually necessary. However, in this embodiment, such a sealing material and a structure for holding the sealing material are not necessary. That is, since the structure for enclosing the plasma medium 6 in the reservoir 20 is simple, a large space 20a in the reservoir 20 can be secured, and a large amount of the plasma medium 6 can be filled in the space 20a. Further, when the filling port 22 is provided on the extension line of the supply path 18, the plasma medium 6 can also be filled into the supply path 18. Since the total number (volume) of residual gas bubbles at the time of melting of the plasma medium 6 can be reduced, the number of bubbles flowing in the supply path 18 can also be reduced, and the waiting time until the plasma light source is in an operating state is reduced.

  The molten plasma medium 6 flows in the supply path 18 by the pressurization by the pressurizing device 50 or by the dead weight of the plasma medium 6 in the reservoir 20. As described above, the inner diameter of the vent hole 19 is smaller than the inner diameter of any position in the supply path 18 including the supply port 18a, and the plasma medium 6 cannot enter the vent hole 19 due to its surface tension. Is set to Therefore, the melted plasma medium 6 reaches the supply port 18a without entering the vent hole 19.

  When the plasma medium 6 is melted, the gas remaining in the gaps between the granular plasma media 6 becomes bubbles. As shown in FIG. 7, the residual gas bubble B also moves toward the supply port 18 a as the plasma medium 6 flows. However, the bubble B reaches the vent hole 19 before reaching the supply port 18a. As described above, the inner diameter of the vent hole 19 is set to a value that prevents the plasma medium 6 from entering the vent hole 19 due to its surface tension. That is, only gas can flow through the vent hole 19. Therefore, the residual gas bubble B flows into the vent hole 19 and is then discharged to the outside of the center electrode 12. That is, by providing the vent hole 19, it is possible to prevent the residual gas bubbles B from being ejected from the supply port 18a. In other words, the residual gas in the supply path for supplying the liquid plasma medium can be appropriately discharged. This also contributes to a reduction in waiting time until the plasma light source is in an operating state.

  When the number of bubbles is large or the volume of the bubbles is large, there is a possibility that the bubbles cannot be discharged with only one vent hole 19 due to the conductance. In that case, a plurality of vent holes 19 are provided. Thereby, sufficient discharge performance can be secured.

  When the plasma medium 6 is melted and the residual gas bubbles are sufficiently discharged, the plasma light source is ready for operation. Hereinafter, main operations of the plasma light source will be described.

  In a state where a discharge voltage is applied to each coaxial electrode 10 in the voltage application device 30, the laser beam 42 is simultaneously irradiated to the supply port 18 a of each coaxial electrode 10. By this ablation by irradiation, the plasma medium 6 remaining in the supply port 18a is discharged as neutral gas or ions. In addition, this ablation generates an initial discharge 2a between the center electrode 12 and each external electrode 14 (see FIG. 1).

  Thereafter, the initial discharge 2a is distributed over the entire circumference of the center electrode 12 while taking in the plasma medium 6 released by ablation, and grows into a planar discharge 2b (see FIG. 1). The planar discharge 2b moves in the direction discharged from the coaxial electrode 10 by the self magnetic field (that is, the direction toward the symmetry plane 1). The planar discharge 2b at this time is distributed in a substantially annular shape when viewed from the central axis Z. The sheet discharge is a sheet discharge current that spreads two-dimensionally and is also called a current sheet or a plasma sheet.

  When the planar discharge 2b reaches the tip of the coaxial electrode 10, the starting point of the discharge current of the planar discharge 2b is forcibly shifted from the circumferential side surface of the center electrode 12 to the tip 12a (see FIG. 1). In other words, the discharge current flows intensively from the tip 12a. Due to the pinch effect due to the current concentration, the current density around the tip 12a rapidly increases, and the plasma medium 6 around the tip 12a sandwiched between the pair of planar discharges 2b becomes high density and high temperature.

  Further, since this phenomenon proceeds at each coaxial electrode 10 across the symmetry plane 1, the plasma medium 6 is pushed out from one coaxial electrode 10 toward the other coaxial electrode 10. As a result, the plasma medium 6 receives electromagnetic pressure from both directions along the central axis Z and moves to an intermediate position where the coaxial electrodes 10 face each other (that is, the symmetry plane 1 of the center electrode 12). A single plasma 3 having a component as a component is formed (see FIG. 1).

  As described above, while the planar discharge 2b is generated, the current of each planar discharge 2b is concentrated on the tip 12a of each center electrode 12. Accordingly, a pinch effect toward the central axis Z acts on the plasma 3 around the tip 12a, and the density and temperature of the plasma 3 increase. That is, ionization of the plasma medium 6 proceeds. As a result, plasma light 8 including extreme ultraviolet light is emitted from the plasma 3. In this state, the voltage application device 30 continues to supply energy corresponding to the light emission energy of the plasma 3. With this energy supply, the plasma light 8 can be generated for a long time with high energy conversion efficiency.

  In addition, this invention is not limited to the above-mentioned embodiment, is shown by description of a claim, and also includes all the changes within the meaning and range equivalent to description of a claim.

  DESCRIPTION OF SYMBOLS 1 ... Symmetry surface, 2a ... Initial discharge, 2b ... Planar discharge, 3 ... Plasma, 6 ... Plasma medium, 8 ... Plasma light, 10 ... Coaxial electrode, 12 ... Center electrode, 12a ... Tip part, 12b ... Side surface, DESCRIPTION OF SYMBOLS 14 ... External electrode 16 ... Insulator, 18 ... Supply path, 18a ... Supply port, 19 ... Vent hole, 20 ... Reservoir, 21 ... Inlet port, 22 ... Filling port, 22a ... Large diameter part, 22b ... Small diameter part, 22c ... Step part, 23 ... Heater, 24 ... Encapsulating member, 24a ... Cylindrical part, 24b ... Head, 24c ... Abutting surface, 30 ... Voltage application device, 32 ... High voltage power supply, 34 ... Energy storage circuit, 36 ... Discharge Current blocking circuit, 40 ... laser device, 42 ... laser light

Claims (3)

A rod-shaped center electrode extending on a single axis and an external electrode provided so as to surround the outer periphery of the center electrode are arranged opposite to each other across a plane of symmetry, and generates plasma that emits extreme ultraviolet light A pair of coaxial electrodes for confining the plasma;
A reservoir provided individually for each coaxial electrode, supporting the center electrode, and having a space for storing the plasma medium and a filling port communicating with the space;
An enclosure member for sealing the filling port of the reservoir;
The center electrode of each of the coaxial electrodes communicates with the reservoir, and opens on the side surface of the center electrode as a supply port of the medium at a position irradiated with laser light for ablating the medium. Has a supply channel,
The inner surface of the filling port in the reservoir has an annular stepped portion,
The tip of the enclosing member abuts on the stepped portion over the entire circumferential direction of the stepped portion,
The electrode structure of the plasma light source, wherein the reservoir and the enclosing member are made of a metal or alloy having low wettability of the medium.   2. The electrode structure of a plasma light source according to claim 1, wherein the filling port is provided on a back surface of the reservoir and is located on an extension line of the supply path along the axis.   3. The plasma light source electrode structure according to claim 1, wherein the reservoir and the enclosing member are made of copper or stainless steel.